Carboxylic acids and related compounds
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What type of titration could I use to analyse the concentration of aspirin in a sample? What chemicals would you need to carry out these titrations?
Igloo suggests
Since aspirin is a carboxylic acid, it neutralises a cold dilute solution of sodium hydroxide to form a salt:
CH3COO.C6H4.COOH + NaOH -> CH3COO.C6H4.COONa + H2O
Having crushed a known mass of your “sample”, dissolved it in aqueous ethanol and made it up to a known volume in a volumetric flask, you could then titrate portions of this solution with a 0.10 mol dm-3 solution of sodium hydroxide, using phenolphthalein as indicator. The titration flask should be buried in crushed ice during the addition of alkali.
The success of this straightforward method assumes that the only reaction taking place is the neutralisation of the carboxyl group. Aspirin is also an ester, so it undergoes hydrolysis in aqueous alkali, and although very little of this will occur under the conditions given above you do need to carry out the titration promptly and at low temperatures in order to be able to make this assumption.
Appropriate calculations would then enable you to make a quantitative estimation of the percentage of pure aspirin in your sample.
Another approach, involving what is known as a 'back titration', is described in several other FAQs on the Re:act website, and you may want to refer to these (by using teh search term 'aspirin') before making a decision as to your choice of method.
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: 18 December 2004
