What is the purpose of soaking pH electrodes? What are the correct procedures for soaking pH combination electrodes?
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 greatly 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 pH 4 buffer solution, with soaking time ranging from 8 hours to 24 hours or longer, depending on the thickness of the bulb 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 potential at the liquid interface will increase or become unstable. Therefore, the soaking solution of the reference electrode must be consistent with the external reference solution of the reference electrode, which is a 3.3mol/L KCl solution or saturated KCL solution. The soaking time is generally a few hours.
Proper soaking of pH composite electrode: Soak it in a pH 4 buffer solution containing KCL so that it can simultaneously affect both the glass bulb and the liquid interface. Special attention should be paid here, because in the past, people used to soak single pH glass electrodes in deionized water or pH 4 buffer solution. Later, when using pH composite electrodes, this soaking method was still used, and even incorrect instructions were provided in the user manual of some incorrect pH composite electrodes. The direct consequence of this incorrect soaking method is that it turns a pH composite electrode with good performance into an electrode with slow response and poor accuracy. Moreover, the longer the soaking time, the worse the performance, because after a long soaking time, the KCL concentration 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 soaked 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 prolonged immersion in such solutions can cause the pH glass film to respond slowly. The correct preparation of pH electrode soaking solution: Take a packet of pH 4.00 buffer (250ml), dissolve it in 250ml of pure water, add 56g of analytical grade KCl, heat appropriately, and stir until completely dissolved.
