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1 EXPERIMENT 5 SPECTROPHOTOMETRIC DETERMINATION OF Fe(III) IN WATER, 07F References: Harris, Chap 18; Lecture notes; R. E. Kitson, Anal. Chem. , 22 , 664 (1950). Spectrophotometry is frequently used for the determination of low concentrations of species in aqueous solutions. Many naturally occurring or industrially added ionic materials are determined spectrophotometrically at the ppm (part per million) level or lower. Many of the transition metals form colored complex ions {CHEM 112, if you are not familiar with them} with very large molar absorptivities that are suitable for spectrophotometric analysis. There are many sources of iron in water and Fe(II) and Fe(III) ions form many highly colored or strongly absorbing complex ions. The samples for this experiment will be aqueous solutions of Fe (III) and other non- complexing ions. A stock solution of Fe (III) with an accurately known concentration as ppm Fe (III) or mg Fe per L or µ g of Fe per mL of solution will be provided. Different volumes of this solution diluted to a known volume will enable you to obtain the needed calibration curve. You will use pipets and volumetric flasks for the preparation of the standard and unknown solutions. Measure the volumes carefully. Take the usual precautions. Do not blow out the last drop. If you use a graduated (or measuring) pipet, you measure the volume by the indicated graduations, not to the tip of the pipet. You will use different pipets for different solutions. DO NOT SWITCH THE PIPETS AND CONTAMINATE THE SOLUTIONS! Wipe the tip of the pipet after you have used it and before you put the pipet into the small reagent bottle. If the pipet tip touches the top of the volumetric flask and if the top of the volumetric flask is wet with one solution, you will contaminate the other solution. There has been much contamination of the SCN - solution in previous years. Care (and perhaps the following order of introducing the solutions to the volumetric flask) may eliminate this problem. The volumetric flask need not be dry, but it must be rinsed with distilled water to remove any Fe (III) from previous use . H 2 O is not a contaminant in this procedure. Clean and dry the top of the volumetric flask before preparing each solution. Absorption spectrum of the Fe 3+ /SCN complex . Add a 1.00 mL aliquot of 5.0 M SCN solution to a clean 10 mL volumetric flask. . (NaSCN, KSCN, NH 4 SCN are all acceptable.) Then add 5.00 mL of acetone and 1.00 mL of the stock solution of Fe (III) to the 10 mL volumetric flask. The concentration of this stock solution will be given on the bottle. Dilute to the mark on the 10 mL volumetric flask with 1.0 M HNO
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This note was uploaded on 04/13/2008 for the course CHEM 119H taught by Professor Munson during the Fall '07 term at University of Delaware.

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