When the pH of a solution is equal to the pKa, the buffer is in its most effective state. In this state, the concentration of the conjugate base is equal to that of the acid, allowing it to effectively neutralize added strong acid or base and maintain a desired pH. For a desired pH different than the pKa, the ratio of the base to the acid is modified.
Buffers are made from a weak acid and its conjugate base or a weak base and its conjugate acid. The conjugate is the species created after the donation of a proton, forming a conjugate base, or the acceptance of a proton, forming a conjugate acid. The greater the difference between the concentration of the acid and the base, the less effective the buffer. Typically, the amount of acid to base does not differ by more than tenfold. The exact ratio of a buffer's base to acid concentration is given by the Henderson-Hasselbalch equation, which states the pH is equal to the pKa minus the logarithm of the concentration of the base divided by the concentration of the acid. When the concentrations are equal, the logarithmic value becomes zero, and the pH equals the pKa.