Practical investigations
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I am finding the % of aspirin in different brands of aspirin tablets and have decided to use colorimetry as one method of analysis. I am really stuck on what a calibration graph is and how to set one up, and also on how much iron(III) chloride to use. I have already hydrolysed the aspirin using alkaline hydrolysis and I was wondering if you could give me some advice on where to go from here. Thanks very much.
Corrie writes ...
A calibration graph shows how the absorbance (of the purple iron(III) - salicylic acid complex here) changes with its concentration on your colorimeter, using an appropriate filter. In an ideal world the relationship between absorbance and concentration would be linear, but practically it is not. Once you have a calibration curve covering the range of concentrations you expect for your unknown samples, you can read off the concentration of a sample from the curve when you have its absorbance.
I think you will find answers to your other questions in our recently revised tutorial on Aspirin Investigations - you'll find it under Practical Investigations on the React Homepage. Come back to us via React if you still have problems.
In the section on Colorimetry you'll find the answers to several previous questions on this topic. The first and last questions are particularly relevant to calibration curves.
If you have used alkaline hydrolysis, you will have to acidify the solution, otherwise the iron(III) ions will form a precipitate of iron(III) hydroxide with the hydroxide ions present.
Iron(III) chloride solutions are usually made up with a little added dilute HCl to prevent hydrolysis (which results in cloudiness). Try to ensure that your final mixtures of iron(III) chloride (in excess) and salicylic acid for the calibration curve in the colorimeter have a pH of 4-5.
Risk assessment
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updated: 26 March 2007
