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221-chapter-16

221-chapter-16 - Redox Titrations Introduction 1 Redox...

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Redox Titrations Introduction 1.) Redox Titration Based on an oxidation-reduction reaction between analyte and titrant Many common analytes in chemistry, biology, environmental and materials science can be measured by redox titrations Biochemistry 2005, 44, 1856-1863 Measurement of redox potentials permit detailed analysis of complex enzyme mechanism Electron path in multi-heme active site of P460
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Redox Titrations Shape of a Redox Titration Curve 1.) Voltage Change as a Function of Added Titrant Consider the Titration Reaction (essentially goes to completion): Ce 4+ is added with a buret to a solution of Fe 2+ Pt electrode responds to relative concentration of Fe 3+ /Fe 2+ & Ce 4+ /Ce 3+ Calomel electrode used as reference E o = 0.767 V Indicator half-reactions at Pt electrode: K ≈ 10 16 E o = 1.70 V
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Redox Titrations Shape of a Redox Titration Curve 2.) Titration Curve has Three Regions Before the Equivalence Point At the Equivalence Point After the Equivalence Point 3.) Region 1: Before the Equivalence Point Each aliquot of Ce 4+ creates an equal number of moles of Ce 3+ and Fe 3+ Excess unreacted Fe 2+ remains in solution Amounts of Fe 2+ and Fe 3+ are known, use to determine cell voltage. Residual amount of Ce 4+ is unknown
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Redox Titrations Shape of a Redox Titration Curve 3.) Region 1: Before the Equivalence Point E o = 0.767 V Use iron half-reaction relative to calomel reference electrode: ) electrode reference E E = 241 0 0 3 2 . Fe Fe E - = + + ] [ ] [ - = + + ] [ ] [ 3 2 05916 0 526 0 Fe Fe log . . E Potential of calomel electrode Simplify
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Redox Titrations Shape of a Redox Titration Curve 3.) Region 1: Before the Equivalence Point Special point when V = 1/2 V e ] [ ] [ + + = 2 3 Fe Fe - = + + ] [ ] [ 3 2 05916 0 526 0 Fe Fe log . . E Log term is zero V . E E . E o 767 0 526 0 = = = + The point at which V= ½ V e is analogous to the point at which pH = pKa in an acid base titration
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Redox Titrations Shape of a Redox Titration Curve 3.) Region 1: Before the Equivalence Point Another special point, when [Ce 4+ ]=0 Voltage can not be calculated [Fe 3+ ] is unknown If [Fe 3+ ] = 0, Voltage = -∞ - Must be some Fe 3+ from impurity or Fe 2+ oxidation Voltage can never be lower than value need to reduce the solvent E o = -0.828 V
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Redox Titrations Shape of a Redox Titration Curve 3.) Region 1: Before the Equivalence Point Special point when V = 2V e ] [ ] [ + + = 4 3 Ce Ce Log term is zero V . E E . E o 70 1 46 1 = = = + The point at which V= 2 V e is analogous to the point at which pH = pKa in an acid base titration - = + + ] [ ] [ 4 3 05916 0 46 1 Ce Ce log . . E
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Redox Titrations
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221-chapter-16 - Redox Titrations Introduction 1 Redox...

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