Metrohm 781 pH/Ion Meter - Manual (Page 139)

8 Troubleshooting – Messages – Maintenance
780/781 pH/Ion Meter, Manual
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8.6.3 Results
The following graph shows the potential of the pH electrode as a func-
tion of time during the electrode test for a buffer in diagram form:
Fig. 14: Diagram showing changes in potential during an electrode test
The evaluation of the pH electrode is made according to the following
criteria. These measured and calculated values are listed in the printed
report:
Streaming potential dU (unstirred - stirred)
This is the absolute difference between the measured potentials in the
stirred (after 3 min) and unstirred solution. It is determined at each pH,
but is initially most significant for the pH 9 buffer.
Drift
The absolute drift in the stirred solution (after 3 min) is determined at
each pH and used to calculate the total.
Slope
Just as in a calibration, for each pH combination (pH 4/7, 7/9 and 9/4)
the electrode slope, pH(0) and the offset potential Uoff are determined
in the stirred solution.
Response time t(1mV)
Time after which the measured potential is within 1 mV of the potential
determined in the stirred solution after three minutes.
Offset potential Uoff
The offset potential Uoff, which is determined together with the slope
and pH(0), is tested once against a preset limit and is not included in
the evaluation.