Solution containing the @[email protected] to which the @[email protected] of the @[email protected] is reversible, e.g. chloride for silver-silver chloride electrode. In the absence of a @[email protected], a high concentration of filling solution comprising cations and anions of almost equal @[email protected] is employed as a means of maintaining the @[email protected] potential small and approximately constant on substitution of @[email protected] for @[email protected]