SPECTROPHOTOMETRIC DETERMINATION OF IRON

A) Spectrum of the iron/1,10 phenanthroline complex: You may work with a partner on this portion of the experiment. Prepare a solution for the determination of your working wavelength by mixing 10 mL of the stock solution provided by the instructor, 2 mL of the hydroxylamine hydrochloride solution, 10 mL of the 1,10 phenanthroline solution, and 8 mL of the sodium acetate solution. Dilute this solution to about 100 mL, and allow the solution to stand for at least ten minutes to allow the orange color (Fe2+ / 1, 10 phenanthroline complex) to develop.

Using water as a reference and your orange solution as your sample, measure the percent transmittance (%T) at different wavelengths in the interval from 400 to 600 nm. Use 20 nm increments initially, and after the region of maximum absorbance (min %T) is determined (40 nm wide region) use a 5 nm interval to refine your measurements. Plot the absorbance (A) vs wavelength points on a graph (A = 2 - log %T) and determine the wavelength of maximum absorbance. This is the wavelength you will use for your unknown determination.

B) Standard preparation: Weigh accurately about 0.07 g of standard iron (II) ammonium sulfate hexahydrate [Fe(NH4)2(SO4)2(H2O)6] (do not dry). Dissolve the salt in deionized water, add 2-3 mL of concentrated sulfuric acid, and quantitatively transfer this solution to a 1000 mL volumetric flask, diluting the solution to the mark with deionized water. Mix well, and calculate the concentration of this stock solution in ppm of iron.

Into five (5) separate 100 mL volumetric flasks pipet exactly 1, 5, 10, 25, and 50 mL portions of the standard iron solution, and add about 50 mL of water to a sixth 100 mL volumetric flask (your blank). To each flask add 2 mL of the hydroxylamine hydrochloride solution, 10 mL of the 1,10 phenanthroline solution, and 8 mL of the sodium acetate solution. Dilute all samples to the mark with deionized water, mix well, and allow them to stand for at least 10 min.

C) Unknown Preparation: Use the solution prepared by dissolving about 0.1 g of the ferrous ammonium sulfate unknown in deionized water, adding 2-3 mL of concentrated sulfuric acid, and quantitatively transferring this solution to a 1000 mL volumetric flask, followed by diluting the solution to the mark with deionized water. Pipet 25.00 mL of this unknown solution into a 100.0 mL volumetric flask, and pipet 50.00 mL of your unknown into a second 100.0 mL volumetric flask. To each flask add 2 mL of the hydroxylamine hydrochloride solution, 10 mL of the 1,10 phenanthroline solution, and 8 mL of the sodium acetate solution. Dilute both solutions to the mark with deionized water, mix well, and allow them to stand for at least 10 min.

D) Calibration Curve and Unknown Determination: Using the wavelength determined in (A), measure the %T for each of the standard solutions (including your blank) and for the unknown solutions. Measure the %T for solutions of increasing concentrations, rinsing the cuvette several times with the next solution to be measured before measurement. Be sure to adjust the 100%T with the blank before each measurement. Calculate the absorbance for each measurement. Calculate the slope, y intercept, and correlation coefficient (least-squares analysis) for the calibration curve prepared from your data. Plot the absorbance vs Fe std in ppm (this is your calibration curve) and determine the concentration of Fe in your unknown solutions using the absorbance value for your unknowns and your least-squares analysis information. From this value calculate the %Fe in your unknown.

E) Report: To view a sample report, you can see the file as an EXCEL file or as a