text: In chemistry and biochemistry, the Henderson–Hasselbalch equation
  relates the pH of a chemical solution of a weak acid to the numerical value of the acid dissociation constant, K, of acid and the ratio of the concentrations,  of the acid and its conjugate base in an equilibrium. 
  For example, the acid may be acetic acid 
  The Henderson–Hasselbalch equation can be used to estimate the pH of a buffer solution by approximating the actual concentration ratio as the ratio of the analytical concentrations of the acid and of a salt, MA. 
  The equation can also be applied to bases by specifying the protonated form of the base as the acid. For example, with an amine,