When considering pH sensor storage, one should evaluate each portion of the electrode assembly: measurement electrode, reference electrode or a combination electrode. The storage time must also be taken into account, i.e. long-term storage for weeks or months, or short storage intervals between measurements.
Storage of Measurement Electrodes
Measuring electrodes can be stored dry for long periods. However, before using the measuring electrode it has to be hydrated for at least 48 hours in normal tap water or a slightly acidic solution in order to establish an outer gel layer at the pH sensitive membrane. Hamilton Cleaning Solution is ideal for this purpose.
Nevertheless, in order that the electrode is ready for immediate use, most manufacturers supply their measurement electrodes hydrated, i.e. a plastic or rubber cap filled with a liquid solution is placed over the membrane. It is essential that this cap is kept filled at all times. The filling liquid is either tap water or a weak acidic solution. This will keep the membrane hydrated and therefore the outer gel layer well developed.
If a measuring electrode has to be stored for short periods between measurements it should be immersed in a container filled with tap water or be fitted with a watertight plastic cap filled with tap water. One should not throw the supplied plastic caps away. They should be kept for re-use. See our dedicated article on pH storage and regeneration for specifics.
The Storage of Reference Electrodes
Reference electrodes should always be stored wet, i.e. the liquid junction should be covered with a similar KCl electrolyte with which the reference electrolyte has been filled. Wet storage can also be applied when the electrode is stored for a long time.
It is not advisable to store reference electrodes dry as the reference electrolyte will slowly penetrate through the diaphragm and crystallize on the outside of the electrode. The salt crystals do not inherently cause a problem but the reference electrode might dry out completely resulting in a substantial increase in the diaphragm resistance. Even when the reference electrode is refilled with its proper KCl electrolyte, the high diaphragm resistance may not disappear immediately and will result in large measurement errors or even cause a measurement to be totally impossible.
For short or long time periods it is essential to store reference electrodes in their respective reference electrolyte. If the sensor is refillable, then the filling port should be closed with a suitable stopper. Storage in tap water or in distilled water should be avoided. Any penetration of these liquids through the diaphragm will increase the diaphragm potential considerably, dilute the electrolyte concentration, and significantly alter the subsequent pH measurement.
The Storage of Combination Electrodes
A combination electrode consists of a measurement electrode and a reference electrode combined into one sensor. The storage conditions must therefore be suitable for a measurement and for a reference electrode. Since every reference electrolyte is an aqueous solution it has been found that the optimum storage liquid is a 3M KCl electrolyte of similar formation to what is used in that combination electrode. If the sensor is a refillable design then the refill aperture has to be closed during storage time. Refer to the Hamilton pH Storage Solution for more information.
All that has been stated about the storage of a reference electrode applies equally to the storage of a combination electrode. Gel-filled combination pH sensors are an exception to the rule. These electrodes have no refill aperture and the drying-out of their diaphragm has to be avoided at all costs. Therefore gel-filled combination electrodes must be stored wet in a 3 mol/l KCl solution. This statement applies to polymer pH sensors as well.