The Best Storage Solutions for pH Sensors

This article is specifically for pH sensors with glass electrodes and silver/silver chloride references.

What is the purpose of storage?

To put it simply, to protect the accuracy and stability of the sensor until it needs to be used.  Improper storage reduces the lifespan of your electrode and prevents you from getting the full value out of it.  

What are the limiting factors for accuracy and stability of pH sensors?

  1. The reference electrolyte, which is neutral pH, 3.0M KCl for the pHionics instruments and most of the industry. As soon as the electrolyte is either depleted or contaminated, the sensor loses a stable reference and cannot hold calibration for long.
  2. The sensing element, which for pH is a hydrogen-ion-sensitive hydrated gel layer on the electrode bulb that must be kept wet.

Let’s look at a few different examples of how storage conditions affect the limiting factors and, therefore, the lifespan of pH sensors.

Storing a pH sensor without storage solution

As the reference electrolyte is exposed to the environment through an outer junction, it evaporates over time and the bulb dries out.  The electrolyte dries out within a few months and the electrode stops working, resulting in wasted money spent on a replacement electrode.

If the sensor is pulled from storage before the electrolyte dries out or if the electrolyte is refilled, then you may attempt to use it and find the measurements drift or stabilize very slowly.  This is a result of the bulb drying out.  pHionics has successfully recovered some electrodes by soaking them in 3.0M KCl overnight after drying out, but results are inconsistent.

Labelled Glass pH Electrode

Image 1. Labelled Glass pH Electrode

Image 2.  Example of ion diffusion caused by storage of electrode in tap water

Storing a pH sensor in RO/DI or tap water

The reference electrolyte and H+-sensitive gel do not dry out.  The potassium and chlorine ions diffuse slowly through the junction and into the water until an equilibrium is reached, resulting in a diluted reference (shown in Image 2.).  Storing a pH sensor in tap water or, worse, RO/DI water, is the same as having it in use, so the time in storage results in an equivalent decline in electrode lifespan.  If left for long enough, the reference becomes unstable and no longer holds calibration, requiring the electrode to be replaced.

Storing a pH sensor in reference solution (3.0M KCl for most.  Check with your manufacturer)

The reference electrolyte and gel do not dry out.  There is no diffusion potential so the potassium and chlorine stay in the reference cell.  The electrode is well preserved and does not have its usable lifespan reduced.  Build-up of salt crystals may occur after long periods of storage which results in slow response times.  These crystals can be removed with a soft cloth or by soaking in 0.1M HCl for up to 20 minutes.

Conclusion

Always store a pH sensor with the electrode submersed in the same solution as the reference electrolyte solution.  If it is not available, a solution of similar ionic content and neutral pH should be used in the meantime.

If you are looking for a pH sensor that can last for a long time, then the pHionics STs Series pH is perfect.  We have combined a fast, accurate, reliable electrode (average lifespan of 3 years) with body made of high quality materials that typically last 10+ years.  The STs Series pH also contains an isolated, 2-wire, 4-20 mA transmitter for excellent noise-resistance and safety.  Find out more by visiting the STs Series pH product page.

Check out our video for a visual demonstration of how improper storage affects pH electrode life.

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