Back titration is an analytical chemistry technique that allows the user to find the concentration of a reactant of unknown concentration by reacting it with an excess volume of another reactant of known concentration. The resulting mixture is then titrated, taking into account the molarity of the excess that was added. This is used as opposed to standard volumetric titration when the substance being analyzed is either too weak to give a valid reaction, or too slow.
A back titration is useful if the endpoint of the reverse titration is easier to identify than the endpoint of the normal titration.
Back titration is also useful when trying to work out the amount of an acid or base in a non-soluble solid.
Take the following experiment as an example for calculation.
1.25 g of crushed limestone was added to 50.0 cm3 of 1.00 mol dm-3 hydrochloric acid (an excess). The mixture was left until all bubbling stopped and was then made up carefully to 250 cm3 with pure water. A 25.0 cm3 sample of this was pipetted into a conical flask and some methyl orange indicator was added. Sodium hydroxide solution of concentration 0.100 mol dm-3 was added from a burette. 30.0 cm3 were needed to reach the end-point of the indicator. We can then calculate the percentage of calcium carbonate in the sample.
The laboratory skills required for back titration are more or less similar to that of common acid-base titration.
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