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.