Metrohm 906 Titrando - Manual (Page 41)

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4 Karl Fischer titration
906 Titrando
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4 Karl Fischer titration
4.1 Volumetric titration
4.1.1 Principle of the volumetric Karl Fischer titration
The volumetric Karl Fischer titration is the classic method of water
content determination. It works with a methanolic solution of iodine, sul-
fur dioxide and a base as buffer substance. If an aqueous sample is titra-
ted, then several reactions take place that can be summarized in the fol-
lowing 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, I
2
reacts quantitatively with H
2
O. This
chemical equation serves as a basis for the water content determination.
The classic Karl Fischer method has undergone continuous further devel-
opment in recent years. This development did not concern itself solely
with the refining and automation of reagent dosing, but also with improv-
ing endpoint indication and reagents. The disadvantage of this method is
that the reagents are not completely stable. This means that the titer has
to be re-determined on a regular basis.
4.1.2 Endpoint determination
The titration endpoint is determined by an electrometric indication
method. Double Pt wire electrodes or double Pt ring electrodes are used
for this purpose. A distinction is made between the following two indica-
tion methods:
Biamperometric indication (Upol)
A constant potential is applied to the electrodes and the resulting current
is measured.
Bivoltametric indication (Ipol)
A direct or alternating current is applied between the two electrodes and
the resulting potential is measured.