What is a Karl Fischer titration?

Since publication of his “New method for the determination of water”, the name of Karl Fischer and his titration technique have been recognised throughout the world. The technique is one of the most widely used and reliable methods for the measurement of water content in a large range of samples. Karl Fischer titration is now employed as a standard method in most laboratories and can be subdivided into two main techniques: volumetric titration & coulometric titration. In 1935 the German scientist, Dr. Karl Fischer, developed a titrimetric determination of water content using a reagent which contained iodine, sulphur dioxide, anhydrous pyridine and anhydrous methanol. This volumetric technique involves dissolving a sample in a suitable solvent and adding measured quantities of a reagent containing iodine until an end point is reached. This end point is determined potentiometrically using a platinum electrode. However, even with the automatic or semi-automatic instruments commercially available there are certain problems associated with the technique. These problems can include long analysis time, reagent calibration required, high reagent consumption rate and large sample amount required. In 1959 Meyer & Boyd were first to apply coulometry to the Karl Fischer principle. In this method the sample is introduced into a mixture of pyridine/methanol which contains iodide ions and sulphur dioxide. Using electrolysis, iodine is produced at the anode which then reacts with any water present. The production of iodine is directly proportional to the amount of electricity. According to the stoichiometry of the reaction, 1 mole of iodine will react with 1 mole of water, and combining this with coulometry, 1 milligram of water is equivalent to 10.71 coulombs of electricity. It is therefore possible to directly determine the amount of water present in a sample by measuring the electrolysis current in coulombs. (Coulombs are a measurement of current multiplied by time). The water present in the anode compartment of the titration cell is coulometrically titrated to a predefined end-point at which there is a minute excess of free iodine present.

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