NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 5

by Alyson Lanciki | Jul 26, 2021 | Analysis, Fundamentals

The history of polyurethanes

In 1937, the German chemist Dr. Otto Bayer (1902–1982) invented the versatile class of plastics we call polyurethanes. Polyurethanes are available in myriad forms—they are used in numerous products, from coatings and adhesives to shoe soles, mattresses, and foam insulation. Despite the variety in their characteristics, the underlying chemistry of these different forms is strikingly similar.

During World War II, the use of polyurethanes became popular as a replacement for rubber, which at the time was expensive and hard to obtain. Around the 1950s, polyurethanes began to be used in adhesives, elastomers, rigid foams, and flexible cushioning foams such as those used today.

Wallace Carothers, inventor of polyamide.

Dr. Otto Bayer was credited with inventing polyurethanes in 1937.

Nowadays, a life without polyurethane is difficult to imagine, as you can easily find it everywhere around you.

How is polyurethane created?

Polyurethanes are formed by reacting polyols (i.e., alcohols containing more than two reactive hydroxyl groups in each molecule) with di-isocyanates or polymeric isocyanates. Suitable catalysts and additives are used wherever necessary. Since both a variety of di-isocyanates and a wide range of polyols can be used to produce polyurethane, a large spectrum of polyurethane materials can be produced to meet the specific requirements for different applications. Polyurethanes can appear in a variety of forms including rigid foams, flexible foams, specialty adhesives, chemical-resistant coatings, sealants, and elastomers.

Figure 1. Molecular structures of isocyanates, polyols, and polyurethane.

Physical and chemical properties of polyurethanes

The properties of polyurethanes are highly dependent on their production process. When the polyol chain (Figure 1) is long and flexible, the final product will be soft and elastic. On the other hand, if the extent of cross-linking is very high, the final polyurethane product will be tough and rigid. The cross-linked structure of polyurethanes generally consists of three-dimensional networks which result in very high molecular weights. This structure also accounts for the thermosetting nature of the polymer since polyurethane typically does not soften or melt when exposed to heat.

One of the most popular forms of polyurethane is foam. This form is created by facilitating the production of carbon dioxide gas during the urethane polymerization process.

Typical applications of polyurethane

The primary application of polyurethane is in the production of foams (rigid and flexible). Other important applications and uses of polyurethane are listed below.

Low-density, flexible polyurethane foams are widely used in mattresses and automobile seats.
Bathroom and kitchen sponges are commonly made from polyurethane. It is also used in the manufacturing process of seat cushions and couches.
Polyurethane is also used to produce textiles used in some clothing and upholstery.
Due to its good insulating properties, polyurethane materials are commonly used in construction work.
Polyurethane moldings are also used in columns and door frames.
Flexible polyurethane is used in the manufacture of partially elastic straps and bands.
The low-density elastomers of polyurethane are widely used in the footwear industry.

In Table 1 a variety of polyurethane properties are compared to other conventional materials like rubber, metal, and plastic.

Table 1. Polyurethane in comparison with rubber, metal, and plastic.

PU vs. Rubber	PU vs. Metal	PU vs. Plastic
High abrasion resistance	Lightweight	High impact resistance
High cut and tear resistance	Noise reduction	Elastic memory
Superior load bearing	Abrasion resistance	Abrasion resistance
Thick section molding	Less expensive fabrication	Noise reduction
Colorability	Corrosion resistance	Variable coefficient of friction
Oil resistance	Resilience	Resilience
Ozone resistance	Impact resistance	Thick section molding
Radiation resistance	Flexibility	Lower cost tooling
Broader hardness range	Easily moldable	Low temperature resistance
Castable nature	Non-conductive	Cold flow resistance
Low pressure tooling	Non-sparking	Radiation resistance

Near-infrared spectroscopy as a tool to assess the quality of polyurethanes

Near-infrared spectroscopy (NIRS) has been an established method for both fast and reliable quality control within the polyurethane industry for more than 30 years. However, many companies still do not consistently consider the implementation of NIRS in their QA/QC labs. The reasons could be either limited experience regarding application possibilities or a general hesitation about implementing new methods.

There are several advantages of using NIRS over other conventional analytical technologies. For one, NIRS is able to measure multiple parameters in just 30 seconds without any sample preparation! The non-invasive light-matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes it an excellent method for the determination of both property types.

In the remainder of this post, a short overview of polyurethane applications is presented, followed by available turnkey solutions for polyurethane analysis developed according the NIRS implementations guidelines of ASTM E1655-17.

Did you miss the first parts in this series? Find them here!

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 1

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 2

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 3

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 4

For more detailed information about NIRS as a secondary technique, read our previous blog posts on this subject.

Benefits of NIRS: Part 1

Benefits of NIRS: Part 2

Benefits of NIRS: Part 3

Benefits of NIRS: Part 4

Applications and parameters for polyurethanes with NIRS

When producing different types of polyurethanes, it is important to check certain parameters to guarantee the quality of the finished products. Typical parameters include hydroxyl number, acid number, moisture, and color in polyols as well as the NCO (isocyanates) content, (total) acid number, and moisture content in polyurethanes. The most relevant applications for NIRS analysis in polyurethane production are listed later in this article in in Table 2.

Where can NIRS be used in the polyurethane production process?

Figure 2 shows the individual steps from plastic producer via plastic compounder and plastic converter to plastic parts and foam producer.

Figure 2. Illustration of the production chain for polyurethanes.

Easy implementation of NIR spectroscopy for plastic producers

Metrohm has extensive expertise with analysis of polyamides and offers a turnkey solution in the form of the DS2500 Polyol Analyzer. This instrument is a ready-to-use solution for the determination of multiple quality parameters in polyols and polyurethanes. For the analysis of polyurethane pellets and parts, the Metrohm DS2500 Solid Analyzer is recommended.

Figure 3. Turnkey solution for polyurethane analysis with the Metrohm DS2500 Polyol Analyzer.

Learn more about the possibilities of polymer analysis with Metrohm DS2500 Analyzers in our free brochure.

DS2500 Analyzer – Boosting efficiency in the QC laboratory with Near-Infrared Spectroscopy (NIRS)

Application example:

Pre-calibrations and starter model for the PU industry on the DS2500 Polyol Analyzer

The determination of the parameters listed below in Table 2 is a lengthy and challenging process with conventional laboratory methods. To measure them all, several different techniques are required which takes a significant amount of time, not only to analyze the sample, but also for the instrument management and upkeep.

Table 2. Primary method vs. NIRS for the determination of various quality parameters in PU samples.

Parameter	Primary method	Time to result (primary method)	Relevant NIRS Application Notes	NIRS benefits
Hydroxyl number in Polyols	Titration	90 min. preparation + 1 min. Viscometer	AN-NIR-068 AN-NIR-065 AN-NIR-035 AN-NIR-007	All three parameters are measured simultaneously within a minute, without sample preparation or the need of any chemical reagents
NCO (Isocyanate) content in PU	HPLC	20 min. preparation + 20 min. HPLC
Moisture content	Karl Fischer Titration	25 min. preparation + 5 min. KF Titration

The NIRS prediction models created for polyols are based on a large collection of real product spectra and are developed in accordance with ASTM E1655-17 Standard practices for Infrared Multivariate Quantitative Analysis. For more detailed information on this topic, download the free white paper.

Near-Infrared Spectroscopy: Quantitative analysis according to ASTM E1655

To learn more about pre-calibrations for polyols, download our brochure and visit our website.

Brochure: Quality control of polyols – Fast results with NIR pre-calibrations

Pre-calibration for the analysis of polyols

One example of a dedicated ASTM standard referring to NIRS is ASTM D6342-12 Standard Practice for Polyurethane Raw Materials: Determining Hydroxyl Number of Polyols by Near Infrared (NIR) Spectroscopy. The following application example demonstrates that the DS2500 Polyol Analyzer operating in the visible and near-infrared spectral region (Vis-NIR) provides a cost-efficient and fast solution for the determination of the hydroxyl number in polyols and the NCO (isocyanate) content in polyurethanes. With no sample preparation or chemicals required, Vis-NIR spectroscopy allows analysis of all three quality parameters listed in Table 2 in less than a minute. The results are shown in Figure 4 and Figure 5.

Figure 4. Turnkey solution for determination of hydroxyl number in polyols using the Metrohm DS2500 Polyol Analyzer. A: Sampling and analysis of polyols. B: NIRS results compared to a primary laboratory method along with the Figures of Merit (FOM).

Figure 5. Turnkey solution for determination of NCO content (Isocyanates) in polyurethane using the Metrohm DS2500 Polyol Analyzer. A: Sampling and analysis of polyurethane. B: NIRS results compared to a primary laboratory method along with the Figures of Merit (FOM).

This application example demonstrates that NIR spectroscopy is excellently suited for the analysis of multiple parameters in polyols and polyurethanes in less than one minute without sample preparation or using any chemical reagents. Visit our website to learn more about our variety of analytical solutions for the polymer industry!

For more information

About spectroscopy solutions provided by Metrohm, visit our website!

We offer NIRS for lab, NIRS for process, as well as Raman solutions

Visit our website

Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.

Fast and fundamental: influences on reliable electrochemical measurements

by Alyson Lanciki | Jul 19, 2021 | Analysis, Evaluating results, Fundamentals

The ultimate goal of any researcher is to contribute to the progress of society by pioneering exploration beyond the known limits. Depending on the research type and application field, one way to fulfill this is to collect reliable experimental data on rapidly occurring processes (less than 1 ms).

Having insight into the fundamentals of these reaction mechanisms can ultimately lead to the discovery of new materials or the improvement of current solutions. In electrochemical research, reaction mechanisms and intermediates are investigated by measuring the kinetics and dynamics of the electrochemical processes happening at the surface of the electrode on a sub-ms timescale.

This article provides a short overview of the factors that have a direct influence on fast and ultra-fast electrochemical measurements from an experimental setup perspective.

Considering the following factors in the experimental design and execution is the first condition to obtain reliable experimental results for such measurements.

Additional challenges which researchers must be aware of when experimenting with «transient electrochemistry», i.e. doing electrochemical measurements at very low time scales, is presented in the featured article from E. Maisonhaute et al. [1].

Transient electrochemistry: beyond simply temporal resolution

Main factors that influence the reliability of fast electrochemical experimental results

The primary components of an electrochemical experimental setup are:

The electrochemical cell including the electrodes and electrolyte
The electrochemical instrument, i.e., the potentiostat/galvanostat (PGSTAT)

To perform reliable electrochemical experiments in general, and fast electrochemical measurements in particular, the specifications of the complete work system must be considered and the optimal settings must be used for all of the individual parts of the experimental setup.

Time constant of the electrochemical cell

The electrochemical cell and its specifications must be taken into account as it is an important element of the experimental setup.

Transient electrochemical experiments are not meaningful unless the cell time constant is small relative to the timescale of the measurement, regardless of the high-frequency characteristics of the control circuitry.

The cell time constant R_uC_dl (s) depends directly on the uncompensated resistance R_u(Ω) (i.e. the resistance of the electrolyte between the reference and the working electrode) and the double-layer capacitance C_dl (F) of the electrode [2].

As a consequence, when the potential is stepped or scanned rapidly, the true measured potential E_true(V) lags behind the applied potential E_appl (V), according to the following equation:

Where R_uC_dl (s) is the time constant of the cell and t (s) is the time at which the measurement is taken.

Figure 1. Theoretical and true waveform applied to a real electrochemical cell [1].

For fast scan rates (i.e. when 𝑡 is much smaller than R_uC_dl ), the exponential term approaches 1 and significant errors in 𝐸_true with respect to 𝐸_appl can arise. For slow scan rates (i.e. when 𝑡 is much larger than R_uC_dl), the exponential approaches 0 and the errors become negligible.

The time constant of the cell can be reduced in three ways:

Reduce R_u via increasing the conductivity of the electrolyte by either increasing concentration of supporting electrolyte or decreasing viscosity
Reduce the size of the working electrode (e.g., by using microelectrodes) so that C_dl will be minimized
Move the reference electrode as close as possible to the working electrode (e.g., by using a Luggin capillary) so that R_uwill be minimized

The electrochemical instrument: potentiostat/galvanostat (PGSTAT)

The potentiostat/galvanostat (PGSTAT) is used to accurately control the applied signal (potential or current) and measure the response (current or potential, respectively) from the electrochemical cell. The accurate control of the applied signals is achieved by using a control loop (or feedback loop) circuit.

Learn more about the working principle of the potentiostat/galvanostat in our free Application Note.

Basic overview of the working principle of a potentiostat/galvanostat (PGSTAT) – electrochemical cell setup

When fast electrochemical measurements are executed, the following specifications will have a direct influence on the results and must be considered.

Bandwidth of the control loop of the PGSTAT

In general terms, bandwidth can be described as the parameter that defines how fast the instrument is able to react to any changes in the signal.

In electrochemical terms, the bandwidth is the frequency beyond which the performance of the system is degraded.

The bandwidth of the control loop of the PGSTAT (i.e. bandwidth of the instrument) indicates how fast the applied signal is controlled through the feedback loop.

Higher bandwidth means that the instrument uses a faster control loop (faster feedback). As a result, the applied signal will reach the desired set point faster, and in ideal circumstances the output signal will be identical to the theoretical waveform. However, depending on the properties of the electrochemical cell connected to the instrument, the applied signal might overshoot. In extreme cases, the instrument feedback loop might get out of control causing the potentiostat to oscillate. This is more likely when high-capacitance electrochemical cells are connected to the PGSTAT.

When a Lower bandwidth is used, the overall stability of the PGSTAT increases by reducing the speed of the control loop. In this case, the consequence is that at very high measurement speeds, the output of the applied signal may be slightly less accurate due to a slower slew rate. Nevertheless, when measuring fast transients is not within the scope of the experiment, using the instrument with a lower bandwidth setting is recommended for highly accurate experimental results.

Figure 2. Schematic representation of the applied signal when Low bandwidth (Low speed) and High bandwidth (High speed) settings are used compared with the theoretical response.

Therefore, it is important to choose the control loop bandwidth settings according to the type of the measurement. For ultra-high speed measurements, a higher bandwidth setting must be used with the following considerations:

The higher the bandwidth, the higher the noise and the probability that the control loop will go out of control and oscillate.
When working with a High bandwidth setting, it is necessary to pay special attention and use adequate cell shielding and electrode connectors. The use of a Faraday cage is recommended in these cases.
The use of a high impedance reference electrode (RE) (e.g., double junction reference electrode, a salt bridge with frit) in combination with a High bandwidth of the control loop might lead to instability of the PGSTAT and even to oscillations.

Bandwidth of the current sensor (current range)

The measurement of the current response of an electrochemical cell (in potentiostatic mode) and the control of the applied current value (in galvanostatic mode) is executed with specially designed current sensors. In order to achieve the best sensitivity and resolution for the measurement, individual current sensors are used depending on the magnitude of the measured (or applied) current.

Each current sensor circuit (which corresponds to a current range) has a specific bandwidth or response time. Therefore for the most accurate results (especially important for fast, time resolved experiments), the current range must be selected so that the bandwidth of the current sensor will not be the limiting factor for the time response (speed) of the measurement.

In general, the lower the measured currents, the lower the bandwidth of the current sensor.

Data sampling interval vs the timescale of the investigated transient signal

The measured electrochemical response can have a complex shape with components at many frequencies. The highest frequency component of the measured or applied signal determines the bandwidth of that signal. The bandwidth of the signal should not be higher than the bandwidth of the measuring device.

If the highest frequency component of the signal is f_S_IGNAL, then according to the Nyquist Theorem [3] the sampling rate f_SAMPLE must be at least 2 f_SIGNAL (i.e. two times higher than the highest frequency component of the signal).

Figure 3. Effect of the sampling frequency of an ideal sinusoidal signal [3]. Shown here are the theoretical signal (dashed line), sample points, and resulting measured signal (orange line).

In other words, the data sampling interval must be lower than the timescale in which the time resolved (transient) measurement from the investigated electrochemical process is expected to occur. There is a practical correlation between the sampling interval and instrument bandwidth. When the sampling interval is:

higher than 100 μs: the 10 kHz (High Stability) bandwidth should be selected.
between 10–100 μs: the 100 kHz (Fast) bandwidth should be selected.
smaller than 10 μs: the 1 MHz bandwidth (Ultra-Fast) should be selected.

Summary

To measure reliable experimental data, all elements of the experimental setup must be considered with their own specifications and limitations. The overview above highlights the main factors and parameters which can have a direct influence on fast electrochemical measurements.

Fast measurements start here!

Visit our website to learn more about the variety of potentiostats/galvanostats from Metrohm Autolab.

Check them out here...

References

[1] Maisonhaute, E.; et al. Transient electrochemistry: beyond simply temporal resolution, Chem.Commun., 2016, 52, 251—263. doi:10.1039/C5CC07953E

[2] Bard, A.J.; Faulkner, L.R. Electrochemical Methods: Fundamentals and Applications, New York: Wiley, 2001, 2nd ed. Russian Journal of Electrochemistry, 2002, 38, 1364–1365. doi:10.1023/A:1021637209564

[3] Keim, R. The Nyquist–Shannon Theorem: Understanding Sampled Systems. All About Circuits, May 26, 2020. https://www.allaboutcircuits.com/technical-articles/nyquist-shannon-theorem-understanding-sampled-systems/

Post written by Dr. Iosif Fromondi, Product Manager and Head of Marketing and Sales Support at Metrohm Autolab, Utrecht, The Netherlands.

Nonaqueous acid-base titrations – Common mistakes and how to avoid them

by Iris Kalkman | Jul 12, 2021 | Analysis, Evaluating results, Fundamentals

Nonaqueous acid-base titrations are widely used in several industries, including the petrochemical and pharmaceutical sectors. Whether you are determining the acid or base number (AN or BN) in oils or fats, titrating substances that are insoluble in water, or quantifying products with different strengths of acidity or alkalinity separately, nonaqueous acid-base titration is the method of choice.

If you already have some experience performing nonaqueous acid-base titrations, you may remember that there are several challenges to overcome in comparison to aqueous acid-base titrations.

In this blog post, I would like to cover some of the most typical issues that could pop up during nonaqueous acid-base titrations and discuss how to best avoid them. An important point to note is that there is no single solution regarding how to perform any nonaqueous acid-base titration correctly. The right procedure depends highly on the solvent and titrant used.

To jump directly to a specific topic, click on the corresponding link below:

What is a nonaqueous acid-base titration?
Electrostatic effects
Blocked diaphragm
Choice of electrolyte and storage solution
Checking the electrode according to ASTM D664
Proper rinsing and cleaning
Conditioning the glass membrane
Maintenance of burets
Thermometric titration as an alternative
Summary

What is a nonaqueous acid-base titration?

Before discussing nonaqueous titrations, first let’s talk a little bit about aqueous acid-base titrations.

Here, a sample is dissolved in water, and depending of the nature of the sample (whether it is acidic or basic) a titration is performed either using aqueous base or aqueous acid as titrant. For indication, a glass pH electrode is used.

However, sometimes due to the nature of the sample, aqueous titration is not possible. Nonaqueous acid-base titration is used when:

the substance of interest is not soluble in water
samples are fats or oils
components of mixtures of acids or bases have to be determined separately by titration

In these cases, a suitable organic solvent is used to dissolve the sample instead of water. The solvent:

should dissolve the sample and not react with it
permits the determination of components in a mixture
if possible, should not be toxic

The solvents that are most often used include ethanol, methanol, isopropanol, toluene, and glacial acetic acid (or a mixture of these). Titrants are not prepared with water but rather in solvent. Frequently used nonaqueous basic titrants are potassium hydroxide in isopropyl alcohol or sodium hydroxide in ethanol, and a common nonaqueous acidic titrant is perchloric acid in glacial acetic acid.

Due to the nature of nonaqueous solvents, they are normally poor conductors and do not buffer well. This makes indication a bit challenging because the electrode must be suitable for such sample types. Therefore, Metrohm offers the Solvotrode which is developed specifically for nonaqueous titrations.

This pH electrode offers the following advantages over a standard pH electrode:

Large membrane surface and a small membrane resistance for accurate reading, also in poorly buffered solutions
A flexible ground-joint diaphragm which can easily be cleaned even when contaminated with oily or sticky samples, additionally it offers a symmetrical outflow for outstanding reproducibility
The electrode is shielded and is therefore less sensitive to electrostatic interferences
It can be used with any nonaqueous electrolyte such as lithium chloride in ethanol

Solvotrode from Metrohm

In the following sections I will discuss the most common mistakes when performing potentiometric nonaqueous acid-base titrations and how you can avoid them.

Electrostatic effects

The influence of electrostatic effects during analysis is normally negligible. However, maybe you have once seen a curve like the one below which looks relatively normal until suddenly a spike occurs.

Figure 1. Titration curve with a spike which might have occurred from an electrostatic interference.

This is then an indication of an electrostatic effect. However, where does it come from and how can we overcome this?

Electrostatic charge can be generated from many sources, such as friction. For example, while walking across a surface you will generate an electrostatic charge which will be stored in your body. You have probably touched the doorknob after walking across a carpeted space in your socks and obtained a small electric shock—this is the discharge of built up electrostatic charge. If we now assume that you are electrostatically charged and then you approach an electrode that is currently measuring (in use), this will result in a spike (Figure 1). Therefore, it is essential to make sure that you are either properly discharged or that you do not approach the electrode during measurement. You can avoid this issue by wearing the appropriate clothes. ESD (electrostatic discharge) clothes and shoes are mostly recommended when performing nonaqueous titrations.

Blocked diaphragm

A blocked diaphragm is another point which occurs more regularly during nonaqueous titrations. Due to the oily and sticky sample, you might have seen that the electrode diaphragm is clogged and cannot be opened anymore. What should you do then?

In most cases, you can place the electrode in a beaker of warm water overnight. This treatment often helps to loosen the diaphragm. To completely prevent the diaphragm from clogging, a Solvotrode with easyClean technology should be used. With this electrode, electrolyte is released by pressing the head ensuring that the diaphragm is not blocked.

Solvotrode easyClean

Choice of electrolyte and storage solution

We recommend two types of electrolyte for nonaqueous titrations.

For titrations with alkaline titrants: tetraethylammonium bromide c(TEABr) = 0.4 mol/L in ethylene glycol

For titrations with acidic titrants: lithium chloride c(LiCl) = 2 mol/L in ethanol

Please make sure to store the electrode in the same electrolyte with which it is filled.

Checking the electrode according to ASTM D664

To check whether the Solvotrode is still in good working condition, perform a test according to ASTM D664 using aqueous buffer solutions of pH 4 and 7. The procedure is as follows:

Measure the potential of buffer pH 4.0 while stirring and note the value after 1 minute
Remove the electrode and rinse it well with deionized water
Measure the potential of buffer pH 7.0 while stirring and note the value after 1 minute
Calculate the mV difference between the reading of buffers 4.0 and 7.0
The difference must be larger than 162 mV (20–25 °C) to indicate an electrode in good shape

If the measured potential difference is less than 162 mV, the electrode requires maintenance. Lift the flexible sleeve of the ground-joint diaphragm to let some electrolyte flow out. Repeat the measurement according to the steps above. If the value is still less than 162 mV, clean the electrode or replace it.

Proper rinsing and cleaning

Proper rinsing is essential if you want to obtain reliable results. Otherwise, the curve might flatten and the equivalence points are no longer recognizable. Figure 2 illustrates this phenomenon well.

Figure 2. Different determinations according to ASTM D664. With time, the start potential of the curves shifts which indicates an unsuitable cleaning procedure.

The sample is the same, however, you see that the equivalence point and starting potential begin to shift and the curves become flatter. This indicates an improper cleaning procedure between measurements. The corresponding electrode is shown in Figure 3.

Figure 3. Appearance of the electrode used in Figure 2 after five measurements.

This electrode was certainly not cleaned properly! Anyone who performs a nonaqueous titration must consider which solvent might best dissolve the residue—this is not an issue that other analysts can easily solve due to the nature of each individual sample. However, do not ignore an electrode with such an appearance.

Conditioning the glass membrane correctly

As you may remember from our previous blog post about pH measurement, it is essential that the hydration layer of the glass membrane stays intact. Nonaqueous solvents dehydrate the glass membrane rather quickly. A change in the hydration layer can have an impact on the measured potential, therefore it is important that the hydration layer is always in the same state before starting a titration to achieve the most reproducible results.

«Analyze This»: Avoiding the most common mistakes in pH measurement

This can be established with a conditioning step of the glass membrane to rebuild the hydration layer. However, if the solvent is able to remove the hydration layer faster than it takes to perform a titration, this can lead to ghost equivalence points. Therefore, the electrode should be completely dehydrated and kept like this for all further titrations.

	Polar solvents (e.g., ethanol, acetone, isopropyl alcohol, or mixtures with toluene)	Water-free solvents (e.g., dimethylformamide, acetonitrile, acetic anhydride, or mixtures of these)
Preparation of electrode	Store only the pH membrane (not the diaphragm) in deionized water overnight to build up a proper hydration layer. Lift the flexible sleeve to allow some electrolyte to flow out.	Dehydrate the pH membrane by placing only the pH membrane (not the diaphragm) in the solvent you will use afterwards for titration. Lift the flexible sleeve to allow some electrolyte to flow out.
Conditioning of glass membrane	Place the pH membrane (bulb only) into deionized water for 1 minute.	Place the pH membrane (bulb only) into the corresponding solvent for 1 minute.
Rinsing procedure	Rinse electrode with 50–70% ethanol. If this does not help, use a suitable solvent to rinse the electrode and then clean afterwards with 50–70% ethanol.	Rinse electrode with glacial acetic acid. If this does not help, use a suitable solvent to rinse the electrode and then clean afterwards with glacial acetic acid.
Remarks	Make sure to always keep the bulb of the electrode in deionized water for the same time duration, otherwise the thickness of the hydrated layer (and therefore the response) may vary.	Avoid any contact of the electrode with water as this can induce a reaction with the solvent causing ghost equivalence points and irreproducible results.

Maintenance of burets

It is not only the electrode that needs some special attention when performing nonaqueous titrations, but also the electrical buret. Some special maintenance is required since alkaline nonaqueous titrants are especially aggressive and they tend to crystallize, therefore leakage of the buret is likely.

The buret must be maintained on a regular basis according to the manufacturer’s instructions. Metrohm recommends the following procedure:

For shorter titration breaks, it is recommended to refill the cylinder with titrant (especially with OMNIS)
Clean the buret with deionized water at the end of the day
Lubricate the cylinder unit on the centering tube and on the cylinder disc

Also check the corresponding manual of the buret. The most important points are mentioned there which will lead to a longer working life of the buret.

Thermometric titration as an alternative

One alternative to using potentiometric nonaqueous acid-base titration is thermometric titration (TET), depending on the sample and analyte to be measured. Thermometric titration monitors the endothermic or exothermic reaction of a sample with the titrant using a very sensitive thermistor.

The benefit of TET over potentiometric titration is clearly the maintenance-free sensor which does not require any conditioning nor electrolyte refilling. More information about thermometric titration can be found in our previous blog posts below.

«Analyze This»: Thermometric titration – the missing piece of the puzzle

«Analyze This»: Fast determination of acid and base number by thermometric titration

«Analyze This»: Multiparameter analysis in fertilizers by thermometric titration

Summary

Hopefully this article has provided you with information about the main problems encountered during nonaqueous titrations. First, make sure that all electrostatic influences are eliminated. This will save a significant amount of troubleshooting. Then prepare and treat your electrode correctly before, during, and after titration. Make sure to condition the electrode right before your first measurement!

Of special importance here is the solvent you plan to use. If it is a polar solvent, the electrode should be conditioned in deionized water. If nonpolar solvents like acetic anhydride are used, the electrode should be dehydrated first. Between measurements, the electrode should be cleaned with a suitable solvent and the diaphragm should be opened on occasion.

Last but not least, take care of your buret. Maintain it regularly and replace it whenever necessary. With this advice, performing nonaqueous titrations should be a breeze!

For more information

about nonaqueous titrations, download our monograph:

Nonaqueous titration of acids and bases with potentiometric endpoint indication

Click here!

Post written by Iris Kalkman (Product Specialist Titration at Metrohm International Headquarters, Herisau, Switzerland) and Dr. Sabrina Gschwind (Head of R&D at Metroglas, Affoltern, Switzerland).

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 4

by Alyson Lanciki | Jul 5, 2021 | Analysis, Fundamentals

Polyamide (Nylon): A brief introduction

Wallace Carothers (1896–1937), the creator of polyamide.

Polyamide, more commonly known as Nylon, was first synthesized by Wallace Hume Carothers, an American organic chemist working for the DuPont chemical company. In 1935, he developed the formula known as PA66, or Nylon 66.

Just a few years later in 1938, Paul Schlack, a German chemist working at IG Farben, developed PA6 (also known as Nylon 6), a different molecule based on the organic compound caprolactam. Both types of polyamides are well-suited for many kinds of applications. The use of PA6 or PA66 depends on the technical requirements needed as well as the economical constraints.

The two most widely used polyamides are by far PA66 and PA6. These polyamides are most often manufactured into fibers for the textile industry or blown into films used for the packaging industry. Polyamides are also used to produce parts for numerous industries.

Polyamides with the highest performances are PPA (Polyphthalamide or high-performance polyamide) and PA46. Polyamides with these qualities are often used as a replacement for metal materials or for very specific applications where the polymer is exposed to extreme conditions, e.g. automotive structural parts or safety helmets.

Differences between Polyamide 6 (PA6 / Nylon 6) and Polyamide 66 (PA66 / Nylon 66)

Polyamide 6 (PA6) is also known as Nylon 6 or Polycaprolactam. It is one of the most commonly used compounds in the polyamide family. PA6 is synthesized via the ring-opening polymerization of caprolactam.

Figure 1. Molecular structure of caprolactam.

Figure 2. Molecular structure of Polyamide 6.

Polyamide 66 (PA66), also known as Nylon 66, is one of the most popular thermoplastics for engineering purposes and is primarily used as a metal replacement for various applications. Nylon 66 is synthesized via the polycondensation of hexamethylenediamine and adipic acid (two monomers containing six carbon atoms each).

Figure 3. Molecular structure of Polyamide 66.

The differences between both PA6 and PA66 come down to a lot of little things. While both are cost effective, Nylon 6 is typically around 30% cheaper than Nylon 66. A comparison of different factors is made for the two polymers in Table 1.

Table 1. Comparison of PA6 and PA66

Parameter	PA6	PA66
Machinability – low tool wear and surface finish	Good	Better
Mold shrinkage	Lower	Higher
Water absorption rate	Higher	Lower
Tensile strength	6.2 × 104 kPa (Good)	8.2 × 104 kPa (Better)
Crystalline melting point	225 °C	265 °C
Density	1.15 g/mL	1.2 g/mL
Typical molding shrinkage ratio	1.2 %	1.5 %

Key properties of PA66 and PA6

As stated earlier, Polyamide 66 (PA66) and Polyamide 6 (PA6) are used in so many different applications because of their excellent performance and relatively low cost. Some of the most important properties of these polyamides are listed below:

High strength and rigidity at high temperatures
Good impact strength, even at low temperatures
Good abrasion and wear resistance
Excellent resistance to fuels and oils
Good fatigue resistance
Very good flow for easy processing
PA6 has excellent surface appearance and better processability than PA66 due to its very low viscosity
Good electrical insulating properties
High affinity for water absorption can limit the applications and usage
Low dimensional stability (water absorption results in dimensional change)

Near-infrared spectroscopy as a tool to assess the quality of polyamides

Near-infrared spectroscopy (NIRS) has been an established method for both fast and reliable quality control within the polyamide industry for more than 30 years. However, many companies still do not consistently consider the implementation of NIRS in their QA/QC labs. The reasons could be either limited experience regarding application possibilities or a general hesitation about implementing new methods.

There are several advantages of using NIRS over other conventional analytical technologies. For one, NIRS is able to measure multiple parameters in just 30 seconds without any sample preparation! The non-invasive light-matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes it an excellent method for the determination of both property types.

In the remainder of this post, a short overview of polyamide applications is presented, followed by available turnkey solutions for polyamide analysis developed according the NIRS implementations guidelines of ASTM E1655-17.

Did you miss the first parts in this series? Find them here!

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 1

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 2

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 3

For more detailed information about NIRS as a secondary technique, read our previous blog posts on this subject.

Benefits of NIRS: Part 1

Benefits of NIRS: Part 2

Benefits of NIRS: Part 3

Benefits of NIRS: Part 4

Applications and parameters for polyamides with NIRS

Polyamide production requires that certain important quality parameters be checked on a regular basis. Typical parameters are relative viscosity as well as the amine and carboxylic end groups, and moisture content. Functional group and viscosity analysis of polyamides is normally a lengthy and challenging process due to the limited solubility of the sample and the need to use different analytical methods. Furthermore caprolactam, an important precursor for polyamide production, is very hygroscopic and water soluble—therefore it is crucial to have a reliable analysis technique for determination of water content. Otherwise the quality of the final product could be compromised.

The most relevant applications for NIRS analysis of PA quality parameters are indicated later in this article in Table 2.

Where can NIRS be used in the production process of polyamides?

Figure 4 shows the individual steps from plastic producer via plastic compounder and plastic converter to plastic parts and textile producer. The first step in which near-infrared lab instruments can be used is when the pure polymers like PA are produced, and their purity needs to be confirmed. NIRS is also a very useful technique during the next step where polymers are compounded into intermediate products to be used for further processing.

Figure 4. Illustration of the production chain for polyamides.

Easy implementation of NIR spectroscopy for plastic producers

Metrohm has extensive expertise with analysis of polyamides and offers a turnkey solution in the form of the DS2500 Polymer Analyzer. This instrument is a ready-to-use solution for the determination of multiple quality parameters in different polyamides.

Figure 5. Turnkey solution for PA analysis with the Metrohm DS2500 Polymer Analyzer.

Application example:

Pre-calibrations available for the polyamide industry on the DS2500 Polymer Analyzer

The determination of the parameters listed below in Table 2 is a lengthy and challenging process with conventional laboratory methods. To measure them all, several different techniques are required which takes a significant amount of time, not only to analyze the sample (which has limited solubility, further complicating the situation), but also for the instrument management and upkeep.

Table 2. Primary method vs. NIRS for the determination of various quality parameters in PA samples.

Parameter	Primary method	Time to result (primary method)	Relevant NIRS Application Notes	NIRS benefits
Relative viscosity	Viscosity	90 min. preparation + 1 min. Viscometer	AN-NIR-077 AN-NIR-060 AN-NIR-005	All four parameters are measured simultaneously within a minute, without sample preparation or the need of any chemical reagents
Carboxyl end groups	Titration	90 min. preparation + 20 min. Titration
Amine end groups	Titration	90 min. preparation + 20 min. Titration
Moisture content	Karl Fischer Titration (oven)	2 min. preparation + 15 min. KF Titration (oven)

The NIRS prediction models created for polyamides are based on a large collection of real product spectra and is developed in accordance with ASTM E1655-17 Standard practices for Infrared Multivariate Quantitative Analysis. For more detailed information on this topic, download the free white paper.

Near-Infrared Spectroscopy: Quantitative analysis according to ASTM E1655

To learn more about pre-calibrations for polyamides, download our brochure and visit our website.

Brochure: Quality control of polymers (PE, PP, PET, Polyamide) – Fast results with NIR pre-calibrations

Pre-calibration for PA6

Figure 6 shows the results of the Metrohm turnkey solution for non-destructive determination of several quality parameters in PA listed in Table 2.

Figure 6. Turnkey solution for relative viscosity (RV), amine end groups, carboxyl end groups, and moisture in nylon (PA6) using the Metrohm DS2500 Polymer Analyzer. A: Sampling and analysis of PA6. B: Results of the four analyses from NIRS compared to a primary laboratory method along with the Figures of Merit (FOM) for each analysis.

This solution demonstrates that NIR spectroscopy is very suitable for the analysis of multiple parameters in polyamide in less than one minute without sample preparation or using any chemical reagents. Learn more about the procedure in our free Application Note!

Quality Control of Polyamides – Determination of viscosity, functional groups, and moisture within one minute using NIR Spectroscopy

The examples shown above refer to PA6 and PA66, but NIRS is undoubtedly a great tool for the rapid screening and QC of polyamides with different chain lengths.

Future installments in this series

This blog is a detailed overview of the use of NIR spectroscopy as the ideal QC tool for Polyamide 6 (PA6) and Polyamide 66 (PA66). The last installment of this blog series will be dedicated to:

Polyols and Isocyanates to produce Polyurethane (PU)

For more information

About spectroscopy solutions provided by Metrohm, visit our website!

We offer NIRS for lab, NIRS for process, as well as Raman solutions

Visit our website

Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.

Chemistry of Fireworks

by Alyson Lanciki | Jun 28, 2021 | Analysis

Developed nearly two millennia ago in ancient China, fireworks are increasingly used in cultural celebrations around the world and enjoyed by nearly all ages. As one of the most entertaining forms of chemistry, fireworks appeal to our senses of sight and sound, offering a staggering variety of colors, sizes, shapes, sounds, and so on. We love to watch fireworks because they take our breath away with their magnificence and mystery.

However it is not all fun and games. The business of fireworks (and the field of pyrotechnics in general) is very serious since they should be made as safe as possible to use and also environmentally friendly. Beyond fireworks, other pyrotechnics are found in all kinds of entertainment, like in concerts, movies, and more serious applications for defense and security (e.g., safety measures like flares and signal lights).

What are fireworks made of?

Early fireworks were quite dangerous and were used for protection rather than for celebrations, and hardly resemble the ones we are now familiar with.

It all began back in Ancient China with the invention of gunpowder, which was created from a mixture of charcoal, sulfur, and saltpeter (potassium nitrate). Eventually, as new developments were made to increase the safety and predictability of using these early fireworks, experimentation with colors began and people started using them more for nonviolent purposes. Now there is an entire industry devoted to the development of all kinds of fireworks for consumers and professionals alike.

Learn more about the history of fireworks in the links below:

Glossary of Firework Terminology

The History of Fireworks

Firework History

A firework, or aerial shell as it is also known, basically consists of three main parts aside from the housing: gunpowder and an igniter to make the rocket explode, and inside of the transported capsule on the top there are small garniture pods usually called «stars» (despite being shaped like spheres or cylinders) that include various chemicals for the desired effects. Stars consist of a colorant, a fuel, an oxidizer (oxygen providing substance, e.g., chlorates or nitrates), and a binder to hold the ingredient mixture together in a compact briquette.

The industry has spent a significant amount of time in development to make fireworks explode in shapes like stars and stripes, hearts, or even more complex forms like a cartoon figure, or letters and numbers if timed correctly.

Cross-sectional diagram of a firework capsule filled with star garnitures (72) and igniter (70). [1]

Forming a rainbow of colors

The vibrant colors of fireworks come from the combustion of metal ions which make up to 20% of the components. Metals have been used to color flames even before the invention of modern fireworks (e.g. Bengal fire). Chemically speaking, these metal ions change their electronic state by heating (addition of energy) and then going back to a lower energy state before emitting light of a certain color.

Table 1. List of metals used in pyrotechnics and their colors [2].

Color	Metal	Example compounds
Red	Strontium (intense red)	SrCO₃ (strontium carbonate)
Red	Lithium (medium red)	Li₂CO₃ (lithium carbonate) LiCl (lithium chloride)
Orange	Calcium	CaCl₂ (calcium chloride)
Yellow	Sodium	NaNO₃ (sodium nitrate)
Green	Barium	BaCl₂ (barium chloride) B₃N₃ (boron nitride)
Blue	Copper halides	CuCl₂ (copper chloride), at low temperature
Indigo	Cesium	CsNO₃ (cesium nitrate)
Violet	Potassium	KNO₃ (potassium nitrate)
Violet	Rubidium (violet-red)	RbNO₃ (rubidium nitrate)
Gold	Charcoal, iron, or carbon black
White	Titanium, aluminum, beryllium, or magnesium powders

Very prominent here is the yellow color from sodium which is also seen in older street lightbulbs in some countries. Unfortunately, the most vibrant colors formed are also the most toxic for the environment, like strontium (red) and barium (green). These contaminants can be measured in the air, water, and even in the soil—but more on that later.

Find out more information about how fireworks get their colors in the links below:

Pyrotechnic colorant

How Do Fireworks Get Their Glorious Colors?

Safety first

Safety is always a critical issue when discussing fireworks, whether concerning their construction, their use, or their storage. Too many serious accidents have happened over the years involving fireworks.

Learn more about how to handle fireworks in a safe manner here:

Safety Tips

Among one of the largest fireworks disasters recorded in Europe was in Enschede (The Netherlands) in 2000. This explosion occurred in the warehouse of the S.E. Fireworks factory, which was located in the center of a residential area as the city grew and continued to build homes around it. An entire neighborhood was razed and the largest of the explosions was felt up to 30 kilometers away.

Because of this incident, sales of larger fireworks in most European countries is only allowed outdoors. Accumulating fireworks at home in preparation for celebrations should be avoided at least in confined environments like basements or apartments. It is better to store them in a ventilated shed or car parking to avoid problems in the case of a fire. Also do not store fireworks for long periods, since most of commercial fireworks are meant to be used within 3–6 months after production because the paper contents can get humid, ionic substances can dissolve and recrystallize, and therefore the likelihood of a failure increases.

In the event of a firework failure: Never have a look immediately! Wait at least 15 minutes at a proper distance and then use a tool to confine it afterwards—never touch it with your bare hands, especially when dealing with exploding fireworks or rockets.

Having said this, fireworks have integrated some safety features over the last several years to work more properly and reliably. For instance, the propellants have been modified from containing black powder to using technology from rockets such as plasticizers for better burning performance during launch, also resulting in less smoke and dust on the ground. A dedicated chain of reactions has to be followed, otherwise it will burn in a harmless way.

Knowledge is power: Prevent accidents with proper analytical testing

In order to help prevent fireworks accidents such as the one in Enschede and countless others, it is crucial to closely monitor different quality parameters including the water content of paper-based fireworks, grain size of the metal particles, and the purity and composition of the colorant, just to mention a few. Adequate quality control provides an entertaining, but safe fireworks experience even in the hands of the general public, when proper protocols are followed.

Metrohm offers several analytical technologies and related applications for this area of research. Analyses can be performed for a wide variety of substances and quality parameters as well as trace materials in the laboratory, on the street, and in the air either via wet chemical methods (e.g., Karl Fischer titration, ion chromatography, voltammetry) or spectroscopic techniques (e.g., near-infrared spectroscopy [NIRS] and Raman spectroscopy).

As mentioned earlier, moisture is an important quality parameter when discussing the safety of explosive materials. Metrohm offers two different techniques for accurate analysis of water content in a variety of matrices which are outlined in the following blog posts.

«Analyze This»: Moisture Analysis – Karl Fischer Titration, NIRS, or both?

«Analyze This»: Moisture analysis in ammonium nitrate

When it comes to determining the individual concentrations of the main constituents, some wet chemical techniques really stand out. Suppressed anion chromatography is ideal for measuring the ionic components of e.g., firecracker powder, other explosive material, and even in explosion residues for forensic purposes. Coupling an ion chromatograph to a mass spectrometer (IC-MS) opens up even more analysis possibilities. Read more about these studies (and more) by downloading our free Application Notes.

Chlorate, nitrate, and perchlorate in firecracker powder

Ten anions in an extract of explosives

Forensic examination analysis with IC – Determining low concentrations of chlorate, thiosulfate, thiocyanate, and perchlorate beside major anions in explosives and explosion residues

The use of several different metal salts to create the vibrant colors of fireworks can be beautiful but also harmful to our health and that of our environment. Voltammetry (VA) is an electrochemical method suitable for the determination of trace and ultratrace concentrations of heavy metals and other electrochemically active substances. Not only is VA excellent at determining these substances in the laboratory, but also in the field such as for measuring the after effects of a fireworks display or an undesired event. Check out our selection of VA instruments and applications on our website.

Metrohm: Experts in voltammetric analysis

Spectroscopic techniques like Raman can help to determine the presence of dangerous explosive materials even when keeping a safe distance by using different instrument attachments. Read our free White Paper about how to use MIRA DS from Metrohm Raman for the purpose of identifying explosives safely.

Creating Custom Libraries Detection of Binary Explosives with Mira DS

Environmentally friendly fireworks – a contradiction?

Although fireworks are a very spectacular form of entertainment, there is quite an environmental impact after big cultural events or national holidays. The general atmospheric pollution after a fireworks display has been set off can be seen in an increase of dust and smoke, but also heavy metal content in the air as most contemporary fireworks use these for coloring.

The unburnt material still contains a significant amount of heavy metals. After falling to the ground, this material can dissolve and enter the ground water after it rains. Plastics materials that covered the fireworks for safety reasons are found again as broken shell shrapnel or as microplastics. The combustion of the compounds inside the fireworks leads to increased air pollution in form of aerosols that can be measured and evaluated resulting in heavy metals in the air, fine dust, and even nanoparticles which are extremely harmful for our lungs.

Metrohm Process Analytics has developed the 2060 MARGA (Monitor for AeRosols and Gases in ambient Air) which is used by official agencies and research bodies worldwide to monitor the air quality fully autonomously. This instrument is based on the analytical technique of ion chromatography and can be used as a dedicated continuous air monitoring device that can be left unattended for several weeks at a time, or as a research instrument that can be used for other projects when not monitoring the air quality.

Learn more about the 2060 MARGA and its capabilities in our blog post.

«Analyze This»: 2060 MARGA

To find out more about the use of Metrohm instruments to monitor the air quality, check out this selection of peer-reviewed articles.

Air monitoring with ion chromatography: An overview of the literature references

The effects of firework regulation on air quality and public health during the Chinese Spring Festival from 2013 to 2017 in a Chinese megacity

A new «green» firework generation is being developed for both professional and indoor use to try to minimize the heavy metal content and also reduce aerosol forming agents. This makes them more suitable for indoor pyrotechnic shows and for movie production. In regular outdoor shows (e.g. at theme parks), the gunpowder for transport of the capsule has mostly been substituted with an air pressure gun mechanism.

A significant amount of research has gone into substituting heavy metal-based colorants with more environmentally benign substances by increasing the luminosity of lithium derivatives by substituting them for strontium, or by using boron instead of barium or chlorinated compounds.

Overview of Green Pyrotechnics (in German)

Finally, the plastic parts commonly used to surround fireworks are planned to be substituted by microcrystalline cellulose mixtures with better plasticizing binders. This leads to a similar stability compared to the current plastic materials, but the cellulose-based containers burn up completely and do not leave harmful materials scattered on the ground.

The future of fireworks shows

All safety measures increase the joy of fireworks not only during, but also after the event—being green and being safe. Foretelling the future, some of these celebrations may now use a cadre of lighted drones in a choreographed dance. This has been happening more steadily as drones fall in price and increase in their handling and programming capabilities. However, fireworks have already been with us for a couple of thousand years, and probably will not disappear any time soon.

Download our free Application Notes

and White Papers related to explosives and propellants

Metrohm Application Finder

Post written by Dr. Norbert Mayr (Ph.D. in the field of HEDM, pyrotechnics, propellants, and oxidizers), Marketing Specialist & Product Training at Metrohm International Headquarters, Herisau, Switzerland.

Green hydrogen, future fuel: Using potentiostats to develop new catalysts for hydrogen production

by Alyson Lanciki | Jun 21, 2021 | Analysis, Fundamentals

Hydrogen – clean and green?

Due to its high gravimetric energy density and zero pollution emission, «green hydrogen» is a clean and sustainable energy carrier which is expected to become one of the fuels of the future. Green hydrogen is produced with renewable energy sources, and it can help to mitigate global warming by using cleaner fuels.

Producing green hydrogen with electrolysis

The most favorable way to produce green hydrogen is via water splitting electrolysis, where water (H₂O) is broken down into its counterparts by using a direct electric current. Electrolysis is a sophisticated technique that has been used for many decades in industry. When using this technique for the production of hydrogen, drawbacks are the sluggish reaction kinetics when using inexpensive catalysts or the high costs for more optimal catalysts (e.g. platinum). To produce hydrogen in an efficient and economical manner, the goal for researchers around the world is to develop catalysts for this purpose which are highly active, inexpensive, and stable over long periods.

This article explains in more detail how Metrohm potentiostats can be used to characterize recently developed catalysts for electrochemical hydrogen production.

Learn more about Metrohm potentiostats on our website

Electrode reactions

Considering alkaline solutions, the water splitting reaction can be described by two half reactions (Figure 1):

Hydrogen Evolution Reaction (HER) at the cathode
Oxygen Evolution Reaction (OER) at the anode

Figure 1. Water splitting reaction with the respective half reactions at the cathode and anode.

A critical issue for water splitting is the slow reaction kinetics of the half reactions. To overcome this, electrocatalysts which decrease the activation energy to an acceptable value need to be designed. For an ideal catalyst, a voltage of 1.23 V (at 20 °C and 1013 mbar) would have to be applied to begin hydrogen and oxygen evolution between the electrodes. Unfortunately, when using catalysts in real life situations, voltages above 1.8 V need to be applied.

Requirements for suitable catalysts

Catalysts for the production of green hydrogen from electrochemical water splitting reactions must be (click to go directly to each segment):

active
stable
highly efficient
cost-effective

The first three properties can be determined by electrochemical measuring techniques using a potentiostat.

Activity

The activity of a catalyst is characterized by three values (click to go directly to each segment):

overpotential
Tafel slope
exchange current density

which can be obtained from the polarization curve as displayed in Figure 2.

Figure 2. Polarization curves of an ideal catalyst (orange) and a real catalyst (dark blue) considering the Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER).

Linear polarization

To record a polarization curve (Figure 2), a potentiostat is used in combination with a three-electrode setup. The working electrode is coated with the catalyst which is to be characterized. A typical setup for this purpose is shown in Figure 3.

Figure 3. Potentiostat from Metrohm Autolab combined with a three-electrode setup using a Rotating Disc Electrode (RDE) as working electrode to keep mass transport under defined conditions.

During the measurement, the potential (beginning at a defined initial potential value) is swept to the end potential in a linear fashion over a certain time interval (Figure 4). For HER-catalysts, the potential is swept in the negative direction compared to the initial potential; catalysts used for OER purposes are swept to positive potentials with respect to the initial potential.

The overpotential (η) is a very important parameter to evaluate the activity of the catalyst; its value is affected by the kinetic barrier for the reaction. To overcome this barrier, a higher potential than the thermodynamic potential (1.23 V for OER, or 0 V for HER) – the overpotential – has to be applied to reach the same current densities. The more active the catalyst, the lower the overpotential.

Figure 4. General diagram of linear sweep voltammetry.

Tafel analysis

Aside from the overpotential, the Tafel slope and exchange current density are two additional parameters that help characterize the activity of the catalyst. They can be accessed by plotting the logarithm of the kinetic current density versus overpotential to create the so-called «Tafel plot» (Figure 5).

Figure 5. Tafel plot of OER for two different catalyst materials.

By evaluating the Tafel plot, these two important kinetic parameters can be extracted. One is the Tafel slope (b) which can be expressed by the following equation:

η = a + b log i

η = overpotential
i = current density

The other parameter is the exchange current density (i₀), which can be obtained by extrapolating the current at zero overpotential (Figure 6).

The Tafel slope is related to the catalytic reaction mechanism in terms of electron-transfer kinetics. For example, faster electrocatalytic reaction kinetics lead to a smaller Tafel slope which is shown in a significant current density increment as a function of the overpotential change (Figure 6).

Koutecký and Levich analysis

The exchange current density (i kin₀) describes the charge transfer under equilibrium conditions. A higher exchange current density means a higher charge transfer rate and a lower reaction barrier. As explained earlier in this article, for a better electrocatalyst, a lower Tafel slope and a higher exchange current density are expected.

When performing real experiments, mass transport is the limiting factor at higher overpotentials (Figure 5) which leads to a nonlinear slope of the Tafel plot.

To overcome the undesirable impacts of mass transport (e.g. diffusion), a Rotating Disc Electrode (RDE) (Figure 3) is used. The current signal is therefore measured at different rotation rates. From this data set, it is possible to extract the pure kinetic current (i kin₀) according to Koutecký and Levich (Figure 6).

Figure 6. Tafel plot of OER for two different catalysts. The exchange current density (i kin₀) is determined by extrapolation to η = 0.

You can find more information about Koutecký and Levich analysis by downloading our free Application Note:

AN-EC-014: Oxygen Reduction Reaction with the Rotating Ring Disk Electrode

Impedance analysis

Another measuring technique can be used to determine the reaction kinetics of catalysts: electrochemical impedance spectroscopy (EIS). One of the most important advantages of impedance spectroscopy is that it is a non-destructive and non-invasive measuring technique. This enables consecutive measurements to be performed on the same sample, such as experiments at different temperatures or at different current densities. Also, the aging effects of catalysts can easily be determined this way.

Impedance spectroscopy is an alternating electric current (AC) technique, where an AC voltage with a very small amplitude of a few millivolts is applied to the electrode which responds with an AC current. The values of the applied voltage signal and the values of the corresponding current signal are used to calculate the AC resistance—the impedance. A broad frequency range of several decades is applied and this enables the identification of the kinetic and transport processes that take place on the electrode over different time scales.

Displaying the impedance spectra in a Nyquist plot (Figure 7) exhibits the charge transfer resistance of the reaction as a semicircle. The diameter of this semicircle corresponds to the reaction kinetics: the smaller the diameter, the faster the kinetics of the reaction.

Figure 7. Example of a Nyquist plot of two different catalysts with different activities.

Electrochemical impedance spectroscopy does not only provide kinetic information to researchers, it also gives insight into mass transport effects and conductivity of electrolytes and membranes.

To find out more about EIS, check out our selection of free Application Notes:

Download Metrohm Application Notes about Electrochemical Impedance Spectroscopy here!

Stability

For industrial use, a catalyst should exhibit an extremely low degradation rate. It needs to be stable for many operating hours. At the development stage, stability is an important factor to determine whether a catalyst has the potential for use in practical applications. Stability can be characterized by the changes of the overpotential or current over time by using chronoamperometry, chronopotentiometry, and cyclic voltammetry. These are described in the following sections.

Chrono methods (chronoamperometry and chronopotentiometry)

When using a chronoamperometry method, a constant voltage is applied to the catalyst and the corresponding current signal is sampled and plotted as an i/t curve (Figure 8).

Figure 8. Chronoamperometry measurement of two different catalysts for OER.

Conversely, when using a chronopotentiometry method, a constant current is applied to the catalyst and the voltage response is measured and plotted as an E/t curve (Figure 9). For this measurement, the longer the tested current or potential remains constant, the better the stability of the catalyst.

Figure 9. Chronopotentiometry measurement of two different catalysts for OER.

Cyclic Voltammetry

Cyclic voltammetry is a technique that measures the current density by cycling the potential of the working electrode linearly versus time (Figure 10) . In contrast to linear sweep voltammetry (Figure 4), after the end potential is reached, the potential in a CV experiment is scanned in the opposite direction to return to the initial potential.

Figure 10. Example of a cyclic voltammetry (CV) diagram with one cycle shown.

To determine the degradation rate of a tested catalyst, usually more than 5000 cycles must be executed with a scan rate between 50–100 mV/s. Before and after CV cycling, linear sweep voltammetry (LSV) is used to examine the overpotential shift at a specific current density.

The smaller the change of the overpotential, the better the electrocatalyst’s stability.

Learn more about cyclic voltammetry (CV) with our free Application Note:

AN-EC-007: Differences between digital scans, analog scans, and signal integration

Efficiency

The efficiency (η) can be determined by the faradaic efficiency (or coulombic / coulombian efficiency) in terms of experimental results compared to theoretical predictions.

To be able to calculate the theoretical hydrogen volume via Faraday’s Law, the total charge is needed. This value is measured by the potentiostat using the chronocoulometry method which records the total charge over time (Figure 11).

Figure 11. Example of a chronocoulometry plot.

Conclusion

Choosing the right technique for the analysis of activity, stability, and efficiency of catalysts depends on the specific research and development project focus. Luckily, Metrohm offers a wide range of solutions that will meet all kinds of research requirements.

For more information about electrocatalysis solutions from Metrohm, download our brochure:

Brochure: Electrocatalysis – Complete solutions for electrocatalysis and forced convection measurements

Visit our website

and learn more about our wide range of solutions for electrochemical research!

Click here

Post written by Sandro Haug, Product Manager Electrochemistry at Deutsche METROHM GmbH & Co. KG, Filderstadt, Germany.

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 3

by Alyson Lanciki | Jun 14, 2021 | Analysis, Fundamentals

Polyethylene terephthalate (PET): A brief introduction

PET is a very common plastic, mostly encountered in our lives as PET bottles and as a food packaging material. In this article you will learn how NIR spectroscopy can improve the efficiency of your PET analysis at different steps along the production cycle. Before getting into this, let’s introduce some background information about PET.

Polyethylene terephthalate (PET)

Polyethylene terephthalate (PET) is a general-purpose thermoplastic polymer which belongs to the polyester family. Polyester resins are known for their excellent combination of properties such as mechanical, thermal, and chemical resistance as well as dimensional stability.

PET is one of the most recycled thermoplastics and has the number 1 as its recycling symbol. Recycled PET can be converted into fibers, fabrics, sheets for packaging and for manufacturing automotive parts. PET is a highly flexible, colorless, and semi-crystalline resin in its natural state. Depending upon how it is processed, it can be semi-rigid to rigid. It exhibits good resistance to impact, moisture, alcohols, and solvents.

The chemical formula of PET is (C₁₀H₈O₄)_n and its molecular structure is shown in Figure 1.

Figure 1. Molecular structure of linear PET.

In addition to linear PET, there is also a branched version of the polymer. Branched PET is typically mixed with a small percent of isophthalic acid (C₈H₆O₄), because purified isophthalic acid (PIA, Figure 2) reduces the crystallinity of PET, serving to improve its clarity and increase the productivity of bottle manufacturing processes.

Diethylene glycol (DEG) as an additive also reduces the rate of crystallization of PET when crystallizing from the melt, isothermally and dynamically.

Figure 2. Molecular structure of isophthalic acid.

The key properties and advantages of PET resin are numerous:

very strong and lightweight, and therefore easy and efficient to transport
has good gas (oxygen, carbon dioxide) and moisture barrier properties, meaning low gas permeability (particularly against CO₂)
exhibits excellent electrical insulating properties
broad range of use temperature (-60 to 130 °C)
high heat distortion temperature (HDT)
suitable for transparent application purposes
practically shatter-resistant – PET does not break or fracture and is used to replace glass in some applications
recyclable material
transparent to microwave radiation
very resistant to alcohols, aliphatic hydrocarbons, oils, greases, and diluted acids
moderately resistant to diluted alkalis, aromatic and halogenated hydrocarbons
PET is approved as safe for contact with foods and beverages by the FDA, Health Canada, EFSA, and other health agencies

What is polyethylene terephthalate (PET) used to make?

Polyethylene terephthalate is used in several types of packaging applications as shown in Figure 3. Due to its strength, light weight, and many other attractive properties, PET excels as a food packaging material.

Figure 3. PET is an ideal choice for many food packaging applications due to its strength to weight ratio.

Polyester makes up nearly two-thirds of synthetic fibers produced. There are many different types of polyester, but the type most often produced for use in textiles is PET. When used in a fabric, it is most often referred to as «polyester» or «poly» (Figure 4). This material costs very little to produce, which is the primary driver for its use in the textile industry.

Approximately 60% of the global PET production is used to make fibers for textiles while about 30% is used to make bottles for various purposes. Its ability to be recycled is especially attractive for manufacturers looking to save costs and operate in a greener manner.

Figure 4. PET makes up a significant portion of produced polyester fabric.

In the electronics industry, PET is chosen to replace less ideal materials due to its excellent electrical insulating properties and resistance to distortion even at high temperatures. PET is also used to manufacture many parts in the automotive industry (Figure 5).

Figure 5. PET is often used in the manufacturing of various automotive parts.

NIRS as a tool to assess the quality of PET

For over 30 years, near-infrared spectroscopy (NIRS) has been an established method for fast and reliable quality control within the PET industry. Despite this, many producers still do not consistently consider the implementation of NIRS in their QA/QC labs. Limited experience regarding application possibilities or a general hesitation about implementing new methods are some of the reasons behind this.

The advantages of using NIR spectroscopy for QA/QC are numerous. One major advantage of NIRS is the determination of multiple parameters in just 30 seconds with no sample preparation! The non-invasive light-matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes NIRS a suitable method for the determination of several critical quality parameters in these polymers and many more.

In the remainder of this article, a short overview of PET applications is presented, followed by available turnkey solutions for PET, developed according the NIRS implementation guidelines of ASTM E1655-17.

Did you miss the first parts in this series? Find them here!

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 1

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 2

For more detailed information about NIRS as a secondary technique, read our previous blog posts on this subject.

Benefits of NIRS: Part 1

Benefits of NIRS: Part 2

Benefits of NIRS: Part 3

Benefits of NIRS: Part 4

Applications and parameters for PET with NIRS

During production of PET it is important to check certain parameters to guarantee the quality. These parameters include the diethylene glycol content, isophthalic acid content, intrinsic viscosity (ASTM D4603), and the acid number (AN). Determination of these parameters is a lengthy and challenging process due to the limited solubility of the sample and the need to use different analytical methods.

The most relevant applications for NIRS analysis of PET are listed in Table 1.

Table 1. Available application notes for use of NIR for PET

Polymer	Parameter	Related NIRS Application Notes
Polyethylene terephthalate (PET)	Diethylene glycol, Intrinsic Viscosity, Acid number, Isophthalic acid	AN-NIR-023

Where can NIRS be used in the production process of PET?

Figure 6 shows the individual production steps from the plastic producer via plastic compounder and plastic converter to the plastic parts producer. The first step in which near-infrared lab instruments can be used is when the pure polymers like PET are produced, and their purity needs to be confirmed. NIRS is also a very useful technique during the next step where polymers are compounded into intermediate products to be used for further processing.

Figure 6. Illustration of the polyethylene terephthalate production chain.

Easy implementation of NIR spectroscopy for plastic producers

Metrohm has extensive expertise with analysis of PET and offers a turnkey solution in the form of the DS2500 Polymer Analyzer. This instrument is a ready-to-use solution to determine multiple quality parameters in PET.

Figure 7. Turnkey solution for PET analysis with the Metrohm DS2500 Polymer Analyzer.

Application example:

Pre-calibrations available for the PET industry on the DS2500 Polymer Analyzer

Due to the limited solubility of polyethylene terephthalate and the need to use several different analytical methods, the determination of the parameters listed in Table 2 is a lengthy and challenging process with conventional laboratory techniques.

Table 2. Primary method vs. NIRS for the determination of various quality parameters in PET samples.

Parameter	Primary method	Time to result (primary method)	NIRS benefits
Diethylene Glycol content	Extraction + HPLC-MS	45 min. preparation + 40 min. HPLC-MS	All four parameters are measured simultaneously within a minute, without sample preparation or the need of any chemical reagents
Isophthalic acid content	Dissolve + HPLC	45 min. preparation + 40 min. HPLC
Intrinsic viscosity	Dissolve + Viscometer	90 min. preparation + 1 min. Viscometer
Acid number	Dissolve + Titration	90 min. preparation + 10 min. Titrator

The NIRS prediction models created for PET are based on a large collection of real product spectra and is developed in accordance with ASTM E1655-17 Standard practices for Infrared Multivariate Quantitative Analysis. For more detailed information on this topic, download the free white paper.

Near-Infrared Spectroscopy: Quantitative analysis according to ASTM E1655

To learn more about pre-calibrations for PET, download our brochure and visit our website.

Brochure: Quality control of polymers (PE, PP, PET, Polyamide) – Fast results with NIR pre-calibrations

Pre-calibration for PET

The result of this turnkey solution for rapid non-destructive determination of the key quality parameters for PET listed in Table 2 is shown in Figure 8.

Figure 8. Turnkey solution for diethylene glycol, isophthalic acid, intrinsic viscosity and acid number in PET using the Metrohm DS2500 Polymer Analyzer. A: Sampling and analysis of PET granulate. B: Results of the four analyses from NIRS compared to a primary laboratory method along with the Figures of Merit (FOM) for each analysis.

This solution demonstrates the feasibility of NIR spectroscopy for the analysis of multiple parameters in PET in less than one minute without sample preparation or using any chemical reagents. Learn more about the procedure in our free Application Note!

Quality Control of PET – Determination of diethylene glycol, isophthalic acid, intrinsic viscosity, and acid number within one minute with NIRS

Future installments in this series

This article is a detailed overview of the use of NIR spectroscopy as the ideal QC tool for the analysis of polyethylene terephthalate (PET). Future installments in this series will be dedicated to:

Polyamide (PA)
Polyols and Isocyanates to produce Polyurethane (PU)

For more information

About spectroscopy solutions provided by Metrohm, visit our website!

We offer NIRS for lab, NIRS for process, as well as Raman solutions

Visit our website

Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.

Supercharge your battery research – Part 2

by Alyson Lanciki | Jun 7, 2021 | Analysis

Battery technology has come a long way since the rudimentary voltaic pile was developed over two centuries ago. The breakthrough innovation of the lithium ion battery and its subsequent improvements has increased the use and accessibility of electronics, particularly in the consumer market. Electronics are more portable, affordable, and thanks to rechargeable or secondary batteries, they are becoming more sustainable.

Expansion of application possibilities is another reason that energy storage research, particularly batteries, is currently a hot topic. For example, only a decade ago drones were the domain of the military industrial complex, and now a drone with a camera is a standard part of nearly any successful photographer or influencer’s gear. Thanks to an improved battery life and more cost-efficient materials, a drone now has an affordable price tag for a larger segment of the civilian population.

This kind of disruption is happening in larger, more profitable markets as well. Tesla, a newsworthy brand thanks to their technological innovations and public relations, still has a small, albeit increasing, market share of the overall automotive market. Their success has challenged other established brands to recognize that a change from conventional combustion engines can be lucrative. Volvo and Ford are committed to be «fully electric» by 2030 [1]. General Motors (GM) has committed to not only be electric by 2035, but for their business to be carbon neutral by 2040 [2].

The automotive market is a high profile example of an industry that will drastically retool their sector—from manufacturing to sales—and this will happen across many other industries as there is a greater focus on climate change and renewable energy sources from governments and consumers alike. Accurate and scalable R&D will be required to make these transformations possible, and the hunt for improved energy storage solutions is at the heart of these changes.

Electrochemistry was the key to the discovery of energy storage and is the natural technique of choice for future innovations.

Electrochemical characterization techniques for lithium ion batteries

In part one of this series, we introduced various techniques to analyze the composition and purity of electrode materials and lithium salts, in addition to accurate water content determination in the battery materials.

«Analyze This»: Supercharge your battery research – Part 1

In this article we highlight techniques that will allow the characterization of multiple attributes of the electrochemical behavior of Li-ion batteries using a high precision potentiostat/galvanostat. In some cases, the difference between techniques is due to performing the experiment in a different mode (i.e., potentiostatic or galvanostatic), and the additional information gathered provides a more complete picture of battery behavior.

Galvanostatic Intermittent Titration Technique (GITT)

One of the first techniques available to researchers exploring the properties of battery electrode materials is the Galvanostatic Intermittent Titration Technique (GITT). Usually conducted on a half-cell, this technique is a series of current perturbations followed by a relaxation time, which provides information about the thermodynamic properties and electrode materials including the critical diffusion co-efficient. All of this information gives a better understanding of the electrochemical behavior that can be expected by the materials.

If you’re looking for more information on this subject, download our free application note AN-BAT-003.

Download: AN-BAT-003 – Galvanostatic Intermittent Titration Technique (GITT)

Potentiostatic Intermittent Titration Technique (PITT)

Potentiostatic Intermittent Titration Technique (PITT) is similar to the GITT technique detailed above, but the PGSTAT is operated in potentiostatic mode. A series of potential step perturbations is applied to the system, and current is measured as a function of time. Both GITT and PITT are capable of accurately determining the diffusion coefficient.

Want to learn more? Download our free application note AN-BAT-004!

Download: AN-BAT-004 – Potentiostatic intermittent titration technique (PITT)

When using a PGSTAT in galvanostatic mode you can also characterize the performance of Li-ion batteries by using different current rates and charging and discharging during various cycles, known colloquially as «cycling». With this technique researchers can understand the rate performance of the Li-ion battery, its capacity, and the associated power and energy density. This is the most commonly used technique in battery research. A constant current constant voltage (CCCV) procedure is usually applied in order to make sure that a battery is fully charged, while avoiding any battery overcharge.

Learn more about characterizing the performance of lithium ion batteries with cycling by downloading our free application note AN-BAT-002.

Download: AN-BAT-002 – Galvanostatic charge-discharge of a Li-ion battery with Autolab

CCCV is the industry standard for Li-ion battery charging, and PGSTAT operates in both galvanostatic and potentiostatic mode for this measurement. Galvanostatic cycling is performed within a safe potential window at which the electrolyte is stable. Any slight deviation from the potential cutoff may result in poor cycle life.

Voltage profile of a 18650 Li-ion battery, cycled at ~ C/15 (left), and its corresponding dQ/dV versus V plot (right). The corresponding peaks and plateaus are marked in the figures.

Electrochemical Impedance Spectroscopy (EIS)

Electrochemical Impedance Spectroscopy (EIS) provides additional data and therefore greater insight into the battery’s behavior and potential performance by using galvanostatic charge/discharge cycling and then adding in the most powerful technique that is being used extensively in current battery research. With EIS, the highly dynamic behavior of a battery and the diffusion of ions at the interfaces can be characterized. In a single experimental procedure encompassing a broad range of frequencies, the influence of the governing physical and chemical phenomena may be isolated and distinguished at a given frequency range and state of charge. With EIS it is possible to measure the internal impedance of the battery and model it using the equivalent circuit and understand the contribution of the battery components to the total impedance of the cell.

For EIS determination of batteries it is important to use 4-terminal sensing to avoid the contribution of wires to overall impedance. This is important for any low impedance electrochemical system. Learn more about this research by downloading our free Application Notes AN-EC-013 and AN-BAT-008.

AN-EC-013 – The importance of using four-terminal sensing for EIS measurements on low-impedance systems

AN-BAT-008 – Metrohm Autolab DuoCoin Cell Holder with EIS measurements on a commercial battery

With EIS, it is possible to determine the through-plane tortuosity of battery electrodes, which along with overall electrolyte conductivity, the transference number of a Li-ion of battery electrolyte, and diffusion coefficient of electrolyte gives a good indication of the practicality of certain battery chemistry for high power applications. In addition, the mass transport limitation of the battery separator and its ionic conductivity plays a crucial role in overall performance of the batteries.

By determining the MacMullin number, researchers can determine the quality of the separators for their application in certain Li-ion cells.

Nyquist plot: Negative imaginary part of impedance as a function of the real part of impedance for an 18650 cell.

Download our free Application Notes below for more information on these subjects.

AN-BAT-011 – Determination of the Through-Plane Tortuosity of Battery Electrodes by EIS in a symmetric Lithium-iron-phosphate cell

AN-BAT-012 – Determination of the Lithium Ion Transference Number of a Battery Electrolyte by VLF-EIS

AN-BAT-009 – Determination of the binary diffusion coefficient of a battery electrolyte

AN-BAT-006 – Determination of the MacMullin number

Download our free white paper «A Guide to Li-ion Battery Research and Development» written by electrochemical instrument innovators at Metrohm Autolab. This white paper provides additional information about applicable electrochemical techniques and provides useful definitions to terminologies that are relevant to Li-ion battery research and development.

Free white paper:

A Guide to Li-ion Battery Research and Development

Download it here!

Post written by Dr. Reza Fathi, Product Specialist at Metrohm Autolab, Utrecht, The Netherlands.

Validation of titration methods

by Alyson Lanciki | May 31, 2021 | Analysis, Evaluating results, Fundamentals

Manufacturing products of the highest quality is a must, especially in the pharmaceutical and food industries. This requires accurate, reproducible, and simple analysis methods that eliminate human errors as much as possible. Automated titration is one such solution that offers additional time and cost savings to laboratories.

After applying automation to a titration method, how can you ensure that the chosen method also delivers a reliable result? And how do you know that it is suitable for the analysis of your analyte(s)? This requires method validation of a titration, which includes standardization of the titrant as well as determination of accuracy and precision, linearity, and specificity.

USP General Chapter <1225> Validation of Compendial Procedures and ICH Guidance Q2(R1) Validation of Analytical Procedures: Text and Methodology define the validation elements – some of the most important ones are described in the following article.

These include (click to go to each section):

Standardization
Accuracy and precision
Linearity
Specificity

Standardization

Dilution and weighing errors as well as the constant aging of all titrants lead to changes in the concentration of the titrant. To obtain results that are as reliable as possible, the most accurate titrant concentration is a prerequisite. Standardization of the titrant is therefore an integral part of a titration method validation. The standardization procedures for various titrants are described in the Volumetric Solution section of USP – NF as well as in the Metrohm Application Bulletin AB-206 regarding the titer determination in potentiometry.

Free download: AB-206 – Titer determination in potentiometry

The titrant to be used in the validation must first be standardized against a primary standard or a pre-standardized titrant. It is important that the standardization step and the sample titration are carried out at the same temperature.

Primary standards are characterized by the following properties:

high purity and stability
low hygroscopicity (to minimize weight changes)
high molecular weight (to minimize weighing errors)

The use of a standard substance (primary standard) allows accuracy to be assessed.

For more information about titrant standardization, check out our blog posts «What to consider when standardizing titrant» (for potentiometric titration) and «Titer determination in Karl Fischer titration».

«Analyze This»: What to consider when standardizing titrant

«Analyze This»: Titer determination in Karl Fischer titration

Accuracy and precision

Accuracy is defined as the proximity of the result to the true value. Therefore, it provides information about the bias of a method under validation. Accuracy should be determined over the entire concentration range.

Precision is usually expressed as the standard deviation (SD) or relative standard deviation (RSD). It expresses how well the individual results agree within an analysis of a homogeneous sample. Here, it is important that not only the analysis itself but also all sample preparation steps are performed independently for each analysis.

Precision is evaluated in three levels:

Repeatability: the precision achieved by a single analyst for the same sample in a short period of time using the same equipment for all determinations.
Intermediate precision: analysis of the same sample on different days, by different analysts and with different equipment, if possible, within the same laboratory.
Reproducibility: precision obtained by analyzing the same sample in different laboratories.

Determination of both accuracy and precision is necessary, as only the combination of both factors ensures correct results (Figure 1).

Figure 1. Only when both precision and accuracy are high can correct results be obtained, as high precision does not necessarily mean good accuracy, and vice versa.

For titration, accuracy and repeatability are usually determined together. At least two to three determinations at three different concentration levels (in total six to nine determinations) are recommended. For assays, the recommendation is to use a concentration range of 80% to 120% of the intended sample weight.

Linearity

Linearity expresses whether a particular method gives the correct results over the concentration range of interest. Since titration is an absolute method, linearity can usually be determined directly by varying the sample size and thus the analyte concentration.

To determine the linearity of a titration method in the range of interest, titrate at least five different sample sizes and plot a linear regression of the sample volume against the titration volume consumed (Figure 2). The coefficient of determination (R²) is used to assess linearity. The recommendation is to use a concentration range of 80% to 120% of the intended sample weight.

Figure 2. Linear regression curve for the assay of potassium bicarbonate.

Specificity

Impurities, excipients, or degradation products are among the many components that may be present in a sample. Specificity is the ability to evaluate the analyte without interference from these other components. Therefore, it is necessary to demonstrate that the analytical procedure is not affected by such compounds. This is the case when either the equivalence point (EP) found is not shifted by the added impurities or excipients, or in the event it is shifted that a second EP corresponding to these added components can be observed when a potentiometric sensor is used for indication.

Specificity may be achieved by using suitable solvents (e.g., non-aqueous titration instead of aqueous titration for acid-base titration) or titration at a suitable pH value (e.g., complexometric titration of calcium at pH 12, where magnesium precipitates as magnesium hydroxide).

How can this be implemented in practice? The titrimetric determination of potassium bicarbonate with hydrochloric acid will serve as an example here.

Download «Recommendations for titration methods validation» here!

In this case, potassium carbonate is expected as an impurity with pk_b values of 8.3 and 3.89. This makes it possible to separate the two species during the acid-base titration. Figure 3 shows the comparison of a curve overlay of the titration curves of potassium bicarbonate with and without added potassium carbonate.

Figure 3. Curve overlay of the specificity test using 1 g KHCO₃ with and without 0.5 g K₂CO₃ (green and orange = no K₂CO₃ added; blue and yellow = K₂CO₃ added). Click to enlarge.

The lower titration curve corresponds to the solution containing both potassium bicarbonate and potassium carbonate. Two EPs are found here: the first EP can be assigned to the added potassium carbonate, while the second corresponds to the sum of potassium bicarbonate and potassium carbonate. The curve at the top of the figure clearly shows only one EP for the potassium bicarbonate solution without impurities.

Find out more about the proper recognition of endpoints (EP) in our previous blog post.

«Analyze This»: Recognition of endpoints (EP)

Conclusion

If you follow the recommendations above, you will be ready for titration method validation – and now it`s time to get started!

Using potentiometric autotitration instead of manual titration increases the accuracy and reliability of your results. In addition, the use of an autotitrator ensures that critical regulatory compliance requirements, such as data integrity are met.

Download: Assay of potassium carbonate and potassium bicarbonate – Reliable, and selective determination of both species

Right from the start, Metrohm products provide peace of mind and confidence in the quality of the data you produce with proper IQ/OQ.

Brochure: Analytical Instrument Qualification - Confidence in quality with IQ/OQ

If you would like to learn more about Metrohm Analytical Instrument Qualification, have a look at our two blog posts dedicated to this important topic.

«Analyze This»: Introduction to Analytical Instrument Qualification – Part 1

«Analyze This»: Introduction to Analytical Instrument Qualification – Part 2

Additional security is also provided, e.g., by Metrohm Buret Calibration which ensures that the accuracy and precision of your dosing device are within the required tolerances. Traceable monitoring of the performance and function of the instrument through regular re-qualifications and tests is therefore a given.

Flyer: Metrohm Buret Calibration

Watch our free webinar

available on demand!

How to convert from manual to automated titration procedures

Register here

Post written by Doris Hoffmann, Product Manager Titration at Metrohm International Headquarters, Herisau, Switzerland.

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 2

by Alyson Lanciki | May 25, 2021 | Analysis, Fundamentals

Polypropylene and polyethylene: A brief introduction

Did you know that polypropylene (PP) and polyethylene (PE) are the most produced plastics in the world? Products made out of PP and PE are so ubiquitous that every single one of us encounters them several times per day. In this article you will learn how NIR spectroscopy can improve the efficiency of your PP and PE analysis along different steps along the production cycle. But first, let’s get a little bit of background information about PP and PE.

Polypropylene (PP)

Polypropylene (also known as polypropene or PP) has a chemical formula of (C₃H₆)_n. It is a thermoplastic polymer mainly produced from propylene monomers. PP is a versatile plastic commodity that also functions as a fiber. In 1954, it was first polymerized simultaneously by the Italian chemist, professor, and Nobel laureate Giulio Natta and Karl Rehn, a German chemist.

Polypropylene has the unique ability that it can be manufactured via several different methods and be utilized in many applications like packaging, injection molding, and fibers. This plastic commodity is the second most popular in the world, only preceded by polythene.

Polyethylene (PE)

Polyethylene (or polythene, PE) is also a polymer, but it is made from ethylene monomers and has the chemical formula (C₂H₄)_n. The first synthesis of PE in 1898 by the German scientist Hans von Pechmann was accidental. Similar to PP, PE is also a thermoplastic.

PE is the most used plastic worldwide. Polythene is very stable and is a good electrical insulator. It has a very low melting point and is used in large amounts for the automotive and food packaging industries. Approximately 70% of PE is utilized in food packages, food containers, pallets, and even in crates and bottles.

Polyethylene is available in many different types:

Ultra-High-Molecular-Weight Polyethylene (UHMWPE)
Ultra-Low-Molecular-Weight Polyethylene (ULMWPE or PE-WAX)
High-Molecular-Weight Polyethylene (HMWPE)
High-Density Polyethylene (HDPE)
High-Density Cross-Linked Polyethylene (HDXLPE)
Cross-Linked Polyethylene (PEX or XLPE)
Medium-Density Polyethylene (MDPE)
Linear Low-Density Polyethylene (LLDPE)
Low-Density Polyethylene (LDPE)
Very-Low-Density Polyethylene (VLDPE)
Chlorinated Polyethylene (CPE)

Figure 1. Molecular structures of PE and PP.

Differences between polypropylene and polyethylene

Which is better, polypropylene or polyethylene? It all depends on the application! For what purpose are they being used? Both polymers are considered «commodity plastics». These are plastics that are used in high volumes for a wide range of applications.

Let’s compare some of the properties of each.

Table 1. Comparison chart of polypropylene vs. polyethylene.

Polypropylene (PP)	Polyethylene (PE)
Chemical Properties
Semi-crystalline Polypropylene bag	Inert, translucent Polyethylene bag
Electrical Properties
High static charge Poor insulator	Low static charge Good insulator
Melting Point
130–171 °C	115–135 °C
Chemical Formula
(C₃H₆)_n	(C₂H₄)_n
Uses
Fibers, films, caps, hinges, synthetic paper	Plastic bags, bottles, food containers, pallets, geomembranes, films made of plastic, crates, etc.
Density
0.855 g/cm³ amorphous 0.946 g/cm³ crystalline	0.88–0.96 g/cm³
Relative Cost
Low	Medium

NIRS as a tool to assess the quality of PP and PE

For over 30 years, near-infrared spectroscopy (NIRS) has been an established method for fast and reliable quality control within the PP/PE industry. Despite this, many producers still do not consistently consider the implementation of NIRS in their QA/QC labs. Limited experience regarding application possibilities or a general hesitation about implementing new methods are some of the reasons behind this.

The advantages of using NIR spectroscopy for QA/QC are numerous. One major advantage of NIRS is the determination of multiple parameters in just 30 seconds with no sample preparation! The non-invasive light-matter interaction used by NIRS, influenced by physical as well as chemical sample properties, makes NIRS a suitable method for the determination of several critical quality parameters in these polymers and many more.

In the remainder of this article, a short overview on PE and PP applications are presented, followed by available turnkey solutions for PE and PP, developed according the NIRS implementations guidelines of ASTM E1655-17.

Did you miss the first part in this series about NIRS as the ideal QC tool for the polymer industry? Find it here!

NIR spectroscopy in the polymer industry: The ideal tool for QC and product screening – Part 1

For more detailed information about NIRS as a secondary technique, read our previous blog posts on this subject.

Benefits of NIRS: Part 1

Benefits of NIRS: Part 2

Benefits of NIRS: Part 3

Benefits of NIRS: Part 4

Applications and parameters for PE and PP with NIRS

During production of PE and PP it is important to check certain parameters to guarantee the quality. These parameters include the density to classify the PE type, copolymer level to enhance certain properties like strength and solvent resistance, and melt flow rate to make sure PP can be formed to the intended shape.

The most relevant applications for NIRS analysis of PE and PP are listed in Table 2.

Table 2. Available application notes for use of NIRS for PE and PP

Polymer

Parameter

Related NIRS Application Notes

Polyethylene (HDPE/LDPE)

Identification, Density, Melt Flow Index, Copolymer level

Polypropylene (PP)

Identification, Melt Flow Index, Additives

Where can NIRS be used in the production process of PE and PP?

Figure 2 shows the individual production steps from the plastic producer via plastic compounder and plastic converter to the plastic parts producer. The first step in which near-infrared lab instruments can be used is when the pure polymers like PE and PP are produced, and their purity needs to be confirmed. NIRS is also a very useful technique during the next step where polymers are compounded into intermediate products to be used for further processing.

Figure 2. Illustration of the polyethylene/polypropylene production chain.

Easy implementation of NIR spectroscopy for plastic producers

Metrohm has extensive expertise with analysis of PE and PP and offers a turnkey solution in the form of the DS2500 Polymer Analyzer. This instrument is a ready-to-use solution to determine multiple quality parameters in PE and PP.

Figure 3. Turnkey solutions for PE and PP analysis with the Metrohm DS2500 Polymer Analyzer.

Application example:

Turnkey solution for the determination of Melt Flow Rate (MFR) of PP

The Melt Flow Rate of polypropylene pellets is an important parameter to measure so that PP can be formed in the intended shape. The model created with the chemometric software is based on a large collection of real product spectra and is developed in accordance with ASTM E1655-17 Standard practices for Infrared Multivariate Quantitative Analysis. For more detailed information on this topic, download the free white paper.

Near-Infrared Spectroscopy: Quantitative analysis according to ASTM E1655

To learn more about pre-calibrations for PP, download our brochure and visit our website.

Brochure: Quality control of polymers (PE, PP, PET, Polyamide) – Fast results with NIR pre-calibrations

Pre-calibration for polypropylene (PP)

The result of this turnkey solution for the non-destructive determination of Melt Flow Rate of PP without rheological tests is shown in Figure 4.

Figure 4. Turnkey solution for Melt Flow Rate of PP using the Metrohm DS2500 Polymer Analyzer. A: Sampling and analysis of PP pellets. B: Results of MFR from NIRS compared to a primary laboratory method along with the Figures of Merit (FOM) for this analysis.

This solution demonstrates the feasibility of NIR spectroscopy for the analysis of MFR in polypropylene samples. The standard procedure (ASTM D1238) requires a significant amount of work including packing the sample, preheating, and cleaning. With no sample preparation or chemicals needed, Vis-NIR spectroscopy allows the analysis of MFR in less than a minute.

Learn more about the procedure in our free Application Note!

Quality Control of Polypropylene – Non-destructive determination of melt flow rate without rheological tests

Future installments in this series

This article is a detailed overview of the use of NIR spectroscopy as the ideal QC tool for the analysis of polypropylene and polyethylene. Future installments in this series will be dedicated to:

Polyethylene terephthalate (PET)

Polyamide (PA)
Polyols and Isocyanates to produce Polyurethane (PU)

For more information

About spectroscopy solutions provided by Metrohm, visit our website!

We offer NIRS for lab, NIRS for process, as well as Raman solutions

Visit our website

Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.

The history of polyurethanes

Dr. Otto Bayer was credited with inventing polyurethanes in 1937.

How is polyurethane created?

Figure 1. Molecular structures of isocyanates, polyols, and polyurethane.

Physical and chemical properties of polyurethanes

Typical applications of polyurethane

Table 1. Polyurethane in comparison with rubber, metal, and plastic.

Near-infrared spectroscopy as a tool to assess the quality of polyurethanes

Applications and parameters for polyurethanes with NIRS

Where can NIRS be used in the polyurethane production process?

Figure 2. Illustration of the production chain for polyurethanes.

Easy implementation of NIR spectroscopy for plastic producers

Figure 3. Turnkey solution for polyurethane analysis with the Metrohm DS2500 Polyol Analyzer.

Application example:

Pre-calibrations and starter model for the PU industry on the DS2500 Polyol Analyzer

Table 2. Primary method vs. NIRS for the determination of various quality parameters in PU samples.

Figure 4. Turnkey solution for determination of hydroxyl number in polyols using the Metrohm DS2500 Polyol Analyzer. A: Sampling and analysis of polyols. B: NIRS results compared to a primary laboratory method along with the Figures of Merit (FOM).

Figure 5. Turnkey solution for determination of NCO content (Isocyanates) in polyurethane using the Metrohm DS2500 Polyol Analyzer. A: Sampling and analysis of polyurethane. B: NIRS results compared to a primary laboratory method along with the Figures of Merit (FOM).

For more information

Main factors that influence the reliability of fast electrochemical experimental results

Time constant of the electrochemical cell

Figure 1. Theoretical and true waveform applied to a real electrochemical cell [1].

The electrochemical instrument: potentiostat/galvanostat (PGSTAT)

Bandwidth of the control loop of the PGSTAT

Figure 2. Schematic representation of the applied signal when Low bandwidth (Low speed) and High bandwidth (High speed) settings are used compared with the theoretical response.

Bandwidth of the current sensor (current range)

Data sampling interval vs the timescale of the investigated transient signal

Figure 3. Effect of the sampling frequency of an ideal sinusoidal signal [3]. Shown here are the theoretical signal (dashed line), sample points, and resulting measured signal (orange line).

Summary

Fast measurements start here!

What is a nonaqueous acid-base titration?

Electrostatic effects

Figure 1. Titration curve with a spike which might have occurred from an electrostatic interference.

Blocked diaphragm

Choice of electrolyte and storage solution

Checking the electrode according to ASTM D664

Proper rinsing and cleaning

Figure 2. Different determinations according to ASTM D664. With time, the start potential of the curves shifts which indicates an unsuitable cleaning procedure.

Figure 3. Appearance of the electrode used in Figure 2 after five measurements.

Conditioning the glass membrane correctly

Maintenance of burets

Thermometric titration as an alternative

Summary

For more information

Polyamide (Nylon): A brief introduction

Wallace Carothers (1896–1937), the creator of polyamide.

Differences between Polyamide 6 (PA6 / Nylon 6) and Polyamide 66 (PA66 / Nylon 66)

Figure 1. Molecular structure of caprolactam.

Figure 2. Molecular structure of Polyamide 6.

Figure 3. Molecular structure of Polyamide 66.

Table 1. Comparison of PA6 and PA66

Key properties of PA66 and PA6

Near-infrared spectroscopy as a tool to assess the quality of polyamides

Applications and parameters for polyamides with NIRS

Where can NIRS be used in the production process of polyamides?

Figure 4. Illustration of the production chain for polyamides.

Easy implementation of NIR spectroscopy for plastic producers

Figure 5. Turnkey solution for PA analysis with the Metrohm DS2500 Polymer Analyzer.

Application example:

Pre-calibrations available for the polyamide industry on the DS2500 Polymer Analyzer

Table 2. Primary method vs. NIRS for the determination of various quality parameters in PA samples.

Future installments in this series

For more information

What are fireworks made of?

Cross-sectional diagram of a firework capsule filled with star garnitures (72) and igniter (70). [1]

Forming a rainbow of colors

Safety first

Knowledge is power: Prevent accidents with proper analytical testing

Environmentally friendly fireworks – a contradiction?

The future of fireworks shows

Download our free Application Notes

Hydrogen – clean and green?

Producing green hydrogen with electrolysis

Electrode reactions

Figure 1. Water splitting reaction with the respective half reactions at the cathode and anode.

Requirements for suitable catalysts

cost-effective

Activity

Figure 2. Polarization curves of an ideal catalyst (orange) and a real catalyst (dark blue) considering the Hydrogen Evolution Reaction (HER) and Oxygen Evolution Reaction (OER).

Linear polarization

Figure 6. Tafel plot of OER for two different catalysts. The exchange current density (i kin₀) is determined by extrapolation to η = 0.