Back Titration Definition

Back titrations are used to determine concentration.
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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 tritrated with another second reagent. The second titration's results show how much of the excess reagent was used in the first titration and the original analyte's concentration can then be calculated.

A back titration may be thought of as a normal titration, except done in reverse. In a regular titration, the original sample is titrated. In a back titration, a known amount of reagent is added to a solution and allowed to react, and the excess is titrated.

A back titration may also be called an indirect titration.

When Is a Back Titration Used?

Basically, you use a back titration when you need to determine the strength or concentration of an analyte and you have a known molar concentration of excess reactant. It's 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), or 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?

Usually, two steps are followed in a back titration. First, the volatile analyte is permitted to react with an excess reagent. Next, a titration is conducted on the remaining quantity of the known solution. This is a way to measure the amount that was consumed by the analyte and thus the excess quantity.