The PH meter is calculated using the electrode potential. So to measure PH, you have to put both electrodes in a solution and form a battery loop. Because the PH value is calculated using only a single electrode potential. The purpose of adding a reference electrode is to calculate the potential of a single electrode. Because the reaction of a single electrode is difficult, a reference electrode is usually denoting zero potential. In this way, the potential between two electrodes is the potential of a single electrode.