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Experiment4-Fall2010 - Chem 207 Experiment No 4...

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Chem 207 -- Experiment No. 4 Spectrophotometric Determination of Iron in Vitamins Fall 2010 Introduction and Background: The basic concepts of an instrumental, spectrophotometric analysis are introduced through a modification of a standard procedure for the colorimetric determination of iron. The red-orange complex that forms between iron(II) and 1,10-phenanthroline (ortho- phenanthroline) is useful for the spectrophotometric determination of iron in various samples. The formation constant for this equilibrium represented by equation 2 below is 2.5 x 10 6 at 25 ° C. Thus, iron(II) is quantitatively complexed in the pH range between 3 and 9. A pH of about 3.5 is ordinarily recommended to prevent precipitation of salts such as phosphates. An excess of a reducing reagent such as hydroxylamine or hydroquinone is needed to maintain the iron in a +2 oxidation state. The complex, once formed, is very stable. This spectroscopic determination is best performed with the spectrophotometer set at a wave length of 508 nm. In this experiment iron from a vitamin supplement tablet or tonic is dissolved in acid, reduced to Fe(II) with hydroquinone, and complexed with o-phenanthroline to form an intensely red-orange colored complex, [Fe(o-phen) 3 ] 2+ . The amount of this complex present in a solution, and consequently the amount of iron, can be determined quantitatively by comparison of its absorption to a calibration curve prepared from similar solutions prepared from known amounts or iron. Objectives: 1) Introduce and familiarize the student with Beer’s Law 2) Record the absorption curve for the compound, [Fe(o-phenanthroline) 3 ] 2+ , in the visible range (400 to 800 nm) 3) Prepare a calibration curve (ppm Fe vs. absorbance) for solutions of [Fe(o-phenanthroline) 3 ] 2+ @ 508 nm 4) Determine the molar absorptivity coefficient for [Fe(o-phenanthroline) 3 ] 2+ @ 508 nm 5) Determine the amount of iron in each pill, capsule, or liquid dose of a commercial vitamin containing iron
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Key Chemical Reactions: 2 Fe 3+ + HO-C 6 H 4 -OH 2 Fe 2+ + O=C 6 H 4 =O + 2 H + (Eq. 1) (Hydroquinone) (Quinone) Fe 2+ + 3 C 12 H 8 N 2 H + [Fe(C 12 H 8 N 2 ) 3 ] 2+ + 3H + (Eq. 2) (o-PhenH + ) ( λ max = 508 nm) where o-PhenH + = o-Phenanthrolinium Ion Procedure: Reagents Hydroquinone: Freshly prepared solution containing 10 g/L in water . Store in an amber bottle. Need ca. 20 ml per lab team for 5 standards and 3 replicates of the unknown.
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