Henderson–Hasselbalch equation

An equation of the form:

$pH = p K a − log 10 ( HA A − )$

for the calculation of the $pH$ of solutions where the ratio $HA A −$ is known.

Source:

PAC, 1994, 66, 1077 (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1120