reference value pH standard 

https://doi.org/10.1351/goldbook.R05234
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 [Terms] [Paper]
Citation: 'reference value pH standard' in IUPAC Compendium of Chemical Terminology, 5th ed. International Union of Pure and Applied Chemistry; 2025. Online version 5.0.0, 2025. https://doi.org/10.1351/goldbook.R05234 RIS BibTex EndNote
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Last revised: February 24, 2014