62
Calibration procedure
-
Select a reference temperature of 25°C on the measuring vessel.
-
Rinse the measuring cell and the measuring vessel several times
with some of the standard solution.
-
Fill the measuring vessel with standard solution.
-
Stabilize the temperature of the standard solution and the
measuring solution at 25°C. For high-accuracy measurements it
is necessary to observe a tolerance of ± 0.1 K, while ± 0.5 K will be
sufficient for routine measurements.
-
Compare the measured value with the theoretical value of the
standard solution.
With WTW plastic cells, the measured value should not deviate from the
theoretical value by more than 1.5 % (2 % in the case of older measuring
cells).
Undesirably low values are caused by measuring cells:
-
with dirty electrodes. Clean the electrodes.
-
with platinized electrodes if the surface has dried out. Replatinize
the electrodes.
-
with polarized electrodes. Cells with a cell constant of k < 0.5/cm
already polarize if the conductivity amounts to > 200 µS/cm.
Use a standard solution with lower conductivity.
The adjustment of a conductivity meter is effected by setting of the cell
constant. This may, e.g., be necessary with a new cell or after platinizing
of the electrodes. The conductometer must have a
potentiometer or a corresponding function for setting of the cell constant.
With new measuring cells it will often be sufficient to enter the value
stated on the cell cable or the operating instructions.
For high-accuracy measurements involving cells with a constant of
k > 0.1/cm it is better to carry out a determination of the cell constant.
LF 1194054
Adjustment
63
Procedure
Use a potassium chloride solution c = 0.01 mol/l as standard solution.
-
Select a reference temperature of 25°C on the measuring vessel.
-
Rinse the measuring cell and the measuring vessel several times
with some of the standard solution.
-
Fill the measuring vessel with standard solution.
-
Stabilize the temperature of the standard solution and the
measuring solution at 25°C. For high-accuracy measurements it
is necessary to observe a tolerance of ± 0.1 K, while ± 0.5 K will
be sufficient for routine measurements.
-
Set the cell constant until the instrument shows a conductivity
of 1413 µS/cm.
It is risky to try compensating errors due to dirty, dried out or polarized
electrodes by adjusting the cell constant.
EN 27888, Determination of Electric Conductivity, European Standard,
9/93
Note
Literature
LF 1194054
