Practical investigations
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I am doing my individual investigation on comparing brands of aspirin and have so far made up my standard solutions of iron(III) chloride and salicylic acid. However, when I use the colorimeter to test the reaction to create my calibration curve, the results do not make sense. For example when my solution is of 0.004 cm3 dm-3 of salicylic acid the absorbance is 1.00 but when the concentration of salicylic acid is 0.002 cm3 dm-3 the absorbance is 1.29. Do I need to add a buffer or something as I cannot find the answers on the internet? If I need a buffer what should it be? I have read that HCl is good, is this correct? Thanks.
Igloo writes
You are quite correct in expecting the absorbance to decrease with a lowering of the concentration of salicylic acid. I presume that you are keeping all the other variables constant, of course.
Iron(III) ions tend to hydrolyse, forming insoluble Fe(OH)3:
Fe3+(aq) + 3H2O(l) <--> Fe(OH)3(s) + 3H+(aq)
and this makes the solution mixture rather murky, which can confuse the results.
Fortunately, the hydrolysis is reversible, so the introduction of external hydrogen ions (e.g. from HCl(aq)), tends to destroy the solid and restore transparency to the solution.
So yes, I suggest the use of a little HCl(aq) – a mixture with a pH between about 3 and 5 should be ideal.
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updated: 09 March 2007
