What are the most frequent causes of calibration problems?

The following three problems occur most often during calibration:

• Zero calibration error
• Electrode slope too low
• Slow response, for example, longer than 3 minutes

There are a variety of causes for the problems named above. The most frequent are:

• The buffer solutions used are either contaminated or out-of-date. It could also be that one of the buffer solutions used is no longer the value labeled on the bottle – for this reason, never store buffer solutions in unmarked or dirty containers.
• The reference electrolyte and / or the diaphragm are contaminated.
• An old or defective electrode is used.
• An electrode is used that has not been hydrated long enough (after dry storage or after cleaning with strong acid solution).
• The pH membrane of the electrode is mechanically damaged, and has cracks.
• The electrode is electrostatically charged (through rubbing of the electrode shaft with a cloth instead of careful dabbing with soft paper).
• The temperature difference between electrode and buffer solution is more than 10°C.
• The connection between electrode and measurement device can also cause problems: for instance, a break in the cable, or a short-circuit caused by moisture in the cable or electrode plug.