In chemistry and biochemistry, the Henderson–Hasselbalch equation pH = p K a + log 10 ⁡ ⁡ ( [ Base ] [ Acid ] ) relates the pH of a chemical solution of a weak acid to the numerical value of the acid dissociation constant, K a, of acid and the ratio of the concentrations, [ Base ] [ Acid ] of the acid and its conjugate base in an equilibrium.