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Basic principle of pH meter electrode sensor

by:BOQU     2023-04-14
The basic principle of pH meter electrode sensor The electrode used in the potentiometric analysis method is called a primary battery. 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 . For example, an electrode is made of a silver wire inserted in a salt solution containing silver ions. At the interface between the wire and the solution, due to the different activities of silver ions in the two phases of the metal and the salt solution, a The charging process of ions, and form a certain potential difference. The silver ions that have lost electrons go into solution. When no external current is applied for reverse charging, that is to say, if there is no current, the process will eventually reach a balance. The voltage that exists in this equilibrium state is called the half-cell potential or electrode potential. Such (as described above) electrodes consisting of a metal and a solution containing ions of the metal are called quasi-electrodes. This potential is measured against a reference electrode whose potential is independent of the composition of the salt solution. Such a reference electrode with independent potential is also called an electrode. For this type of electrode, the metal wires are covered with a slightly soluble salt of the metal (eg Ag/AgCl) and inserted into an electrolyte solution containing anions of the metal salt. At this time, the half-cell potential or electrode potential depends on the activity of the anion. The voltage between these two electrodes follows the Nernst (NERNST) formula: where: E—potential E0—the standard voltage of the electrode R—gas constant (8.31439 joules/mole and °C) T——Kelvin temperature (Example: 20°C=273+293 Kelvin) F——Faraday’s constant (96493 coulombs/equivalent) n——the valence of the ion to be measured (silver=1, hydrogen=1) aMe——the activity of the ion standard hydrogen electrode is the reference point for all potentiometric measurements. The standard hydrogen electrode is a platinum wire, which is electrolytically plated (coated) with platinum chloride, and filled with hydrogen around it (fixed pressure is 1013hpa). Immerse this electrode in a solution with a H3O+ ion content of 1 mol/l at 25°C to form the half-cell potential or electrode potential referenced by all potential measurements in electrochemistry. Among them, it is difficult to realize the hydrogen electrode as a reference electrode in practice, so a quasi-electrode is used as a reference electrode. The most commonly used of these is the silver/silver chloride electrode. The electrode reacts to changes in chloride ion concentration through dissolved AgCl. The electrode potential of this reference electrode is kept constant by a saturated kcl storage pool (eg: 3mol/l kcl). The electrolyte solution in liquid or gel form communicates with the solution to be measured through the diaphragm. The silver ion content in the film processing solution can be measured by using the above electrode combination - silver electrode and Ag/AgCl reference electrode. It is also possible to replace the silver electrode with a platinum or gold electrode to measure the redox potential. Example: Oxidation phase of a certain metal ion. The most familiar and commonly used pH indicating electrode is the glass electrode. It is a glass tube with a pH-sensitive glass membrane blown at the end. The tube is filled with a 3 mol/l kcl buffer solution containing saturated AgCl, and its pH value is 7. The potential difference reflecting the pH value present on the two sides of the glass membrane is derived using the Ag/AgCl conductive system, such as electrodes. This potential difference also follows the Nernst formula: E=59.16mv/25°C per pH where R and F are constants, n is the valence, and each ion has its fixed value. For hydrogen ions, n=1. The temperature 'T' plays a large role in the Nernst formula as a variable. As the temperature rises, the potential value will increase accordingly. For every 1°C increase in temperature, a change in potential of 0.2 mv/per pH will be induced. Expressed by pH value, the pH value changes by 0.0033 per 1°C per 1 pH. That is to say: for the measurement between 20 ~ 30 ℃ and about 7pH, there is no need to compensate the temperature change; but for the application of the temperature> 30 ℃ or < 20 ℃ and the pH value> 8pH or 6pH Compensate for temperature changes.
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