Transcript Slide 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
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
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
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Before the Equivalence Point
At the Equivalence Point
After the Equivalence Point
3.) Region 1: Before the Equivalence Point
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Each aliquot of Ce4+ creates an equal
number of moles of Ce3+ and Fe3+

Excess unreacted Fe2+ remains in solution
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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

[ Fe2  ] 
E  0.767  0.05916log
  0.241
3
[ Fe ] 

Simplify
 [ Fe2  ] 

E  0.526  0.05916log
 [ Fe3  ] 


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
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Another special point, when [Ce4+]=0
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Voltage can not be calculated
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[Fe3+] is unknown
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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+
-
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From Reaction:
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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
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
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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
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Opposite Situation Compared to Before the Equivalence Point
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Equal number of moles of Ce3+ and Fe3+
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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

[Ce3  ] 
E  1.70  0.05916log
  0.241
4
[Ce ] 

Simplify
 [Ce3  ] 

E  1.46  0.05916log
 [Ce 4  ] 


Redox Titrations
Shape of a Redox Titration Curve
6.) Titration Only Depends on the Ratio of
Reactants
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Independent on concentration and/or
volume
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Same curve if diluted or concentrated by
a factor of 10
Redox Titrations
Shape of a Redox Titration Curve
7.) Asymmetric Titration Curves
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Reaction Stoichiometry is not 1:1
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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
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Electrochemical measurements (current or potential) can be used to determine
the endpoint of a redox titration
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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:
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
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.) 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
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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
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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
brown
pH strolngly alkaline
Eo = 0.56 V
Violet
green
Before
Near
After
Equivalence point
Redox Titrations
Common Redox Reagents
2.) 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.