Hydrogen ion exponent (pH) is the measure of active reaction (acidity or alkalinity) environment. Hydrogen ion exponent equal to negative decimal logarithm of the concentration of hydrogen ions [H + ], expressed in gram-ions per liter (pH = -lg[H + ]), i.e. represents taken with the opposite sign an indicator of the extent to which it is necessary to build 10 to get [H + ]. For example, if the pH=9 [H + ]=10-9. At temperature 22 degrees pH neutral environment is 7, acid - less than 7 and alkaline - more than 7. The pH value vary the speed and direction of biochemical processes. Hydrogen indicator of human blood in norm is in the range of 7.35-7,45 is the main characteristic of acid-base balance (see) of blood. Cm. also, Buffer solutions, Indicators.