Perhaps the most common questions received by technical support involve the shelf life and expiration date of pH sensors.
What is the shelf life of your sensors? - Hamilton’s formal shelf life statement is provided in the instruction manual and shown in the graphic below.
Why do pH sensors have a shelf life? The sensor is an electrochemical device. The glass membrane is continuously reacting to hydrogen ions in the liquid it is exposed to and producing a potential voltage. Over time, as the sensor ages, the voltage response to changing pH becomes less and less. This effect is even true if the sensor is in storage.
Does the sensor have an expiration date? Expiration implies that the sensor is at end of life. In reality, sensors don't simply "die" from sitting in storage. The decline is very slow, over time. Ageing sensors will exhibit the following characteristics:
- Increased response time to changes in pH
- Increased impedance of the glass membrane
- A declining slope value
- A shift in the asymmetry potential
Many of these changes can be corrected for by performing a proper two-point buffer calibration. Best practices should be followed including using fresh buffers, allowing adequate stabilization time in each buffer, and rinsing between buffers to avoid cross contamination.
Slow response time can also be witnessed by testing the sensor in buffer solutions prior to calibration. The response may be improved by exposing the sensor to NaOH and HCl for 10 minutes then a final soak in storage solution for 15 minutes (see Hamilton cleaning solution set Ref 238290). Re-test the sensor in the same buffer solutions to see if response time has improved prior to calibration.
What speeds up the ageing of the sensor? Ageing is significantly accelerated by:
- Measurement in hot liquids above 60 °C
- Continuous measurement in high acidity or high alkalinity liquids
- Exposure to strong chemicals which can poison the reference (sulfide, heavy metals, hydrocarbons).
- Improper handling such as letting the sensor sit dry for long period of time (>24 hours), cleaning with abrasives, or improper storage solutions.
What slows the ageing of sensors? Follow the opposite of what ages the sensor. Avoid exposure to high temperatures. Use the sensor in clean liquids close to 7pH so that no voltage potential is generated. Always store the sensor in 3M KCl (or use Hamilton storage solution Ref 238931).
My sensor is quite old, can it still be used? This question should be considered in terms of the requirements of your process. Certainly for high value processes a new sensor will always provide the best measurement response and accuracy.
For non-critical application or processes close to neutral with slowly changing pH, a old sensor with proper calibration may work just fine.