Practical investigations
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Hi, I'm really stuck. I'm doing my individual investigation on methods for testing the purity of aspirin. I did colorimetry with copper(II) nitrate. I got the method from school. I have been trying to find out about the Cu-aspirin complex and how the method works, but I can only find information on colorimetry with iron(III) chloride. Is my method correct? And what does the Cu-aspirin complex look like?
Igloo writes ...
Aspirin itself does not normally form stable complexes with metal ions. It is the de-ethanoylated form (salicylic acid) which is involved instead. Since you already know about the iron(III) complex (with salicylic acid), it is easy to visualise that copper(II) ions are also capable of complexing in a rather similar way.
I would expect that salicylic acid acts as a bidentate ligand here and Cu(II) therefore forms a complex ion with two, or possibly three, salicylic acid ligands, with the formula [Cu(HO.C6H4.COOH)x]2+ (where x = 2 or 3), though I have not been able to find any confirmation for this on the web.
As for procedure, this must be similar to that for iron(III) ions. On this occasion, the pale blue copper(II) aquo ions form a deeper blue complex with the salicylic acid, so choose a filter with the appropriate complementary colour.
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updated: 13 January 2007
