Nuffield Advanced Chemistry - For my A2 individual investigation, I am investigating the techniques available for assessing the quantity of ASA in aspirin tablets. In one method, the back titration, I hydrolyse a sample of ASA-ethanol solution with NaOH by heating. I realise that the stoichiometry of this reaction is in fact 2:1 and not 3:1. However, in another method, the straightforward acid-alkali titration, I am titrating NaOH directly against a sample of ASA-ethanol solution. In this instance, the stoichiometry between NaOH and ASA appears to be 1:1. I wondered why the ratio should be different in this instance compared with the back titration. I suspect heating had something to do with the discrepancy. Please could you help me?

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For my A2 individual investigation, I am investigating the techniques available for assessing the quantity of ASA in aspirin tablets. In one method, the back titration, I hydrolyse a sample of ASA-ethanol solution with NaOH by heating. I realise that the stoichiometry of this reaction is in fact 2:1 and not 3:1. However, in another method, the straightforward acid-alkali titration, I am titrating NaOH directly against a sample of ASA-ethanol solution. In this instance, the stoichiometry between NaOH and ASA appears to be 1:1. I wondered why the ratio should be different in this instance compared with the back titration. I suspect heating had something to do with the discrepancy. Please could you help me?

Igloo writes ...
Yes, it’s all to do with heat. Under cold conditions the acid (ASA) simply forms a salt (the carboxyl group, -COOH becomes a carboxylate ion, COO^-). This is an ionic acid-base reaction, so occurs virtually instantaneously even under cold conditions.

However when heated, not only is a salt formed, but the covalent ester linkage is also broken by hydroxide ion, producing sodium salicylate and sodium ethanoate. “Covalent processes” are usually slower, and need higher temperatures to encourage a reaction.

When carrying out the direct titration it is advisable to carry out the technique using an ice-bath, so as to minimise any possibility of the ester group being affected.

Always carry out a risk assessment and check with your teacher before starting any practical work.

Risk assessment
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updated: 20 December 2006

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