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 ionization 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 ionized form of the buffer compound, and [HA] is the molar concentration of the nonionized form of the buffer compound.