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27.11.2018

Save money – store your pH electrodes properly!

How correct storage affects the lifetime of pH electrodes.

Combined pH electrodes (commonly pH electrodes) are electrochemical sensors which are in interaction with the sample. The strongest exposure concerns the most important parts of the electrodes, glass membrane and junction with the reference system in its back. For the storage of pH electrodes are some points to be considered. But first we give for better understanding of the principle a short view on the functionality.

The principle of a pH electrode is that H+ ions penetrate the surface of special glasses and cause a shift of the charge in comparison to the reference system. This electrical charge delivers a weak potential which can be detected by using sensitive electronics.  

 A close look on the pH electrode reveals that a pH electrode is nothing but an ion selective electrode with a high selectivity for hydrogen ions.  As common to all ion selective electrodes interferences are existing for ions with similar characteristics (charge, size).

What is the reason for this? Glasses consist of a framework of silicon tetrahedrons which have a non-crystalline ordering but are amorphous in different orientations. Particularly in pH sensitive glasses are lithium ions embedded which are very small and from a chemical view of the periodic system are in the alkali group close to hydrogen.  Hydrogen ions penetrate in the glass surface (gel layer) into the gaps occupied by lithium ions and cause the charging of the membrane. Size matters: Only hydrogen ions, lithium ions and sodium ions at a certain concentration and pH value are able to create or to simulate a pH value.  The term “sodium error” describes exactly this phenomenon, but comes only to exist when the pH is very high; correspondingly the concentration of hydrogen ions is very low. The same applies to lithium which is in daily lab routine much less common.  

And so we build the bridge to the storage of pH electrodes. Sometimes it is stated that solutions containing potassium ions (commonly 3 mol/l KCl solutions) are not suitable for the storage of pH electrodes because they supposed to interact with the membrane and reduce the selectivity or sensitivity for hydrogen ions. This is not the case. A reduced sensitivity results in a reduced slope. Practical experience shows: highly reduced slope is a result of mechanical or chemical treatment, scratches or long term exposure to extreme pH values and/or temperatures. Electrodes which are only subject to a “normal” daily lab routine show over their complete life time only little change of their slope.

But: Electrodes show a drift of the asymmetry. It is caused by the reference system which is today almost exclusively a silver/silver chloride system embedded in highly concentrated potassium chloride solutions. In order to maintain the life time particularly for gel and polymer electrodes it is important that during storage the conditions inside of the electrode and in the storage solution are identically.  

A lower concentration of potassium chloride or the use of other solutions (particularly cleaning of electrodes with polymer or gel systems with pepsin/hydrochloride acid should be very short) will not only result in a slower response because of so-called memory effects but also lead to a decrease of the concentration in the inner electrolyte which is irreversible in case of gel or polymer electrodes and may result in a drift of the asymmetry outside the standard tolerance of +/- 30 mV.

Because of this the long term storage of pH electrodes should always take place in an electrolyte which corresponds to the type and the concentration of the electrolyte of the reference system, typically 3 mol/l KCl solutions. Is it not available, e.g. during an outdoor measurement the watering cap was incidentally emptied, a pH buffer can be used.   In case of need the membrane can also be moistened by some drinking or river water, but never use distilled or deionized water. Basically put the electrode as fast as possible into 3 mol/l KCl solution in order to avoid a washing of the KCl and to extend the lifetime of the electrodes for many additional measurements.  So it is possible to increase the operation time with only little efforts. 

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