Practical investigations
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Why is the back titration method more reliable than the acid-base titration method when it comes to determining the purity of an aspirin sample?
Igloo writes ...
The direct titration method is relatively easy to set up and to carry out, and the ensuing calculation is straightforward too, but there is one serious problem in that the added alkali not only interacts with the carboxyl group producing a carboxylate:
(1) CH3COO.C6H4.COOH + NaOH ----> CH3COO.C6H4.COO-Na+ + H2O
It can also hydrolyse the ester group:
(2) CH3COO.C6H4.COOH + 2NaOH ----> CH3COO-Na+ + HO.C6H4.COO-Na+ + H2O
and the phenol produced can then form a phenoxide with excess alkali:
(3) HO.C6H4.COO-Na- + NaOH ----> Na+ -OC6H4.COO-Na+ + H2O
These reactions are the same ones which occur when aspirin is boiled with an excess of sodium hydroxide solution, but the ester linkage breaks relatively slowly. At room temperature, or preferably in an ice bath, the ester linkage is largely unaffected. This of course is what you assume when you carry out the direct acid-alkali titration – you only want the carboxyl group to react with the alkali.
However, some reactions of types (2) and (3) are bound to occur, which is why the technique can give unpredictable results. The reliability of the method depends largely on the way it is carried out. The use of an ice bath is recommended and it is advisable to carry out the titration as rapidly as possible.
The back titration method is more reliable overall since no side reactions occur. However, the technique is lengthier, as are the calculations which follow!
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Risk assessment
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updated: 02 April 2007
