Metrohm 899 Coulometer - Manual (Page 38)

4.1 Principle of coulometry according to Karl Fischer
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899 Coulometer
4 Coulometric titration
4.1 Principle of coulometry according to Karl Fischer
The coulometric Karl Fischer titration is a variation of the classic
water content determination method according to Karl Fischer. The con-
ventional method works with a methanolic solution of iodine, sulfur diox-
ide and a base as buffer substance. If an aqueous sample is titrated, then
several reactions take place that can be summarized in the following sum
equation:
H
2
O + I
2
+ [RNH]SO
3
CH
3
+ 2 RN ⇄ [RNH]SO
4
CH
3
+ 2 [RNH]I
According to the equation above the I
2
reacts quantitatively with H
2
O.
This chemical equation serves as a basis for the water content determina-
tion.
With the coulometric Karl Fischer titration, the necessary iodine is
directly and electrochemically generated in the electrolyte containing
iodine ("electronic buret"). Between the amount of electric charge and the
amount of generated iodine, there is a strictly quantitative relationship,
which is used for high-precision dosing of the iodine. Because the coulo-
metric Karl Fischer method is an absolute determination, no titer needs
to be determined. It must only be ensured that the reaction generating
the iodine runs with a 100% current efficiency. All of the reagents availa-
ble today ensure this.
The endpoint indication is effected voltametrically by modulating an alter-
nating current of constant strength to a double Pt electrode. This results in
a voltage differential between the Pt wires. This is drastically reduced as
soon as even the slightest amounts of free iodine are present. This circum-
stance is used for detecting the endpoint of the titration.