Food and Dairy analysis and Chemistry (for level 3 Food technology program Students ) - lectuer 3

water determinations

Moisture determination with Karl Fischer titration

Principle:

The Karl Fischer method is used for many substances as a reference method. It is a chemical analysis procedure which is based on the oxidation of sulfur dioxide by iodine in a methanolic hydroxide solution. In principle, the following chemical reaction takes place:

H₂O + I₂ + SO₂ + CH₃OH + 3RN -> [RNH]SO₄CH₃ + 2[RNH]I

The titration can be performed volumetrically or coulometrically.
In the volumetric method a Karl Fischer solution containing iodine is added until the first trace of excess iodine is present. The amount of iodine converted is determined from the burette volume of the iodine-containing Karl Fischer solution.
In the coulometric procedure, the iodine participating in the reaction is generated directly in the titration cell by electrochemical oxidation of iodide until again a trace of unreacted iodine is detected. Faraday's law can be used to calculate the amount of iodine generated from the quantity of electricity required.

Application:

The Karl Fischer titration is a moisture determination method specific for water and is suitable for samples with a high moisture content (titrimetry) and also for those with water contents in the ppm range (coulometry). It was originally developed for nonaqueous liquids, but is also suitable for solids if these are soluble or if the water they contain can be removed by heating in a stream of gas or by extraction.

Advantages:

Accurate reference method, coulometry also suitable for trace analysis and water detection.

Restriction:

The working method must be adapted to the specific sample.