Certain substances which meet the criteria of:
reproducible preparation in a highly pure state;
stability of solution over a reasonable period of time;
having a low value of the residual liquid junction potential;
having pH betwen 3 and 11.
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook