Back titration (Indirect titration) is a type of complexometric titration used for analyzing substances that are either insoluble or partially soluble in water. Two steps are usually followed in a back titration: measuring the amount consumed by the analyte and calculating the excess quantity.
The solid can be made to react with an excess of acid in these conditions. Because excess is used, the reaction can be quite rapid. After the reaction is completed, titration with alkali can be used to detect how much acid is remaining. The composition/purity of the original solid can be determined based on the volume and concentration of alkali used.
In this example, a mixture of calcium oxide (CaO) and silicon dioxide (SiO2) is analyzed to estimate the % by mass of calcium oxide present. As calcium oxide reacts with acids, but silicon dioxide does not, the method can be used.
Back titration is used when the endpoint is easier to see than with a normal titration, which is applied to several precipitation reactions.When the interaction between the analyte and the titrant is very slow, or when the analyte is in a non-soluble solid, back titrations are also useful.
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