Operating instructions

METTLER TOLEDO DL36 5
Measurement principle
3. Measurement principle
In the Karl Fischer method for determination of the moisture content, water (H
2
O) reacts with
iodine (I
2
) and sulfur dioxide (SO
2
) in the presence of methanol (CH
3
OH) and a base (RN).
H
2
O + I
2
+ SO
2
+ CH
3
OH + 3RN → [RNH] SO
4
CH
3
+ 2[RNH]I (1)
In the volumetric titration, iodine is added as the titrant. In the coulometric titration, iodine is
generated electrolytically by an iodide-containing anolyte.
2I
–
→ I
2
+ 2e
–
(2)
As long as water is present in the titration cell, the generated iodine reacts according to
reaction (1). As soon as all the water has been consumed by the reaction, there is a small
excess of iodine in the anolyte. The double-pin platinum electrode detects this iodine excess
and the iodine generation is stopped. According to Faraday's law, the amount of iodine
generated is proportional to the current which has flowed. In reaction (1), I
2
and H
2
O react
with each other in proportion 1:1. One mole of water (18 g) thus corresponds to
2 x 96 500 coulomb, in other words per mg H
2
O a quantity of electricity of 10.72 coulomb is
consumed. The total current consumption is a measure of the amount of moisture present.
Inner burette
Catholyte
Cathode
Membrane
Anode
Detection electrode
(double-pin platinum electrode)
Titration cell
Anolyte
Magnetic stirrer
Detection/
control unit
Microcomputer
Display