hydrogen-ion concentration
a measure of the acidity or alkalinity of a substance. When acid and alkaline substances dissociate in water, hydrogen ions (H+) and hydroxyl ions (OH–) are released. Acidity and alkalinity are associated with the relative concentrations of H+. The hydrogen-ion concentration can be measured using the pH scale, on which values represent the negative logarithm of the H+ ion concentration. Thus, the greater the excess of H+ ions, the lower the pH value. A pH of 0 represents the greatest possible excess of H+ ions, or a “pure” acid. At the other extreme, a pH of 14 represents the maximum degree of alkalinity. A pH of 7 represents a neutral solution. Human body fluids have an average pH of about 7.4, that is, slightly alkaline. If the pH rises above 7.8 or falls below 6.8, enzymes and other biochemical substances in the body malfunction (see acidosis; alkalosis).
See also homeostasis.