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 V_{1} ml of an acid solution of strength S_{1}(N) is titrated with V_{2} ml of an alkali solution of strength S_{2} (N). So, at the end point of titration V_{1} ml of S_{1}(N) acid solution neutralizes V_{2} ml of S_{2}(N) alkali solution. We know that V_{1} ml of S_{1}(N) acid solution = V_{1} x S_{1} ml (N) acid solution and V_{2} ml of S_{2}(N) alkali solution = V_{2} x S_{2} 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.