A reference electrode is combined with an ion-selective electrode to give a potential difference that depends on the ion concentration. Its reference potential has to be stable. In general, a silver-silver chloride electrode is used as the internal electrode.
The liquid junction area of reference electrodes should always be kept clean since they readily tend to become dirty, resulting in fluctuation of the liquid junction potential. It is important to use a reference electrode for which the fluctuation in liquid junction potential is kept as small as possible.