What are buffers and how do they work?

Buffers are solutions of acid and base which work to minimise changes in pH following the addition of small amounts of acid or base. Ideal buffers are made of a weak acid (i.e. one which does not fully dissociate in water), and its conjugate salt (which will fully dissociate in water). Take, for example, the weak acid CH₃COO^-H⁺ and its salt CH₃COO^-Na⁺. The equation for the dissociation of ethanoic acid is: CH₃COO^-H^{+ <-->} CH₃COO- + H⁺ And since this is a weak acid, the equilibrium lies well to the left. When the salt is added, it fully dissociates, adding a lot of CH₃COO- , which by Le Chetalier's principle shifts the equilibrium even further to the left. Upon the addition of acid, the concentration of H⁺ ions increases, and equlibrium is shifted even further to the left. Upon the addition of an alkali, the OH^- ions react with the H⁺ ions present in the buffer. [H⁺] drops, and the equilibrium shifts to the right to replace them. Thus, when small amounts of either alkali or acid are added, the buffer minimizes any changes in pH.