According to this law, the number of equivalents of the sub!>lance to be titrated is equal to the number of equivalents of the titrant used.

Derivation of the normality equation. Consider an acid alkali neutralization reactions. Let V _{1} cm3 of an acid solution of N_{1} normality require V _{2} cm^{3} of base of N_{2} normality.

We know that 1000 cm^{3} of 1 N acid solution contains acid = 1 gram equivalent.

V_{ 1} cm^{3} of 1 N acid contains acid = 1 /1000 x V_{1} gram equivalents. Thus number of gram equivalents of acid in V1 equivalents of acid in V_{1}cm3 of N_{1} acid solution = V_{1} N_{1} / 1000

Similarly, number of gram equivalents of base in V _{2} cm3 of its N_{2} solution = V_{2} N_{2 }/ 1000

By the law of equivalents, at the end point,

V_{1} N_{1 }/ 1000 = V_{2} N_{2} / 1000

N_{1} V_{1} = N_{2} V_{2}