Reaction of the Medium

REACTION OF THE MEDIUM, a term used in chemistry and characterized by the ratio of hydrogen and hydroxyl ions. The reaction of the medium is acidic if hydrogen ions predominate in the solution; the solution in this case exhibits acidic properties. In the case of predominance of hydroxyl ions, the reaction of the medium is alkaline. It is neutral if hydrogen and hydroxyl ions are completely absent or are present in equal amounts. The reaction of the medium was formerly characterized by titratable acidity or alkalinity, i.e., by the amount of hydrogen or hydroxyl ions that the solution gives upon its neutralization. In contrast to this, at present the reaction of the medium is determined by the concentration of actually present in the solution at a given moment, actively acting H- and OH-ions and is accordingly designated as 'active reaction.' The doctrine of the reaction of the medium has been developed mainly for aqueous solutions, which are of greatest interest since water is the main solvent in which chemical reactions occur and the only medium in which life processes are possible. Due to the partial dissociation of water itself, all aqueous solutions contain both H- and OH-ions, and in various reactions only their ratio changes. The product of the concentrations of both these ions represents in aqueous solutions at each temperature a constant value. Therefore, knowing the concentration of one of these ions, it is easy to calculate the concentration of the other. To characterize the reaction of the medium, the concentration of only the hydrogen ions or their decimal logarithm is usually indicated (see Active Reaction_T Hydrogen Ions).