primary pH standards 

https://doi.org/10.1351/goldbook.P04837
Certain substances which meet the criteria of:
  1. reproducible preparation in a highly pure state;
  2. stability of solution over a reasonable period of time;
  3. having a low value of the residual liquid junction potential;
  4. having pH betwen 3 and 11.
Source:
Orange Book, 2nd ed., p. 19 [Terms] [Book]
Citation: 'primary pH standards' 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.P04837 RIS BibTex EndNote
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Last revised: February 24, 2014