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Redox Titrations

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Many common analytes in chemistry, biology, environmental and materials science ... Each aliquot of Ce4 creates an equal. number of moles of Ce3 and Fe3 ... – PowerPoint PPT presentation

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Title: Redox Titrations


1
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, 1856-1863
2
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 0.767 V
Eo 1.70 V
3
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

4
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 0.767 V
Potential of calomel electrode
Simplify
5
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
6
Redox Titrations
  • Shape of a Redox Titration Curve
  • 3.) Region 1 Before the Equivalence Point
  • Another special point, when Ce40
  • Voltage can not be calculated
  • Fe3 is unknown
  • If Fe3 0, Voltage -8
  • Must be some Fe3 from impurity
  • or Fe2 oxidation
  • Voltage can never be lower than value need
  • to reduce the solvent

Eo -0.828 V
7
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
8
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

9
Redox Titrations
  • Shape of a Redox Titration Curve
  • 4.) Region 2 At the Equivalence Point
  • Dont Know the Concentration of either Fe2 or
    Ce4
  • Cant solve either equation independently to
    determine E
  • Instead Add both equations together

Add
Rearrange
10
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
11
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

12
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
13
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

14
Redox Titrations
  • Shape of a Redox Titration Curve
  • 7.) Asymmetric Titration Curves
  • Reaction Stoichiometry is not 11
  • Equivalence point is not the center of the steep
    part of the titration curve

Titration curve for 21 Stoichiometry
2/3 height
15
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

16
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
17
Redox Titrations
  • Finding the End Point
  • 2.) Redox Indicators
  • In order to be useful in endpoint detection, a
    redox indicators range of color change should
    match the potential range expected at the end of
    the titration.

Relative to calomel electrode (-0.241V)
18
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

19
Redox Titrations
  • Common Redox Reagents
  • 2.) Example
  • A 50.00 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 0.006363 M KMnO4.
  • Calculate the molarity of La3 in the unknown.
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