Solution of high concentration of @[email protected] @[email protected], preferably comprising cations and anions of equal @[email protected], optionally interposed between the @[email protected] filling and both the test and @[email protected], when the test solution and filling solution are chemically incompatible. This procedure introduces into the operational cell a second @[email protected] formed usually in a similar way to the first. (This definition refers to a @[email protected] solution of a double junction @[email protected])