Cause analysis: The pH electrode must be soaked before use, because the pH bulb is a special glass membrane. There is a very thin hydrated gel layer on the surface of the glass membrane. It can respond well to H ions 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 pH 4 buffer solution. It is usually better to use a pH 4 buffer, with a soaking time of 8 to 24 hours or longer, depending on the thickness of the bubble glass film and the degree of electrode aging. Meanwhile, the liquid interface of the reference electrode also needs to be soaked. Because if the liquid interface dries up, it 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 generally a few hours.
Correct soaking of pH composite electrode: Soak in a pH 4 buffer solution containing KCL, so as 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 buffer. However, this soaking method is still used when using pH composite electrodes, and even incorrect instructions may be provided in the user manual of some pH composite electrodes. The direct consequence of this incorrect soaking method is to transform a pH composite electrode with good performance into an electrode with slow response and poor accuracy, and the longer the soaking time, the worse the performance. This is because after a long soaking time, the concentration of KCL inside the liquid interface (such as inside the sand core) has greatly decreased, resulting in an increase and instability of the liquid interface potential. 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 should not 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 pack of pH 4.00 buffer (250ml) and dissolve it in 250ml of pure water. Then add 56 grams of analytical pure KCl, heat appropriately, and stir until completely dissolved.
