Nuffield Advanced Chemistry - Hello again. I am doing a back-titration of aspirin. I will be hydrolysing the aspirin with NaOH. What I am confused about is working out how much NaOH to add to the aspirin to hydrolyse it. If I were to use a 200 mg aspirin tablet, the moles would work out to be: 1.11x 10-3 In what ratio do aspirin aand alkali react? Is the stoichiometry of this reaction 2:1 and how? By multiplying the moles by 3, so I would have 3.33 x 10-3 moles of NaOH to add. Would that be adequate? This would mean I would add 30 ml of 0.1 mol dm-3 NaOH to the crushed aspirin. Is that right? 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.? Thankyou

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Hello again.
I am doing a back-titration of aspirin. I will be hydrolysing the aspirin with NaOH. What I am confused about is working out how much NaOH to add to the aspirin to hydrolyse it. If I were to use a 200 mg aspirin tablet, the moles would work out to be: 1.11x 10^-3

In what ratio do aspirin aand alkali react? Is the stoichiometry of this reaction 2:1 and how?
By multiplying the moles by 3, so I would have 3.33 x 10^-3 moles of NaOH to add. Would that be adequate? This would mean I would add 30 ml of 0.1 mol dm^-3 NaOH to the crushed aspirin. Is that right?
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.? Thankyou

Corrie writes ...

In the direct titration of aspirin the alkali only reacts with the acidic -COOH group on the aspirin molecule, hence the 1:1 ratio.

If you want to see why the ratio of NaOH to aspirin is different in the back titration method, have you read our Re:act tutorial on Aspirin Investigations? You can find it via the Practical Investigations link on the Re:act Homepage.

Also look at some of the FAQs and answers that appear in the tutorial - some of them ask similar questions to yours.

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updated: 07 January 2008

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