PH meter electrode and its common problems
PH electrode, PH meter electrode type, PH electrode structure, PH electrode type, PH electrode use, what are the common problems of PH electrode and their solutions?
1. What is a pH indicating electrode?
The electrode that responds to the activity of hydrogen ions in the solution and the electrode potential changes accordingly is called a pH indicating electrode or a pH measuring electrode. There are several kinds of pH indicating electrodes, such as hydrogen electrode, antimony electrode and glass electrode, but the most commonly used one is glass electrode. The glass electrode is composed of a glass strut and a hydrogen ion-sensitive glass membrane composed of a special composition. The glass membrane is generally in the shape of a bulb. The bulb is filled with an internal reference solution, inserted into an internal reference electrode (usually silver/silver chloride electrode), sealed with an electrode cap to lead out wires, and fitted with a socket to become a pH indicator. electrode. A single pH indicating electrode cannot be measured, it must be measured together with a reference electrode.
2. What is a reference electrode?
An electrode that does not respond to the activity of hydrogen ions in solution and has a known and constant electrode potential is called a reference electrode. There are several reference electrodes such as mercurous sulfate electrode, calomel electrode and silver/silver chloride electrode. The most commonly used are the calomel electrode and the silver/silver chloride electrode. The role of the reference electrode in the measurement battery is to provide and maintain a fixed reference potential. Therefore, the requirements for the reference electrode are stable and reproducible potential, small temperature coefficient, and small polarization potential when current passes.
3. What is a pH composite electrode?
An electrode that combines a pH glass electrode and a reference electrode is called a pH composite electrode. The case with plastic shell is called plastic shell pH composite electrode. The one with glass shell is called glass pH composite electrode. The biggest advantage of the composite electrode is that it combines two into one and is easy to use. The structure of pH compound electrode is mainly composed of electrode bulb, glass support rod, internal reference electrode, internal reference solution, shell, external reference electrode, external reference solution, liquid junction, electrode cap, electrode wire, socket and so on.
(1) Electrode bulb: It is made of molten lithium glass with hydrogen function and is spherical in shape, with a film thickness of about 0.1-0.2mm and a resistance value of <250 megohms (25°C).
⑵Glass support tube: It is the glass tube body that supports the electrode bulb. It is made of lead glass with excellent electrical insulation, and its expansion coefficient should be consistent with that of the electrode bulb glass.
(3) Internal reference electrode: It is a silver/silver chloride electrode, the main function is to lead out the electrode potential, which requires stable potential and small temperature coefficient.
(4) Internal reference solution: The internal reference solution with a zero potential of 7pH is a mixed solution of neutral phosphate and potassium chloride. The glass electrode and the reference electrode form a battery to establish a pH value of zero potential, which mainly depends on the pH of the internal reference solution value and chloride ion concentration.
⑸Electrode molded case: The electrode molded case is a shell that supports the glass electrode and the liquid junction, and holds the external reference solution, and is molded by PPS plastic pressure.
⑹External reference electrode: silver/silver chloride electrode, the function is to provide and maintain a fixed reference potential, which requires stable potential, good reproducibility and small temperature coefficient.
⑺External reference solution: 3.3mol/L potassium chloride gel electrolyte, not easy to lose, no need to add.
⑻Sand core liquid junction: The liquid junction is the connecting part that connects the external reference solution and the measured solution, and requires stable penetration.
⑼ Electrode wire: It is a low-noise metal shielded wire, the inner core is connected to the inner reference electrode, and the shielding layer is connected to the outer reference electrode.
4. Why should the pH electrode be soaked? How to properly soak the pH combination electrode?
Reason analysis: The pH electrode must be soaked before use, because the pH bulb is a special glass membrane, and there is a very thin hydrated gel layer on the surface of the glass membrane, which can only interact with the H ions in the solution under fully wet conditions. There are good responses. At the same time, the glass electrode is soaked, which can greatly reduce the asymmetric potential and tend to be stable. The pH glass electrode can generally be soaked in distilled water or pH4 buffer solution. Usually it is better to use pH4 buffer solution, and the soaking time is 8 hours to 24 hours or longer, depending on the thickness of the bulb glass membrane and the aging degree of the electrode. At the same time, the liquid junction of the reference electrode also 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 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 will do.
Soak the pH composite electrode correctly: Soak it in the pH4 buffer solution containing KCL, so that it can work on 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 and were used to soaking it in deionized water or pH4 buffer solution. Later, they still used this soaking method when using a pH composite electrode, even in some incorrect pH composite electrodes. This kind of erroneous guidance is also carried out in the instruction manual of the electrode. The direct consequence caused by this wrong soaking method is to turn a pH composite electrode with good performance into an electrode with slow response and poor precision, and the longer the soaking time, the worse the performance, because after a long time soaking, the liquid junction The concentration of KCL inside the boundary (such as inside the sand core) has been greatly reduced, making the potential of the liquid junction increase and unstable. Of course, the electrodes will recover with just a few hours of re-soaking in the correct soaking solution.
In addition, the pH electrode should not be soaked in neutral or alkaline buffer solutions. Long-term immersion in such solutions will cause the pH glass membrane to respond sluggishly. Preparation of the correct pH electrode soaking solution: take a pack of pH4.00 buffer (250ml), dissolve it in 250ml of pure water, add 56 grams of analytically pure KCl, heat properly, and stir until completely dissolved.
