An aqueous solution of 0.05 mol potassium hydrogen phthalate per kg water which is the reference value for the definition of pH(RVS) values in the temperature range 0-95°C.
See also: operational pH standard, primary pH standard
Source:
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)