A2 Chemistry
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I am confused with my aspirin investigation. For the acid-base titration and colorimetry I have dissolved my aspirin tablet in ethanol. I thought this was a good idea as aspirin is not very soluble in water. However, I am now confused as to whether the ethanol affects the structure of the aspirin. Is it converted to salicylic acid?
I have also carried out iron (III) colorimetry. However, I think I have done this wrong because I used a variety of concentrations of salicylic acid to plot a calibration curve. I then dissolved the aspirin tablet in ethanol and added the iron(III) chloride. This formed a coloured complex, but I did not expect this as the aspirin would have needed to have a phenol group to form the purple complex.
Any help would be appreciated.
Igloo writes ...
Ethanol could be used as a solvent instead of water, but perhaps it is not a good idea to use this solvent since it might react (slowly) with the free carboxyl group of the aspirin to form an ester. However, this is unlikely to happen very much at room temperature and in the absence of a catalyst. No salicylic acid is formed when ethanol and aspirin are mixed together. On balance, I think that you should stick to water and use more dilute solutions.
As for your colorimetry, you are quite correct. The formation of the Fe(III) complex depends on the presence of a phenol group. This is present in salicylic acid of course, but not in aspirin. You were correct to calibrate the colorimeter with solutions of salicylic acid.
You have not mentioned whether your aspirin is from tablets or has been synthesised by you in the laboratory.
If it is in the form of tablets you will need to completely hydrolyse it first to convert it into salicylic acid and then carry out the colorimetry. Eventually you can argue that one mole of aspirin forms one mole of salicylic acid, so it will be easy to find the original quantity of aspirin in the tablets and anything else can be assumed to be insoluble “filler”.
If the aspirin is a sample you have made, then the most likely impurity is unconverted salicylic acid, so you need to carry out the colorimetry on a known mass of your sample, determine the quantity of salicylic acid present and subtract to find the quantity of aspirin. There won’t be any “filler” in a laboratory-prepared sample.
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updated: 06 December 2006
