Why do pH electrodes need to be soaked? How to properly soak pH composite electrodes?
The pH electrode must be soaked before use, because the pH bulb is a special glass membrane. There is a very thin gel layer on the surface of the glass membrane. It can have a good effect on the hydrogen ion in the solution only when it is fully wetted. Meanwhile, soaking the glass electrode can significantly reduce the asymmetric potential and stabilize it. PH glass electrodes can generally be soaked in distilled water or a buffer solution with pH=4. It is usually better to soak in a pH=4 buffer solution, with a soaking time of 24 hours or more, depending on the thickness of the bubble glass film and the degree of electrode aging. At the same time, the liquid interface of the reference electrode also needs to be soaked. Because if the liquid interface dries up, the liquid interface potential will increase or become unstable. The soaking solution of the reference electrode must be consistent with the external reference solution of the reference electrode, that is, 3.3mol/L KCl solution or saturated KCl solution, and the soaking time is usually a few hours.
Therefore, for pH composite electrodes, they must be immersed in a pH=4 buffer containing KCl in order to simultaneously act on the glass bulb and liquid interface. Special attention should be paid here, as in the past, people used to soak single pH glass electrodes in deionized water or pH=4 groups of flushing solution. However, when using pH composite electrodes, this soaking method was still used, and even incorrect guidance was provided in the user manual of some pH composite electrodes. The direct consequence of this incorrect soaking method is that a pH composite electrode with good performance becomes a slow response and poor accuracy electrode, and the longer the soaking time, the worse the performance. Because after a long time of soaking, the KCl concentration inside the liquid interface (such as inside the sand core) has greatly decreased, causing the liquid interface potential to increase and become unstable. Of course, as long as the electrode is re immersed in the correct soaking solution for a few hours, it will still recover.
In addition, pH electrodes cannot be immersed in neutral or alkaline buffer solutions, as long-term immersion in such solutions can cause the pH glass film to respond slowly.
The correct preparation of pH electrode soaking solution: Take a bag of pH=4.00 buffer (250mL), dissolve it in 250mL of pure water, add 56 grams of analytical pure KCl, heat appropriately, and stir until completely dissolved.
