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
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):
K ≈ 1016

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
Indicator half-reactions at Pt electrode:
Eo = 0.767 V
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
Use iron half-reaction relative to calomel reference electrode:
Eo = 0.767 V
E  E  ( indicator electrode )  E  ( reference electrode )
Potential of
calomel
electrode

[ Fe 2  ] 
E  0.767  0.05916 log
  0.241
3
[ Fe ] 

Simplify
 [ Fe 2  ] 

E  0.526  0.05916 log 
 [ Fe 3  ] 


Redox Titrations
Shape of a Redox Titration Curve
3.) Region 1: Before the Equivalence Point

Special point when V = 1/2 Ve
[ Fe 3  ]  [ Fe 2  ]
 [ Fe 2  ] 

E  0.526  0.05916 log 
 [ Fe 3  ] 


Log term is zero
E  0.526  E   E o  0.767 V
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
[Ce 3  ]  [Ce 4  ]
 [Ce 3  ] 

E  1.46  0.05916 log 
 [Ce 4  ] 


Log term is zero
E  1.46  E   E o  1.70V
The point at which V= 2 Ve is analogous to the point at
which pH = pKa in an acid base titration
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+
-

From Reaction:

Primarily only Ce3+ and Fe3+ present
Tiny amounts of Ce4+ and Fe2+ from equilibrium
[Ce3+] = [Fe3+]
[Ce4+] = [Fe2+]
Both Reactions are in Equilibrium at the
Pt electrode
 [ Fe 2  ] 

E   0.767  0.05916 log 
3

 [ Fe ] 


 [Ce 3  ] 

E   1.70  0.05916 log 
 [Ce 4  ] 


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
 [ Fe 2  ] 

E   0.767  0.05916 log 
 [ Fe 3  ] 


 [Ce 3  ] 

E   1.70  0.05916 log 
 [Ce 4  ] 


Add
 [ Fe 2  ] 
 [Ce 3  ] 
  0.05916 log 

2 E   0.767  1.70  0.05916 log 
 [ Fe 3  ] 
 [Ce 4  ] 




Rearrange
 [ Fe 2  ] [Ce 3  ] 

2 E   2.47  0.05916 log 
 [ Fe 3  ] [Ce 4  ] 


Redox Titrations
Shape of a Redox Titration Curve
4.) Region 2: At the Equivalence Point

Instead Add both equations together
 [ Fe 2  ] [Ce 3  ] 

2 E   2.47  0.05916 log 
3

4

 [ Fe ] [Ce ] 


[Ce 3  ]  [ Fe 3  ]
[Ce 4  ]  [ Fe 2  ]
Log term is zero
2 E  2.47V  E  1.23V
Cell voltage
E  E  E ( calomel )  1.23  0.241  0.99V
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
Use iron half-reaction relative to calomel reference electrode:
Eo = 1.70 V
E  E  ( indicator electrode )  E  ( reference electrode )
Potential of
calomel
electrode

[Ce 3  ] 
E  1.70  0.05916 log
  0.241
4
[Ce ] 

Simplify
 [Ce 3  ] 

E  1.46  0.05916 log 
 [Ce 4  ] 


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

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
Redox Titrations
Finding the End Point
2.) Redox Indicators

Color Change for a Redox Indicator occurs mostly over the range:
0.05916 

E   Eo 
volts
n


where Eo is the standard reduction potential for the indicator
and n is the number of electrons involved in the reduction
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:
0.05916 

E   1.147 
volts  1.088 to 1.206 V
1


Relative to SCE is:
0.05916 

E   1.147 
  E ( calomel )  1.088 to 1.206 V   ( 0.241 )  0.847 to 0.965V
1


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.) 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
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
-

Reagents for prereduction or preoxidation must:
-

This step in the procedure is known as prereduction or preoxidation
Totally convert analyte into desired form
Be easy to remove from the reaction mixture
Avoid interfering in the titration
Examples:
-
Preoxidation:
a)
Peroxydisulfate or persulfate (S2O82-) with Ag+ catalyst
Powerful oxidants
Oxidizes Mn2+, Ce3+, Cr3+, VO2+
excess S2O82- and Ag+ removed by boiling the solution
Redox Titrations
Common Redox Reagents
2.) Adjustment of Analyte Oxidation State

Examples:
-
Preoxidation:
b)
Silver(II) oxide (AgO) in concentrated mineral acids also yields Ag2+
excess removed by boiling
c)
-
Hydrogen peroxide (H2O2) is a good oxidant to use in basic solutions
Oxidizes Co2+, Fe2+, Mn2+
Reduces Cr2O72-, MnO4excess removed by boiling
Prereduction:
a)
Stannous chloride (SnCl2) in hot HCl
Reduce Fe3+ to Fe2+
excess removed by adding HgCl2
b)
Jones reductor (Zn + Zn amalgam – anything in mercury)
Redox Titrations
Common Redox Reagents
3.) Common Titrants for Oxidation Reactions

Potassium Permanganate (KMnO4)
-
Strong oxidant
Own indicator
Titration of VO2+ with KMnO4
pH ≤ 1
Eo = 1.507 V
Violet
colorless
pH neutral or alkaline
Eo = 1.692 V
Violet
Before
Near
After
Equivalence point
brown
pH strolngly alkaline
Eo = 0.56 V
Violet
green
Redox Titrations
Common Redox Reagents
3.) Common Titrants for Oxidation Reactions

Potassium Permanganate (KMnO4)
-
Application of KMnO4 in Redox Titrations
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
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
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
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
K = 7 x 102
-
Either starch of Sodium Thiosulfate (Na2S2O3) are used as indicator
I3-
I3- + S2O32-
I3- + Starch
Before
Before
At
endpoint endpoint endpoint
Redox Titrations
Common Redox Reagents
3.) Common Titrants for Oxidation Reactions

Iodine (Solution of I2 + I-)
-
Application of Iodine in Redox Titrations
Redox Titrations
Common Redox Reagents
3.) Common Titrants for Oxidation Reactions

Iodine (Solution of I2 + I-)
-
Application for Redox Titrations that Produce I3-
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
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.