Metrohm 899 Coulometer - Manual (Page 44)

4.4 Optimum working conditions
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899 Coulometer
When you work with a Karl Fischer oven, a drift ≤ of 10 µg/min is all right.
The drift depends on the gas flow (the smaller the gas flow, the lower the
drift) and on the humidity of the surroundings.
4.4.3 Reagent replacement
The electrolyte solutions must be replaced in the following cases:
￭ The coulometer cell is too full.
￭ The KF reagent has reached its capacity limit.
￭ The drift is too high, and cannot be reduced by shaking the coulometer
cell.
￭ A two-phase-mixture is being formed in the coulometer cell; in this
case it is also possible to aspirate the sample phase only.
Exhausted electrolyte solution is best disposed of by aspiration. To do this,
you can use, for example, an 803 Ti Stand with built-in membrane pump.
An advantage is, that the coulometer cell does not have to be disassem-
bled.
In the event of severe contamination, the coulometer cell can be rinsed
with a suitable solvent which is also aspirated.
In the case of the generator electrode with diaphragm, the catholyte
should be replaced once per week. Longer use can cause blackening and
yellow precipitates in the cathode chamber. A discernable stench is also a
sign of having used the catholyte for too long.
4.4.4 Indicator electrode
A new indicator electrode can take a certain warm-up time for forming
the surface. During this time unexpectedly long titration times and high
measurement results can occur. This phenomenon will, however, disap-
pear after a short time of use. In order to accelerate the setting of a new
indicator electrode, the 899 Coulometer can be conditioned e.g. over
night.
A contaminated indicator electrode can be carefully cleaned with an abra-
sive agent (6.2802.000 polishing set or toothpaste). After the cleaning,
rinse with ethanol.
The two Pt wires of the indicator electrode should run as parallel as possi-
ble to one another. Check the Pt wires before inserting the electrode.