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.
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.