Karl Fischer titration
Karl Fischer titration (KFT) is a method for determining the amount of water in a sample.
Table of contents
- Karl Fischer titration at Quality Analysis
- How does Karl Fischer titration work?
- Which methods of Karl Fischer titration are available?
- Karl Fischer titration in practice
Karl Fischer titration at Quality Analysis
For many years, we at Quality Analysis have used, both reliably and practically, the method invented by Karl Fischer for the quantitative analysis of water. With our analysis systems Titrando 852 and 874 Oven Sample Processor manufactured by Metrohm, we analyse a wide range of samples for their exact water content. Here we use both the coulometric method with oven technology, as well as the volumetric method, as a result measurement in the range of 1 ppm to 100 percent is possible. The gas extraction or oven method can, in principle, be used for all samples that give up their water when heated. The oven method is indispensable in those cases in which direct volumetric or coulometric Karl Fischer titration is not possible, either because the sample contains interfering components, or because the sample cannot be transferred to the titration vessel due to its consistency, or can only be transferred with difficulty.
Thanks to our fast, modern analysis systems, the measurement result is available in a very short time. The results of the KF method are very precise, independent of the type of sample and are not affected by the presence of other volatile substances.
Do you have any questions about Karl Fischer titration? Our experts would be pleased to advise you! Simply call us: 07022 2796-831
How does Karl Fischer titration work?
A solution with methanol is used for the titration in the Karl Fischer method; this solution contains iodine and sulphur dioxide as well as a buffer (e.g. imidazole). A chemical reaction occurs between iodine and sulphur dioxide only in the presence of water, during this reaction the brown iodine is reduced to a colourless iodide. If there is no more water available, the reduction of the iodine stops and the colour ceases to change.
In this way the end point of the reaction can, in theory, be determined from the colour change of the solution. In practice this colour change is, however, difficult to detect with the naked eye, for this reason the end point is determined using a pair of electrodes. The potential necessary to maintain the flow of electrical current between the electrodes reduces sharply with the presence of free iodine.
Which methods of Karl Fischer titration are available?
A differentiation is made between coulometric and volumetric Karl Fischer titration.
1. Volumetric Karl Fischer titration
During volumetric KF titration, the sample must first be pretitrated in alcohol, the actual measurement using the Karl Fischer method described above then follows. Both one and two-component reagents can be used for the titration. One-component reagents contain, along with the iodine, also the necessary nitrogenous base and the sulphur dioxide. With two-component reagents, these two components must be added first to the titration cell, but then the reaction is faster.
The water is measured based on the consumption of the standard solution using which the amount of water converted can be calculated stoichiometrically.
2. Coulometric Karl Fischer titration
Coulometric KF titration does not require a standard solution because the necessary iodine is produced in the titration cell with this method. For this action to be possible, iodide must be available in the cell dissolved in alcohol, together with sulphur dioxide. The iodide is converted into iodine by applying a voltage to a pair of platinum electrodes. The end of the reaction is also measured here using a (second) pair of electrodes.
Instead of the consumption of the standard solution, with this method the amount of current that has flowed up to the end of the reaction is measured and in this manner the quantity of iodine produced is calculated.
3. Which of the two methods is used when?
The selection of the most suitable method for the measurement of the water depends, above all, on the H2O content of the sample. Using volumetric titration, substances with a water content from 0.01 to 100 percent can be analysed. Coulometric titration is suitable, on the other hand, for samples with a water content from 1 ppm to 5 percent and is therefore appropriate for the verification of very small quantities of water.
Karl Fischer titration in practice
The water content plays a major role in the quality of many raw materials and products made from a very wide range of materials. The Karl Fischer method is therefore of great economic importance, for example in plastics analytics and in petrochemistry.
During the processing of plastics, a certain water content is not allowed to be exceeded. Typically this water content is in the range between 0.02 and 0.1 percent and varies from plastic to plastic. If this so-called processing moisture is exceeded, the quality of the products is no longer ensured. Bubbles or smearing then often occur in the plastic. As a rule, Karl Fischer titration is used to safeguard the permissible limits.
Determination of the water content of crude oil
A further important application is the determination of the water content of crude oil. The water content fluctuates depending on the type of oil and has a direct effect on the value of the oil. Expressed simply: the more water in the crude oil, the lower its value. Both methods of Karl Fischer titration are also used here.