pH electrodes are inherently very sensitive temperamental devices. However bearing in mind that hydrogen ion activity, is measured as a potential across a glass membrane 0.2 mm thick, with a resistance in excess of 20 Megohms, it is hardly surprising.
The decision as to when a pH electrode requires changing has always been a moot point. The life of a pH electrode can vary dramatically, from hours to several years. The oldest one that we know of was 14 years when it finally expired, until then it worked perfectly.
pH Electrodes fail for many reasons, listed below are some examples:
- Mechanically damaged (cracked glass)
- Chemically damaged (etched glass)
- Loss of internal insulation between glass and reference electrodes
- Loss of insulation in coaxial cable or plug
- Poisoning of reference electrode
- Coating of glass.
The Rule of Thumb
Experience is an important factor in deciding when to change a pH electrode, in general we look for, (1) speed of response, (2) stability, (3) slope, and (4) Eº potential (check reading).
- Speed of response, between two buffer solutions 3.0 pH apart, equilibration to within 0.1 pH in less than 3 minutes.
- Stability, in a standard buffer solution the reading should not drift more than 0.05 pH over several minutes.
- Slope, an electrode with a slope of less than 80% should be rejected.
- Eº potential, the normal acceptable range is ± 58 mV (6.0pH – 8.0pH) although if other parameters are satisfactory ± 70 mV could be accepted.