Solution of high concentration of @I03026@ @S05447@, preferably comprising cations and anions of equal @M03955@, optionally interposed between the @R05229@ filling and both the test and @S05924@, when the test solution and filling solution are chemically incompatible. This procedure introduces into the operational cell a second @L03584@ formed usually in a similar way to the first. (This definition refers to a @B00736@ solution of a double junction @R05229@.)