A buffer solution is one in which the pH of the solution is "resistant" to small additions of either a strong acid or strong base. Buffers usually consist of a weak acid and its conjugate base, in relatively equal and "large" quantities. Calculations are based on the equation for the ionization of the weak acid in water forming the hydronium ion and the conjugate base of the acid. "HA" represents any weak acid and "A-" represents the conjugate base.
HA(aq) + H2O(l) --> H3O+(aq) + A-(aq)
Ka = [H3O+][A-]
[HA]
A buffer system can be made by mixing a soluble compound that contains the conjugate base with a solution of the acid such as sodium acetate with acetic acid or ammonia with ammonium chloride. The above equation for Ka can be rearranged to solve for the hydronium ion concentration. By knowing the Ka of the acid, the amount of acid, and the amount of conjugate base, the pH of the buffer system can be calculated.
[H3O+] = Ka[HA]
[A-]
pH = -log[H3O+]