A pH meter consists of three 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 electrode that measures pH is a glass bulb sensitive to hydrogen ions, with an output in millivolts that varies according to changes 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 the hydrogen ions.
The pH electrode has a very high internal resistance, which makes it difficult to measure the voltage variation with pH. Therefore, the impedance of the pH meter input and leakage resistances are important factors.