Solution containing the @A00358@ to which the @R05229@ of the @O04298@ is reversible, e.g. chloride for silver-silver chloride electrode. In the absence of a @B00740@, a high concentration of filling solution comprising cations and anions of almost equal @M03955@ is employed as a means of maintaining the @L03584@ potential small and approximately constant on substitution of @T06286@ for @S05924@.