An 'inner' reference electrode which is contained inside an ion-selective electrode assembly.
Note:
The system frequently consists of a silver/silver chloride electrode in contact with an appropriate solution containing fixed concentrations of chloride and the ion for which the @MT06878@ is selective. This 'inner' filling solution is in contact with the ion-selective @MT06878@.
Source:
PAC, 1994, 66, 2527. 'Recommendations for nomenclature of ionselective electrodes (IUPAC Recommendations 1994)' on page 2533 (https://doi.org/10.1351/pac199466122527)