Principle of Acidimetry and Alkalimetry

The law of normality can be stated as – “Equal volumes of solutions of two reacting substances (acids and bases) of equal strength expressed in normality exactly neutralize each other“.

Let us consider an acid-base titration in which V1 ml of an acid solution of strength S1(N) is titrated with V2 ml of an alkali solution of strength S2 (N). So, at the end point of titration V1 ml of S1(N) acid solution neutralizes V2 ml of S2(N) alkali solution. We know that V1 ml of S1(N) acid solution = V1 x S1 ml (N) acid solution and V2 ml of S2(N) alkali solution = V2 x S2 ml of (N) alkali solution.

or, Volume of Acid x Strength of Acid = Volume of Alkali x Strength of Alkali.

This is an important relationship in acidimetry and alkalimetry and is known as Normality Equation. The whole of volumetric analysis is based on this equation and all the numerical problems in acidimetry and alkalimetry can be solved with the help of the normality equation.