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Frequently asked questions in Karl Fischer titration – Part 1

Frequently asked questions in Karl Fischer titration – Part 1

Since I started working at Metrohm more than 15 years ago, I have received many questions about Karl Fischer titration. Some of those questions have been asked repeatedly from several people in different locations around the world. Therefore, I have chosen 20 of the most frequent questions received over the years concerning Karl Fischer equipment and arranged them into three categories: instrument preparation and handling, titration troubleshooting, and the oven technique. Part 1 will cover instrument preparation and handling, and Part 2 will cover the other two topics.

Summary of questions in the FAQ (click to go directly to each question):

Instrument preparation and handling

1.  How can I check if the electrode is working correctly?

I recommend carrying out a volumetric or coulometric Karl Fischer titration using a certified water standard as sample. In volumetry, you can carry out a threefold titer determination followed by a determination of a different standard. Then, you can calculate the recovery of the water content determination of the standard.

To check a coulometric system, carry out a threefold determination with a certified water standard and calculate the recovery. If the recovery is between 97–103%, this indicated that the system, including the electrode, is working fine.

The color of the working medium is an additional indicator as to whether the indication is working properly.

Pale yellow is perfect, whereas dark yellow or even pale brown suggests indication problems. If this happens, then the indicator electrode should be cleaned.

Check out questions 7 and 8 for tips on the cleaning of the indicator electrode.

2.  How long can an electrode be stored in KF reagent?

Karl Fischer electrodes are made from glass and platinum. Therefore, the KF reagent does not affect the electrode. It can be stored in reagent as long as you want.

3.  Can the molecular sieve be dried and reused, or should it be replaced?

The molecular sieve can of course be dried and reused. I recommend drying it for at least 24 hours at a temperature between 200–300 °C. Afterwards, let it cool down in a desiccator and then transfer it into a glass bottle with an airtight seal for storage. 

4.  How long does conditioning normally take?

Conditioning of a freshly filled titration vessel normally takes around 2–4 minutes for volumetry, depending on the reaction speed (type of reagent), and around 15–30 minutes for coulometry. In combination with an oven, it might take a bit longer to reach a stable drift owing to the constant gas flow. I recommend stabilizing the entire oven system for at least 1 hour before the first titration.

Between single measurements in the same working medium, conditioning takes approximately 1–2 minutes. Take care that the original drift level is reached again.

5.  When conditioning, many bubbles form in the coulometric titration cell with a very high drift, also when using fresh reagent. What could be the reason for this effect?

At the anode, the generator electrode produces iodine from the iodide-containing reagent. The bubbles you see at the cathode are the result of the reduction of H+ ions to hydrogen gas.

After opening the titration cell or after filling it with fresh reagent, the conditioning step removes any moisture brought into the system, avoiding a bias in the water content determination of the sample. Removing the water results in an increased drift level. During conditioning, the aforementioned H2 is generated. The gas bubbles are therefore completely normal and not a cause for concern. Generally, the following rule applies: The more moisture present in the titration vessel, the higher the drift value will be, and the more hydrogen will form.

6.  What is the best frequency to clean the Karl Fischer equipment?

There is no strict rule as to when you should clean the KF equipment. The cleaning intervals strongly depend on the type and the amount of sample added to the titration cell. Poor solubility and contamination of the indicator electrode (deposition layer on its surface) or memory effects due to large amounts of sample can be good reasons for cleaning the equipment.

The drift can be a good indicator as well. In case you observe higher and unstable drift values, I would recommend cleaning the titration cell or at least refilling the working medium.

7.  How do I clean the Karl Fischer equipment?

For a mounted titration vessel, it can be as simple as rinsing with alcohol. For an intense cleaning, the vessel should be removed from the titrator. Water, solvents like methanol, or cleaning agents are fine to clean the KF equipment. Even concentrated nitric acid can be used as an oxidizing agent, e.g. in case of contaminated indicator electrodes or coulometric generator electrodes.

All of these options are fine, but keep in mind that the last cleaning step should always be rinsing with alcohol followed by proper drying in a drying oven or with a hair dryer at max. 50 °C to remove as much adherent water as possible.

You should never use ketones (e.g., acetone) to clean Karl Fischer equipment, as they react with methanol. This reaction releases water. If there are still traces of ketones left in the titration cell after cleaning, they will react with the methanol in the KF reagent and might cause the drift to be too high to start any titration.

8.  Is it also possible to use a cleaning agent like «CIF» or toothpaste to clean the double Pt electrode?

Normally, rinsing with alcoholic solvents and polishing with paper tissue should be enough to clean the indicator electrode. You may also use detergents, toothpaste, or the polishing set offered by Metrohm! Just make sure that you rinse the electrode properly after the cleaning process to remove all traces of your chosen cleaning agent before using the electrode again.

Cleaning instructions can also be found in our video about metal and KF electrode maintenance:

9.  How do I clean a generator electrode with a diaphragm?

After removing the generator electrode from the titration vessel, dispose the catholyte solution, then rinse the electrode with water. Place the generator electrode upright (e.g., in an Erlenmeyer flask) and cover the connector with the protection cap to prevent corrosion. Fill the generator electrode with some milliliters of concentrated nitric acid, and let the acid flow through the diaphragm. Then fill the cathode compartment with water, and again allow the liquid to flow through the diaphragm. Repeat the rinsing step with water several times to make sure that all traces of nitric acid are washed out of the diaphragm.

Please note that the nitric acid treatment can be left out if the level of contamination does not require it.

Finally, pour some methanol into the generator electrode to remove the water. Repeat this step a few times to remove all traces of water. The last step is properly drying the electrode in a drying oven or with a hair dryer at max. 50 °C. After this cleaning procedure, the electrode is as good as new and can be used again for titrations.

Keep on the lookout for our next installment in this two-part series, or subscribe to the blog below so you’re sure not to miss it! In Part 2, I will cover the topics of KF titration troubleshooting and the Karl Fischer oven technique.

Post written by Michael Margreth, Sr. Product Specialist Titration (Karl Fischer Titration) at Metrohm International Headquarters, Herisau, Switzerland.

Comprehensive water analysis: combining titration, IC, and direct measurement in one setup

Comprehensive water analysis: combining titration, IC, and direct measurement in one setup

If you perform water analyses on a regular basis, then you know that analyzing different parameters for drinking water can be quite time-consuming, expensive, and it requires significant manual labor. In this article, I’d like to show you an example of wider possibilities in automated sample analysis when it comes to combining different analytical techniques, especially for our drinking water.

Water is the source and basis of all life. It is essential for metabolism and is our most important foodstuff.

As a solvent and transporting agent it carries not only the vital minerals and nutrients, but also, increasingly, harmful pollutants, which accumulate in aquatic or terrestrial organisms.

Within the context of quality control and risk assessment, there is a need in the water laboratory for cost-effective and fast instruments and methods that can deal with the ever more complex spectrum of harmful substances, the increasing throughput of samples, and the decreasing detection limits.

Comprehensive analysis of ionic components in liquid samples such as water involves four analytical techniques:

  • Direct measurement
  • Titration
  • Ion chromatography
  • Voltammetry

Each of these techniques has its own particular strengths. However, applying them one after the other on discrete systems in the laboratory is a rather complex task that takes up significant time.

Back in 1998, Metrohm accepted the challenge of combining different analytical techniques in a single fully automated system, and the first TitrIC system was introduced.

What is TitrIC?

The TitrIC system from Metrohm combines direct measurement, titration, and ion chromatography in a fully automated system.

Direct measurements include temperature, conductivity, and pH. The acid capacity (m and p values) is determined titrimetrically. Major anions and cations are quantified by ion chromatography. Calcium and magnesium, which are used to calculate total hardness, can be determined by titration or ion chromatography.

The results are displayed in a common table, and a shared report is given out at the end of the analysis. All methods in TitrIC utilize the same liquid handling units and a common sample changer.

For more detailed information about the newest TitrIC system, which is available in two predefined packages (TitrIC flex I and TitrIC flex II), take a look at our informative brochure:

Efficient: Titrations and ion chromatography are performed simultaneously with the TitrIC flex system.

Figure 1. Flowchart of TitrIC flex II automated analysis and data acquisition.

How does TitrIC work?

Each water sample analysis is performed fully automated at the push of a button—fill up a sample beaker with the sample, place it on the sample rack, and start the measurement. The liquid handling units transfer the required sample volume (per measurement technique) for reproducible results. TitrIC carries out all the work, and analyzes up to 175 samples in a row without any manual intervention required, no matter what time the measurement series has begun. The high degree of automation reduces costs and increases both productivity and the precision of the analysis.

Figure 2. The Metrohm TitrIC flex II system with OMNIS Sample Robot S and Dis-Cover functionality.

To learn more about how to perform comprehensive water analysis with TitrIC flex II, download our free application note AN-S-387:

Would you like to know more about why automation should be preferred over manual titration? Check out our previous blog post on this topic:

Calculations with TitrIC

With the TitrIC system, not only are sample analyses simplified, but the result calculations are performed automatically. This saves time and most importantly, avoids sources of human error due to erroneously noting the measurement data or performing incorrect calculations.

Selection of calculations which can be automatically performed with TitrIC: 

  • Molar concentrations of all cations
  • Molar concentrations of all anions
  • Ionic balance
  • Total water hardness (Ca & Mg)
  • … and more

Ionic balances provide clarity

The calculation of the ion balance helps to determine the accuracy of your water analysis. The calculations are based on the principle of electro-neutrality, which requires that the sum in eq/L or meq/L of the positive ions (cations) must equal the sum of negative ions (anions) in solution.

TitrIC can deliver all necessary data required to calculate the ion balance out of one sample. Both anions and cations are analyzed by IC, and the carbonate concentration (indicative of the acid capacity of water) is determined by titration.

If the value for the difference in the above equation is almost zero, then this indicates that you have accurately determined the major anions and cations in your sample.

Advantages of a combined system like TitrIC

  • Utmost accuracy: all results come from the same sample beaker

  • Completely automated, leaving analysts more time for other tasks

  • One shared sample changer saves benchtop space and costs

  • Save time with parallel titration and IC analysis

  • Flexibility: use titration, direct measurement, or IC either alone or combined with the other techniques

  • Single database for all results and calculation of the ionic balance, which is only possible with such a combined system, and gives further credibility to the sample results

Even more possibility in sample analysis

TitrIC has been developed especially for automated drinking water analysis but can be adapted to suit any number of analytical requirements in food, electroplating, or pharmaceutical industries. Your application determines the parameters that are of interest.

If the combination of direct measurement, titration, and IC does not suit your needs, perhaps a combination of voltammetry and ion chromatography in a single, fully automatic system might be more fitting. Luckily, there is the VoltIC Professional from Metrohm which fulfills these requirements.

Check out our website to learn more about this system:

As you see, the possibility of combining different analysis techniques is almost endless. Metrohm, as a leading manufacturer of instruments for chemical analysis, is aware of your analytical challenges. For this reason, we offer not only the most advanced instruments, but complete solutions for very specific analytical issues. Get the best out of your daily work in the laboratory!

Discover even more

about combined analytical systems from Metrohm

Post written by Jennifer Lüber, Jr. Product Specialist Titration/TitrIC at Metrohm International Headquarters, Herisau, Switzerland.

What to consider during back-titration

What to consider during back-titration

Titrations can be classified in various ways: by their chemical reaction (e.g., acid-base titration or redox titration), the indication method (e.g., potentiometric titration or photometric titration), and last but not least by their titration principle (direct titration or indirect titration). In this article, I want to elaborate on a specific titration principle – the back-titration – which is also called «residual titration». Learn more about when it is used and how you should calculate results when using the back-titration principle.

What is a back-titration?

In contrast to direct titrations, where analyte A directly reacts with titrant T, back-titrations are a subcategory of indirect titrations. Indirect titrations are used when, for example, no suitable sensor is available or the reaction is too slow for a practical direct titration.

During a back-titration, an exact volume of reagent B is added to the analyte A. Reagent B is usually a common titrant itself. The amount of reagent B is chosen in such a way that an excess remains after its interaction with analyte A. This excess is then titrated with titrant T. The amount of analyte A can then be determined from the difference between the added amount of reagent B and the remaining excess of reagent B.

As with any titration, both involved reactions must be quantitative, and stoichiometric factors involved for both reactions must be known.

Figure 1. Reaction principle of a back-titration: Reagent B is added in excess to analyte A. After a defined waiting period which allows for the reaction between A and B, the excess of reagent B is titrated with titrant T.

When are back-titrations used?

Back titrations are mainly used in the following cases:

  • if the analyte is volatile (e.g., NH3) or an insoluble salt (e.g., Li2CO3)
  • if the reaction between analyte A and titrant T is too slow for a practical direct titration
  • if weak acid – weak base reactions are involved
  • when no suitable indication method is available for a direct titration

Typical examples are complexometric titrations, for example aluminum with EDTA. This direct titration is only feasible at elevated temperatures. However, adding EDTA in excess to aluminum and back-titrating the residual EDTA with copper sulfate allows a titration at room temperature. This is not only true for aluminum, but for other metals as well.

Learn which metals can be titrated directly, and for which a back-titration is more feasible in our free monograph on complexometric titration.

Other examples include the saponification value and iodine value for edible fats and oils. For the saponification value, ethanolic KOH is added in excess to the fat or oil. After a determined refluxing time to saponify the oil or fat, the remaining excess is back-titrated with hydrochloric acid. The process is similar for the iodine value, where the remaining excess of iodine chloride (Wijs-solution) is back-titrated with sodium thiosulfate.

For more information on the analysis of edible fats and oils, take a look at our corresponding free Application Bulletin AB-141.

How is a back-titration performed?

A back titration is performed according to the following general principle:

  1. Add reagent B in excess to analyte A.
  2. Allow reagent B to react with analyte A. This might require a certain waiting time or even refluxing (e.g., saponification value).
  3. Titration of remaining excess of reagent B with titrant T.

For the first step, it is important to precisely add the volume of reagent B. Therefore, it is important to use a buret for this addition (Fig. 2).

Figure 2. Example of a Titrator equipped with an additional buret for the addition of reagent B.

Furthermore, it is important that the exact molar amount of reagent B is known. This can be achieved in two ways. The first way is to carry out a blank determination in the same manner as the back-titration of the sample, however, omitting the sample. If reagent B is a common titrant (e.g., EDTA), it is also possible to carry out a standardization of reagent B before the back-titration.

In any case, as standardization of titrant T is required. This then gives us the following two general analysis procedures:

Back-titration with blank
  1. Titer determination of titrant T
  2. Blank determination (back-titration omitting sample)
  3. Back-titration of sample
Back-titration with standardizations
  1. Titer determination of titrant T
  2. Titer determination of reagent B
  3. Back-titration of sample

Be aware: since you are performing a back-titration, the blank volume will be larger than the equivalence point (EP) volume, unlike a blank in a direct titration. This is why the EP volume must be subtracted from the blank or the added volume of reagent B, respectively.

For more information on titrant standardization, please have a look at our blog entry on this topic.

How to calculate the result for a back-titration?

As with direct titrations, to calculate the result of a back-titration it is necessary to know the involved stoichiometric reactions, aside from the exact concentrations and the volumes. Depending on which analysis procedure described above is used, the calculation of the result is slightly different.

For a back-titration with a blank, use the following formula to obtain a result in mass-%:

VBlank:  Volume of the equivalence point from the blank determination in mL

VEP Volume at the equivalence point in mL

cTitrant:  Nominal titrant concentration in mol/L

fTitrant Titer factor of the titrant (unitless)

r:  Stoichiometric ratio (unitless)

MA Molecular weight of analyte A in g/mol

mSample Weight of sample in mg

100:  Conversion factor, to obtain the result in %

The stoichiometric ratio r considers both reactions, analyte A with reagent B and reagent B with titrant T. If the stoichiometric factor is always 1, such as for complexometric back-titrations or the saponification value, then the reaction ratio is also 1. However, if the stoichiometric factor for one reaction is not equal to 1, then the reaction ratio must be determined. The reaction ratio can be determined in the following manner:

 

  1. Reaction equation between A and B
  2. Reaction equation between B and T
  3. Multiplication of the two reaction quotients
Example 1

Reaction ratio: 

Example 2

Reaction ratio: 

Below is an actual example of lithium carbonate, which can be determined by back-titration using sulfuric acid and sodium hydroxide.

The lithium carbonate reacts in a 1:1 ratio with sulfuric acid. To determine the excess sulfuric acid, two moles of sodium hydroxide are required per mole of sulfuric acid, resulting in a 1:2 ratio. This gives a stoichiometric ratio r of 0.5 for this titration.

 For a back-titration with a standardization of reagent B, use the following formula to obtain a result in mass-%:

VB Added volume of the reagent B in mL

cB:  Nominal concentration of reagent B in mol/L

fB:  Titer factor of reagent B (unitless)

VEP:  Volume at the equivalence point in mL

cT:  Nominal concentration of titrant T in mol/L

fT Titer factor of the titrant T (unitless)

sBT Stoichiometric factor between reagent B and titrant T

sAB:  Stoichiometric factor between analyte A and reagent B

MA:  Molecular weight of analyte A in g/mol

mSample:  Weight of sample in mg

100:  Conversion factor, to obtain the result in %

Modern titrators are capable of automatically calculating the results of back-titrations. All information concerning the used variables (e.g., blank value) are stored together with the result for full traceability.

To summarize:

Back-titrations are not so different from regular titrations, and the same general principles apply. The following points are necessary for a back-titration: 

  • Know the stoichiometric reactions between your analyte and reagent B, as well as between reagent B and titrant T.
  • Know the exact concentration of your titrant T.
  • Know the exact concentration of your reagent B, or carry out a blank determination.
  • Use appropriate titration parameters depending on your analysis.

If you want to learn more about how you can improve your titration, have a look at our blog entry “How to transfer manual titration to autotitration”, where you can find practical tips about how to improve your titrations.

If you are unsure how to determine the exact concentration of your titrant T or reagent B by standardization, then take a look at our blog entry “What to consider when standardizing titrant”.

Post written by Lucia Meier, Product Specialist Titration at Metrohm International Headquarters, Herisau, Switzerland.

Titer determination in Karl Fischer titration

Titer determination in Karl Fischer titration

In a recent post, we have discussed the importance of titer determinations for potentiometric titrations.

Without a titer determination, you will not obtain correct results. The same applies for volumetric Karl Fischer (KF) titrations. In this blog post, I will cover the following topics (click to jump directly to each):

Why should I do titer determinations?

Why is a titer determination necessary? Well, the answer is quite simple. Without knowing the titer of a KF titrant, the water content of the sample cannot be calculated correctly. In Karl Fischer titration, the titer states how many mg of water can be titrated with one mL of titrant. Therefore, the KF titer has the unit «mg/mL».

You might say: “Now, ok, let’s determine the titer. That isn’t too much work and afterwards, I know the titer value and I don’t need to repeat the titer determination.

I agree this would be very nice. However, reality is somewhat different. You must carry out titer determinations on a regular basis. In closed bottles, KF titrants are very stable and the titer does not change appreciably. Once you open the bottle, the KF titrant starts to change significantly. Air will enter the bottle, and considering that 1 L of air contains several milligrams of water, you can imagine that this moisture has an influence on the titer. To prevent moist air from getting into the titrant, the bottle must be either tightly closed after use with the original cap, or should be protected with an absorber tube filled with a molecular sieve (0.3 nm).

Please be aware that temperature changes also have an influence on the titer. A temperature increase of the titrant by 1 °C leads to a titer decrease of approximately 0.1% due to volume expansion. Consider this, in case the temperature in your laboratory fluctuates during the working day.

Do not forget: if your titration system is stopped overnight, the reagent in the tubes and in the cylinder is affected and the titer is no longer equal to the titrant in the bottle. Therefore, I recommend first running a preparation step to flush all tubes before the first titration.

How often should I perform titer determinations?

This question is asked frequently, and unfortunately has no simple answer. In other words, I cannot recommend a single fixed interval for titer determinations. The frequency depends on various factors:

  • the type of reagent (two-component titrants are more stable than single-component titrants)
  • the tightness of the seals between the titration vessel and the titrant bottle
  • how accurate the water content in the sample must be determined

In the beginning, I would recommend performing a titer determination on a daily basis. After a few days, it will become apparent whether the titer remains stable or decreases. Then you can decide to adjust the interval between successive titer determinations.

What equipment do I need for a titer determination?

You need a fully equipped titrator for volumetric KF titration, as well as the KF reagents (titrant and solvent). Another prerequisite for accurate titer determinations is an analytical balance with a minimal resolution of 0.1 mg. Last but not least, you need a standard containing a known amount of water and some tools to add the standard to the titration vessel. These tools are discussed in the next section.

How to carry out a titer determination

Three different water standards are available for titer determinations. There are both liquid and solid standards available from various reagent suppliers. The third possibility is available in every laboratory: distilled water. Below, we will take a closer look at the individual handling of these three standards. For determination of appropriate sample sizes, you can download our free Application Bulletin AB-424, Titer determination in volumetric Karl Fischer titration.

1. Liquid water standard

For the addition of a liquid water standard, you need a syringe and a needle.

There are two possibilities to add liquid standard. One is to inject it with the tip of the needle placed above the reagent level. In this case, aspirate the last drop back into the syringe. Otherwise, it will be dropped off at the septum. The droplet is included in the sample weight, but the water content in the drop is not determined. This will lead to false results.

If the needle is long enough, you can immerse the tip in the reagent during the standard addition. In this case, there is no last droplet to consider, and you can pull the needle out of the titration vessel without any additional aspiration step.

Step-by-step – how to carry out a titer determination:

  1. Open the ampoule containing the standard as recommended by the manufacturer.
  2. Aspirate approximately 1 mL of the standard into the syringe.
  3. Remove the tip of the needle from the liquid and pull the plunger back to the maximum volume. Sway the syringe to rinse it with standard. Then eject the 1 mL of standard into the waste.
  4. Aspirate the remaining content of the ampoule into the needle.
  5. Remove any excess liquid from the outside of the needle with a paper tissue.
  6. Place the needle on a balance, and tare the balance.
  7. Then, start the determination and inject a suitable amount of standard through the septum into the titration vessel. Please take care that the standard is injected into the reagent and not at the electrode or the wall of the titration vessel. This leads to unreproducible results.
  8. After injecting the standard, place the syringe again on the balance.
  9. Enter the sample weight in the software.
2. Solid water standard

It is not possible to add the solid water standard with a syringe. For this, different tools are required. Here, examples are shown of a weighing boat and the Metrohm OMNIS spoon for paste.

Place the weighing boat on the balance, then tare the balance. Weigh in an appropriate amount of the solid standard, and tare the balance again. Start the titration, quickly remove the stopper with septum, add the solid standard and quickly replace the stopper. When adding the standard, take care that no standard sticks to the electrode or the walls of the titration vessel. In case that happens, gently swirl the titration vessel to wash down the standard. After the addition of the standard, place the weighing boat on the balance again and enter the sample weight in the software.

3. Pure water

Pure water can be added to the titration vessel either by weight or by volume.

For a titer determination with pure water, only a few drops are required. Such small volumes can be difficult to add precisely, and results strongly depend on the user. Moreover, addition by weight requires a balance capable of weighing a few milligrams. I personally prefer using water standards, and suggest that you use them as well.

By weight

Fill a small syringe (~1 mL) with water. Due to the very small amounts of pure water added for the titer determination, I recommend using a very thin needle to more accurately add small volumes. After filling the syringe, place it on a balance and tare the balance. Then start the titration, and inject an appropriate amount of water through the septum into the titration vessel. Aspirate the last droplet back into the syringe. Remove the needle, place the syringe on the balance again, and enter the sample weight in the software.

By volume

Fill a microliter syringe with an appropriate volume of water. Make sure there are no air bubbles in the syringe, as they will falsify the result. Begin the titration and inject the syringe contents through the septum into the titration vessel. Enter the added sample size in the software.

Acceptable results

During trainings, I am often asked if the obtained result is acceptable. I recommend carrying out a threefold titer determination. Ideally, the relative standard deviation of those three determinations is smaller than 0.3%.

How long can the reagent be used?

As long as you carry out regular titer determinations, the titer change will be considered in the calculation, and the results will be correct. Just keep in mind: the lower the titer, the larger the volume needed for the determination.

I hope I was able to convince you that titer determination is essential to obtain correct results in volumetric Karl Fischer titration, and that it is not that difficult to perform.

In case you still have unanswered questions, please download Metrohm Application Bulletin AB-424 to get additional information, tips, and tricks on performing titer determination.

Still have questions?

Check out our Application Bulletin: Titer determination in volumetric Karl Fischer titration.

Post written by Michael Margreth, Sr. Product Specialist Titration (Karl Fischer Titration) at Metrohm International Headquarters, Herisau, Switzerland.

FAQ: All about pH calibration

FAQ: All about pH calibration

In a recent blog post, we discussed how to avoid the most common mistakes in pH measurement:

Here, I want to discuss in a bit more detail how you can correctly calibrate your pH electrode and what you have to consider to obtain the best measurement results afterwards by answering some of your most frequently asked questions.

Let’s get right into it! If you want to jump directly to a question, click on one of these links:

When do I have to calibrate my pH electrode?

Performing regular calibration of your pH electrode is important to get accurate results. The pH electrode can change its properties (e.g., by contamination of the reference electrolyte) which then leads to deviating calibration results. If you do not freshly calibrate your electrode, you obtain precise but inaccurate results of your pH measurement. Therefore, the more accurate the results need to be, the more often you have to calibrate.

Depending on the number of measurements and the sample matrix, I recommend calibrating at least weekly. If the sensor is used often, or if the sample matrix contaminates the sensor, then you should calibrate daily or even more frequently. If the pH electrode is not used often, then always calibrate it prior to a new set of measurements. Also make sure that you always calibrate your sensor if you have received a new one, or after maintenance.

How do I select the correct buffers?

Any time you perform a calibration, it is essential that appropriate buffers are used.

First, you have to select the pH values that you would like to use for calibration. Use at least two different buffers, though it is even better to perform a multi-point calibration. Furthermore, make sure that the pH of your sample is of course within the calibration range! For example, if you want to measure a sample at pH 9, your calibration should not be within pH 4 and 7, but at least up to pH 10. In the graph, you can see that errors become large especially outside of the calibrated range.

 

In addition, the quality of your buffer solutions is essential, as your calibration can only be as good as the buffers used. Never use expired calibration buffers! If the buffer solutions are meant for single use only, do not reuse them. Microbial growth in the buffer can alter its properties quickly. Always mark your buffer solution bottle with the opening date, and especially ensure that alkaline buffers above pH 9 are not used for too long (< 1 month), as CO2 will enter and change the pH value slowly. Moreover, never pour the standards back into the bottle, as they might have been contaminated!

How should I set up my instrument?

Not only is the right choice of calibration buffers essential, it is also very important that you set up your instrument correctly. It’s not only the pH measurement that is sensitive to temperature, pH buffers are as well, and the measured pH value can change with the temperature. This temperature dependency of the pH buffers is usually depicted with buffer tables.

Most instruments already include buffer table templates from various buffer manufacturers. Several tables are available that contain the information about the exact pH value at various temperatures for a certain buffer. These tables are unique for each manufacturer.

The instrument will then select the correct pH value according to the measured temperature. If your buffer is not available with a table, make sure you enter the correct pH value or use a custom buffer table to store the information. As seen here, a temperature change of only 5 °C can have an influence of > 0.04 pH units.

Therefore selecting the manufacturer of your buffer solutions within the calibration parameters is important to obtain an accurate calibration.

Why do I have to measure the temperature?

You might wonder why you should always measure the temperature when you perform pH measurements. Most pH electrodes used for pH measurement have a temperature sensor directly included. This is because the pH value is temperature-dependent. Let me digress for a moment:

In 1889 the Nernst equation was established, describing the potential of an electrochemical cell as a function of concentrations of the ions taking part in the reaction. The relationship between potential and pH [-log(H+)] is given by the formula:

Where U is the measured potential, U0 the standard electrode potential, R the universal gas constant, T the absolute temperature, n the charge (here, +1), and F the Faraday constant. The central term

is called the Nernst slope and gives the mV change per pH unit. As you can see, this term includes the absolute temperature, meaning the slope of your calibration is temperature-dependent. The higher the temperature, the steeper the slope.

Modern pH meters will correct the slope for this temperature variation when the calibration and measurement are not performed at the same temperature.

However, there is an effect that cannot be corrected by the instrument: samples do not have the same pH value at different temperatures! This can already be seen when looking at the example buffer table above. This temperature dependence is different for each sample. Therefore: Always measure your samples at the same temperature if you want to compare their pH values. Also be sure to carry out the pH calibration at the same temperature at which you are measuring your samples. This will greatly reduce the error of your pH measurement.

How do I perform my calibration?

First, prepare your electrode for calibration: open the refilling plug to ensure proper electrolyte outflow, rinse the electrode well with deionized water, and place the sensor into the buffer solution. An important note: both glass membrane and diaphragm must be covered with the buffer solution.

Additionally, assure that you position the electrode in the beaker for maximum reproducibility, especially when stirring. Never place the sensor haphazardly into the beaker where the glass membrane is touching the glass of the beaker; this can cause scratches on the glass membrane, leading to erroneous results.

Do you even have to stir at all? No, you do not! However, as there can be effects on the measured potential depending on the stirring speed, make sure that you always choose the same stirring speed among all buffer solutions, but also for calibration and subsequent measurements. Also, make sure that you do not stir so strongly that a vortex is formed, and avoid any splashing of the solution.

Now, you can start your calibration. Most instruments decide autonomously when the reading is stable by monitoring the drift (mV change per minute). Sometimes it is also possible to stop the buffer measurement after a fixed time interval. However, this requires enough time for the electrode to reach a stable potential as otherwise the calibration will be biased.

Between the buffer solutions, the electrode is rinsed with deionized water. Never dry the electrode afterwards with a tissue, paper towel, or a cloth! This can lead to electrostatic charges on the electrode or even scratches on the glass membrane. Both will lead to longer response times, and in the latter case – to irreversible damage.

What do «slope» and «offset» mean?

Once you’ve finished the calibration, the instrument will display the calibration result. The calibration results usually consists of a slope and offset value. In this section, I want to explain their meaning.

The slope is normally given in % and is calculated from

the measured slope of the calibration divided by the theoretical slope (Nernst Slope) which is equal to 59.16 mV per pH unit at 25 °C. This is done in order to be able to correct the slope for temperature differences between calibration and measurement.

The second parameter that is evaluated is the pH(0), which is the pH value measured at 0 mV. In an ideal case, 0 mV corresponds to a pH value of 7. However, reality usually does not stick to the ideal case. Sometimes, the offset potential (Uoff) is also given, which corresponds to the potential at pH 7.

After calibration, always check the slope and the pH(0). The slope should fall between 95 and 103% and the pH(0) should lie between pH 6.8 and 7.2 (Uoff within ± 15 mV).

If you would like to get more information about your pH electrode, you can either perform a pH electrode test, which is implemented in some instruments from Metrohm, or a test according to Metrohm application bulletin AB-188.

If the pH(0) is outside the recommended range, this can be caused by a contaminated electrolyte or your probe may require a general cleaning.

If the slope is lower than 95%, this can be related to expired or contaminated buffer solutions. However, old and slow electrodes can also exhibit slopes outside of the limits. Therefore, always use fresh buffers.

If the slope is still too low even with fresh buffer, or the pH(0) is outside the recommended range after cleaning and subsequent reconditioning, it is time to replace the electrode.

To summarize

  • Select the calibration frequency and buffer types according to your samples.
  • Make sure that you always use fresh, high quality, and certified buffers as your calibration can only be as good as your buffers.
  • Set up your instrument correctly and use a fixed electrode position for the best reproducibility.
  • Measure the temperature for calibration and subsequent measurement. Moreover, only compare pH values of samples measured at the same temperature.
  • After calibration, check that your data for slope and pH(0) are within the optimal limits.

Would you like to learn even more about pH measurement? Come to our website and check out our informative webinars!

Metrohm webinars

Available below for pH measurement

Post written by Dr. Sabrina Gschwind, Jr. PM Titration (Sensors) at Metrohm International Headquarters, Herisau, Switzerland.

Moisture Analysis – Karl Fischer Titration, NIRS, or both?

Moisture Analysis – Karl Fischer Titration, NIRS, or both?

In addition to the analysis of the pH value, weighing, and acid-base titration, measurement of water content is one of the most common determinations in laboratories worldwide. Moisture determination is important for nearly every industry, e.g., for lubricants, food and feed, and pharmaceuticals.

Figure 1. Water drops in a spider web

For lubricants, the water concentration is very important to know because excess moisture expedites wear and tear of the machinery. For food and feed, moisture content must be within a narrow range so that the food does not taste dry or stale, nor that it is able to provide a breeding ground for bacteria and fungi, resulting in spoilage. For pharmaceuticals, the water content in solid dosage forms (tablets) and lyophilized products is monitored closely. For the latter, the regulations state that the moisture content needs to be below 2%.

Karl Fischer Titration

Karl Fischer (KF) Titration for water determination was introduced back in the 1930’s, and to this day remains one of the most tried and trusted methods. It is a fast and highly selective method, which means that water, and only water, is determined. KF titration is based on the following two redox reactions.

In the first reaction, methanol and sulfur dioxide react to form the respective ester. Upon addition of iodine, the ester is oxidized to the sulfate species in a water-consuming reaction. The reaction finishes when no water is left.

Figure 2. Manual sample injection for volumetric KF Titration

KF titration can be used for the determination of the water content in all sample types: liquids, solids, slurries, or even gases. For concentrations between 0.1% and 100%, volumetric KF titration is the method of choice, whereas for lower moisture content between 0.001% and 1%, coulometric KF titration is recommended.

Depending on the sample type, its water content, and its solubility in the KF reagents, the sample can either be added directly to the titration vessel, or would first need to be dissolved in a suitable solvent. Suitable solvents are those which do not react with the KF reagents — therefore aldehydes and ketones are ruled out. In case the sample is dissolved in a solvent, a blank correction with the pure solvent also needs to be performed. For the measurement, the sample is injected directly into the titration vessel using a syringe and needle (Fig. 2). The endpoint is detected by a polarized double Pt pin electrode, and from this the water concentration is directly calculated.

Insoluble or hygroscopic samples can be analyzed using the gas extraction technique with a KF Oven. Here, the sample is sealed in small vial, and the water is evaporated by heat then is subsequently carried to the titration cell.

Figure 3. Fully automated KF Titration with the Metrohm 874 KF Oven Sample Processor

For more information, download our free Application Bulletins: AB-077 for volumetric Karl Fischer titration and AB-137 for coulometric Karl Fischer analysis.

If you would like some deeper insight, download our free monograph: “Water determination by Karl Fischer Titration”. 

Near-infrared spectroscopy

Near-infrared spectroscopy (NIRS) is a technique that has been used for myriad applications in the areas of food and feed, polymers, and textiles since the 1980’s. A decade later, other segments began using this technique, such as for pharmaceutical, personal care, and petroleum products.

NIRS detects overtones and combination bands of molecular vibrations. Among the typical vibrations in organic molecules for functional groups such as -CH, -NH, -SH, and -OH, it is the -OH moiety which is an especially strong near infrared absorber. That is also the reason why moisture quantification is one of the key applications of NIR spectroscopy.

For a further explanation, read our previous blog entry on this subject: Benefits of NIR spectroscopy: Part 2.

NIR spectroscopy is used for the quantification of water in solids, liquids, and slurries. The detection limit for moisture in solids is about 0.1%, whereas for liquids it is in the range of 0.02% (200 mg/L), However, in special cases (e.g., water in THF), moisture detection limits of 40–50 mg/L have been achieved.

This technique does not require any sample preparation, which means that samples can be used as-is. Solid samples are measured in high quality disposable sample vials, whereas liquids are measured in high quality disposable cuvettes. Figure 4 displays how the different samples are positioned on the analyzer for a measurement.

Detailed information about the NIRS technique has been described in our previous blog article: Benefits of NIR spectroscopy: Part 1.

Figure 4. Solid (left) and liquid (right) sample positioning for NIR measurements

NIRS is a secondary technique, meaning it can only be used for routine analysis for moisture quantification after a prediction model has been developed. This can be understood by an analogy to HPLC, for which measuring standards to create a calibration curve is among the initial steps. The same applies to NIRS: first, spectra with known moisture content must be measured and then a prediction model is created.

The development of prediction models has been described in detail in our previous blog article: Benefits of NIR spectroscopy: Part 3.

The schematic outline is shown in Figure 5.

Figure 5. Workflow for NIR Method implementation for moisture analysis

For creation of the calibration set, around 30–50 samples need to be measured with both NIRS and KF titration, and the values obtained from KF titration must be linked to the NIR spectra. The next steps are model development and validation (steps 2 and 3 in Figure 5), which are quite straightforward for moisture analysis. Water is a strong NIR absorber, and its peaks are always around 1900–2000 nm (combination band) and 1400–1550 nm (first overtone). This is shown in Figure 6 below.

Figure 6. NIR Spectra of moisturizing creams, showing the absorptions related to H2O at 1400–1550 nm and 1900–2000 nm

After creation and validation of the prediction model, near-infrared spectroscopy can be used for routine moisture determination of that substance. The results for moisture content will be obtained within 1 minute, without any sample preparation or use of chemicals. Also, the analyst does not need to be a chemist, as all they need to do is place a sample on the instrument and press start.

You can find even more information about moisture determination by near-infrared spectroscopy in polyamides, caprolactam, lyophilized products, fertilizers, lubricants, and ethanol/hydrocarbon blends below by downloading our free Application Notes.

Your choice for moisture measurements: KF Titration, NIRS, or both!

As summarized in Table 1, KF Titration and NIR Spectroscopy each have their advantages. KF Titration is a versatile method with a low level of detection. Its major advantage is that it will always work, no matter if you have a sample type that you measure regularly or whether it is a sample type that you encounter for the first time.

Table 1. Overview of characteristics of moisture determination via titration and NIR spectroscopy

NIR spectroscopy requires a method development process, meaning it is not suitable for sample types that always vary (e.g., different types of tablets, different types of oil). NIRS however is a very good method for sample types that are always identical, for example for moisture content in lyophilized products or for moisture content in chemicals, such as fertilizers.

For the implementation of a NIR moisture method, it is required that samples are measured with KF titration as the primary method for the model development. In addition, during the routine use of a NIR method, it is important to confirm once in a while (e.g., every 50th or every 100th sample) with KF Titration that the NIR model is still robust, and to ensure that the error has not increased. If a change is noticed, extra samples need to be added to the prediction model to cover the observed sample variation.

In conclusion, both KF Titration and NIR spectroscopy are powerful techniques for measuring moisture in an array of samples. Which technique to use depends on the application and the individual preference of the user.

For more information

Download our free whitepaper:

Karl Fischer titration and near-infrared spectroscopy in perfect synergy

Post written by Dr. Dave van Staveren (Head of Competence Center Spectroscopy), Dr. Christian Haider (Head of Competence Center Titration), and Iris Kalkman (Product Specialist Titration) at Metrohm International Headquarters, Herisau, Switzerland.