In 1935, the German scientist Karl Fischer published a method for determining the water content in samples.
Karl Fischer discovered that if sulfur dioxide is added in excess, the same reaction can be used to determine water by titration of the acids produced.
His base of choice was pyridine which was supposedly “just standing on the shelf.” In his honor, the titration is called the Karl Fischer titration (KF titration for short).
The Karl Fischer test is a prevalent method for quantifying the water content in solid and liquid samples and, in certain circumstances, in gas samples.
In the following years, the original stoichiometry and the reagents were revised.
Fischer presented the reaction that gave an incorrect molar ratio, assuming an aqueous Bunsen reaction in which methanol functions only as a solvent.
Titration was done manually at first. The endpoint was marked by the persistence of the brown color of excess iodine added. This was slow, but it was also unsuitable for colored samples.
Today, KF titration is automated and widely used for water determination in various industries.
Karl Fischer degree techniques
Karl Fischer has two different techniques for the determination of water:
- The volumetric titration of KF.
- The coulometric titration of KF.
Volumetric determination is suitable for determining water content up to 1% water.
The sample is dissolved in KF solvent (generally methanol-based). The iodine is added as part of a KF reagent containing sulfur dioxide and iodine dissolved in pyridine and methanol. The endpoint is determined potentiometrically.
Coulometric KF analysis requires only a solution containing iodide. The iodine necessary for the KF reaction is produced by anodic oxidation of the iodide in the key, and the endpoint is electrochemically detected.
Coulometric determination is best suited for samples with less than 1% water.
Some samples are not soluble in any suitable solvent, and others can cause side reactions with Karl Fischer reagents. This is when the volumetric or coulometric titrator can be coupled with the oven.
The sample is heated in the oven, and the water released from the model is transported by a dry gas flow to a titration cell, where it reacts with the KF reagent.
This is a perfect solution for poorly soluble samples, samples that can cause secondary reactions with the solvent, and highly hygroscopic samples. When handling the selection under laboratory conditions can give falsely higher results.
Reagents for Karl Fischer titration are available in a wide range, depending on the purpose and ecological awareness of the user.
Pyridine can be replaced with a more robust (and less odorless) imidazole base, and ethanol can be used instead of methanol as a “greener” solvent option.
Different auxiliaries are available, such as additives for fats and oils, reagents to determine low water content, solubilizers for poorly soluble substances, and buffer solutions for strongly alkaline or highly acidic samples.
Uses of the KF titration
As a reliable and robust method, Karl Fischer’s titration is widely used to analyze water content in various industries directly.
In the food industry, it is used to determine the water content in:
- Fruit juice.
- Cocoa powder.
In the oil industry for all types of:
- Different oils.
In the cosmetic industry for the determination of water in:
In the pharmaceutical industry for:
- Raw Materials.
- Active substances.
- Freeze-dried substrates.
It is used to determine the water in:
- Construction materials (zeolite and cement).
This is just proof that with the right choice of technique and the proper reagents, Karl Fischer’s titration can be adequate for determining water content in almost every imaginable sample.
It is not surprising that this titration is a technique for use in any laboratory, in any industry.
- Safe: low toxicity and pyridine free.
- Speed: fast titrations and time savings.
- Reliable – Stable endpoints ensure accurate and reliable results.
- Long shelf life: up to 5 years depending on the reagent.