The pH meter or peachmeter[1]​ is an electroanalytical instrument used to measure the pH of a solution. It was created by Arnold Orville Beckman in 1934. It is basically a direct current millivoltmeter with a measurement range or interval generally between -2000 and +2000 mV (approx. depending on the model) connected to an ion-selective electrode sensitive to pH variations. Combined electrodes are usually used that incorporate the sensor and reference electrodes in the same body. The reference electrode can be calomel (Hg/HgCl) or more commonly silver/silver chloride (Ag/AgCl) in a 3.5 M hydrochloric acid solution, while the measurement electrode, sensitive to pH variations, is a glass electrode, so called because it is formed by a thin special glass membrane, sensitive to the concentration, or more correctly, activity, of the hydrogen ions in the solutions.

The measurement of the pH of a solution is carried out by introducing the reference electrode and the glass electrode into it, creating a potential difference between the two that is measured by the millivoltmeter, which is calibrated to be able to read directly in pH units, the equivalent of 59 mV being equal to one pH unit. This potential difference between both electrodes is proportional to the negative logarithm of the hydrogen ion activity.[2][3]​.

The glass electrode is relatively immune to interference from color, turbidity, colloidal material, free chlorine, oxidant and reducer.

The measurement is affected when the surface of the glass membrane is dirty with grease or water-insoluble organic material, which prevents it from making contact with the sample, therefore, scrupulous cleaning of the electrodes is recommended.

The electrodes have to be rinsed with distilled water between samples. They should not be dried with a cloth because they could become electrostatically charged. To remove excess water, they should be placed on lint-free paper.

Since glass pH electrodes measure H+ concentration relative to their references, they must be calibrated periodically to ensure accuracy. That is why calibration buffers (regulating solutions of known pH) are used to read substances.

pH electrode → Input amplifier → Filter → Digital analog converter → Display (screen).