filling solution

of a reference electrode
Solution containing the anion to which the reference electrode of the operational pH cell is reversible, e.g. chloride for silver-silver chloride electrode. In the absence of a bridge solution, a high concentration of filling solution comprising cations and anions of almost equal mobility is employed as a means of maintaining the liquid junction potential small and approximately constant on substitution of test solution for standard solution(s).
PAC, 1985, 57, 531. 'Definition of pH scales, standard reference values, measurement of pH and related terminology (Recommendations 1984)' on page 541 (