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Talk about the principle and structure of pH electrode

by:BOQU     2023-03-26
pH electrode, also known as pH probe, pH sensor, English name pH electrode or pH sensor, is the part of the pH meter that is in contact with the substance to be measured, and is a device that characterizes the pH value by measuring the electrode potential. Most of the pH measurement in daily work and production revolves around liquids. For example, most of the food, medicines and liquids ingested by humans, even if the final product is not liquid, its source environment is also liquid. For example, there are many liquid foods for humans: liquor, beer, carbonated drinks, fruit juice, coffee and tea. The pH value affects the flavor of food. And quality, pH value is an important indicator in drug production. The pH value detection is not simple. Industrial pharmaceuticals produce much more liquid substances than solid substances (it is not difficult to understand the importance of industrial water supply). Common industrial water includes cooling water, cleaning water, heating water, steam and solvents. In addition to water use, it also includes sewage that needs further environmental protection treatment. Then, in industrial production, substances undergo various changes, and a considerable part of the reaction is accompanied by changes in the concentration of hydrogen ions. Therefore, it is particularly important to detect the concentration of hydrogen ions in the aqueous phase solution, that is, the pH value. The structure of the pH electrode: Before the glass electrode is used, it must be soaked in the aqueous solution to form a three-layer structure, that is, the dry glass layer in the middle and the hydrated silica gel layer on both sides. The hydrated silica gel layer has an interface and constitutes a single phase, and its thickness is generally 0.01-10 μm. In the hydration layer, Na+ and H+ on the glass exchange ions to generate a phase boundary potential, that is, the Taonan potential. The surface of the hydration layer can be regarded as a cation exchanger. The H+ in the solution diffuses to the dry glass layer through the hydration layer, and the cations in the dry glass layer diffuse outward to compensate for the dissolved ions, and the relative movement of ions generates a diffusion potential. The sum of the two forms the membrane potential. Generally, there are two methods for measuring the pH value in an aqueous solution: colorimetric method (pH test paper, cuvette) and potentiometric method. Among them, only the potentiometric method can realize continuous online measurement and process monitoring, and can obtain accurate and reproducible pH values. The core theory of pH electrode measurement is the Nernst equation. Electrodes used in potentiometric analysis are called primary cells. A primary battery is a system whose function is to convert chemical reaction energy into electrical energy. The voltage of this battery is called electromotive force (EMF). This electromotive force (EMF) is formed by two half-cells. One of the half-cells is called the measuring electrode, and its potential is related to a specific ion activity; the other half-cell is the reference half-cell, usually called the reference electrode, which is generally connected to the measuring solution and connected to the measuring instrument . The most familiar and commonly used pH indicating electrode is the glass electrode. An industrial on-line pH measurement system usually consists of four parts: a pH sensor, namely a pH electrode, a pH transmitter, an electrode sheath, and a cable.
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