Every glass measurement electrode undergoes an aging process, even when it is not in use. The aging process is continuous and starts immediately after manufacture. Primary reasons for decreased pH probe lifespan are as follows:
- The chemical composition of the glass membrane
- The steady growth of the internal gel layer of the membrane
- Chemical and mechanical attack of the outer gel layer of the membrane during measuring and cleaning of sensors
Aging is significantly accelerated by:
- Continuous pH measurement in hot solutions above 60°C
- Measurements in high acidity and high alkalinity solutions
- Incorrect handling of electrode assemblies when not in use, i.e. cleaning and sensor storage
Typical symptoms of an aged measuring electrode are:
- An increased response time
- An increased membrane glass resistance
- A declining slope, especially in the alkaline region
- A shift of the asymmetry potential
It is of course impossible to state the lifetime of a glass measurement electrode, especially as a combination of the above list of reasons for aging may cause the deterioration of the sensor performance. For example, a pH sensor, used continuously in aqueous solution of 4 to pH 8 at ambient temperature and good handling may have an operating life of 18 months or longer. The same sensor could only last only 2 months if operated above at 90°C. Furthermore, the same electrode may stop performing after 2 to 3 weeks if it is subjected to high alkalinity of > 13 pH and simultaneously at high temperature above 90°C.
The increasing membrane resistance, the declining slope and the zero point drift (shift of the asymmetry potential) may all be compensated for, within limits, by the pH meter / transmitter during the calibration process.
After proper pH calibration, a deteriorating response time is a certain indication of aging. New pH sensors typically have a response time of 20 to 30 seconds when checked in pH buffers with a difference of 3 pH units or more. If the response time is unacceptable to the user, then there is no other alternative, but to replace the electrode assembly with a new one.