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 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 (https://doi.org/10.1351/pac199466051077)