Aqueous electrolyte solution, which may be gelled, containing fixed concentration of hydrogen ions, e.g. HCl
or a buffer solution, and a fixed concentration of the ion to which the internal reference electrode is reversible, e.g. chloride ion in the case of silver-silver chloride, or calomel electrodes.
PAC, 1985, 57, 531. 'Definition of pH scales, standard reference values, measurement of pH and related terminology (Recommendations 1984)' on page 540 (https://doi.org/10.1351/pac198557030531)