Metrohm 899 Coulometer - Manual (Page 43)

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4 Coulometric titration
899 Coulometer
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￭ the sample releases its moisture completely.
￭ no side reaction with the Karl Fischer reagent takes place.
￭ the surfaces of the electrodes are not covered by the sample substance
(incomplete KF reaction).
￭ the Pt grid of the generator electrode does not become damaged.
￭ the Pt wires of the indicator electrode do not become damaged.
4.4 Optimum working conditions
4.4.1 General
When a coulometer cell that has been well dried-out beforehand is put
into operation with a generator electrode without diaphragm, the basic
drift will be reached within approx. 30 minutes. It is recommended that
the coulometer cell be repeatedly and carefully shaken during this time.
For generator electrodes with diaphragm, you should expect a preparation
time of approx. 2 hours.
To obtain precise determinations of amounts of water smaller than 100
µg, it may also be of advantage to condition the coulometer cell overnight
before using it.
If the Coulometer is switched off for extended periods with the coulome-
ter cell filled, it will take some time for it to become conditioned again
after it is switched back on.
With continuous use, the Coulometer should not be switched off over-
night.
4.4.2 Drift
A constant drift in the range of ≤ 4 µg/min is all right. Lower values, how-
ever, are quite possible. Higher but stable values will still produce good
results, because the drift can be compensated.
A constantly high drift can be caused by water-containing deposits in
inaccessible parts of the coulometer cell. In these cases, shaking the cell
can reduce the value. Ensure that there are no drops above the liquid level
in the coulometer cell.
If you are working with a generator electrode with diaphragm, shake the
cell only hard enough that the catholyte and anolyte do not mix with one
another. If the drift remains too high for a prolonged time, even after
shaking the cell, then the electrolyte solutions should be replaced. The
catholyte should be replaced once per week.
A wet catholyte can be another reason for the excessively high drift. The
wet catholyte can be dried with a KF single-component reagent.