The activity of most enzymes is largely dependent upon the exact acidity or alkalinity of the medium. This is now usually expressed in terms of hydrogen ion concentration. Thus a normal solution of hydrochloric acid would contain, if the HC1 were completely ionized, 1 gram of hydrogen ions per liter; and in a thousandth-normal solution in which the ionization actually is almost complete (actually about 99 percent of the HC1 in such a solution is ionized at ordinary temperatures) the concentration of hydrogen ions is 0.001 gram per liter or 1 x 10-3. Pure water, according to the usually accepted estimates, has a hydrogen ion concentration of 1 x 10-7 and the same concentration of hydroxyl ions. Thus water which is pure and strictly neutral may also be regarded as being equivalent to a ten-millionth-normal acid and at the same time a ten-millionth-normal alkali. In order to avoid cumbersome numbers Sorensen has proposed to indicate hydrogen ion concentration by writing the negative exponent as a whole number, e.g. in the case of pure water PH+ = 7.0; in thousandth-normal hydrochloric acid PH+ = 3.0. Thus according to the Sorensen notation, generally indicated by the use of the symbol PH+, a number lower than 7 shows acidity and the more acid the solution the lower the number; a number higher than 7 shows alkalinity and the greater the alkalinity the higher the PH+ number, since this is the negative exponent of the hydrogen ion concentration.

It must be remembered that the Sorensen exponent, or PH+ number, varies with the hydrogen ion concentration not arithmetically but logarithmically: 1 x 10-6 = PH+ 6.0; 2 X 10-6 = Ph+ 5.7. The hydrogen ion concentrations most favorable to the action of certain well-known enzymes have recently been measured with the following results: