Water determination according to Karl Fischer

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Water determination for quality assurance of sample

The determination of water content is of great importance in regards to quality assurance. The water content reduces the lubricating properties of oils and leads to corrosion damage to components. In the case of pharmaceutical substances, the water content affects their stability. Where other methods such as the drying method determine all volatile components, or methods such as gas chromatography or infrared spectroscopic methods are very costly in terms of equipment, Karl Fischer Titration (KF Titration) offers a highly selective and rapid method of water determination for almost all types of samples.

The Karl Fischer titration for the determination of water content for almost all sample types

The Karl Fischer titration is used to determine the water content of a sample. It is therefore part of the comprehensive range of titration solutions from Xylem Analytics, which can be expanded modularly according to your individual needs. If you currently only need to carry out titrations manually, or only very few water determinations for the time being, but later wish to switch to semi-automatic or fully automatic procedures for water determination, this is possible at any time. Please get in touch with our customer care team - we will be happy to advise in selecting the right products for your needs.

If you need more information, the blog articles on the titration handbook, accurate titration using a concrete application example and the FAQ on titration will certainly be very helpful for you. Here, customer questions are answered and our experts provide you with practical tips for the correct handling, care and application of our measuring technology.

The advantages of Karl Fischer titration

• Fast and accurate determination
• Selective
• Method recognized in many standards and regulations
• Low equipment requirements

KF titration has the advantage over the drying method by giving faster determination and is also highly selective. The stoichiometry of the KF reaction was not clear for a long time. In principle, this would make titration impossible, however recent investigations of the mechanism have resulted in the following reaction equation:

ROH + SO₂ + R´N → [R´NH]SO₃R
H₂O + l₂ + [R´NH]SO₃R + 2 R´N → [R´NH]SO₄R + 2 [R´NH]l
Meaning:
ROH   An alcohol, e.g. methanol, ethanol, ethylene glycol monoethyl ether
R´N    A base, e.g. imidazole (formerly pyridine)

The oxygen of the sulfate ester comes from the water molecule. The studies of the mechanism show a change in stoichiometry when working in other solvents. Therefore, the addition of other solvents should not exceed 50% by volume. Four components (an alcohol, sulfur dioxide, a base and iodine) are involved in the reaction in addition to water: these must be present for the reaction to take place.

KF titration uses a double platinum electrode for detection, to which a voltage of 40-220 mV is applied. Titration is carried out with an iodine solution. In the KF reaction, the iodine reacts directly to form iodide, so that no current flows at the double platinum electrode of the detection system. However, as soon as the first drop of an iodine solution is present in excess, a reversible redox system of iodine and iodide is present. A current flows which is measured and indicates the end of the titration.

A distinction is made between the volumetric and coulometric methods. In coulometric titration, the iodine is generated directly in the titration vessel, which enables the determination of very low water concentrations. Another advantage is that the concentration of the titrant does not have to be determined.

In volumetric titration, the iodine is added precisely via a piston burette. The volumetric method is the more general method, suitable for almost all types of samples - from chemicals, petrochemicals, plastics to pharmaceuticals as well as the food industry. The samples can be either liquid, solid or gas.

The volumetric titration

In volumetric KF titration, the iodine required for the reaction is dosed into the titration cell via a burette. In the classical Karl Fischer titration, all components are offered as a combined reagent and are available in sufficient stability with suitable bases and alcohols. In addition, 2-component reagents are also offered today, in which the solvent component contains an alcohol, a base and SO2.

The titration solution then consists of an iodine solution in an alcohol. This reagent has the advantage of pH buffering and a higher concentration of all components on the left side of the reaction equation. As a result, the reaction is much faster and the reagents have a much longer shelf life. With the one-component reagent, it is possible to adapt the solvent to the solubility of the sample.

The appropriate titrator for this application is the TitroLine® 7500 KF for volumetric water determination.

The coulometric titration

Volumetric reagent addition is preferably replaced by coulometrically generated iodine at low water contents. Coulometry is based on the same chemical reaction, but the iodine is not dosed by means of a burette, but is generated in situ at the anode of a generator electrode by oxidation of iodide. Hydrogen is produced at the cathode by reduction. The amount of iodine generated is calculated by the titrator according to Coulomb's law.

Coulometry is an absolute method, therefore a titer determination is not necessary (it is also not possible!) KF volumetry and coulometry are the same except for the iodine addition.

Today, a generator electrode is used in the majority of cases without a diaphragm. Only in the case of very small amounts of water, difficult samples and when very high accuracy is needed, electrodes with a diaphragm are used. In this case, an additional catholyte is required for the cathode compartment.

The appropriate titrator for this application is the TitroLine® 7500 KF trace for coulometric water determination.

Comparison of volumetric and coulometric titration

Today, practically all applications can be carried out easily, quickly and accurately with the coulometric and volumetric Karl Fischer titration instruments. Because of its selectivity and accuracy, Karl Fischer titration has become the most important method for water and moisture determination.

We would like to make the decision between a coulometric and a volumetric KF titrator a little easier for you.

In direct comparison, coulometric KF titration is simpler, since conditioning is performed automatically in the background, meaning the titration cell is kept dry.

In volumetric titration, conditioning is required for each sample before titration. Both methods complement each other and rarely can one be completely replaced by the other. Coulometry has its advantages in simple operation and the determination of very small amounts of water, while volumetry can be used far more flexibly.

In practice, there are some differences between the two methods, which are shown in the following table. The advantages of volumetry lie in the more flexible application possibilities due to different sample feeds and solvent variations. Coulometry, on the other hand, can score with lower detection limits and even easier handling.

If you need to measure difficult samples that are problematic to dissolve in the Karl Fischer solvent or lead to unwanted reactions, the oven method is the right choice for you.

With the oven method, the sample is weighed into a glass vial and heated in the TO 7280 headspace oven. The water is transferred to the titration cell via a carrier gas stream. Either dried air or nitrogen can be used as carrier gas. Contamination of the cell is therefore avoided.

Depending on your daily number of samples, the oven method can be carried out semi-automatically or fully automatically. In the case of semi-automatic determination, the sample is manually inserted into the oven and removed after the titration has been completed. After confirmation, the head automatically moves up or down and the titration starts. The semi-automatic titration can be carried out as a standalone titration, but also PC-controlled via the TitriSoft titration software.

If your number of samples exceeds 10 to 15 per day, a fully automatic system is the best solution. In this case, the TW 7650 sample changer will take over the transfer of the samples into the oven for you. The entire measuring cycle is controlled fully automatically and conveniently via the TitriSoft PC titration software. The pharmaceutical version of the TitriSoft PC software, which is included in the standard scope of delivery of the changer, also allows working in a regulated environment.

FAQ - Water determination according to Karl Fischer

• Can a Karl Fischer coulometer also be used up to 100% water content?
  The coulometric method can also determine up to 100% water. However, at 10 .000 µg (10 mg) the titration already takes quite a long time. Therefore, no more than 5,000 µg (= 5 mg) of water should be weighed in. For comparison: one drop of water corresponds to approx. 35-50 mg of water!

• What are the applications of coulometric or volumetric KF titration?
  In general, coulometry is used to determine µg amounts of water in liquid samples without side effects. If an oven is used, solid samples such as plastics or pharmaceutical products can also be measured using a coulometer. Volumetry is used to determine mg amounts of water in liquid and solid samples.

• When should coulometry with a diaphragm be performed?
  The most common applications are with the cell without diaphragm. For very small amounts of water (< 50 ppm), the diaphragm cell may have advantages depending on the application. Special anolytes used, for example, in the determination of the water content of mineral oils, also require the use of a generator cell with diaphragm.

Which KF titrator is right for me?

For a large number of applications, the use of both a coulometric and volumetric system is possible. For some special samples such as mineral oils or plastic samples, the right choice can be clear. Nevertheless, the selection of the right system for you depends on many factors. Contact us today and we will be happy to advise you on the system that is right for your specific application.