Question

THE DETERMINATION OF IRON BY SPECTROPHOTOMETRY INTRODUCTION In this experiment, the red-orange colored complex formed between iron(II) and 1,10- phenanthroline (Eqn.) is used in determination of iron by spectrophotometry. Fe+3PhenH Fe(Phen)2 +3H red-orange (A 512 nm) An excess of reducing reagent, such as hydroxylamine or hydroquinone, is often used to reduce and maintain iron in +2 oxidation state. The complex, once formed, is very stable, and can be stored for a long time. Required Reading: Skoog and West (9E): Chapter 24 REAGENTS 1.2 M sodium acetate. Hydroxylamine Hydrochloride solution. 1,10-phenanthroline (o-phenanthroline) solution (0.02M). Concentrated Sulfuric Acid Fe(NH4)2(SO4)2 6H20 PROCEDURES 1. Preparation of Standard Iron Solution Weigh (to the nearest 0.2 mg) 0.17xx g of reagent-grade Fe(NH4)2(SO4)2 6H2O into a 250 mL volumetric flask. Dissolve in 50 mL of distilled water containing 1 to 2 mL of concentrated sulfuric acid; Dilute to the mark, and mix well. Transfer 25.00 mL of the prepared solution to a 250 mlL volumetric flask. Dilute with distilled water to the mark, and mix well. 2. Preparation of a Calibration Curve Transfer 25.00 mL of standard iron solution to a 100 mL volumetric flask and 10 mL of distilled water to a second 100 mL volumetric flask, as the blank. Add 1 mL of hydroxylamine, 5 mL of sodium acetate, and 5 mL of 1,10-phenanthroline to each flask Allow the mixtures to stand for 5 minutes; dilute to the mark and mix well. Clean a pair of matched cells for the instrument. Rinse each cell with at least three nortions of the solution it is to contain. Determine the absorbance of the standard with respect to the blank. Repeat the procedure with at least three other volumes of the standard iron solution: attempt to encompass an absorbance range between 0.1 and 0.7. Plot a calibration curve 3. Determination of Iron in an Unknown Transfer 10.00 mL of the unknown solution to a 100 mL volumetric flask; treat in exactly the same way as the standards, measuring the absorbance with respect to the blank. The eorbance of unknown sample should be within the range of the calibration curve. If not alter the volume of unknown and repeat the measurement. Report the parts per million of iron in the unknown.

Answer 1

1) The concentration in mol / L is calculated:

M = 0.5 mg / L * (1 g / 1000 mg) * (1 mol Fe / 55.8 g) = 9x10 ^ -6 M

2) The curve presented is only useful for determining the concentration while the line is straight, and then the accuracy of the method is lost.

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