The Principle of Measurement of pH

A pH meter is an instrument that determines the acidity or basicity of aqueous solutions.

The measurement of pH is achieved by utilizing a glass electrode that generates an electric potential when solutions of differing pH are placed on the two sides of its membrane.

The glass electrode is a member of the class of ion-specific electrodes that react to any extent only with a specific ion.

The approach of a Hydrogen ion to an outside of the membrane causes the silicate structure of the glass to conduct a positive charge (hole) into the ionic solution inside the electrode. The Nernst equation applies so that voltage across the membrane changes by 60 mV/pH unit. Because the range of physiological pH is only 0.06 pH units, the pH meter must be capable of accurately measuring changes of 0.1 mV.

The basic method used is to place a solution of known pH on the inside of the membrane and the unknown solution on the outside. Hydrochloric acid is generally used as the solution of known pH. A reference electrode, normally an Ag/AgCl or a saturated calomel electrode is placed in this solution. A second reference electrode is placed in the specimen chamber.

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 A salt bridge is included within the reference to prevent the chemical constituents of the specimen from affecting the voltage of the reference voltage. The potential developed across the membrane of the glass electrode is read by a pH meter. This pH meter must have extremely high input impedance because the internal impedance of the pH electrode is in the 10 to 100 Mꭥ range.