Karl Fischer titration

What is Karl Fischer titration used for?

Karl Fischer (KF) titration finds use in any industry where maintaining a specific moisture level is essential. This includes the food and drink, petrochemical, and pharmaceutical industries. In addition to liquids and high-moisture solids, KF can be used to test dry construction materials, such as cement and crushed rock, for trace amounts of water.

Other common uses of Karl Fischer titration include testing organic solvents to ensure they remain water-free for synthesis purposes and monitoring the progression of polymerization reactions, where water content is indicative of progression.

How does the Karl Fischer titration method work?

Karl Fischer titration is performed using an auto-titrator. The underlying principle involves reacting a titration solution of sulfur dioxide and elemental iodine with water to produce sulfuric acid and hydrogen iodide. By measuring the amount of iodine that is consumed in this reaction, it is possible to calculate the amount of water that was originally in the sample.

Volumetric vs. coulometric KF titration

The calculation can be done in two ways. Volumetric KF analysis involves slowly adding the titration solution to the sample while a set of electrodes measures the current that can flow through the solution. Once all of the water has reacted, there will be a significant voltage drop as the iodine becomes "in excess". Volumetric Karl Fischer titration is ideal for measuring water content in relatively high concentrations.

Alternatively, coulometric KF analysis can be performed. Here the titration solution is premixed in the chamber, already containing all of the components, including iodine in the form of potassium iodide. This is slowly converted into iodine by applying a small electric current, allowing titration to occur. Coulometric Karl Fischer titration can be used to measure very low water concentrations more accurately than is possible with volumetric analysis.

Sample requirements and preparation

Suitable liquid and solvent samples can be tested as is, through injection into the KF titration chamber. Many solid samples can be dissolved in a suitable solvent of known water content, allowing them to be tested in the same way. Even some insoluble solid samples are suitable for Karl Fischer titration if they are sufficiently dispersed in a suitable solvent before being added to the titration chamber.

Advantages and limitations of Karl Fischer titration

Karl Fischer titration has a high throughput and can be used to test samples in quick succession. With some setups, KF can identify water in the parts per million range, providing information on even the smallest traces of water. Very little preparation is generally required, and KF analysis can be performed using only a few milliliters or grams of sample material.

Certain sample types will react with the chemicals in the titration solution in unwanted ways, which makes them unsuitable for Karl Fischer titration. Furthermore, the apparatus can experience drift, where it will require a longer period between samples to achieve a baseline before further testing. When the drift becomes too large, the device will have to be cleaned and fully dried before further use.