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
Similar to Acid-Base Titrations
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
Similar to acid-base indicators that change color with a change in protonation state

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

17. Redox Titrations Finding the End Point 2.) Redox Indicators
Color Change for a Redox Indicator occurs over a potential range
Illustration:
For Ferroin with Eo = 1.147V, the range of color change relative to SHE:
Relative to SCE is:

18. 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)

19. Redox Titrations Common Redox Reagents 1.) Starch
Commonly used as an indicator in redox titrations involving iodine
Reacts with iodine to form an intensely blue colored complex
Starch is not a redox indicator
Does not undergo a change in redox potential
I6 bound in center of starch helix
Repeating unit

20. Redox Titrations Common Redox Reagents
2.) 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
Examples:
Preoxidation:
Peroxydisulfate or persulfate (S2O82-) with Ag+ catalyst
Powerful oxidants
Oxidizes Mn2+, Ce3+, Cr3+, VO2+
excess S2O82- and Ag+ removed by boiling the solution

21. Redox Titrations Common Redox Reagents
2.) Adjustment of Analyte Oxidation State
Examples:
Preoxidation:
Silver(II) oxide (AgO) in concentrated mineral acids also yields Ag2+
excess removed by boiling
Hydrogen peroxide (H2O2) is a good oxidant to use in basic solutions
Oxidizes Co2+, Fe2+, Mn2+
Reduces Cr2O72-, MnO4-
Prereduction:
Stannous chloride (SnCl2) in hot HCl
Reduce Fe3+ to Fe2+
excess removed by adding HgCl2
b) Jones reductor (Zn + Zn amalgam – anything in mercury)

22. Redox Titrations Common Redox Reagents
3.) Common Titrants for Oxidation Reactions
Potassium Permanganate (KMnO4)
Strong oxidant
Own indicator
pH ≤ 1
Titration of VO2+ with KMnO4
Eo = V
Violet colorless
pH neutral or alkaline
Eo = V
Violet brown
Before Near After
Equivalence point
pH strolngly alkaline
Eo = 0.56 V
Violet green

23. Redox Titrations Common Redox Reagents
3.) Common Titrants for Oxidation Reactions
Potassium Permanganate (KMnO4)
Application of KMnO4 in Redox Titrations

24. Redox Titrations Common Redox Reagents
3.) Common Titrants for Oxidation Reactions
Cerium (IV) (Ce4+)
Commonly used in place of KMnO4
Works best in acidic solution
Can be used in most applications in previous table
Used to analyze some organic compounds
Color change not distinct to be its own indicator
Yellow colorless
Ce4+ binds anions very strongly results in variation of formal potential
1.70V in 1 F HClO4
1.61V in 1 F HNO3
1.47V in 1 F HCl
1.44V in 1 F H2SO4
Formal potential
Measure activity not concentration

25. Redox Titrations Common Redox Reagents Eo = 1.36 V
3.) Common Titrants for Oxidation Reactions
Potassium Dichromate (K2Cr2O7)
Powerful oxidant in strong acid
Not as Strong as KMnO4 or Ce4+
Primarily used for the determination of Fe2+
Not an oxidant in basic solution
Color change not distinct to be its own indicator
Eo = 1.36 V
orange green to violet

26. Redox Titrations Common Redox Reagents K = 7 x 102
3.) Common Titrants for Oxidation Reactions
Iodine (Solution of I2 + I-)
I3- is actual species used in titrations with iodine
Either starch of Sodium Thiosulfate (Na2S2O3) are used as indicator
K = 7 x 102
I3- + Starch
I3-
I3- + S2O32-
Before
endpoint
Before
endpoint At
endpoint

27. Redox Titrations Common Redox Reagents
3.) Common Titrants for Oxidation Reactions
Iodine (Solution of I2 + I-)
Application of Iodine in Redox Titrations

28. Redox Titrations Common Redox Reagents
3.) Common Titrants for Oxidation Reactions
Iodine (Solution of I2 + I-)
Application for Redox Titrations that Produce I3-

29. Redox Titrations Common Redox Reagents
3.) Common Titrants for Oxidation Reactions
Periodic Acid (HIO4)
Commonly used in titration of organic compounds (especially carbohydrates)
4.) Titrations with Reducing Agents
Not as common as titrations using oxidizing agents
Available titrants are not very stable in the presence of atmospheric O2
Reagents can be generated directly in solution by means of chemical or electrochemical reactions

30. Redox Titrations Common Redox Reagents 5.) 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.