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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 Electron path in multi-heme active site of P460 Measurement of redox potentials permit detailed analysis of complex enzyme mechanism Biochemistry 2005, 44,

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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): Ce4+ is added with a buret to a solution of Fe2+ Pt electrode responds to relative concentration of Fe3+/Fe2+ & Ce4+/Ce3+ Calomel electrode used as reference K ≈ 1016 Indicator half-reactions at Pt electrode: Eo = V Eo = 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 Ce4+ creates an equal number of moles of Ce3+ and Fe3+ Excess unreacted Fe2+ remains in solution Amounts of Fe2+ and Fe3+ are known, use to determine cell voltage. Residual amount of Ce4+ is unknown

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**Redox Titrations Shape of a Redox Titration Curve**

3.) Region 1: Before the Equivalence Point Use iron half-reaction relative to calomel reference electrode: Eo = V 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 Ve Log term is zero The point at which V= ½ Ve 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 [Ce4+]=0 Voltage can not be calculated [Fe3+] is unknown If [Fe3+] = 0, Voltage = -∞ Must be some Fe3+ from impurity or Fe2+ oxidation Voltage can never be lower than value need to reduce the solvent Eo = V

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**Redox Titrations Shape of a Redox Titration Curve**

3.) Region 1: Before the Equivalence Point Special point when V = 2Ve Log term is zero The point at which V= 2 Ve 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**

4.) Region 2: At the Equivalence Point Enough Ce4+ has been added to react with all Fe2+ Primarily only Ce3+ and Fe3+ present Tiny amounts of Ce4+ and Fe2+ from equilibrium From Reaction: [Ce3+] = [Fe3+] [Ce4+] = [Fe2+] Both Reactions are in Equilibrium at the Pt electrode

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**Redox Titrations Shape of a Redox Titration Curve**

4.) Region 2: At the Equivalence Point Don’t Know the Concentration of either Fe2+ or Ce4+ Can’t solve either equation independently to determine E+ Instead Add both equations together Add Rearrange

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**Redox Titrations Shape of a Redox Titration Curve**

4.) Region 2: At the Equivalence Point Instead Add both equations together Log term is zero Cell voltage Equivalence-point voltage is independent of the concentrations and volumes of the reactants

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**Redox Titrations Shape of a Redox Titration Curve**

5.) Region 3: After the Equivalence Point Opposite Situation Compared to Before the Equivalence Point Equal number of moles of Ce3+ and Fe3+ Excess unreacted Ce4+ remains in solution Amounts of Ce3+ and Ce4+ are known, use to determine cell voltage. Residual amount of Fe2+ is unknown

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**Redox Titrations Shape of a Redox Titration Curve**

5.) Region 3: After the Equivalence Point Use iron half-reaction relative to calomel reference electrode: Eo = 1.70 V Potential of calomel electrode Simplify

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**Redox Titrations Shape of a Redox Titration Curve**

6.) Titration Only Depends on the Ratio of Reactants Independent on concentration and/or volume Same curve if diluted or concentrated by a factor of 10

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**Titration curve for 2:1 Stoichiometry**

Redox Titrations Shape of a Redox Titration Curve 7.) Asymmetric Titration Curves Reaction Stoichiometry is not 1:1 Equivalence point is not the center of the steep part of the titration curve Titration curve for 2:1 Stoichiometry 2/3 height

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**Redox Titrations Finding the End Point 1.) Indicators or Electrodes**

Electrochemical measurements (current or potential) can be used to determine the endpoint of a redox titration Redox Indicator is a chemical compound that undergoes a color change as it goes from its oxidized form to its reduced form

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**Redox Titrations Finding the End Point 2.) Redox Indicators**

Color Change for a Redox Indicator occurs mostly over the range: where Eo is the standard reduction potential for the indicator and n is the number of electrons involved in the reduction For Ferroin with Eo = 1.147V, the range of color change relative to SHE: Relative to SCE is:

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**Relative to calomel electrode (-0.241V)**

Redox Titrations Finding the End Point 2.) Redox Indicators In order to be useful in endpoint detection, a redox indicator’s range of color change should match the potential range expected at the end of the titration. Relative to calomel electrode (-0.241V)

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**Redox Titrations Common Redox Reagents**

1.) Adjustment of Analyte Oxidation State Before many compounds can be determined by Redox Titrations, must be converted into a known oxidation state This step in the procedure is known as prereduction or preoxidation Reagents for prereduction or preoxidation must: Totally convert analyte into desired form Be easy to remove from the reaction mixture Avoid interfering in the titration Potassium Permanganate (KMnO4) Strong oxidant Own indicator Titration of VO2+ with KMnO4 Before Near After Equivalence point Eo = V Violet colorless pH ≤ 1 Eo = V pH neutral or alkaline Violet brown pH strolngly alkaline Eo = 0.56 V Violet green

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**Redox Titrations Common Redox Reagents 2.) Example**

A mL sample containing La3+ was titrated with sodium oxalate to precipitate La2(C2O4)3, which was washed, dissolved in acid, and titrated with 18.0 mL of M KMnO4. Calculate the molarity of La3+ in the unknown.

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