A pH meter (also known as an acidity meter) is an instrument used for precise measurement of the acidity or alkalinity of a solutionThe core principle is based on electrochemical effects and Nernst equation, which converts pH value by measuring the electromotive force generated between the electrode system and the measured solution.

　　1、 Core measurement principle: electromotive force of primary battery and Nernst equation

The working principle of a pH meter (acidity meter) essentially utilizes a primary battery system. The system consists of two electrodes - an indicator electrode (usually a glass electrode) and a reference electrode (such as a calomel electrode or a silver chloride electrode) - simultaneously immersed in the test solution. The key role of a reference electrode is to provide a known and stable electrode potential as a reference for measurement. The uniqueness of glass electrodes lies in the special glass film at the front end that is sensitive to hydrogen ions. When it comes into contact with a solution, its membrane potential changes in response to changes in hydrogen ion activity in the solution.

The total electromotive force (E-cell) generated by these two electrodes in a solution is the algebraic sum of the reference electrode potential (E-reference) and the glass electrode potential (E-glass), i.e. E-cell=E-reference+E-glass. The relationship between this electromotive force and the activity of hydrogen ions in solution follows the Nernst equation: E=E ₀+(RT/F) * ln [H ⁺]

Among them, E is the electrode potential, E ₀ is the standard electrode potential, R is the gas constant, T is the thermodynamic temperature, F is the Faraday constant, and [H ⁺] is the hydrogen ion concentration. After converting the equation, it can be concluded that at 25 ℃, for every 1 pH unit change in the solution, the electrode potential changes by approximately 59.16 millivolts. The high impedance precision potentiometer (ammeter) inside the pH meter is responsible for measuring this small potential difference, amplifying it, and ultimately converting it directly into a pH reading for display.

　　2、 The Importance of Temperature Compensation

The potential change value in the Nernst equation is directly related to temperature (T). Therefore, in order to ensure the accuracy of measurements at different temperatures, modern pH meters are equipped with temperature compensation functions (such as temperature compensation knobs or automatic temperature sensing probes) to correct measurement deviations caused by temperature changes.

　　3、 Key components of instruments and modern development

Traditional instrument separated electrodes have been largely replaced by composite electrodes. The composite electrode integrates a glass indicator electrode and a reference electrode, and is filled with a saturated KCI solution as an internal reference solution, making it more convenient to use. According to application scenarios and precision requirements, various types of instruments have been developed, including pen based, portable, laboratory desktop, and high-precision industrial online. Its accuracy level ranges from 0.2 to 0.001, with smaller numbers indicating higher accuracy.

　　4、 The necessity of calibration during operation

Due to the slow variation of electrode potential over time, in order to obtain accurate results, the pH meter must be calibrated before measurement, usually using a two-point calibration method. First, use a standard buffer solution with pH=7 for "positioning" calibration, and then select a standard buffer solution with pH=4 (for acidic solutions) or pH=9.18/10 (for alkaline solutions) for "slope" calibration based on the acidity or alkalinity of the test solution. Calibration can effectively eliminate systematic errors caused by factors such as electrode asymmetry potential and liquid contact potential.

In summary, the pH meter (acidity meter) is based on the principle of electrochemistry, which precisely measures the electrode potential difference caused by changes in hydrogen ion activity and converts it into pH value using the Nernst equation. Its design cleverly integrates chemistry, physics, and electronics technologies, making it a crucial analytical tool in both scientific research and industrial fields.