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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
Biochemistry 2005, 44, 1856-1863
Measurement of redox potentials permit detailed
analysis of complex enzyme mechanism
Electron path in multi-heme active site of P460
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
Eo = 0.767 V
Indicator half-reactions at Pt electrode:
K ≈ 1016
Eo = 1.70 V
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
Redox Titrations Shape of a Redox Titration Curve
3.) Region 1: Before the Equivalence Point
Eo = 0.767 V
Use iron half-reaction relative to calomel reference electrode:
)electrodereference(E)electrodeindicator(EE
241005916076703
2.
Fe
Felog..E
][
][
][
][3
20591605260
Fe
Felog..E
Potential of calomel electrode
Simplify
Redox Titrations Shape of a Redox Titration Curve
3.) Region 1: Before the Equivalence Point Special point when V = 1/2 Ve
][][ 23 FeFe
][
][3
20591605260
Fe
Felog..E
Log term is zero
V.EE.E o 76705260
The point at which V= ½ Ve is analogous to the point at which pH = pKa in an acid base titration
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 = -0.828 V
Redox Titrations Shape of a Redox Titration Curve
3.) Region 1: Before the Equivalence Point Special point when V = 2Ve
][][ 43 CeCe
Log term is zero
V.EE.E o 701461
The point at which V= 2 Ve is analogous to the point at which pH = pKa in an acid base titration
][
][4
3059160461
Ce
Celog..E
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
][
][3
20591607670
Fe
Felog..E
][
][4
3059160701
Ce
Celog..E
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
][
][3
20591607670
Fe
Felog..E
][
][4
3059160701
Ce
Celog..E
][
][
][
][4
3
3
205916005916070176702
Ce
Celog.
Fe
Felog...E
Rearrange
][
][
][
][4
3
3
20591604722
Ce
Ce
Fe
Felog..E
Add
Redox Titrations Shape of a Redox Titration Curve
4.) Region 2: At the Equivalence Point Instead Add both equations together
][
][
][
][4
3
3
20591604722
Ce
Ce
Fe
Felog..E
][][
][][
24
33
FeCe
FeCeLog term is zero
V.EV.E 2314722
Cell voltage
V...)calomel(EEE 9902410231
Equivalence-point voltage is independent of the concentrations and volumes of the reactants
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
Redox Titrations Shape of a Redox Titration Curve
5.) Region 3: After the Equivalence Point
Eo = 1.70 V
Use iron half-reaction relative to calomel reference electrode:
)electrodereference(E)electrodeindicator(EE
][
][4
3059160461
Ce
Celog..E
Potential of calomel electrode
Simplify
24100591607014
3.
Ce
Celog..E
][
][
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
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
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
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
voltsn
.EE o
059160
V.to.volts.
.E 206108811
0591601471
V.to.).(V.to.)calomel(E.
.E 965084702410206108811
0591601471
For Ferroin with Eo = 1.147V, the range of color change relative to SHE:
Relative to SCE is:
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)
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 AfterEquivalence point
Eo = 1.507 VViolet colorless
pH ≤ 1
Eo = 1.692 VpH neutral or alkaline
Violet brown
pH strolngly alkalineEo = 0.56 V
Violet green
Redox Titrations Common Redox Reagents
2.) ExampleA 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.
