internal filling solution

of a glass electrode
https://doi.org/10.1351/goldbook.I03105
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 @[email protected] is reversible, e.g. chloride ion in the case of silver-silver chloride, or calomel electrodes.
Source:
PAC, 1985, 57, 531. (Definition of pH scales, standard reference values, measurement of pH and related terminology (Recommendations 1984)) on page 540 [Terms] [Paper]