Dosing water by the Karl Fischer method

Karl Fischer method

The Karl Fischer method is a chemical titration method based on the oxidation of sulfur dioxide by iodine in the presence of water.

There are two kinds of apparatus for implementing these assays: those based on a volumetric approach and those based on a coulometric determination.

Volumetric dosing

The volumetric method involves measuring the water content of a sample by gradually adding reagent to the sample until the equivalence point is detected.

Coulometric dosing

With the coulometric method, the dosing reaction is controlled electrically. The instrument used for this type of dosing is a Karl Fisher coulometer. To achieve this, a small quantity of the sample to be analyzed is placed in a reactive mixture containing, in particular, sulfur dioxide and diiodine in excess. The end of dosing is marked by a drop in the measured voltage. This method is particularly suitable for samples containing very small amounts of water.

The coulometric assay is very effective for samples that contain very little water: from a few ppm to a few %.

Mickaël Simond, CEO and Sales Director, Calnesis

Karl Fischer water dosing requirements

Water can be dosed by the Karl Fischer method:

• for pH solutions between 5.5 and 8. Outside this zone, a pH correction must be made prior to analysis.
• For compounds with no ketone or aldehyde function. Otherwise, a parasitic reaction occurs, requiring the use of specifically adapted reagents.
• For soluble solids in the reagent, mainly composed of methanol

A feasibility test is often proposed to ensure the feasibility of the assay on your samples.