Practical investigations
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I am currently planning my individual investigation, synthesising my own aspirin and comparing it to chemist-purchased aspirin. I am partly analysing the concentrations of aspirin samples with iron(III) colorimetry. I know you have to hydrolyse the aspirin to salicylic acid, but how do you then transfer this salicylic acid to a cuvette for testing? Surely the sample has ethanoic acid and excess NaOH in it, which could interfere with the iron complex made?
Igloo writes ..........
The sample cannot contain both an excess of ethanoic acid and NaOH since these react with each other via a neutralisation process. In any case, there is no need to carry out any sort of hydrolysis on your aspirin. It’s far easier and far more precise to carry out the following type of procedure.
First you need to construct a calibration curve using solutions of pure laboratory salicylic acid of varying concentrations together with known concentrations of iron(III) ions (an excess) and HCl, ensuring that the concentrations of salicylic acid are of the same order as that you expect to find in your aspirin sample.
Next you carry out colorimetry on a known mass of your prepared aspirin, assuming of course that it does contain some unreacted salicylic acid. This experiment enables you to determine the mass of salicylic acid in the tablet and hence to deduce the mass of aspirin.
Always carry out a risk assessment and check with your teacher before starting any practical work.
Risk assessment
Before attempting any practical work based on the advice and suggestions on this website, you must do the following. Identify any hazards, assess the risks from these hazards, and then decide appropriate control measures to reduce the risks. You must have these approved by those in authority in your school or college laboratory. Do not rely on what is said on this website.
For further guidance see our tutorial on Risk Assessment.
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updated: 11 November 2007
