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 (https://media.iupac.org/publications/analytical_compendium/)