Silver Chloride Electrode

Silver/Silver Chloride Electrode is by far the most common reference type used today because it is simple, inexpensive, very stable and non-toxic. It is mainly used with saturated potassium chloride electrolyte, but can be used with lower concentrations such as 1 M potassium chloride. Note that changing the electrolyte concentration changes the potential. Silver chloride is slightly soluble in strong potassium chloride solutions, so it is sometimes recommended the potassium chloride be saturated with silver chloride to avoid stripping the silver chloride off the silver wire.

The silver-silver chloride electrode is the most common due to its ease of manufacture and its superior temperature range, actually usable even above 130°C. The electrode is a silver wire that is coated with a thin layer of silver chloride either by electroplating or by dipping the wire in molten silver chloride. When the electrode is placed in a saturated potassium chloride solution it develops a potential of 199 mV vs the standard hydrogen electrode. The potential developed is determined by the chloride concentration of the solution, as defined by the Nernst equation. The potential of the electrode remains constant as long as the chloride concentration remains constant. The silver-silver chloride reference electrode develops a potential proportional to the chloride concentration, whether it is sodium chloride, potassium chloride, ammonium chloride or some other chloride salt.

The concentration of chloride in the reference electrolyte determines the potential of the reference element. Potassium chloride is the most widely used electrolyte because it does not generally interfere with pH measurements and the mobility of the potassium and chloride ions are nearly equal. This equal transference minimizes junction potentials since the (+) potassium and (-) chloride ions move at the same rate. These ions provide the conductive path between the reference element and the sample, commonly referred to as a salt bridge.