at zero electric current between two identical salt bridges, usually saturated KCl
bridges (conveniently measured by linking them to two identical electrodes) inserted
into two solutions in Donnan equilibrium.
Source:
PAC, 1972, 31, 577
(Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix
II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry)
on page 619