Back Titration Definition

Young women testing titration systems
Back titrations are used to determine concentration. Sandra Mu/Getty Images

A back titration is a titration method where the concentration of an analyte is determined by reacting it with a known amount of excess reagent. The remaining excess reagent is then titrated with another, second reagent. The second titration's result shows how much of the excess reagent was used in the first titration, thus allowing the original analyte's concentration to be calculated.

A back titration may also be called an indirect titration.

When Is a Back Titration Used?

A back titration is used when the molar concentration of an excess reactant is known, but the need exists to determine the strength or concentration of an analyte.

Back titration is typically applied in acid-base titrations:

  • When the acid or (more commonly) base is an insoluble salt (e.g., calcium carbonate)
  • When direct titration endpoint would be hard to discern (e.g., weak acid and weak base titration)
  • When the reaction occurs very slowly

Back titrations are applied, more generally, when the endpoint is easier to see than with a normal titration, which applies to some precipitation reactions.

How Is a Back Titration Performed?

Two steps are typically followed in a back titration:

  1. The volatile analyte is permitted to react with an excess reagent
  2. A titration is conducted on the remaining quantity of the known solution

This is a way to measure the amount consumed by the analyte, thus calculate the excess quantity.