Karl-Fischer-titrimetry

The only way to chemically determine whether water is present in a sample or not, is by Karl-Fischer titration. This method is based on the oxidation of SO2 by elemental I2 in the presence of water. The reaction taking place in the titration vessel is called Bunsen reaction and follows this equation:

 

SO2 + I2 + H2O ↔ 4 H+ + 2 I- + SO42-

 

It is obvious that one mole water is consumed for each mole I2, thus by titrating against an iodine indicator, the endpoint of the reaction can be determined. This reaction is highly selective for water and can be used on any sample which can be dissolved in the reaction medium (normally methanol based). However, since this is a chemical reaction, compounds which can decompose by consuming or releasing water (e.g. β-lactams) can give wrong high or low water contents.

Karl-Fischer-titrimetry has been developed to be performed coulometrically, where the I2 is produced in situ by the anode. The endpoint can be detected here with a second pair of electrodes, which measure the electrical potential of the solution. This will drop considerably at the endpoint, when a surplus of I2 is produced from I- ions. The possibly biggest advantage of this variation of Karl-Fischer-titrimetry is the possibility to use very small samples. Normally, approximately 1 mg of sample is sufficient to run a coulometric tritration, while normal titrations use 50 to 100 mg of sample. Both kinds of methods have the potential to be automated.

Karl-Fischer-titrimetry can be used in combination with thermogravimetry, where the evolving gas of the TG furnace is bubbled through the tritration medium. At the end of the measurement, the Karl-Fischer-titration is carried out and an exact amount of released water can be determined. This coupling has advantages when mass spectrometry coupling is not possible.