Practical investigations
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I am currently doing my A2 coursework and am comparing the techniques used to assess the purity of aspirin, and I was wondering what the chemistry is behind the purification by recrystallisation of aspirin (as in what happens to the substance, as I am recrystallising it).
Igloo says
The technique of recrystallisation depends on finding a solvent in which the substance dissolves well at high temperatures but poorly at lower temperatures.
In the case of aspirin, water is such a solvent (though there are others). Aspirin is much more soluble in hot water than it is in cold.
A solution of impure aspirin can be made, using the minimum volume of hot water. All the aspirin should be in solution whilst the solution is hot, but it can now be filtered (whilst still hot) to remove insoluble impurities, such as bits of paper and other foreign matter.
The filtered solution is then cooled down (usually in an ice-bath). Crystals of aspirin will appear, since they are much less soluble in the cold water, and these crystals can be filtered off and dried between sheets of clean filter paper. Any soluble impurities should remain in solution at this point.
Several successive recrystallisations can be carried out to obtain aspirin crystals of increasingly higher purity.
Unfortunately, although recrystallisation improves the quality of the crystals, their yield diminishes with each recrystallisation procedure, because some aspirin will always be lost in solution at the final stage. This is the reason why the minimum volume of hot water is used to dissolve the crystals in the first place - using larger volumes of hot water will cause even greater quantities of aspirin to be lost in solution.
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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.
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updated: 17 January 2006
