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A pH meter measures the voltage between two electrodes and converts it to a pH value. The core component is the glass electrode, which has a thin glass bulb sensitive to hydrogen ion activity. The potential difference, E, between the glass electrode and a stable reference electrode is linearly proportional to the pH according to a form of the Nernst equation: [latex]E = K + frac{2.303RT}{F}pH[/latex].
The glass electrode functions as an ion-selective electrode. Its tip is a thin membrane of a special glass (typically a silicate glass doped with alkali metal ions) that has been hydrated. This hydration creates a gel layer on both the inner and outer surfaces. An ion-exchange equilibrium is established between the alkali metal ions in the glass and the hydrogen ions in the solution. This process creates a boundary potential across the membrane that is dependent on the difference in ion hydrogène activity between the internal solution (which has a fixed pH) and the external sample solution.
This potential is measured against a reference electrode, such as a silver/silver chloride (Ag/AgCl) or calomel electrode, which maintains a constant potential. The complete device, often a single combination probe, is calibrated using buffer solutions of known pH. The high internal resistance of the glass electrode requires a high-impedance voltmeter (the pH meter itself) to accurately measure the small potential difference without drawing significant current, which would alter the system’s equilibrium.
Électrode de verre pH
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