Select Page
NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 1

NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 1

Introduction to the petrochemical and refining industry

Oil and gas for fuel are produced in nearly every corner of the globe, from small private wells generating around 100 barrels a day, to the large bore wells producing upwards of 40 times that volume. Despite this great variation in size, many parts of the refining process are quite similar.

Chemicals derived from petroleum or natural gas, so-called «petrochemicals», are an essential part of the contemporary chemical industry. The field of petrochemistry became increasingly popular around the early 1940’s during the second world war. At that time there was a growing demand for synthetic products which was a great driving force for the development of petrochemical products.

Oil refining aims to provide a defined range of products according to agreed specifications. Simple refineries use a distillation column (Figure 1) to separate crude oil into different fractions based on their chemical properties, and the relative quantities are directly dependent on the crude oil used. Therefore, it is necessary to obtain a range of crudes that can be blended into a suitable feedstock to produce the required quantity and quality of end products.

The basic products from fractional distillation are shown in Figure 1.
Wallace Carothers, inventor of polyamide.
Figure 1. Illustration of a fractionating distillation column used for the purposes of refining crude oil into several desirable end products.
Near-infrared (NIR) spectroscopy is a technique that is particularly suited for making quality control of these end products more efficient and cost-effective for manufacturers. Furthermore, NIRS is recognized and accepted by ASTM as an alternative method to other techniques. Dedicated ASTM methods for method development, method validation, and results validation are presented later in this article.

Read on for a short overview on NIR spectroscopy followed by application examples for the petrochemical and refinery industry to learn how petrochemical producers and refineries alike can benefit from NIRS.

NIR technology: a brief overview

The interaction between light and matter is a well-known process. Light used in spectroscopic methods is typically not described by the applied energy, but in many cases by the wavelength or in wavenumbers.

A NIR spectrometer such as the Metrohm NIRS DS2500 Petro Analyzer measures this light-matter interaction to generate spectra such as those displayed in Figure 2. NIRS is especially sensitive to the presence of certain functional groups like -CH, -NH, -OH, and -SH. Therefore, NIR spectroscopy is an ideal method to quantify different QC parameters like water content (moisture), cetane index, RON/MON (research and motor octane numbers), flash point, and cold filter plugging point (CFPP), just to name a few. Furthermore, the interaction is also dependent upon the matrix of the sample itself, which also allows the detection of physical and rheological parameters like density and viscosity.

Figure 2. Diesel spectra resulting from the interaction of NIR light with the respective samples.
All of this information is contained in a single spectrum, making this method suitable for quick multiparameter analysis. Liquid samples such as oils are secured within an appropriate container or vial (Figure 3), then placed as-is on the smart vial holder.
Figure 3. Liquid sample placement for NIR spectra measurement on the smart vial holder from Metrohm.
The measuring mode is referred to as «transmission», generally an appropriate procedure for analyzing liquids. For transmission measurement (Figure 4), the NIR light will travel through the sample while being absorbed. Unabsorbed NIR light passes to the detector. In less than 60 seconds the measurement is completed and the results are displayed.
Figure 4. A. Measurements of liquids are typically done with disposable vials. B. The NIRS measurement mode is known as transmission, where light travels through the sample while being absorbed (from left to right in the illustration).
The procedure to obtain NIR spectra already highlights two major advantages of NIR spectroscopy compared to other analytical techniques: simplicity regarding sample measurement, and speed:

  • Fast technique with results in less than a minute.
  • No sample preparation required – measure samples as-is.
  • Low cost per sample – no chemicals or solvents needed.
  • Environmentally friendly technique – no waste generated.
  • Non-destructive – precious samples can be reused after analysis.
  • Easy to operate – inexperienced users are immediately successful.

Read our previous blog posts to learn more about NIRS as a secondary technique.

Where can NIRS be used in the refining process?

The refining process can be divided into three different segments:

  • Upstream
  • Midstream
  • Downstream

Upstream describes the process of converting crude oil into intermediate products. Refineries are usually very large complexes with several hazardous explosive areas. Therefore, operators are reluctant to transport samples from the different processes to the laboratory. Even the process of obtaining samples for analysis at external QC laboratories is laborious and can require significant paperwork and certified transport services. For obvious reasons, in most cases inline measurements are preferred. These types of measurements are typically done by process NIRS analyzers.

Read more about the difference between atline, online, and inline analyses in our blog post.
Curious about NIRS analyzers for inline process measurements, even in explosive areas? Visit our website to learn more.
Midstream, shown here in Figure 5, offers many more opportunities for the Metrohm DS2500 Petro Analyzer to assist in quality control.
Figure 5. Flowchart of how crude oil becomes gasoline at the local gas station, and where NIRS can perform quality checks during the process.
Fuel is constantly checked for quality when it is received as well as supplied, and in addition to this many terminals also test fuel quality prior to offloading the trucks. The total time for receiving and offloading fuel into a storage tank is approximately 30 minutes, so a fast analysis technique like NIRS is very advantageous.

Downstream at fuel depots and gas stations, the regulatory agencies require measurement of many of the same quality parameters as in the production of gasoline and diesel, and this can also be accomplished with NIRS. There is a significant advantage if the analysis can be done on-site using fresh samples and without the hassle of needing to transport them to testing laboratories.

Mobile NIRS fuel testing using the Metrohm NIRS XDS RapidLiquid Analyzer (XDS-RLA) has been successfully implemented in a number of countries where they enjoy the benefits of having instantaneous on-site results for gasoline and diesel testing. The calibrations developed on the XDS-RLA are easily transferrable to the DS2500 Petro Analyzer. The DS2500 Petro Analyzer does not require trained analysts, and the calibrations do not require constant maintenance, making this an ideal way to monitor different fuels at service stations and more.
Figure 6. Examples of mobile fuel testing with the Metrohm DS2500 Petro Analyzer.
Learn more about the possibilities of petrochemical analysis with Metrohm DS2500 Analyzers in our free brochure.

NIRS as an ASTM compliant tool for QC

Method development

ASTM E1655: Standard Practices for Infrared Multivariate Quantitative Analysis

«These practices cover a guide for the multivariate calibration of infrared spectrometers used in determining the physical or chemical characteristics of materials. These practices are applicable to analyses conducted in the near infrared (NIR) spectral region (roughly 780 to 2500 nm) through the mid infrared (MIR) spectral region (roughly 4000 to 400 cm-1).»

Multivariate analysis of petroleum products

ASTM D8321: Standard Practice for Development and Validation of Multivariate Analyses for Use in Predicting Properties of Petroleum Products, Liquid Fuels, and Lubricants based on Spectroscopic Measurements

«This practice covers a guide for the multivariate calibration of infrared (IR) spectrophotometers and Raman spectrometers used in determining the physical, chemical, and performance properties of petroleum products, liquid fuels including biofuels, and lubricants. This practice is applicable to analyses conducted in the near infrared (NIR) spectral region (roughly 780 nm to 2500 nm) through the mid infrared (MIR) spectral region (roughly 4000 cm-1 to 40  cm-1).»

Method validation

ASTM D6122: Standard Practice for Validation of the Performance of Multivariate Online, At-Line, Field and Laboratory Infrared Spectrophotometer, and Raman Spectrometer Based Analyzer Systems

«This practice covers requirements for the validation of measurements made by laboratory, field, or process (online or at-line) infrared (near- or mid-infrared analyzers, or both), and Raman analyzers, used in the calculation of physical, chemical, or quality parameters (that is, properties) of liquid petroleum products and fuels.»

Results validation

ASTM D8340: Standard Practice for Performance-Based Qualification of Spectroscopic Analyzer Systems

«This practice covers requirements for establishing performance-based qualification of vibrational spectroscopic analyzer systems intended to be used to predict the test result of a material that would be produced by a Primary Test Method (PTM) if the same material is tested by the PTM.»

Typical NIRS applications and parameters for the petrochemical and refinery industry

Petrochemicals are subject to standardized test methods to determine their chemical, physical, and tribological properties. Laboratory testing is an indispensable part of both research and development and quality control in the production of petrochemicals. The following test parameters are typically important to measure in the petrochemical and refinery industry (Table 1).

Table 1. Examples for use of NIRS for selected petrochemical QC parameters.
Specific Gravity (API) Gravity meter ASTM D298

AN-NIR-022

AN-NIR-024

AN-NIR-025

AN-NIR-041

AN-NIR-053

AN-NIR-071

AN-NIR-075

AN-NIR-080

AN-NIR-086

AN-PAN-1052

Boiling Point Distillation ASTM D2887
Cold Filter Plugging Point (CFPP) Standardized filter device ASTM D6371
Pour Point Pour Point analyzer ASTM D97
Cloud Point Cloud Point analyzer ASTM D2500
Flash Point Flash Point tester ASTM D93
Viscosity Viscometer ASTM D445
Color Colorimeter ASTM D1500
Density Densimeter ASTM D792
Fatty Acid Methyl Ester (FAME) FTIR ASTM D7806
Reid Vapor Pressure RVP analyzer ASTM D323
PIANO (Paraffins, Isoparaffins, Aromatics, Naphthenes, Olefins) Gas chromatograph ASTM D6729
Octane Number (RON/MON) CFR Engine ASTM D2699

ASTM D2700

Cetane Number CFR Engine ASTM D613
Diene value / MAV index Titration UOP 327-17
Parameter Conventional method ASTM method Relevant NIRS Application Notes

Future installments in this series

This article is a general overview of the use of NIR spectroscopy as the ideal QC tool for the petrochemical / refinery industry. Future installments will be dedicated to the most important applications and will include much more detailed information. Don’t miss our next blogs on the topics of:

 

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

Post written by Wim Guns, International Sales Support Spectroscopy at Metrohm International Headquarters, Herisau, Switzerland.
Best practice for separation columns in ion chromatography (IC) – Part 1

Best practice for separation columns in ion chromatography (IC) – Part 1

The high performance ion chromatography (IC) separation column is often referred to as the «heart» of the ion chromatograph. The reason for this denomination is straightforward: the column is responsible for the separation of the analytes of interest from each other as well as from interfering sample matrix ions. The unique separation capabilities of IC columns allow the determination of multiple analytes within one run. In this blog series, we will share what is required to ensure the proper operation of an IC column and how to maximize the column lifetime.

Standard operating conditions

To begin with, the standard operating conditions should be considered, such as the eluent (mobile phase) composition, the eluent flow rate, the column oven temperature, and the detection method. These standard conditions are specific for each individual column type and correspond to the conditions that work best with the application the column is intended for. Every analytical column sold by Metrohm is delivered with a certificate of analysis (CoA) which is recorded under standard operating conditions. In the following table, you can find the standard operating conditions for three different Metrohm IC columns.

Column Metrosep C 6 – 150/4.0 Metrosep A Supp 17 – 150/4.0 Metrosep Carb 2 – 150/4.0
Eluent composition 1.7 mmol/L nitric acid

1.7 mmol/L dipicolinic acid

5.0 mmol/L sodium carbonate

0.2 mmol/L sodium bicarbonate

100 mmol/L sodium hydroxide

10 mmol/L sodium acetate

Flow rate 0.9 mL/min 0.6 mL/min 0.5 mL/min
Oven Temperature 30 °C 25 °C 30 °C
Detection Non-suppressed conductivity Suppressed conductivity Amperometric detection
Analytes Lithium, sodium, ammonium, potassium, calcium, magnesium Fluoride, chloride, nitrite, bromide, nitrate, sulfate, phosphate Inositol, arabitol, sorbitol, glucose, xylose, fructose, lactose, sucrose

Equilibration

Next to the standard operating conditions, the start-up parameters play an important role for the lifetime of the separation column. High mechanical and thermal stress is a frequent reason for decreased column lifetime. It is therefore recommended to slowly increase the eluent flow rate to the column and to avoid thermal and pressure shocks. For more information regarding specifics, please refer to the recommended equilibration conditions in the corresponding column leaflet.

Operating limits

Based on the different stationary phase designs that are required to achieve different selectivities and guarantee a wide range of IC applications, the chemical and physical properties of the ion exchangers can vary. Therefore, different operating limits are recommended for various column types and set according to stress tests performed during the development of a product. Before beginning to optimize an application, please refer to the respective column leaflet for the corresponding operating limits to achieve optimal results and guarantee a long column lifetime.

The chemistry of the ion exchanger defines the limits of operating temperature, eluent pH, and organic modifiers that can be present in the eluent. These values are valid for every dimension of a column type (e.g., the Metrosep C 6). Exceeding these limits can strongly affect column performance and, in the worst case, lead to irreversible damage. Tolerated flow rates and maximum pressure correlate with physical properties and of course column dimension—therefore these limits are set for every column dimension (e.g., Metrosep C 6 – 100/4.0 vs. Metrosep C 6 – 250/2.0).

Are you searching for a specific IC column for your research? Then check out the Metrohm Column Finder!

Information leaflet

In addition to the CoA, a lot of necessary valuable information can be found in the respective column leaflet. Instructions about equilibration, regeneration, operating limits, and much more can be found in the column leaflet which is provided for every IC column type offered by Metrohm.

You can download an example of a column leaflet here, where the information is provided in several different languages (DE, EN, FR, and ES).

Storage

Depending on the operation conditions and the properties of the ion exchanger material, different storage conditions are recommended for different column types. These conditions (storage temperature and storage eluent) are described on the respective column leaflet and should be strictly followed.

Please note that storing a new column for long time periods as well as frequently switching between storage and operation can actually be more stressful to the column than operating it 24/7! It is therefore not recommended to stock a column for a long time without using it.

Column guards

To protect your separation column from sample contamination and to extend its lifetime, it is critical to use a guard column. The guard column should be exchanged at regular intervals—a general rule of thumb is that about four guard columns will be used over the lifetime of an IC column.

Guard columns are offered in two designs: «on column» (e.g. Metrosep A Supp 17 Guard/4.0, on the left) and as a separate guard that is connected to the IC column with a capillary (e.g. Metrosep A Supp 17 S-Guard/4.0, on the right). For microbore separation columns, the corresponding microbore column guards are recommended (e.g. Metrosep A Supp 10 Guard/2.0).

By default, a guard column with the same material as the column should be used. However, for special applications, combining different ion exchange materials by using a different guard column can help to optimize the separation. One such example is in the case of sulfate and sulfite, as shown below.

Pulsation absorber

With the exception of the Eco IC product line, all Metrohm ion chromatography instruments are equipped with a pulsation absorber. For the Eco IC, it is strongly recommended to add the pulsation absorber as an option. As mentioned earlier, IC columns do not like repeated mechanical stress, especially those based on polyvinyl alcohol or polymethacrylate stationary phases. Therefore, the pulsation absorber is a useful tool to protect the column from possible pressure fluctuations in the system and enhance the column lifetime.

Find out more about the full line of Metrohm ion chromatography products and accessories on our website!

Chemical quality

In ion chromatography, the ratio between the volume of sample and the volume of eluent that flows through the column is very small, usually in the range of 1:1000. Therefore the quality of the chemical reagents used in the eluent preparation plays a crucial role for the column lifetime. To guarantee optimal performance, use chemicals denominated as «for IC» whenever possible as they are tested particularly on impurities (e.g. metals) that could harm the chromatographic column.

For dilution of the eluent components, ultrapure water is typically used in ion chromatography. To ensure good chromatographic results, the ultrapure water should have a specific resistance greater than 18 MΩ · cm and be free from particles. The ultrapure water is filtered through a 0.45 µm filter and treated with UV. Modern ultrapure water sources for laboratory use guarantee this level of water quality (Type I).

Sample preparation

With the Metrohm Inline Sample Preparation (MISP) options, Metrohm provides a large number of sample preparation techniques that are beneficial to the separation column, as well as analysts. Instead of loading the full sample onto the column, these techniques ensure the reduction of several sample matrix effects, thus avoiding potential harm to the column.

One of the most prominent MISP techniques is Inline Ultrafiltration (illustrated here), which efficiently removes particles from the sample in a fully automated way, before they ever reach the column. In that way, column blockage from dirty samples can be avoided without any manual effort.

iColumn features

All Metrosep columns offered by Metrohm are equipped with an intelligent chip containing useful information about various column operation conditions (e.g., equilibration, standard operation conditions, operation limits, and more) and tracks certain parameters (e.g., set to work, number of injections, number of working hours, and maximum working values such as pressure and flow rate) over the column lifetime. It is beneficial to attach the column chip to the chip reader as illustrated here for proper monitoring as well as support.

Column shelf life

How many injections are possible on a specific column? Unfortunately, it is not possible to deliver an exact answer to this question. This is because the column lifetime strongly depends on the sample matrix and elution conditions. Due to the multitude of different applications and samples that can be covered with a single IC column, it is not possible to predict the column life for every application and sample type.

During column development, several endurance tests are performed under standard conditions with a guard column using appropriate standards. Under these conditions, the column must withstand at least 2000 injections.

The History of Metrohm IC

Metrohm ion chromatography: bringing top quality and exceptional analytical performance to the lab since 1987. 
Post written by Dr. Vincent Diederich (Jr. Product Manager IC Columns) and Dr. Anne Katharina Riess (Head of Column Division) at Metrohm International Headquarters, Herisau, Switzerland.
Does counter electrode (CE) size matter?

Does counter electrode (CE) size matter?

To begin, let’s go back to the year 1950 when metallurgists and chemists tried to shine a light on a fascinating electrochemical phenomenon originally discovered in the 17th century by the chemist Sir Humphry Davy [1].

Sir Humphry Davy (1778–1829) was credited with many discoveries in the field of electrochemistry.

If you dip a wire made of iron (or as electrochemists say: an iron electrode) into diluted sulfuric acid (which is considered the electrolyte), it instantly starts to dissolve—it corrodes. If you then insert another electrode which does not corrode (e.g. platinum), and connect the iron electrode to the negative pole of a current source, and the platinum wire (electrode) to the positive pole, the iron dissolution will slow down or even stop, depending on the voltage applied.

On the other hand, if you connect the iron electrode to the positive pole and raise the voltage from very low values to higher ones, the dissolution grows exponentially with the increasing voltage.

However, above a certain current limit (and depending on the electrode area, electrolyte composition, and temperature), the current suddenly drops to very low values, and the iron electrode stops dissolving. This phenomenon was detected by Michael Faraday, and he called it «passivation». This phenomenon has been subject of controversy and disputes until the 1950s when a better understanding was possible with the invention of the modern potentiostat (Figure 1).

Figure 1. Basic line diagram of a potentiostat/galvanostat.

In experiments where the ohmic drop may be high (e.g., in large-scale electrolytic or galvanic cells or in experiments involving nonaqueous solutions with low conductivities), a three-electrode electrochemical cell is preferable. In this arrangement, the current is passed between the working electrode (WE) and a counter (or auxiliary) electrode (CE).

Learn more about the ohmic drop in our free Application Notes.

The counter electrode can be made of any available electrode material because its electrochemical properties do not affect the behavior of the working electrode of interest. It is best to choose an inert electrode so that it does not produce any substances by electrolysis that will reach the working electrode surface and cause interfering reactions there (e.g., platinum or carbon). Because the current flows between the WE and the CE, the total surface area of the CE (source/sink of electrons) must be larger than the area of the WE so that it will not be a limiting factor in the kinetics of the electrochemical processes under investigation.

Sometimes it is placed in a compartment separated from the working electrode by a sintered-glass disk or other separator (Figure 2). Bulk electrolysis experiments typically require much longer times than electroanalytical experiments, so separation of the counter electrode is required.

Figure 2. Gastight electrochemical cell for CO2 reduction study (click to enlarge).

The potential of the working electrode is monitored relative to a separate reference electrode (RE), positioned with its tip nearby (with a Luggin capillary as shown in Figure 3). The potentiostat used to control the potential difference between the working electrode and the reference electrode has a very high input impedance so that a negligible current flows through the reference electrode. Consequently, the potential of the reference electrode will remain constant and equal to its open-circuit value. This three-electrode arrangement is used in most electrochemical experiments.

The counter electrode is used to close the current circuit in the electrochemical cell. It is usually made of an inert material (e.g., Pt, Au, graphite, or glassy carbon) and it hosts a redox reaction which occurs at the CE surface that balances the redox reaction at the surface of the WE. The products of this reaction can diffuse to the WE and interfere with the redox reaction occurring at that site. However, in electroanalytical experiments such as cyclic voltammetry (CV), the time scale of the experiment is too short for this diffusion to be able to cause significant interferences. Therefore, in most cases there is no need to place the CE in a separate compartment, as shown in the electrochemical cell in Figure 3.

Figure 3. Metrohm Autolab 1 liter corrosion cell labelled with electrodes.

The potential profile in an actual cell depends on the electrode shapes, geometry, solution conductance, and more. If the reference electrode is placed anywhere except precisely at the electrode surface, some fraction of an uncompensated potential, iRu (due to the uncompensated resistance, Ru) will be included in the measured potential. Even when the tip of the reference electrode is designed for very close placement to the working electrode by use of a fine tip (Luggin capillary, also known as a Luggin-Haber capillary), some uncompensated resistance usually remains. Modern electrochemical instrumentation includes circuitry for electronic compensation of the iRu term [2].

As mentioned earlier, the electrochemical reaction of interest takes place on the working electrode, and the electron transfer there will generate the measured current which flows between the WE and CE. As a general rule for accurate current measurements and an unhindered flow of electrons in the cell, the counter electrode should be three times larger than the working electrode. The kinetics of solvent electrolysis are generally slow, so the only way to ensure the CE reaction can sustain the required cell current is to significantly increase the surface area (e.g., with Pt gauze or a Pt rod/sheet).

However, in poorly conductive media (or high currents), when positioning the counter electrode in the cell (i.e., for the cell geometry), it is worth considering where the field lines between the WE and CE will go, and whether the WE will experience a uniform field. It is generally recommended to move the counter electrode away from the working electrode, or even to put it behind a separator, unless you are certain that the WE reaction is insensitive to pH. The CE reaction sustains the current flow, but it is often unknown exactly what kind of reactions are occurring at the counter electrode. For most aqueous electrochemistry experiments this is likely to be solvent electrolysis, which leads to a change in the pH.

When the application requires, the potential of the counter electrode can be monitored during the experiment with the S2 electrode of VIONIC. Find out more in our free Application Note.

Alternative counter electrode materials

Platinum is expensive, and it is too costly to use for large CE areas. Suitable alternative electrode materials include nickel-based alloys or carbon. Be aware that nickel-based alloys may passivate, and carbon will be oxidized at high potentials of the CE. Therefore, those materials should have a sufficient area to avoid strong polarization.

Platinized titanium is a good choice when larger counter electrode areas are required. Platinized titanium is produced either in the form of sheets or mesh grids. Mesh grids (Figure 4) have very desirable properties: a large active area compared to the geometric area, and the electrolyte is able to flow through the counter electrode.

Figure 4. Example of wire mesh grid suitable for use as a counter electrode.

 Also, at the counter electrode, a voltage drop occurs across the metal-electrolyte interface. To reduce this drop, it may be useful to use platinum covered with platinum black to increase the roughness, and consequently, the active surface area.

Final notes

Size matters in typical electrochemical measurements. Larger counter electrodes create an unconstrained current flow between WE and the CE, leading to more stable and insightful experiments. It is equally important that the overall cell configuration provides the required current density distribution. Thus, a counter electrode of the same size as the working electrode, mounted parallel to it, may optimize the current distribution without significant adverse effects from the size of the counter electrode.

The reaction occurring at the CE should be fast so that the potential drop between the counter electrode and the electrolyte does not limit the polarization that can be applied.

The products of the reaction occurring at the CE should not contaminate the solution. In practice, there will always be an electrochemical reaction at the counter electrode, and the products of that reaction should be harmless or able to be easily removed. Inert electrodes such as platinum or graphite are often used, in which case the reaction products are usually gases (oxygen or chlorine when anodic, or hydrogen when cathodic) that can be removed by bubbling air or nitrogen past the counter electrode (although there may also be a pH change at the CE).

Curious about electrochemistry?

Metrohm has you covered.

References and suggested further reading

[1] Knight, D. Humphry Davy: Science and Power (Volume 2 of Cambridge Science Biographies); Cambridge University Press: Cambridge, UK, 1998.

[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] Yang, W., Dastafkan, K., Jia, C., et al. Design of Electrocatalysts and Electrochemical Cells for Carbon Dioxide Reduction Reactions. Adv. Mater. Technol. 2018, 3, 1700377. doi:10.1002/admt.201700377

[4] Cottis, R. A. 2.30 – Electrochemical Methods. In Shreir’s Corrosion; Cottis, B., Graham, M., Lindsay, R., Lyon, S., Richardson, T., Scantlebury, D., Stott, H., Eds.; University of Manchester: Manchester, UK, 2010; Vol. 2, pp 1341–1373. doi:10.1016/B978-044452787-5.00068-8

[5] Vanýsek, P. Impact of electrode geometry, depth of immersion, and size on impedance measurements. Canadian Journal of Chemistry 1997, 75(11), 1635–1642. doi:10.1139/v97-194

Post written by Martijn van Dijk, Area Manager Electrochemistry at Metrohm Autolab, Utrecht, The Netherlands.

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

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

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!

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

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.

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.

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

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

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

Fast and fundamental: influences on reliable electrochemical measurements

Fast and fundamental: influences on reliable electrochemical measurements

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].

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 RuCdl (s) depends directly on the uncompensated resistance Ru (Ω) (i.e. the resistance of the electrolyte between the reference and the working electrode) and the double-layer capacitance Cdl (F) of the electrode [2].

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

Where RuCdl (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 RuCdl ), the exponential term approaches 1 and significant errors in 𝐸true with respect to 𝐸appl can arise. For slow scan rates (i.e. when 𝑡 is much larger than RuCdl), the exponential approaches 0 and the errors become negligible.

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

  • Reduce Ru 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 Cdl will be minimized
  • Move the reference electrode as close as possible to the working electrode (e.g., by using a Luggin capillary) so that Ru will 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.

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 fSIGNAL, then according to the Nyquist Theorem [3] the sampling rate fSAMPLE must be at least 2 fSIGNAL (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.

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

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

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.

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

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.

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.

Proper electrode immersion depth

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.

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

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).