Practical investigations
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I am planning an investigation to determine the concentration of aspirin in urine. I need to create a calibration curve using iron(III) chloride and a colorimeter. The aspirin solution has been dyed yellow, but it is still not the same colour as the urine. How can I eliminate the factor of the colour of the urine from the experiment? Is it possible to place the urine sample in the colorimeter and subtract the percentage of light not transmitted from the reading obtained after the iron(III) chloride has been added? Also what is the equation for the reaction between aspirin and iron(III) chloride?
Igloo writes ...
The (artificial) yellow dye in your aspirin solution is bound to differ from the yellow natural organic material in urine. You would far better off using aspirin solution without the dye and finding a means of removing the colour from the urine sample. One possibility is to try using activated charcoal. If you return to the chem-react website and search the FAQs using ‘activated charcoal’ as your search phrase, you will come up with several references to this method of removing colour. Once you have decolourised your urine samples, your problem should be resolved.
If removing colour isn’t an option, it might be worth trying to find a colorimeter filter which absorbs and blocks out the yellow colour of both your solutions. Fortunately, the colour of the complex is violet-purple – the complementary colour of yellow-green.
Aspirin itself doesn’t react with solutions of iron(III) chloride. First the aspirin has to be hydrolysed under alkaline conditions to form an anion of salicylic acid (2-hydroxybenzoic acid), and it is this salicylate ion which forms a violet-coloured complex with an acidified solution of iron(III) chloride. The hydrated iron(III) ions – more accurately hexa-aqua-iron(III) ions - in this solution are the ones responsible for forming the complex:
Hydrolysis step:
CH3COO.C6H4.COOH + 3OH- -> -O.C6H4.COO- + CH3COO- + 2H2O
aspirin salicylate ion ethanoate ion
Formation of complex ion:
-O.C6H4.COO- + [Fe(H2O)6] 3+ -> [(O.C6H4.COO)Fe(H2O)4] + + 2H2O
salicylate ion hexa-aqua-iron(III) ion iron(III) salicylate complex ion
Try drawing the structures of these molecules and ions for yourself.
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Risk assessment
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updated: 20 December 2006
