According to the Brønsted-Lowry theory of acids and bases, an acid (HA) is capable of donating a proton (H+) and a base (B) is capable of accepting a proton. After the acid (HA) has lost its proton, it is said to exist as the conjugate base (A-). Similarly, a protonated base is said to exist as the conjugate acid (BH+).
The dissociation of an acid can be described by an equilibrium expression:
Consider the case of acetic acid (CH3COOH) and acetate anion (CH3COO2):
Acetate is the conjugate base of acetic acid. Acetic acid and acetate are a conjugate acid/base pair. We can describe this relationship with an equilibrium constant:
In this simulation, we will use KA for the acid dissociation constant. Taking the negative log of both sides of the equation gives:
This can be rearranged:
By definition, pKA = -logKA and pH = -log[H+], so
This equation can then be rearranged to give the Henderson-Hasselbalch equation:
The Henderson-Hasselbalch equation can be used to prepare buffer solutions and to estimate charges on ionizable species in solution, such as amino acid side chains in proteins. Caution must be exercized in using this equation because pH is sensitive to changes in temperature and salt concentration in the solution being prepared.