Metrohm 781 pH/Ion Meter - Manual (Page 141)

8 Troubleshooting – Messages – Maintenance
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8.6.4 Messages and measures
For the test results ' passing electrode ' and ' electrode out of
limits ' the following recommendations for improving the quality of the
electrode can be given:
Result Measure
passing electrode Clean diaphragm
electrode out of limits Clean diaphragm and/or re-
generate the glass membrane;
check reference system
The following general information should be taken into consideration
when evaluating the electrode test results:
• With separate measuring and reference electrodes the pH elec-
trode is not always the cause of unexpectedly poor test results.
This is why you should also check the condition of the reference
electrode (see Section 8.6.5).
• If the tested pH electrode has been given a lower evaluation only
because of its slope being inadequate then you should compare
the individual slopes at pH 4/7 and pH 7/9. If these values are
different then repeat the electrode test using a fresh pH 9 buffer.
Only then can you conclude that there is an electrode problem.
• A correct outflow of the inner electrolyte causes only a minimum
potential drop when the stirrer is turned of (see Fig. 14). A large
drop indicates a too small outflow - the diaphragm is blocked. If
you obtain a potential step-up, the outflow is too high and should
be corrected.
• Under certain conditions, e.g. very low relative humidity, plastic
floor coverings or clothing made from synthetic materials, the pH
electrode can be affected by electrostatic discharges. The re-
sults are high drift values and correspondingly poor electrode
test results. Such problems can be avoided by the user wearing
e.g. an antistatic bracelet.
In addition to the evaluation of the electrode quality described above,
the measured values are also checked to see if they have exceeded
certain tolerances. In such cases the electrode test is concluded with
the following message on the display:
electrode test not passed
More specific error messages will also be shown and included in the
report. The following table lists these tests, the resulting messages and
recommended measures for remedying their possible causes. These
measures are described in detail in the following Section.
The first two tests are carried out after the measurement in the pH 9
buffer and the electrode test then stopped if necessary.