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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.

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.

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.