The Henderson-Hasselbalch equation is the equation: PH = pK + log ([A-]/[HA]) Where pH is the acidity or alkalinity of the buffer solution, pK is the negative logarithm of the equilibrium constant (Kd) for the ionisation of the acid form (A-) of the compound used to buffer the solution for the reaction HA = H+ +A-, [A-] is the molar concentration of the ionised form of the buffer compound, and [HA] is the molar concentration of the nonionised form of the buffer compound.