Metrohm 756 KF Coulometer - Manual (Page 12)

2.1 Principle of coulometric KF determinations
756/831 KF Coulometer, Instructions for Use 4
2 The wet chemistry workplace
2.1 Principle of coulometric KF determinations
The coulometric Karl Fischer titration is a version of the classical
water determination method developed by Karl Fischer. The
traditional method utilises a methanolic solution of iodine, sulphur
dioxide and a base as buffer. Several reactions run in the titration of
a water-containing sample and can be summarised by the following
overall equation:
H
2
O + I
2
+ [RNH]SO
3
CH
3
+ 2 RN ⇔ [RNH]SO
4
CH
3
+ 2 [RNH]I
According to the above equation, I
2
reacts quantitatively with H
2
O.
This chemical relation forms the basis of the water determination.
The classical Karl Fischer method has undergone constant
development in the past years. This further development has
involved not only refinement and automation of the reagent
dispensing, but also improvement of the end point indication and the
reagents. Despite the progress made, the classical, volumetric Karl
Fischer method suffers from the disadvantage that the reagents are
not completely stable resulting in the need to redetermine the titer at
intervals.
In the coulometric Karl Fischer titration, the iodine needed is
generated directly in the electrolyte by electrochemical means
("electronic buret"). The rigorously quantitative relationship between
the electric charge and the amount of iodine generated is used for
high-precision dispensing of the iodine. As the coulometric Karl
Fischer method is an absolute determination no titer need be
determined. It is necessary only to ensure that the reaction which
generates the iodine runs with 100% current efficiency. With the
reagents available today this is always the case.
The end point is indicated voltametrically by applying an alternating
current of constant strength to a double Pt electrode. This results in
a voltage difference between the Pt wires of the indicator electrode
which is drastically lowered in the presence of minimal quantities of
free iodine. This fact is used to determine the end point of the
titration.