The basic principle of electrometric pH measurement is determination of the
activ ity of the hydrogen ions by potentiometric measurement using a standard hydrogen
electrode and a reference electrode. The hydrogen electrode consists of a platinum electrode
across which hydrogen gas is bubbled at a pressure of 101 kPa. Because of difficulty in its use
and the potential for poisoning the hydrogen electrode, the glass electrode commonly is used.
The electromotive force (emf) produced in the glass electrode system varies linearly with pH.
This linear relationship is described by plotting the measured emf against the pH of different
buffers. Sample pH is determined by extrapolation