Henderson–Hasselbalch equation

https://doi.org/10.1351/goldbook.H02781
An equation of the form: \[\text{pH} = \text{p}K_{\text{a}}- \log _{10}(\frac{\left[\text{HA}\right]}{\left[\text{A}^{-}\right]})\] for the calculation of the \(\text{pH}\) of solutions where the ratio \(\frac{\left[\text{HA}\right]}{\left[\text{A}^{-}\right]}\) is known.
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1120 [Terms] [Paper]