All glass membranes used in pH electrodes react with water to form a hydrated gel layer. This gel layer is invisible, since it is only 50-5000 Å thick. The thickness and composition of the gel layer determine the response time, slope and alkali error of the glass electrodes. Thus, the gel layer is of decisive importance for the performance of a glass electrode. Normally, 1 or 2 days hydration is sufficient to form a complete gel layer.Over a range of pH 1 to 10, the condition of the gel layer remains virtually constant. However, it tends to break down at an increasing rate in alkaline solution above pH 10, thus leading to different equilibrium conditions. This manifests itself by an initial drift of electrode potential in alkaline solutions. Also, with fluctuating temperature, the gel layer attempts to establish a new equilibrium, thus leading to sluggish electrode response. If a glass electrode is to be kept out for long period (weeks or months), the question arises of whether wet or dry storage is best. Both alternatives have their advantages and drawbacks. Wet stored electrodes are always ready for immediate use. Dry stored electrodes require to be soaked in storage solution for a few hours before being re-used; but on the other hand they show less tendency to ageing than wet stored electrodes. On the basis of the foregoing, we can indicate the most suitable storage conditions for glass electrodes in different circumstances.
General storage: In Storage solution, HI70300, at ambient temperature.
Optimum storage: The storage solution should have the same temperature and similar pH value to the sample solution being measured.
Long term storage: Dry, or in a neutral or slightly acidic solution.