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.
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.
Your proposed procedure is a valid approach to determine the concentration of Fe^3+ in the given solution range using a spectrophotometric method. Here is a more detailed outline of the procedure:
1. Prepare a series of 5 test tubes (or cuvettes) labeled 1 to 5.
2. In each test tube, add a fixed volume of Na(SCN) solution. This will serve as the constant amount of SCN^- in all tubes.
3. Prepare a Fe(NO3) solution with a known concentration of 0.001 M. This will be the highest concentration used.
4. In tube 1, add a fixed volume of Fe(NO3) solution (e.g., 1 mL) to the fixed volume of Na(SCN) solution (e.g., 5 mL). The final concentration of Fe(NO3) in tube 1 will be 0.001 M.
5. In tube 2, add a lower volume of Fe(NO3) solution (e.g., 0.8 mL) to the fixed volume of Na(SCN) solution. Calculate the final concentration of Fe(NO3) in tube 2 accordingly (e.g., 0.0008 M).
6. Repeat step 5 for tubes 3, 4, and 5, using progressively lower volumes of Fe(NO3) solution to achieve concentrations of 0.0006 M, 0.0004 M, and 0.0002 M, respectively.
(Note: Adjust the volumes based on the desired concentrations and the known concentration of the Fe(NO3) solution.)
7. Mix the contents of each tube thoroughly to ensure complete reaction between Fe^3+ and SCN^- to form Fe(SCN)^+2 complex.
8. Allow the reactions to reach equilibrium, usually by waiting for a few minutes.
9. Set the spectrophotometer to a wavelength of 450 nm, which corresponds to the maximum absorbance of the Fe(SCN)^+2 complex.
10. Use a cuvette containing only the Na(SCN) solution as a blank, and set the spectrophotometer to zero absorbance at 450 nm using this blank.
11. Take the absorbance readings of each test tube using the spectrophotometer, one by one.
12. Plot a graph of absorbance versus concentration of Fe(NO3). The graph should show a linear relationship between absorbance and concentration.
13. Measure the absorbance of the unknown solution at 450 nm using the same procedure.
To determine the concentration of Fe^3+ in the unknown solution, interpolate its absorbance value on the calibration curve obtained from the graph. The corresponding concentration can then be read off and reported as the concentration of Fe^3+ in the unknown solution.
Remember to carefully handle chemicals and follow appropriate safety procedures while conducting the experiment.