In many chemical reactions, especially in biological systems, it is important to keep the pH or acidity within defined limits. The bloodstream of humans is kept at pH = 7.4 + / - 0.2, and acidosis or alkalosis outside this range can be fatal. (Those of you who read Michael Crichton's novel The Andromeda Strain may recall that the plot hinged on the alien life form in the victims' bloodstreams being even more sensitive to pH than the infected humans were.)

pH control also is important in rontrolling enzyme activity. Most enzymes have an optimum pH range in which they function best, with sharply reduced efficiency outside this range. At too high or too low pH, acidic and basic groups on the enzyme or on its substrate molecules may pick up or lose extra protons, thereby altering the charge distribution at the molecular surface and possibly making a mating of enzyme and substrate difficult or impossible (see above).

Many industrial processes go most efficiently, or most rapidly, at certain pH values. It is important to have some way of keeping pH changes to a minimum. The answer is the use of acid-base buffers.