A pH meter 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 if it were 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 as the relative concentration of hydrogen ions within and outside the bulb changes.
The reference electrode output does not change with the activity of the hydrogen ions. The pH electrode has a very high internal resistance, which makes it difficult to measure the voltage variation with the pH. Therefore, the input impedance of the pH meter and the dispersion resistances are important factors.