This method is used to determine quantity of a substance which cannot be analysed by direct titration. It involves the following steps:
1. A known but excess quantity of a suitable reagent is reacted with the substance to be analysed.
2. A part of the reagent reacts with the substance and the amount of the excess reagent is determined by titration.
3. The difference between the amounts of the reagent before and after the reaction gives the amount of the reagent that reacted with the substance.
4. Knowing the amount of the reagent used and the chemical equation for the reaction between the substance and the reagent, it is possible to calculate the amount of the substance.
The amount of nitrogen or ammonia in an ammonium salt can be calculated by back titration method