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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).

Validation of titration methods

Validation of titration methods

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

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

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

These include (click to go to each section):

Standardization

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

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

Primary standards are characterized by the following properties:

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

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

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

Accuracy and precision

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

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

Precision is evaluated in three levels:

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

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

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

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

Linearity

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

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

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

Specificity

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

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

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

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

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

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

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

Conclusion

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

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

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

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

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

Watch our free webinar

available on demand!

How to convert from manual to automated titration procedures

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

How much do pipes rust in a year?

How much do pipes rust in a year?

Why is corrosion important?

According to the Association of Materials Protection and Performance (AMPP) the total estimated annual cost of corrosion is as high as 3.5% of a country’s GDP [1]. An AMPP international study [2] found that in the United States alone, the corrosion related cost can be as high as $1.4 billion USD annually in the oil and gas exploration and production sector. This figure climbs even higher, up to $40 billion USD for gas and drinking water distribution plus sewer systems. This is an unavoidable problem with a high cost to bear.

Even though the corrosion itself isn’t unavoidable, it can be controlled by using the right material in the right place. Using a reliable test method that evaluates the material’s resistance against corrosion and predicts its potential failure is of the utmost importance. This test method should also be cost-effective and practicable.

What is corrosion?

Corrosion refers to a naturally occurring process that involves the deterioration or degradation of metals and alloys through a chemical reaction. The corrosion rate is highly dependent on the type of material, ambient temperature, contaminants/impurities, and other environmental factors. Most corrosion phenomena are electrochemical in nature and consist of at least two reactions on the surface of the metals or alloys.

For example:

These electrochemical process require three main elements:

  • Anode: where the metal corrosion occurs.
  • Cathode: the electrical conductor, which is not consumed during the corrosion process in the real-life electrochemical cell configuration.
  • Electrolyte: the corrosive medium that enables the transfer of electrons between the anode and the cathode.

Depending on the materials and environment, corrosion can occur in different ways, such as uniform corrosion, pitting corrosion, crevice corrosion, galvanic corrosion, or microbiologically induced corrosion to name just a few. Learn more about the different types of corrosion in our free white paper.

This white paper also includes details about relevant electrochemical techniques including Linear Sweep Voltammetry (LSV), Electrochemical Impedance Spectroscopy (EIS), and Electrochemical Noise (ECN or ZRA). These techniques allow for the exploration of corrosion mechanisms, the behavior of different materials, the rate at which corrosion occurs, and also to determine the suitability of the corrosion protection solutions such as protective coatings and inhibitors, among others.

Find out more about these subjects individually with our selection of free Application Notes (AN).
Calculation of corrosion parameters with NOVA – Tafel plot corresponding to corrosion behavior of iron in seawater. (Click to enlarge)

Creating pipe-flow conditions in your corrosion laboratory

Internal corrosion is the most problematic cause of pipeline failure. To understand the fundamentals about corrosion failure and its root causes within pipelines, a similar environment should be created in the lab.

The Rotating Cylinder Electrode (RCE) is an integral part of creating hydrodynamic electrochemical experiments in the lab that create turbulent flow conditions which realistically simulate the situation for liquids flowing through pipes. The RCE can be used with most electrochemical techniques such as chronoamperometry, chronopotentiometry, and potential sweep.

Study of the corrosion rate as a function of rotation speed (convective flux) is one of the most common applications for the RCE. Corrosion studies can be performed using linear or cyclic polarization measurements (LP, DPD, CP), electrochemical impedance spectroscopy (EIS), and electrochemical noise (ECN) with respect to the rotation speed.

Results obtained by electrochemical methods are more accurate and are obtained much faster than conventional corrosion investigation methods (e.g. salt spray), providing more efficiency and productivity to any corrosion measurement laboratory. Learn about the RCE and how to simulate realistic pipe-flow conditions in the lab combined with electrochemical corrosion techniques in our free white paper.

One typical method in electrochemical corrosion studies is linear polarization (LP). With this method, it is possible to evaluate the corrosion behavior of a sample under pipe-flow (i.e. turbulent flow) conditions and learn about the corrosion rate of the sample at a specific flow rate.

Metrohm offers two Application Notes that use this technique specifically:

The Tafel plot obtained from LP measurement gives an indication of the corrosion potential. Using dedicated analysis tools in the NOVA software from Metrohm Autolab, the corrosion rate analysis can be performed and corrosion rate can be calculated, giving an indication of how much the pipe will rust in a year (in mm/year) under given conditions. Once this information is available for a certain material, a more corrosion resistive environment can be developed by applying a certain coating or a corrosion inhibitor.

Tafel plot created by Metrohm Autolab’s NOVA software. Blue line is measured without corrosion inhibitor and red line is measured with corrosion inhibitor.
Tafel plot created by Metrohm Autolab’s NOVA software corresponding to the measurements done in quiescent electrolyte (blue) and under 500 RPM rotation rate (red). All other experimental parameters were kept the same.

A second evaluation can be performed to learn how much the pipe will rust in a year, under these resistive conditions. In the example below, under standard conditions, the corrosion rate of carbon steel is measured at 0.25 mm/yr. However, when a specific corrosion inhibitor is used (tryptamine in this case), the performance is significantly improved and the corrosion rate drops to 0.065 mm/yr. These results can be achieved in a matter of minutes by using electrochemical methods, whereas by conventional methods (e.g., salt spray chamber combined with weight loss analysis), it takes up to a few months to conclude the results. That is a huge difference in efficiency!

Corrosion Parameter No Inhibitor With Inhibitor
Ecorr (V) from linear regression -0.479 -0.392
Ecorr (V) from Tafel analysis -0.482 -0.396
Rp (Ω) from linear regression 42.62 135.96
Rp (Ω) from Tafel analysis 43.32 136.39
Corrosion rate (mm/year) from Tafel analysis 0.25 0.065
Linear regression and Tafel analysis data resulting from experiments with and without corrosion inhibitor.

Summary

Understanding the corrosion behavior of a material under real-life conditions helps manufacturers to more quickly optimize the material design in terms of corrosion resistance, either by using a more suitable material for the pipes or by using adequate corrosion protection methods (i.e., coatings or corrosion inhibitors), which results in significant cost savings and safer operation.

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

Recognition of endpoints (EP)

Recognition of endpoints (EP)

Like many of you, I gained my first practical titration experience during my chemistry studies in school. At this time, I learned how to perform a manual visual endpoint titration – and I can still remember exactly how I felt about it.

Using a manual buret filled with titrant, I added each drop individually to an Erlenmeyer flask that contained the sample solution (including the analyte to be measured) and the indicator which was added prior to the titration. With each drop and even slight color change of my sample solution, minutes passed with increasing uncertainty. I asked myself, «Have I already reached the true endpoint, should I add another drop, or have I even over-titrated?» You have probably been in the same situation yourself!

Sound familiar to you? Don’t forget to check out our other blog post about the main error sources in manual titration!

Several years have passed since then, and I am glad that I no longer have to face the challenges of performing a manual titration because Metrohm offers the possibility of automated titrations.

If you want to know how to determine the endpoint in an automated titration, I will give you all the answers you need. In the following article I will cover these topics (click to go directly to each):

    Different detection principles – an overview

    At this point you may ask yourself—if not visually, how the endpoint (EP) can be detected in an automated titration? Well, aside from the visual endpoint recognition (e.g., by a color change, the appearance of turbidity, or appearance of a precipitate), a titration EP can also be detected by the automated monitoring of a change in a chemical or physical property which occurs when the reaction is complete.

    As shown in the table below, there are many different detection principles:

    Table 1. Determination principles for various EP detection methods.

    Now, let’s discuss the potentiometric and photometric EP determination in comparison to a visually recognized EP detection as they are the most commonly used determination principles for automated titrations. If you’d like to learn more about the principles of thermometric titration, read our blog post about the basics!

    Potentiometric principle

    As shown in the table above, in the potentiometric principle the concentration dependent potential (mV) of a solution is measured against a reference potential. Therefore, a silver-silver chloride (Ag/AgCl) reference electrode is used in combination with a measuring electrode (pH sensitive glass membrane or metal ring). In general, a combined sensor (electrode) including both measuring and reference electrode is used.

    Figure 1 illustrates with a simple example how a manual titration with a color change looks when being converted to an automatic system.

    Figure 1. Illustration of the same titration performed manually (left) and automatically (right).

    Step 1: Beginning of the titration before titrant is added.

    Step 2: Addition of titrant – as the titration approaches the endpoint you begin to see signs of the color change. At this point in an automatic titration the sensor will detect a change in mV signal and the titrator begins dosing the titrant in smaller volumes and at a slower rate.

    Step 3: Finally, the EP is reached with a faint pink color which corresponds with the inflection point in the titration curve.

    Step 4: Titrating beyond the endpoint leads to over titration, and here the mV signal is fairly constant.

    This is how you achieve the characteristic S-shaped titration curve you see when performing an automated titration.

    Not only acid-base titrations can be converted. Figure 2 shows how a simple chloride titration can be converted. The titrant, titrant concentration, sample size, and sample preparation remain the same.

    Figure 2. Illustration of a chloride titration – conversion from manual to automatic analysis.

    Only the indicator is replaced by the Ag Titrode, a silver ring electrode, and we get a titration curve (Figure 2, right side) with a clearly defined endpoint.

    For more examples of possible potentiometric titrations, download our free monograph «Practical Titration» or check out our Application Finder where you can find several examples for all endpoint recognition principles.

    Photometric Principle

    Titrations using color indicators are still widely used e.g. in pharmacopeias. When performed manually, the results depend, quite literally, on the eye of the beholder. Photometric titration using the Optrode makes it possible to replace this subjective determination of the equivalence point with an objective process that is completely independent of the human eye.

    The advantage here is that the chemistry does not change – that is, the standard operating procedure (SOP) generally does not have to be adapted. 

    The basis of photometric indication is the change in intensity at a particular wavelength of a light beam passing through a solution. The transmission is the primary measured variable in photometry, and is given by the light transmission (mV or % transmission) of a colored or turbid solution that is measured with a photometric sensor such as the Optrode from Metrohm.

    There are eight possible wavelengths to choose from that span nearly all color indicators used for titrations (see table below). The shaft is solvent resistant and there is no maintenance required. It connects directly to the titrator and improves accuracy and repeatability of color indicated titrations.

    Table 2. The Optrode – an optical sensor for photometric titrations.

    I’ve also picked an example to show you how to convert an EDTA titration of manganese sulfate from manual titration to automated titration. Like in the example above, the procedure remains the same.

    Are you ready to take the leap and switch to using an automated titration system? Read our other blog post to learn more about how to transfer manual titration to autotitration.

    One advantage of automated titration is that a lower volume of chemicals is needed, resulting in less waste. With the same indicator Eriochrome Black TS, the Optrode is used at a wavelength of 610 nm. The titration curve (Figure 3, right side) shows a large potential change of the mV signal indicating a clearly defined titration endpoint.

    Figure 3. Illustration of the photometric EDTA titration of manganese sulfate according to USP.

    If you are not sure what the optimal wavelength for your titration is, then have a look at our blog post about photometric complexometric titration to learn more!

    Comparison: Optrode vs. potentiometric electrodes

    When you decide to make the switch to automated titration, there are some points to consider when comparing the Optrode with other Metrohm potentiometric electrodes. The following table lists the main criteria.

    Table 3. Comparison of photometric and potentiometric measurement techniques for a selection of factors.
    1Optrode has a working life of tens of thousands of hours.

    You see, an autotitration is quite simple to perform and has the great advantage that a clearly defined endpoint is given.

    Believe me, whenever I`m working with such a device including a suitable electrode for an automatic titration, I have a big smile on my face thinking back to my university days: Bye bye subjectivity, time-consuming procedure, economic inefficiency and non-traceability!

    Maybe you are now also convinced to make the change in your laboratory.

    Save more money

    with automated titration

    Read our blog post to find out more.

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

    Introduction to Analytical Instrument Qualification – Part 2

    Introduction to Analytical Instrument Qualification – Part 2

    Welcome back to our blog, and happy 2021! We hope that you and your families had a safe and restful holiday season. To start the year, we will conclude our introduction to Analytical Instrument Qualification. 

    Metrohm’s approach to Analytical Instrument Qualification (AIQ)

    Metrohm’s answer to Analytical Instrument Qualification is bundled in our Metrohm Compliance Services. The most thorough level of documentation offered for AIQ is the IQ/OQ.

    Metrohm IQ/OQ documentation provides you with the required documentation in strict accordance to the major regulations from the USP, FDA, GAMP, and PIC/S, allowing you to document the suitability of your Metrohm instruments for your lab’s specific intended use.

    With our test procedures (described later in more detail), we can prove that the hardware and software components function correctly, both individually and as part of the system as a whole. With Metrohm’s IQ/OQ, you are supported in the best possible way to integrate our systems into your current processes.

    Our high quality documentation will have you «audit ready» all the time.

    The flexibility of a modular document structure

    Depending on the environment you work in and your specific demands, Metrohm can offer a tailored qualification approach thanks to documentation modularity. If you need a lower level of qualification, only the required modules can be executed. Our documentation consists of different modules, each of which documents the identity of the Metrohm representative along with the qualification reviewer, combined with the details of each instrument, software, and document involved in the qualification.  Thanks to this, each module is independent, which guarantees both full traceability and reliability for your system setup.

    Cost-effective qualification from Metrohm

    Metrohm supports you by implementing a cost-effective qualification process, depending upon your requirements and the modules needed. This means that a qualification is not about performing unnecessary actions, qualification is about completing the required work.

    The risk assessment analysis defines the level of qualification needed and based on it, we focus on testing only what needs to be tested. In case you relocate your device to another lab, which qualification steps (DQ, IQ, OQ, PQ) are really needed in order to fulfill your requirements? Contact your local Metrohm expert for advice on this matter.

    A complete Metrohm IQ/OQ qualification includes…

    Metrohm IQ/OQ documentation is based on the following documentation tree, beginning with the first module, the Master Document (MD), followed by the Installation Qualification (IQ) and eventually the Operational Qualification (OQ). The OQ is then divided again into individual component tests (Hardware and Software) and a holistic test to validate your complete system.

    Master Document (MD)

    Each qualification starts with the Master Document (MD) – the central organizing document for the AIQ procedures. It not only describes the process of installing and qualifying the instruments, but also the competence and education level of the qualifying engineer. The MD identifies all other components to be added to the qualification, resulting in a flexible framework on which to build up a set of documentation.

    Installation Qualification (IQ)

    Once the content of the documentation is defined in the MD, the Installation Qualification (IQ) follows. This set of documentation is designed to ensure that the instrument, software, and any accessories have been all delivered and installed correctly. The IQ protocol additionally specifies that the workplace is suitable for the analytical system as stipulated by Metrohm.

    Operational Qualification (OQ)

    After a correct installation comes the main part of the qualification: the Operational Qualification (OQ). In the first part of the OQ, the functionality of the single hardware components is tested and evaluated according to a set of procedures. This is to ensure that the instrument is working perfectly as designed, and is safe to use. Rest assured that you can rely on the expertise of our Metrohm certified engineers to conduct these comprehensive tests on your instruments using the necessary calibrated and certified tools.

    The second part of the OQ consists of a set of Software Tests to prove that the installed Metrohm software functions correctly and reliably on the computer it was installed on. The importance of maintaining software in a validated state is also related to the data integrity of your laboratory. Therefore these software tests can be repeated periodically or after major changes. In particular, these functionality tests cover verifications on user management, database functionalities, backups, audit trail review, security policy, electronic signatures, and so on.

    At Metrohm, we constantly work to improve our procedures and use state of the art tools and technologies.  For this reason, we have implemented a completely automated test procedure for validating the software of our new OMNIS platform. This ensures full integrity in the execution and delivers consistent results with a faster and completely error-free test execution. This innovative and automated software validation eliminates manual activities that are labor intensive and time consuming. This therefore expedites testing and removes the inefficiencies that plague the paper-based software validation.

    Your benefit is clear: save valuable time and reduce unnecessary laboratory start-up activities during qualification. That’s time you can spend on other work in your lab!

    Holistic Test (Performance Verification, PV)

    Once each individual component has been separately tested, the performance of the system as a whole is proven by means of a holistic test (OQ-PV).

    This includes a series of «wet-chemical» tests, performed using certified reference materials, to prove the system is capable of generating quality data, i.e. results that are accurate, precise, and above all fit for purpose. Based on detailed, predefined instructions (SOPs), a series of standard measurements are performed, statistically evaluated, and compared to the manufacturer’s specifications.

    Differences between Performance Verification (PV) and Performance Qualification (PQ)

    The Performance Verification (PV) is a set of tests offered by Metrohm in order to verify the fitness for purpose of the instrument. As mentioned in the previous paragraph, the PV includes standardized test procedures to ensure the system operates as designed by the manufacturer in the selected environment.

    On the other hand, the Performance Qualification (PQ) is a very customer specific qualification phase (see the «4 Q’s» Qualification Phases found in Part 1). PQ verifies the fitness for purpose of the instrument under actual condition of use, proving its continued suitability. Therefore, PQ tests are defined depending on your specific analysis and acceptance criteria.

    Now my questions to you—is your analytical instrument qualified for its intended use? Is your lab in compliance with the latest regulations for equipment qualification and validation? Get expert advice directly from your local Metrohm agency and request your quote for Metrohm qualification services today!

    Check out our online material:

    Metrohm Quality Service

    Post written by Lara Casadio, Jr. Product Manager Service at Metrohm International Headquarters, Herisau, Switzerland.