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11. Appendices
11.1 Checking the reference junction of a pH electrode.
If pH readings are inaccurate or unstable, the reference junction of the electrode
may be blocked. The following test can be performed to determine if the reference
junction of a pH electrode is making adequate contact with the sample solution.
1. Calibrate the miniCHEM-pH, as per section 4.1.
2. Dilute 1 part of pH6.88 buffer with 9 parts of distilled water.
3. Measure the pH of the diluted buffer. The result should be 7.06 ±0.02 pH.
4. If the value obtained is outside of these limits, then clean the reference junction,
as per the instructions supplied with the pH electrode.
5. Re-calibrate the miniCHEM-pH and repeat the test.
6. If the value obtained is still outside 7.06 ±0.02 pH, then the electrode may need
to be replaced, depending on the level of accuracy required.
11.2 Determining if an instrument or electrode is faulty
The following test can be performed to help determine if the miniCHEM-pH or the
pH electrode is faulty.
1. Disconnect the pH electrode.
2. Connect a short piece of wire or a paper clip etc. between terminals 5 and 6.
3. The ZERO control should have sufficient range to obtain a reading of exactly
7.00 on the display.
4. If the miniCHEM-pH is operating correctly, the reading should be totally
stable with the wire in place. If not, the meter requires servicing.
5. Now carefully disconnect the wire from terminal 6 only (make sure the other
end of the wire remains connected to terminal 5).
6. The reading should steadily drift away from 7.00 (either up or down) at a rate
of approximately 1 pH or less every 3 seconds. If the drift rate is faster than
this, then input circuitry of the miniCHEM-pH is faulty and requires servicing.
7. Disconnect the wire from terminal 5. Either re-connect a new pH electrode, or
return the miniCHEM-pH to the factory or your distributor for servicing.
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