Practical investigations
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I've done an investigation on the purity of aspirin and completed a colorimetry procedure with ammonium iron(III)sulphate. If you could help by outlining the calculation steps for deducing the purity of the aspirin from salicylic acid that would be great!
Igloo replies ...
You do not say what procedure you used. Did you use colorimetry on the solution produced by using a known mass of your product, so as to determine the quantity of salicylic acid present? I presume that you did, so here are the steps, briefly:
1 Use your calibration curve to convert readings of absorbance or percentage transmission into concentration of salicylic acid.
2 Knowing the volume of solution made up determine the total number of moles of salicylic acid in this solution. This will be the number of moles of salicylic acid in your weighed sample, and can be converted to grams by using its molar mass.
3 Divide this by the number of grams of sample which you used, multiply by 100 and you have the percentage of salicylic acid in your aspirin.
4 This can be subtracted from 100 to find the percentage of aspirin by mass in your product.
This method assumes that salicylic acid is the only impurity in your aspirin sample.
The reason why this method works is that only salicylic acid forms the complex with iron(III) ions – aspirin does not.
I hope that you have a calibration curve. Without it you cannot continue!
Risk assessment
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updated: 17 December 2006
