Why do pH electrodes need to be soaked? How to soak pH compound electrode correctly?
Reason analysis: pH electrode must be soaked before use, because the pH bulb is a special glass film, there is a very thin hydrated gel layer on the surface of the glass film, it is only under the condition of sufficiently wet to have a good response with the H ion in solution. At the same time, the glass electrode can be immersed so that the asymmetric potential is greatly reduced and stabilised. pH glass electrodes can generally be immersed in distilled water or pH4 buffer solution. Usually it is better to use pH4 buffer solution, and the soaking time is from 8 hours to 24 hours or longer, depending on the thickness of the bulb glass film, and the aging degree of the electrode. Also, the liquid junction of the reference electrode needs to be soaked. Because if the liquid junction dries up, the potential of the liquid junction will increase or become unstable. The soaking solution of the reference electrode must be the same as the external reference solution of the reference electrode, i.e., 3.3mol/L KCL solution or saturated KCL solution, and the soaking time is generally a few hours.
Correct immersion of the pH compound electrode: immersed in a pH4 buffer solution containing KCL, so that it can act on both the glass bulb and the liquid junction at the same time. Special attention should be paid here, because in the past, people used to use a single pH glass electrode has been accustomed to use deionised water or pH4 buffer soaking, and later use pH compound electrode still adopts such a soaking method, and even in some incorrect pH compound electrode instruction manuals will also carry out this kind of incorrect guidance. The direct consequence of this wrong soaking method is to make a good performance of the pH composite electrode into a slow response, poor accuracy of the electrode, and the longer the soaking time, the worse the performance, because after a long time of soaking, the liquid junction of the internal (such as the sand core inside the) KCL concentration has been greatly reduced, so that the liquid junction potential increases and instability. Of course, the electrode will still recover if it is re-soaked in the correct soaking solution for a few hours.
Also, pH electrodes should not be immersed in neutral or alkaline buffer solutions; prolonged immersion in such solutions will dull the response of the pH glass membrane. The preparation of the correct pH electrode soaking solution: take a packet of pH4.00 buffer (250ml), dissolve it in 250ml of pure water, then add 56g of analytically pure KCl, heat it appropriately and stir it until it is completely dissolved.
