Practical investigations
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I have read your posts and was wondering if you could help me. I am planning an investigation into determining the purity of an aspirin sample I have made. A member of your team suggested that it may be difficult to use titrations to determine the purity of aspirin produced in a lab because of the salicylic acid contaminant which affects the concentrations produced overall. To overcome this problem I believe that fractional distillation could be used to remove a large amount of salicylic acid from the sample, then if a colorimeter is used to determine the concentration of salicylic acid in the sample, I could then determine the overall concentration of acid required to neutralise sodium hydroxide in a titration and subtract the known concentration of salicylic acid in the sample, thus giving an overall concentration of aspirin. Do you believe that this is a logical method to use?
Igloo writes ...
Fractional distillation is not going to work unfortunately. Aspirin and salicylic acid are solids, at room temperature, melting at about 135°C and 160°C respectively. In order to use fractional distillation the liquid mixture is going to have to be heated to their respective boiling points, and this means to a temperature between about 150°C and 200°C, at which one or both of these components is most likely to decompose or burn.
I suggest that you determine the total acidity by titration with an ice-cold solution of sodium hydroxide (why ice-cold?), and separately determine the concentration of salicylic acid via the colorimetric method you suggest.
Alternatively, assuming that salicylic acid is the only contaminant, you can use the following method. Prepare a fixed volume of three solutions containing a fixed mass (say 1.00 g) of pure salicylic acid, pure aspirin (from your chemical store) and your sample, using volumetric flasks. Carry out titrations with portions of each, and deduce by interpolation the composition of your mixture. For example, if your titration readings for pure salicylic acid, pure aspirin and your sample of aspirin are 20.0 cm3, 10.0 cm3 and 12.5 cm3, you could deduce that your mixture contains 25% salicylic acid and 75% aspirin by mass. A molar composition could then be deduced by calculation.
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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: 07 March 2006
