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
• 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
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)
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-
• excess removed by boiling
• 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 1.507 V
Violet colorless
pH neutral or alkaline
Eo 1.692 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
Measure activity not concentration
Formal potential
25
Redox Titrations

• Common Redox Reagents
• 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
• 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 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.