Practical investigations
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I am doing an experiment on recrystallisation of aspirin. I would like to know what could be the reasons for a sample with higher and the reasons for a sample with lower melting point than expected (140°C)?
Also, what are the safety precautions we have to take note while doing the experiment?
Thanks.
Igloo writes ..........
A high melting point could simply result from inaccuracies in measurement, for example heating too rapidly whilst taking the measurement, so that the thermometer records the temperature before the aspirin has been able to conduct heat throughout the solid and melt.
A low reading could result from trace impurities in the aspirin, e.g. unreacted salicylic acid. A common problem is measuring the melting point of aspirin which has been insufficiently dried, resulting in dissolving (at about 115°-110°C), rather than melting.
Incidentally, reliable sources quote the actual melting point at anything between 135° and 140°C, which seems somewhat surprising in view of the fact that the true melting point must surely be in a much narrower range than 5°C. The fact is that aspirin tends to decompose at its melting point, which makes a really accurate measurement that much harder.
You do not mention the method you used to prepare the aspirin. On the assumption that this involved the reaction between ethanoic anhydride and salicylic acid, one precaution is to cope with the exothermic nature of the reaction which might possibly cause the mixture to “spit”. Another is to realise that ethanoic acid is a by-product. This has harmful fumes. Finally ethanoic anhydride itself is a hazardous material, being lachrymatory amongst other things.
We are not qualified to give health and safety advice on React. Use appropriate sources of information. 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: 31 May 2007
