Metrohm 756 KF Coulometer - Manual (Page 20)

2.8 Optimal working conditions
756/831 KF Coulometer, Instructions for Use 12
2.8 Optimal working conditions
If a thoroughly dry titration vessel with a generator electrode without
diaphragm is used then the basic drift is reached within approx. 30
minutes. It is recommended that the titration vessel is carefully
shaken several times during this time.
For generator electrodes with diaphragm a preparation period of
approx. 2 hours must be expected.
If the 768 KF oven is used it is recommended that the oven is al-
lowed to run overnight with the oven valve set to "purge".
For precise determination of amounts of water below 100 µg it may
also be an advantage to condition the instrument overnight before
use.
If the instrument is switched off for a longer period of time with a
filled titration vessel then a certain time is required for it to become
dry again after it is switched on.
During continuous operation the instrument should not be switched
off overnight.
2.8.1 Drift
A constant drift of the order of about ≤ 4 µg/min is good. However,
lower values are certainly possible. If higher, stable values occur
then the results are normally still good as the drift can be compen-
sated (drift correction see page 29).
The drift is shown together with the "drift trend":
⇔ constant drift and drift below the start drift, see page 32.
↑ drift increasing
↓ drift falling
A drift which remains high may be caused by water-containing de-
pots in inaccessible locations inside the cell. In such cases a reduc-
tion in the value would be achieved by shaking the titration vessel.
Take care that no drops above the level of the liquid are formed in
the titration vessel.
For generator electrodes with diaphragms shaking must not be so
vigorous as to cause the catholyte and anolyte to become mixed
with each other.
If even after shaking the drift remains too high over longer periods of
time then the electrolyte solution must be exchanged.
When working with the oven a drift
≤ 10 µg/min is good. The drift de-
pends on the gas flow (the smaller the gas flow the lower the drift).