Metrohm 840 PC Control 5.0 / Touch Control - Manual (Page 277)

4 Parameters
PC Control / Touch Control
265
4.9 Electrode test for pH electrodes (ELT)
For pH electrodes you can carry out an automatic electrode test which
allows the quality of the electrode used to be evaluated. As the result
you will obtain various information about the characteristic measuring
properties of your pH electrode (slope, response time etc.). The con-
cluding evaluation of the electrode is carried out by checking the results
obtained against preset limit values. If the electrode test has been fin-
ished successfully then the calibration data is stored together with the
sensor data of the used sensor.
These limit values are stored in the software for the three most fre-
quently used types of Metrohm pH electrodes: standard electrodes,
gel electrodes and non-aqueous electrodes. You can also define
your own limits (see Section 3.14.8). The specific classification of the
electrode system is defined by the reference system used. You find an
overview for these electrode types and the corresponding limit values
on page 268.
Faulty electrodes frequently show certain combinations of measured
results of the electrode test which lie outside the preset limits. This per-
mits more detailed information to be given about possible causes and
recommended measures.
Buffers
You need faultless pH 4, pH 7 and pH 9 buffers for carrying out the
electrode test. We recommend the use of suitable Metrohm buffers. In
principle the electrode test can also be carried out with any of the
stored sets of buffers.
The buffers must be measured in the following order:
1. pH 9
2. pH 4
3. pH 7
Note!
Particularly with buffer pH 9 you should make certain that it is in a
faultless condition. By absorption of CO
2
from the ambient air this
buffer can easily vary from its specified pH value and therefore pro-
duce incorrect test results.
Stirrer
A stirrer must be connected for the electrode test.
Temperature
Please ensure that the whole electrode test is carried out at a constant
temperature. As the temperature has a great influence on the response
time you should work at room temperature if possible. The preset limit
values are set for 25 °C.