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Perhaps the most common questions received by technical support involve the shelf life and expiration date of pH sensors.

Do pH sensors have a shelf life?

Hamilton pH sensors can be stored for a period of 3 years if maintained properly. Storage beyond 3 years is possible as long as the sensor’s response time, slope and asymmetry potential are within the user's acceptable limits. Refreshing the sensor before use in a process is also advised if the storage time exceeds 1 year. A formal statement on this can be downloaded here.

How can I refresh my pH sensor?

The refresh process involves exposing the glass membrane to a base and acid solution to activate the alkaline metal compounds embedded within the glass. The procedure is listed below:

  • Rinse the sensor with clean water and then immerse it in a 0.1 to 1.0M NaOH (Sodium Hydroxide) solution (0.4% to 4% by weight) for 10 minutes. Heating to 50°C enhances regeneration.
  • Remove the sensor and rinse it thoroughly with tap water.
  • Immerse the sensor in a 0.2 to 1.0M HCl (Hydrochloric Acid) solution (0.73% to 3.65% by weight) for 10 minutes. Heating to 50°C enhances regeneration. A fume hood is advised if the HCl is heated.
  • Remove the sensor and rinse it thoroughly with tap water.
  • Immerse the sensor in Hamilton storage solution, (238931) for a minimum of 60 minutes. Overnight immersion is recommended.

Do pH sensors have an expiration date?

No, 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 following the refresh procedure listed above then 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.

What speeds up the ageing of the sensor?

Ageing is significantly accelerated by:

  • pH measurement in hot liquids above 60 °C
  • Continuous pH measurement in high acidity or high alkalinity liquids
  • Exposing the pH probe 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 pH 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 pH sensor will always provide the best measurement response and accuracy.

For non-critical application or processes close to neutral (7pH) with slowly changing pH, a old sensor with proper calibration may work just fine.

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