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

24100591607670 3

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

2410059160701 4

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

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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.