Assume that it is necessary to determine the [FE^3+ in a solution that may be 0.0001 to 0.0005 M range . Outline the spectrophotometric method that would accurately give the required data. Be spectific about the solutions that you would use.
I know I have to work up a procedure using SCN^-. It forms a complex with iron(III) that is VERY colored. Fe(SCN)^+2.
My idea of answering this question is:
I will prepare 5 tubes of excess Fe3+ and a limited amount of SCN^-. Under these conditions i can assume there is so much FE3+ that all SCN- has been converted to FESCN2+. For FE3+ I would use a FE(NO3) solution and for SCN- I would use a Na(SCN-) solution. I would set the spectrophotometer at 450 nm. I would keep the concentration of Na(SCN) in all 5 tubes constant and vary the Fe(NO3) solution from 0.001 to 0.0005 M.
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I believe you have the right idea but what you have written will not work. Using SCN^- as the limiting reagent will assure that all of the SCN^- is used but all of the Fe^3+ will NOT be converted. (Depending upon the amount of SCN and Fe used, some of the samples of Fe may be completely converted but not all of them. That means that the graph you prepare will not be the absorbance for that concn of Fe for all of the Fe may not have reacted. I would have chosen KSCN because that's what I've seen that done throughout my career; however, I think that NaSCN will be just as effective.
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