Buffer

Name:
Buffer
Detailed information:
A solution containing both a weak acid and its conjugate weak base, whose pH changes only slightly on addition of acid or alkali. The weak acid becomes a buffer when alkali is added, and the weak base becomes a buffer when acid is added. This action is explained by the reaction A + H₂O → B + H₃O_n in which the base B is formed by the loss of a proton from the corresponding acid A. The acid may be a cation such as NH₄⁺, a neutral molecule such as CH₃COOH, or an anion such as H₂PO₄⁻. When alkali is added, hydrogen ions are removed to form water, but as long as the added alkali is not in excess of the buffer acid, many of the hydrogen ions are replaced by further ionization of A to maintain the equilibrium. When acid is added, this reaction is reversed as hydrogen ions combine with B to form A. The pH of a buffer solution may be calculated by the mass-law equation, pH = pK′ + log C_b/C_a in which pK′ is the negativelogarithm of the apparent ionization constant of the buffer acid and the concentrations are those of the buffer base and its conjugate acid.