A pHmeter consists of three essential parts: a pH measuring electrode, a reference electrode and a high input impedance meter. The reference electrode has a known, constant and stable potential.
The pH electrode can be considered as a battery, with a voltage that varies according to the pH of the measured solution. The pH-determining electrode is a hydrogen ion sensitive glass bulb, with a millivolt output that varies according to alterations in the relative concentration of hydrogen ions inside and outside the bulb. The output of the reference electrode does not change with the activity of hydrogen ions. The pH electrode has a very high internal resistance, which makes it difficult to measure voltage variation with pH. Therefore, the impedance of the pH meter input and the dispersion resistances are important factors.