Electrochemical Cells and Titrations

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Electrochemical Cells and Titrations
Read Chapter 15-1 to 15-5 and 16.

• In this presentation you will learn how one can
  use electrochemical methods to monitor
  concentrations during titrations. I will
• describe the cells that are used
• describe two specific titrations to connect
  theory to experiment.

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Consider the following voltaic cell.
Reduce part within dashed line substantially.
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Silver-Silver Chloride Reference Electrode
Eo
0.222V E(saturated KCl) 0.197V at 25oC
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Note that porous plugs can become clogged.
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Double-junction reference electrode
Inner part is the same as previous
transparency. Solution in outer part is
compatible with analyte Solution. May contain
KNO3 solution.
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Here is another important electrode.
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The Calomel Electrode
Eo
0.268V E(saturated KCl) 0.241V
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Titrations
In this presentation we are seeing how we can
use electrochemical methods to monitor
concentrations during titrations. So far I
have describe the cells that are used to monitor
the potential during a titration. Now I will
describe two specific titrations to connect
concentrations to observed potentials.
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Consider the following titration. The goal is to
calculate potential vs. Vt.
Ce4 in HClO4
Fe2 in HClO4
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Calculating Titration Curves (Read 16-1)
Consider the reaction
made up of the following two half reactions
Eo1.700V Eo0.767V
Since Eo is greater for cerium this reaction will
be the reduction reaction. The standard
potential for the galvanic cell would be
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calculating titration curves continued
Continuing with the reaction
In a galvanic cell we would have
At equilibrium E0 and
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Before the equivalence point
A key point is that each half cell would have the
same potential if there is an equilibrium. So we
can use either Fe2/Fe3 or Ce3/Ce4 to
calculate E.
Set up an ICE table.
0 0
0
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Before the equivalence point continued
0
If
If Vt0.5Vi then E.526V If Vt(10/11)Vi then
E(.526.059)V.585V If Vt(100/101)Vi then
E(.5262x.059)V.644V
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At the equivalence point
initial c c
0 0 final x
x c-x c-x
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Titration of Fe2 with MnO4- (Demonstration 16-1)
Goal is to calculate theoretical titration curve
as function of Vt. The first step is to write
down the balanced redox reaction.
The second step is to write down the two half
reactions.
Before the equivalence point one uses the Fe2
reaction. How does one calculate the potential at
the equiv. pt.
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Write down two Nernst eqns. for the above half
reactions
Add these two equations together.
At the equivalence pt. the above terms cancel.
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vmE where v velocity mmobility Epotential
5.19 x 10-8
7.91 x 10-8
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EobservedEcellEjunction
The junction potential puts a fundamental
limitation on the accuracy of direct
potentiometric measurements.
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Summary

• In this presentation we
• described two cells that are often used as
  reference electrodes,
• described two specific examples to connect theory
  to experimental titration curves,
• discussed junction potentials and their effects.

Skills
You should be able to explain and sketch these
two reference cells. You should be able to
construct a curve of potential versus E for many
redox reactions. Finally you should know what a
junction potential is.