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Best practice for separation columns in ion chromatography (IC) – Part 2

Best practice for separation columns in ion chromatography (IC) – Part 2

The second part of this blog series about best practice for IC separation columns focuses on application related topics that have an impact on the column suitability and stability. First, there is the proper choice of the column that best suits the intended application. Then we turn to the operating parameters which can be modified in order to optimize the separation between analytes, and what the respective effects and possibilities are.

Choice of column length and diameter

Metrohm offers a broad range of columns that contain different stationary phases, have different lengths and/or inner diameters. The choice of the stationary phase has a great impact on the selectivity between the individual analytes on the one hand, as well as the stability against different sample matrices on the other hand. Instead, the column length has no impact on the selectivity, but rather on the separation efficiency between the individual peaks.

Find out more about Metrohm’s wide selection of separation columns for ion chromatography in our Column Catalog.
Effects of column length

In the following chromatograms (Figure 1), the effect of the column length on the separation efficiency for the Metrosep A Supp 17 column series is shown. Whenever choosing a column length, you should take the complexity of the intended separation and the presence of matrix components that could disturb the ions of interest into account.

Figure 1. Effect of column length on the retention times of the standard anions on the Metrosep A Supp 17 column (1: fluoride, 2: chloride, 3: nitrite, 4: bromide, 5: nitrate, 6: sulfate, 7: phosphate). Click image to enlarge.
Effects of column diameter

In addition to providing different lengths of IC separation columns, Metrohm also offers most columns in both in 4 mm inner diameter and 2 mm inner diameter (known as «microbore») versions. With regard to this, there are several criteria to distinguish:

  • If you use online systems in a continuous mode (i.e. systems which run unattended for several days in a row such as the Metrohm Process Analytics MARGA system – Monitor for AeRosols and Gases in Ambient air), we recommend using 2 mm IC columns. Due to the reduced flowrate for microbore columns (only 25% of the flowrate for 4 mm columns), the eluent and the regenerant solutions last much longer, which increases the time the instrument can be left unattended.
  • There are applications that require hyphenated techniques such as IC-MS for higher analyte selectivity and sensitivity. In this case, the use of 2 mm columns is ideal. The low flowrate is optimal for the electrospray process, and thus no flow splitter is required before entering the mass spectrometer.
  • Sometimes, only a limited amount of sample is available for injection. In these situations, 2 mm columns are preferred. This is because less dilution/diffusion occurs during the separation process and therefore higher signals are obtained.
  • On the other hand, if your sample contains a high load of matrix components, then selecting a suitable 4 mm IC columns will be a better choice because of the higher capacity available to separate the desired analytes from the matrix.
Find out more about MARGA and its capabilities for continuous air quality monitoring in our blog post.

Optimizing the analyte separation

Next to the column itself, several other parameters can be modified to optimize the selectivity of the separation. These parameters include temperature, eluent components and strength, and organic modifiers.

Effects of modifying the temperature

One of the simplest ways to fine tune the separation selectivity in IC is by modifying the temperature of the analysis. This is accomplished by using the integrated column oven in the instrument (if available). Multiple effects can be observed, for instance in anion analysis. As an example, the impact of the temperature on the selectivity is shown in the chromatogram overlay (Figure 2) for the Metrosep A Supp 17 column line.

Figure 2. Effect of temperature variation on the retention times of a suite of standard anions on the Metrosep A Supp 17 column (1: fluoride, 2: chloride, 3: nitrite, 4: bromide, 5: nitrate, 6: sulfate, 7: phosphate). Click image to enlarge.
  • The monovalent ions such as fluoride, chloride, nitrite, bromide, and nitrate are all accelerated with increasing temperature, indicating that fewer interactions with the stationary phase happen.
  • The behavior of multivalent ions such as phosphate or sulfate is more complicated to describe and will vary with each stationary phase. In general, multivalent ions are retarded more at higher temperatures, which causes the retention times to increase, as can be seen for sulfate. Phosphate on the other hand behaves differently, because of the temperature induced change of the eluent pH in a range close to the pKa value of phosphate. Due to this pH change, the effective charge of the phosphate ion changes as well (in this example, the effective charge is reduced with increasing temperature).
  • The peak shape of the polarizable ions such as nitrite, bromide, and in particular nitrate, is significantly improved at higher temperatures. The reason for this behavior is the reduction of secondary interactions with the stationary phase.
Effects of modifying the eluent composition and strength

Eluent composition and strength can be used to change the elution order of several analytes while using the same separation column. In cation chromatography, a retention model was developed by P.R. Haddad and P.E. Jackson, which allows researchers to predict retention times when changing the eluent composition [1].

Considering that the column remains identical in each determination, no change of ion exchange equilibrium and column capacity is to be expected. Therefore, when changing only the eluent concentration, the following correlation can be used:
Where:

  • k’ is the retention factor of the analyte of interest
  • c is a constant
  • x is the charge of the analyte
  • y is the charge of the eluent
  • Ey+M is the concentration of the eluent in the mobile phase
If nitric acid is used as the eluent, y = 1, and the model can be simplified to:
Applying this formula to practical situations in the laboratory means the following: with increasing the eluent strength, alkaline earth metals are accelerated much faster (x = 2) in comparison with alkali metals (x = 1), and thus it is possible to elute magnesium before potassium. This effect is called electroselectivity.

Multivalent metal ions are capable of forming complexes with dedicated complexing agents. Therefore, selectivities can be modified by adding complexing agents to the eluent. As an example, dipicolinic acid (DPA) is often used to complex calcium, which leads to a reduction of the effective charge of calcium. As a consequence, the retention time of calcium is reduced and calcium elutes before magnesium in the chromatogram (Figure 3).

The retention of monovalent cations can be influenced by the addition of crown ether to the mobile phase.

Figure 3. Effect of DPA concentration in the eluent on the retention times of several cations measured using the Metrosep C 6 column.
Anion systems are more complex regarding the retention time model, although the same electroselectivity effect can be observed to some extent for anions. However, when changing the eluent strength, the eluent pH also frequently changes, leading to different deprotonation equilibria of multivalent anions (e.g. phosphate). This influences the effective charge of the analyte, and by doing so, also influences its retention in a similar way as previously described for the effects of changing temperature.

In some cases, the use of a small amount of an organic modifier such as methanol, acetonitrile, or acetone in the eluent can make sense:

  • If bacterial contamination has been an issue before, the addition of 5% methanol to the eluent can help prevent future bacterial growth.
  • When samples containing a lot of organic solvent(s) need to be injected and no sample pretreatment such as extraction or matrix elimination (MiPCT-ME) is possible, it is recommended to add a suitable organic modifier to the eluent to ensure that the organic solvent(s) can be properly flushed out of the chromatographic column.
  • When using IC-MS, it is also recommended to add an organic modifier to the eluent to improve the electrospray process.

Be aware that the addition of organic modifiers will also affect the separation selectivities. For the standard anions, the effect is similar to that observed with increased temperatures: the peak shapes of the polarizable ions such as nitrite, bromide, and nitrate are improved.

Organic acids on the other hand may react very differently compared to the standard anions, and their reaction also strongly depends on the type of organic modifier used. Sample chromatograms that show the effect of the organic modifier on retention of analytes are shown in the manual for the Metrosep A Supp 10 column.

Download the Metrosep A Supp 10 column manual here to see example chromatograms showing the effects of organic modifier on analyte retention time.
For more information about column care, check out our blog post for different tips and tricks.

The History of Metrohm IC

Metrohm ion chromatography: bringing top quality and exceptional analytical performance to the lab since 1987. 
Reference

[1Haddad, P. R.; Jackson, P. E. Ion Chromatography: Principles and Applications; Journal of chromatography library; Elsevier; Distributors for the U.S. and Canada, Elsevier Science Pub. Co: Amsterdam, Netherlands; New York: New York, NY, USA, 1990.

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.
Best practice for electrodes in Karl Fischer titration

Best practice for electrodes in Karl Fischer titration

Have you ever asked yourself why you need an electrode for the endpoint detection in Karl Fischer (KF) titration? Theoretically, the endpoint of a Karl Fischer titration could be determined based on the color change of the reagent. However, if accuracy and reproducibility are important, endpoint detection with a double Pt electrode is a much better choice.

As the indicator electrode detects the endpoint, you can imagine that the results depend highly on the condition of the electrode. In coulometry, an additional electrode (generator electrode) is used to generate the iodine needed for the titration. Both electrode types (i.e. indicator and generator electrode) need to be kept in good shape to guarantee the correct results. It goes without saying that cleaning, storage, maintenance, and checks of the KF electrodes are important factors for success. This blog post takes a closer look at these topics.

Did you catch our series about frequently asked questions in Karl Fischer titration? Find them here!

Cleaning

Indicator electrode

Double  Pt-wire or double Pt-ring electrodes can be easily cleaned with an abrasive cleaning agent like aluminum oxide powder or toothpaste. After cleaning, rinse the electrode well with water and let it dry before mounting it in a titration cell. Check out our video below for more tips and tricks about the proper cleaning procedure for Karl Fischer titration indicator electrodes.

Take special care not to bend the Pt pins of the double Pt-wire electrode. Bending the pins can lead to tiny cracks in the glass body of the electrode. Over time, reagent can flow into the electrode and lead to corrosion (short circuit). If this happens, the electrode is beyond repair and needs replacement. Alternatively, a double Pt-ring electrode can be used instead. Problems with bent pins are then a thing of the past.
Generator electrode
Without diaphragm
Rinse generator electrodes without diaphragms with water, or if the contaminant is not water soluble, then rinse with a suitable organic solvent. If the anode or the cathode of the generator electrode shows discoloration or deposits that cannot be removed with rinsing, the electrode can then be cleaned with concentrated nitric acid (65%). Be aware that nitric acid is a strong oxidizing agent and must be handled carefully according to relevant safety regulations and instructions. Remember to first mount the green protection cap on the connector to avoid corrosion caused by fumes of nitric acid. Afterwards, rinse the electrode with water and finally with methanol.
With diaphragm
To remove salt-like residues, the generator electrode with diaphragm can be rinsed with water. Oily contamination can be rinsed off with an organic solvent (e.g. hexane). Sticky residues on the diaphragm can be removed in the following way: 

  1. Mount the green protection cap on the connector of the electrode.
  2. Place the electrode in an upright position (e.g. in an Erlenmeyer flask) and add a few milliliters of concentrated nitric acid (65%) in the cathode chamber. Let the acid flow through the diaphragm.
  3. Fill the cathode chamber with water and let it flow through the diaphragm to remove the nitric acid. Repeat this step two or three times. A simple way to see whether another rinsing step is required is by performing a quick check of the pH value at the cathode using pH indication paper.
  4. Finally, fill the cathode chamber with methanol and let it flow out.

Now the generator electrode is as good as new and ready for use in a titration cell again.

Maintenance

Except for the generator electrode with diaphragm, KF electrodes are maintenance free. However, the catholyte filled in the generator electrode with diaphragm can decompose over time. To avoid any influence of the decomposition products on the results, exchange the catholyte on a regular basis according to the manufacturer’s recommendations.

Storage

Unlike pH electrodes, KF electrodes do not contain a glass membrane that could potentially dry out. Therefore, no special solution is required in which to store KF electrodes. If you use the electrodes frequently, it is recommended to keep the electrodes mounted in the titration cell and immersed in the KF reagent. Alternatively, all KF electrodes (indicator and generator electrodes) can be stored dry.

What to check for

It is recommended to check the complete titration setup instead of only the electrode(s).

Volumetry

Carry out a threefold titer determination using either a liquid or a solid water standard suitable for volumetry and calculate the mean value of the titer. Then, determine the water content of a water standard (also via triplicate determination). Make sure that you do not use the same standard as for the titer determination but use a different batch of the standard or even a completely different standard. Calculate the water content and compare it to the certified water content of the standard.

If the recovery is determined to be in the range of 97–103%, the titration system (including the electrode) is working fine. Finding values outside this range means that there is something wrong with the titration system or with the determination procedure. Results of the sample analysis would very likely also deviate from the real water content. Therefore, it is important to find the reason for values that are too high or too low. Sometimes the reason for deviations is just an air bubble in the dosing cylinder or due to an exhausted molecular sieve. However, if you do not find the reason, do not hesitate to contact your local Metrohm agency.

Coulometry

Water standards with lower water contents (0.1%) are available to properly check the health of coulometric titration systems. Carry out a water content determination in triplicate with such a standard. Calculate the recovery with the obtained results and the certified water content of the standard.

A recovery value in the range between 97–103% means that everything is fine with the system and that the electrodes work as expected. As with volumetry, in coulometry it is important to find the reason for any deviating recovery values. Make sure that you find and eliminate the problem to obtain correct results for your samples.

What you should avoid

  • Do not use solvents that contain ketones or aldehydes (e.g. denatured ethanol) to clean KF electrodes or any KF accessories.
  • Do not treat KF electrodes in an ultrasonic bath. This might destroy the electrode.
  • For drying, use a maximum temperature of 50 °C. Higher temperatures might damage the electrode.
  • Do not bend the Pt pins of the double Pt-wire electrode.

Summary

As you can see, keeping your KF electrodes in good shape is actually very simple. Regular cleaning helps to avoid erroneous results and ensures that your Karl Fischer electrodes will work for a long time.

Best practice for electrodes in titration

Treat your sensors right!
Post written by Michael Margreth, Sr. Product Specialist Titration (Karl Fischer Titration) at Metrohm International Headquarters, Herisau, Switzerland.
NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 2

NIR spectroscopy in the petrochemical and refinery industry: The ASTM compliant tool for QC and product screening – Part 2

Differences between gasoline, diesel, and jet fuel

Gasoline is a fuel made from crude oil and other petroleum-based liquids, containing carbon numbers generally between 4 and 12, and exhibiting boiling points of up to 120 °C. Gasoline is primarily used as a fuel for vehicles. Petroleum refineries and blending facilities produce motor gasoline for sale at gas (or petrol) stations. Most of the gasoline that petroleum refineries produce is unfinished gasoline. This unfinished product requires blending with other liquids to control parameters such as octane rating and volatility to make gasoline meet the basic requirements for fuel that is suitable for use in spark ignition engines.

Diesel fuel is refined from crude oil at petroleum refineries. «Diesel» is the common term for the petroleum distillate fuel oil sold for use in motor vehicles that use the compression ignition engine, invented by the German engineer Rudolf Diesel (1858–1913). He patented his original design in 1892. One of the fuels that Rudolf Diesel originally considered for his engine was vegetable seed oil, an idea that eventually contributed to the biodiesel production process of today. Prior to 2006, most diesel fuel contained high quantities of sulfur. Sulfur emissions from combusting diesel fuel leads to air pollution that is quite harmful to human health. Therefore, the U.S. Environmental Protection Agency issued requirements to reduce the sulfur content of diesel fuel to be as low as 15 mg/L. Diesel fuel contains components with a carbon number range from 8 to 21 (though mainly between 16–20) and is the fraction that boils between 200 °C and 350 °C.

Jet fuels (or aviation fuels) are one of the basic products used by aircraft. Jet fuel is comprised of refined petroleum products with carbon numbers between 10 to 16 (although they can range from 6 to 16), and it boils between 150 °C and 275 °C. This type of fuel is heavily regulated by national and international bodies. There are two main types of jet fuel: Jet A and Jet B. The main difference between the two is the freezing point. Jet B is usually used for military operations and locations with inclement weather. Jet A is mainly used to fuel commercial airplanes.

Near-infrared spectroscopy—an ASTM compliant tool to assess the quality of gasoline, diesel, and jet fuel

Near-infrared spectroscopy (NIRS) has been an established method for both fast and reliable quality control within the petrochemical 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, available turnkey solutions for gasoline, diesel, and jet fuel are outlined which have been developed according the NIRS implementation guidelines of ASTM E1655 (method development), ASTM D6122 (method validation), and ASTM D8340 (results validation). Afterward is a discussion about the return on investment (ROI) of using NIRS as an alternative to the CFR Engine.

Did you miss Part 1 in this series about NIRS as a QC tool for the petrochemical industry? Check it out below!

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

NIRS expedites and simplifies fuel quality control

 Without high quality fuels (e.g., gasoline, diesel, and jet fuel), our daily lives would look much different. At the end of the production process as well as at various steps in the distribution chain, the quality of the product needs to be determined. Typically, key quality parameters such as RON/MON (research and motor octane numbers), cetane index, and flash point are determined in the laboratory by chemical and physical methods. These methods not only incur high running costs but they are also quite time consuming.

NIRS on the other hand requires neither chemicals nor sample preparation. This technique can even be used by non-technical people (no chemistry degree necessary) and it provides results in less than a minute. Furthermore, multiple chemical and physical parameters can be determined simultaneously. The combined benefits of this technology make NIRS the ideal solution for many daily QA/QC measurements or ad-hoc at-line analysis.

Metrohm offers the NIRS DS2500 Petro Analyzer for quality control and routine analysis of fuels and is compliant with ASTM D6122. Resistant to dust, moisture, and vibrations, this instrument is not only suitable for laboratory use, but also use in direct production environments. Learn more on our website

Turnkey solutions: available pre-calibrations for gasoline, diesel, and jet fuel

Table 1 lists all constituents covered by the pre-calibrations for these different fuels. Click on the fuel type in the table to learn more about its pre-calibrations offered by Metrohm.

Table 1. Pre-calibrations available for a variety of key quality parameters in gasoline, diesel, and jet fuel.
Fuel type Parameters Range SECV
Gasoline RON 81–100 0.68 0.958
MON 81–88 0.53 0.889
Anti-Knock Index 85–94 0.45 0.948
Aromatics 20–45% 0.011 0.959
Benzene 0.15–0.70 % 0.0004 0.902
Density 0.74–0.76 g/cm3 0.0024 g/cm3 0.797
Olefins 0–25 % 0.013 0.909
Oxygen 0.2–2.0 % 0.00045 0.994
Diesel Cetane index 46–77 0.62 0.987
Cetane number 45–60 0.942 0.942
Density 0.82–0.89 g/cm3 0.0021 g/cm3 0.968
CFPP -22–(+19) °C 2.8 °C 0.963
T95 325–410 °C 7.04 °C 0.799
Flash Point 56–120 °C 2.7 °C 0.97
Viscosity 2–5.5 cSt 0.15 0.91
Kerosene / Jet Fuel Cetane index 36–50 1.1 0.871
API gravity 38–48 ° 0.56 ° 0.931
Aromatics 10–25 % 0.01 0.851
T10 158–200 °C 4.1 °C 0.801
T20 165–205 °C 3.1 °C 0.88
T50 180–220 °C 4.1 °C 0.789
Density 0.78–0.83 g/cm3 0.003 g/cm3 0.936
Flash Point 38–65 °C 4.3 °C 0.62
Freeze Point -65–(-40) °C 3. 5°C 0.576
Hydrogen 13.2–14.2 % 0.0005 0.934
Saturates 75–90 % 0.009 0.888
Viscosity at 20 °C 3–7 cSt 0.33 cSt 0.804

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

Application example: quality control of diesel with the NIRS DS2500 Petro Analyzer

The cetane index (ASTM D613), flash point (ASTM D56), cold filter plugging point (CFPP) (ASTM D6371), D95 (ISO 3405), and viscosity at 40 °C (ISO 3104) are among some of the key parameters to determine the quality of diesel. The primary test methods for these parameters are labor intensive and challenging due to the need for multiple analytical methods.

In this turnkey solution, diesel samples were measured in transmission mode with a NIRS DS2500 Petro Analyzer over the full wavelength range (400–2500 nm). The built-in temperature-controlled sample chamber was set to 40 °C to provide a stable sample environment. For convenience reasons, disposable vials with a pathlength of 8 mm were used (Figure 1), which made a cleaning procedure unnecessary.

Figure 1. Quality control of diesel fuel as performed by the Metrohm NIRS DS2500 Petro Analyzer.

The obtained Vis-NIR spectra (Figure 1) were used to create prediction models for the determination of key diesel parameters. The quality of the prediction models was evaluated using correlation diagrams which display the correlation between Vis-NIR prediction and primary method values. The respective figures of merit (FOM) display the expected precision of a prediction during routine analysis (Figure 2).

Figure 2. Correlation plots and figures of merit (FOM) for the different constituents tested in diesel.

This solution demonstrates that NIRS is excellently suited for the analysis of multiple parameters in diesel fuel, providing results in less than one minute without the need for sample preparation or any chemical reagents.

Want to learn more? Download our free Application Note.

Return on investment: CFR Engine vs. NIRS

Gasoline requires intensive checks on several quality parameters which must be within certain specifications before commercialization. These parameters which can also be controlled by NIRS analysis include the research octane number (ASTM D2699) and motor octane number (ASTM D2700), also known as RON/MON.

The importance of measuring these values precisely is not only to comply with regulations, but also because of the further potential to save costs for manufacturers. As an example, RON values exceeding the stated requirements will still be accepted by the market, but these products will then include a higher amount of lucrative long-chain organic molecules. This so-called «RON giveaway» is estimated at approximately 0.5 RON per barrel, resulting in $2.25 million USD/month in lost revenue for a production process of 100,000 barrels per day.

Figure 3. CFR® F1/F2 Octane Rating Unit Combination Research & Motor Method. (Source: CFR Engines Inc.)

The Combination Cooperative Fuel Research (CFR) octane rating engine (model F1/F2) is used to determine the octane quality of gasoline and fuel blending components. This unit is recognized and approved by ASTM D2699 and D2700. The engine is equipped with a heavy-duty crankcase, variable compression cylinder, carburetor with adjustable fuel to air ratio, and knock measurement equipment (Figure 3).

Ready-to-use NIRS systems are also available for monitoring several gasoline quality parameters which cover varied ranges and their respective precisions (Table 1). Additionally, the manufacturers of NIRS analyzers usually offer application support to extend these ranges or improve upon the precision.

An overview of estimated costs for the analysis of RON and MON with a CFR Engine compared to the Metrohm NIRS DS2500 Petro Analyzer is shown in Table 2. The full payback is achieved within two years if considering only 50% of the primary analysis method (CFR Engine) is replaced by NIRS. This calculation is based on 2000 analyses per year (1000 RON + 1000 MON), with total running costs of approximately $32.50 per analysis (chemicals, maintenance, and labor).

Table 2. Cost of ownership CFR engine vs. DS2500 Petro Analyzer.
Total analyses RON + MON per year 2000 2000
Cost of operator per hour $25.00 $25.00
Cost of Analyzer CFR Engine NIRS DS2500 Petro Analyzer
Analyzer $500,000.00 $55,000.00
Total initial costs $0.001 $55,000.00
Running costs consumables / chemicals / maintenance
Chemicals per year (ASTM D2699/D2700) $20,000.00 $0.00
Maintenance cost per year $20,000.00 $1,500.00
Chemicals plus maintenance cost per analysis $20.00 $0.75
Total running costs per year $40,000.00 $1,500.00
Time spent per analysis  30 minutes < 1 minute
Labor cost of 1000 analyses of RON (ASTM D2699) $12,500.00 $416.50
Labor cost of 1000 analyses of MON (ASTM D2700) $12,500.00 $416.50
Labor cost per analysis $12.50 $0.42
Total labor costs per year $25,000.00 $833.00
Total running costs per year $65,000.00 $2,333.00
1Assumption that the instrument had previously been purchased, and therefore this cost is not included in the ROI calculation.

More information about the analysis of RON/MON and other parameters in gasoline can be found in our free Application Notes below.

In this example, RON/MON analysis was used to show cost savings and ROI when using NIRS to supplement a primary method. However, when expanding this to consider other key quality parameters such as the ones indicated in Table 1, the financial incentives for such an investment are even more compelling.

Summary

Near-infrared spectroscopy is very well suited for the analysis of key quality parameters in gasoline, diesel, and jet fuel. Available pre-calibrations are developed and validated in accordance with the ASTM guidelines. Positive aspects of using NIRS as an alternative technology are the short time to result (less than one minute), no chemicals or other expensive equipment needed, and ease of handling so that even shift workers and non-chemists can perform these analyses in a safe manner.

Future installments in this series

This blog article was dedicated to the topic of gasoline, diesel, and jet fuel and how NIR spectroscopy can be used as the ideal QC tool for the petrochemical / refinery industry. Future installments will be dedicated to other important applications in this industry. These topics will include:

 

  • Pyrolysis gasoline (Pygas)
  • Lubricants
  • ASTM Norms

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.
Staircase or linear scans: two options for reliable electrochemical experiments

Staircase or linear scans: two options for reliable electrochemical experiments

Electrochemical experiments are performed by delivering and controlling a potential or current signal to the electrochemical cell/device under test and measuring its response through a potentiostat/galvanostat (PGSTAT). Here, two different options for performing different electrochemical experiments are discussed: linear and staircase scans, as well as some applications where one may be preferred over another.

From analog to digital

Before modern digital electronics were widely available, PGSTATs worked with analog electronics, and therefore delivered analog signals. Analog boards were expensive and time-consuming to produce and test. Moreover, controlling the equipment from a computer is done via digital communication and requires digital electronics.

An analog signal is continuous, and it has virtually infinite resolution. On the other hand, a digital signal is written in discrete bits (0 and 1), so it is not continuous.

Linear scans

To better explain an analog signal, consider a linear sweep voltammetry (LSV) in potentiostatic mode, performed with an analog scan. Here, the applied voltage E versus time plot consists of a straight line between the initial and end potentials. The potential interval between two consecutive data points and the scan rate define the interval time—the slope of the E versus time plot (Figure 1).

Figure 1. A typical linear scan from an initial and an end potential. The interval time and the measurement time are also shown here.

The current resulting from the application of the LSV is measured at the end of the interval time. The measurement time defines the duration of the sampling. The current is composed of a capacitive part ic (given by the charging of the double layer), and a faradaic part if.

A double layer is formed when a potential is applied to an electrode. Then, current flows to the electrode, which becomes charged. Ions from the bulk solution migrate to the surface to balance this charge. Therefore, a layer of ions at the interface between the electrode and the electrolyte builds up, forming the equivalent of a capacitor.

Learn more about the principles and characterization of capacitors in our free Application Note.

The faradaic current is the result of the electrochemical reactions occurring at the working electrode | electrode and counter electrode | electrolyte interfaces, and it changes with the scan rate either linearly or with the square root of the scan rate, depending on the type of electron transfer.

The capacitive current resulting from a linear scan iC,ls is a constant value given by the product of the double layer capacitance Cdl and the scan rate.

Find out more about the differences between linear and staircase cyclic voltammetry on a commercial capacitor in our free Application Note.

Staircase scans

When modern digital electronics became more commercially available and economically feasible, PGSTAT manufacturers adopted them, together with personal computers, to control PGSTATs. This allowed researchers to create more complex procedures as well as to perform data analysis via software. Metrohm Autolab was the first company to deliver computer controlled PGSTATs to the market back in 1989.

In the case of a digital scan, the applied potential versus time plot between an initial and end potential has the typical «staircase» shape of a digitized signal. The interval time tint defines the duration of each step, while the step potential Estep defines the potential difference between two consecutive steps (Figure 2).

Figure 2. A typical staircase scan profile. The step potential Estep is shown along with the interval time and the measurement time.

The resulting current is measured at the end of the step. The measurement time defines the sampling time.

In a digital scan, the step potential  Estep results in a capacitive current iC,ss which rises almost immediately up to the maximum value allowed by the current range, lim(CR), and then it decays exponentially as time constant t = RC. After the decay of the capacitive current, the faradaic current if is predominant.

The current is measured at the end of the step in order to remove the capacitive current and collect only the faradaic one (Figure 3).

Figure 3. Potential step (in blue) and current profile for a staircase scan. The current profile is divided into capacitive current iC,ss (black line) and faradaic current if (purple line). Total current itot is shown in dark red. The measurement (sampling) time is also shown here. For clarity, the decay of the capacitive current and the decay of the faradaic current are not in scale.

Application examples: staircase or linear scan?

Capacitors

Some electrochemical processes can result in a capacitive current, having a characteristic time comparable to the charging of the double layer. In such cases a digital scan would neglect such capacitive currents and all of the information contained within them.

This is the case of highly capacitive cells, such as capacitors and supercapacitors.

In Figure 4, cyclic voltammograms at different scan rates of a commercial 1 µF capacitor are shown. The diagram on the left shows the results from the digital scan, and on the right are the results from the analog scan [1].

Figure 4. Cyclic voltammograms of a 1 µF capacitor at different scan rates. Left: the cyclic voltammogram resulting from a digital scan. Right: the cyclic voltammogram resulting from an analog scan.

It is pretty clear that the digital scan results do not contain any information about the charge/discharge of the capacitor, while the analog scan results have the expected shape of a capacitor’s cyclic voltammograms.

Adsorption/desorption processes

Another application example includes cells in which fast adsorption/desorption of species at the electrode surface occurs in a short time, like the adsorption/desorption of hydrogen as part of the electrochemical behavior of platinum submerged in an aqueous solution of sulfuric acid (Figure 5).

Figure 5. The cyclic voltammogram of a Pt working electrode immersed in a 0.5 mol/L H2SO4 aqueous solution. The reference is a Ag/AgCl 3M KCl electrode, while the counter electrode is a Pt electrode.

Here, the fast adsorption/desorption of hydrogen occurs at time scales that are similar to the charging of the double layer in the capacitor example. Therefore, the linear scan is the preferred option, compared to the digital staircase scan which is unable to capture the hydrogen adsorption/desorption (Figure 6) [2].

Figure 6Top: linear cyclic voltammograms of a Pt working electrode immersed in a 0.5 mol/L H2SO4 aqueous solution at different scan rates. Bottom: staircase cyclic voltammograms of the same setup at the same scan rates.

Redox reactions

Another example of experiments in which the capacitive current should not be neglected are redox reactions in which the electron transfer is very fast. In these situations, the charge transfer resistance is very small and the cyclic voltammogram results in redox peaks which are symmetrical over the potential axes. Examples include redox reactions on species adsorbed on the working electrode surface [3].

VIONIC: the future of electrochemistry

The most recent generation of PGSTATs, such as VIONIC, are equipped with electronics that allow to users perform analog scans without the drawbacks mentioned earlier. This gives researchers the opportunity to choose the type of scan according to the type of systems being studied, the materials, and the importance of the capacitive current for the outcomes of the research.  

Learn more about VIONIC

Maximum experimental possibilities, intelligent software, and the most complete data.

References

[1] Locati, C. Comparison between linear and staircase cyclic voltammetry on a commercial capacitor, Metrohm AG: Herisau, Switzerland, 2021. AN-EC-026

[2] Locati, C. Study of the hydrogen region at platinum electrodes with linear scan cyclic voltammetry – How VIONIC powered by INTELLO can be used to characterize processes at the Pt-electrolyte interface, Metrohm AG: Herisau, Switzerland, 2021. AN-EC-025

[3] Chi, Q.; Zhang, J.; Andersen, J. E. T.; Ulstrup, J. Ordered Assembly and Controlled Electron Transfer of the Blue Copper Protein Azurin at Gold (111) Single-Crystal Substrates. J. Phys. Chem. B 2001, 105 (20), 4669–4679. https://doi.org/10.1021/jp0105589

Post written by Dr. Corrado Locati, Application Specialist at Metrohm Autolab, Utrecht, The Netherlands.

Coffee: serving up chemistry in every cup

Coffee: serving up chemistry in every cup

International Coffee Day is October 1st, not that many of us need a day to celebrate the drink we enjoy all year round. What makes a high quality cup of coffee? There are several factors at hand from the origin of the beans and the climate they grow in, to how the beans are processed, roasted, and packaged, and finally how the roasted beans are ground and brewed. In this blog post, I will discuss a bit of the history of coffee, how it is processed, and how to accurately determine the quality parameters in order to brew the most flavorful cup.

Origins of our favorite brew

The word «coffee» was introduced in 1582, derived from the Dutch «koffie». This traces back even further to the Arabic word for coffee, «qahwah», which has been speculated to come from «quwwa» (defined as power or energy), or even from Kaffa (also spelled as Kefa) which was a medieval Ethiopian kingdom that exported coffee plants to Arabia. It is believed that coffee was first discovered by a goat herder in Ethiopia who noticed the energy of his goats increase after consuming the coffee fruit (known as «cherries»). From Ethiopia, coffee consumption spread through the Arabian Peninsula and Middle East during the 15th and 16th centuries.

Coffee cherries ripening.
Coffee is now the most consumed beverage, other than water, around the globe. From the thickest espressos all the way to transparent drip coffee, the world is truly hooked! This beverage is so deeply ingrained in all cultures that the top places for annual World Barista Championships (2019) were taken by contestants from South Korea, Greece, and Canada!

Good coffee: not as straightforward as you might think

Coffee comes in many forms, with niche roasters looking to discover new flavors daily. Since the gourmet coffee market is growing more, specialty coffee is also in high demand. Global Brands Magazine reported the price of Black Ivory Coffee at $500 per pound in 2020.

Why so expensive — is the taste that good? In order to make Black Ivory Coffee, the coffee cherries are fed to and digested by elephants, in a similar manner to Kopi Luwak (or civet coffee), another expensive coffee type created by fermentation of coffee cherries in the gut of civet cats. The resulting coffee beans are then cleaned, dried, and roasted.

Civet cats can digest coffee beans to create a unique coffee experience with a heavy price tag.
Aside from these high-priced small batches, other types of coffee beans are roasted in large quantities for mainstream consumption (arabica, robusta, and liberica). Arabica beans make up 60% of the global market with 2.5 million tons exported per year from Brazil alone. Robusta beans account for a bit less than 40% of the market and are mostly produced in Vietnam. Robusta beans exhibit more bitter flavors, contain more caffeine, and are used more often to create instant coffee. Liberica beans have high levels of sugars but low concentrations of caffeine compared to the other two major species. Very low yields (between two and four times lower than the others) and larger plant size make this type more difficult to mass produce and therefore it only accounts for approximately 2% of the global coffee market. From these major coffee species, several varieties have been produced with a large range of different flavor characteristics and caffeine content.

Typically, coffee is grown in (sub)tropical areas, but the ideal climate differs depending on the species. Some prefer higher altitudes and are more suitable for mountainous regions. Others need hot and dry conditions to produce the best quality beans. Now there are over 70 countries that produce coffee. That’s a good thing, because global coffee consumption in 2020/2021 is estimated to be 167.23 million 60 kg bags, which is more than 10 million tons of coffee!

Map showing the different coffee-producing countries around the world.

Changes in coffee consumption practices

The adoption of the pod coffee machine (e.g., Keurig, Nespresso) over the past decade has pushed the consumption of coffee from something generally enjoyed in a café, restaurant, or on the go, to a much higher rate of consumption at home. With this significant shift to pod coffee, the ability to adjust grind size, water temperature, or extraction time used by the best baristas to counter changes in flavor and strength is no longer a possibility. In fact, the ease of pressing a single button and receiving hot, fresh coffee within seconds is exactly why pod coffee is so popular. This puts new pressures on coffee roasters to maintain the flavor and caffeine strength expected of their brand and varieties.

Though many people may think that the largest contribution to a good cup of coffee is due to the coffee brewing process, many other quality parameters such as acidity, roast temperature, and water quality contribute even more. Two of the main factors in an optimal cup of coffee, the acidity (taste) and the amount of caffeine, are mainly affected by the bean type, region of origin, and roasting temperature.
Progression of the coffee bean roasting process.

Science—brewing up your perfect cup

Not all coffee beans are created equal, but luckily science allows us to define many of the key quality parameters that result in the taste and caffeine strength we expect from our favorite brand of coffee. Coffee is generally acidic with a pH of around five. Highly acidic coffee displays a sour, harsh flavor. While there are ways to counteract this on the consumer side, for manufacturers it is even more important to identify that there is an issue to begin with. A simple identifier is the titratable acidity of the coffee, and this has a direct correlation with the taste you associate with your favorite brew.

Of equal importance is the «kick» you may get from your preferred caffeine fix. Whether you drink one cup per day, or four, the recommended daily limit for adults is suggested at 400 mg caffeine. Of course, decaffeinated coffee is also an option for those who are sensitive to its effects or are looking for ways to reduce their intake (but can’t stay away from coffee).

Traditionally, caffeine has been analyzed by titration, liquid chromatography (LC), or spectrophotometry after a long sample preparation procedure. Now, the analysis of key coffee quality parameters like caffeine content can be done simply and effectively using a single titration system.

Example titration curve for caffeine analysis with OMNIS (click to enlarge).
The pH and acidity of coffee samples are analyzed using a robust pH electrode during titration against standardized sodium hydroxide. Caffeine is determined through a redox back titration after a known excess of iodine is added to the sample and left to react. After the reaction period the sample is filtered and titrated with sodium thiosulfate.

Find out more about this analysis in our free Application Note!

Metrohm has the solution for your analysis needs

The OMNIS platform from Metrohm provides laboratory analysts the automation they need to make each sample determination significantly simpler, faster, and more reproducible thanks to minimal manual sample preparation steps. Key steps in the analysis process that have required manual interactions, reagent addition, filtration, and accurate volume transfers are now completed accurately and automatically. Learn more about the OMNIS titration platform on our website.
Hopefully this article has given you some insight into coffee’s long journey from the farm to your cup, and that you have learned about the chemistry behind the way your coffee tastes! Enjoy International Coffee Day, whether you celebrate on October 1st or every other day of the year.

Download our free Application Note:

Analysis of caffeine, pH, and acidity in coffee – Fully automated determination including filtering, reagent addition, and sample pipetting using OMNIS
Post written by Isaac Rogers (Titration Product Manager at Metrohm Australia & New Zealand) and Dr. Alyson Lanciki (Scientific Editor at Metrohm International Headquarters).