Practical investigations
Read our general notes on Risk Assessment
I am planning my A2 chemistry coursework, and I have decided to investigate the different methods used to determine the purity of aspirin. The sheet suggests the use of colorimetry and iron(III) chloride. I am familiar with colorimetry, but don't know why I need to use iron(III) chloride in colorimetry nor do I know its relation to aspirin. If you could tell me how I relate it to colorimetry and/or aspirin, it would be a great starting point for me.
Igloo writes
When aspirin is hydrolysed using, for example, a dilute acid such as hydrochloric acid, salicylic acid, HO.C6H4.COOH, is formed (together with ethanoic acid).
Salicylic acid forms an intensely purple complex with iron(III) ions, e.g. from a solution of iron(III) chloride, hence the use of a colorimeter to measure the intensity of this colour with a yellow-green filter. Using a calibration curve (from a range of salicylic acid concentrations + an excess of iron(III)) chloride), the concentration of salicylic acid in the hydrolysed solution can be found. Thus the original concentration of aspirin in solution, and finally the mass of “pure aspirin” in the tablet can be deduced.
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.
back to Practical investigations
Rate this page or react
Share your views on this page, 51 ratings so far
, rated at 
updated: 08 March 2007
