Aging of pH probes

Keep in mind that all pH probes age with time, even if it is not in use.

Main reasons for probe aging

• The decomposition process that occurs when glass surfaces are in contact with aqueous solutions, will change the chemical composition of the glass membrane.
• As aging progresses, the hydrated gel layer becomes thicker, resulting in an increase in the gel layer resistance.
• Damage and degradation of the hydrated gel layer due to chemical or mechanical cleaning and demanding measurement conditions.

Typical symptoms of an aging probe

• Response will become slower (increase in gel layer resistance).
• During calibration, you will observe an increasing zero point offset and a declining slope value.
• As the probe aging process continues you will need to calibrate more frequently.
• Finally, erratic readings. Now it's definitely time to replace your pH sensor.

Probe aging is considerable accelerated by:

• incorrect handling of the pH sensor when not in use. Proper storage procedure is important.
• measuring strong acids and strong alkaline solutions. Strong acids and, to a greater extent, strong alkaline solutions attack the pH sensitive glass membrane. Even neutral solutions containing high concentrations of alkali ions, and sodium ions attack the glass membrane.
• using in high-temperature environments (> 60°C or 140°F). Increasing temperature will increase the rate of the chemical attack.

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