Back titration

Back titration is an analytical chemistry technique which 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 back, taking into account the molarity of the excess which was added.

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Back titrations can be used for many reasons, including: when the sample is not soluble in water, when the sample contains impurities that interfere with forward titration, or when the end-point is more easily identified than in forward titration.

Example

Consider using titration to measure the amount of aspirin in a solution. Using titration it would be difficult to identify the end point because aspirin is a weak acid and reactions may proceed slowly.

Using back titration the end-point is more easily recognised in this reaction, as it is a reaction between a strong base and a strong acid. This type of reaction occurs at a high rate and thus produces an end-point which is abrupt and easily seen.

The titration curve for a strong acid with a strong alkali shows that the equivalence point occurs at pH 7. This means that the indicator phenolphthalein can be used. The end-point will be seen when the pink solution produced by the adding of phenolphthalein fades to colourless.

The first stage of this reaction is that of alkaline hydrolysis. This involves reacting the aspirin solution with a measured amount of sodium hydroxide; an amount that will exceed the amount of aspirin present. Because the hydrolysis reaction occurs at a very low rate at room temperature it will be heated to increase the reaction rate.

CH3COOC6H4COOH   +   2NaOH   --->    CH3COO.Na   +   HOC6H4COO.Na  +     H2O
                                                        Sodium Ethanoate + Sodium-2-hydroxybenzoate + Water

The second stage then involves back titration of the hydrolysed sodium hydroxide solution with hydrochloric acid. This process reacts the excess sodium hydroxide with hydrochloric acid.

NaOH     +     HCl     ---->     NaCl     +     H2O 		     
 
Sodium     Hydrochloric --->    Sodium        Water
Hydroxide     acid             Chloride

By the method of back titration the amount of hydrochloric acid needed to neutralise the unreacted sodium hydroxide in the solution can be determined. Knowing this and the amount of sodium hydroxide that was added the amount of aspirin that reacted with the sodium hydroxide can be determined.

See also

• Titration