Indicators

Indicators, Chemical, in volumetric analysis, are materials which are added to reacting substances to indicate when the reaction is completed. Thus, for example, if the quantity of alkali has to be estimated in a solution, a standard solution of acid is run in until the liquid becomes exactly neutralised, this point being ascertained by an indicator such as litmus, which, although of a blue colour in solutions of alkalies, turns red immediately in acid solutions, even though very dilute. The change in colour thus indicates the completion of the reaction, and from the quantity of acid added the quantity of the alkali present is known. Many substances are used as indicators, the mode of use being in all cases essentially similar.