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,

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 = 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 = 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 [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

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
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

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. 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

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. 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)

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
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

19. 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.