Voltammetry

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Voltammetry
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Read pp. 716 753 Problems
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Basic principle of voltammetry
Voltammetry A group of
electrochemical methods based on measuring
current (i)- applied potential curve during
electrolysis - only a small amount
of sample (analyte) is used
Polarography Invented by J. Heyrovsky
(Nobel Prize 1959). Differs from voltammetry in
that it employs a dropping mercury electrode
(DME) as Working electrode to continuously renew
the electrode surface.
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1. Polarographic analysis process and the
conditions for polarographic wave formation
Polarographic analysis Electrolytic analysis
carried out under special conditions.
specific characteristics A?a polarized electrode
and a depolarized electrode are used as working
electrode B?No stirring Incomplete
electrolysis (only a small amount of analyte is
consumed)
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Polarized electrode and depolarized electrode
If the electrode potential has great changes when
infinite small current flow through the
electrode, such electrode is referred to as
polarized electrode. eg. DME If the electrode
potential does not change with current , such
electrode is called ideal depolarized electrode.
eg. SCE
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• Three electrode cell Working
• Reference
• Counter/auxilliary
• current flows between working and counter
  electrodes.
• Potential controlled by potentiostat between
  working and reference electrodes.

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Two special electrodes
Supporting electrolyte Usually relatively
higher concentration of strong electrolytes
(alkali metal salts) serves as supporting
electrolyte
Dissolved oxygen is usually removed by bubbling
nitrogen through the solution
Voltage scanning Under unstirred state, recording
voltage - current curve
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? ?residual current ? electrolytic
current ??limiting diffusion current
Cd 2 2e Hg Cd(Hg) 2Hg 2Cl- -2e Hg2Cl2
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0.001 M Cd2 in 0.1 M KNO3 supporting electrolyte
i (?A)
E½
id
Base line of residual current
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
V vs SCE
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Limiting current Related to concentration
E½ at ½ i
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0.5mmol??????
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Limiting diffusion current -- A basis of
polarographically quantitative analysis
When the applied voltage exceeds the
decomposition voltage, diffusion-controlled
current is expressed as
i K(C-C0)
When the applied voltage gets more negative, C0
?0, current becomes only diffusion limited,
then id KC
Id reaches a limiting value proportional to ion
concentration C in bulk solution, and do not
changes with applied voltage longer
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Half-wave potential polarographic qualitative
analysis
The potential at which the current is equal to
one half the limiting current is called the
half-wave potential and given the symbol E1/2.
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How it works? ? The applied voltage is gradually
increased, typically by going to a more positive(
more negative decomposing potential) ? A small
residual current is observed. ? When the voltage
becomes great enough, reduction occurs at the
analytical electrode causing a current. ? The
electrode is rapidly saturated so current
production is limited based on diffusion of the
analyte to the small electrode.
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How it works ? The reduced species alters the
surface of the mercury electrode. To prevent
problems, the mercury surface is renewed by
knocking off a drop providing a fresh
surface. This results in an oscillation of the
data as it is collected.
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2. The diffusion current theory and
polarographic wave equation
We have already known
id KC
In above equations, K is called Ilkovic constant,
it is expressed as follows
K 607 n D1/2m2/3t1/6
Thus,
id 607nD1/2m2/3t1/6C
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From above equation, we can find that when
temperature, matrix solution and capillary
characteristic are kept constant, id is
proportional to C
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polarographic wave equation
When i ½ id , log term in above equation is
equal to zero, corresponding potential is called
halfwave potential E1/2
?E1/2 independent on the concentration ?basis of
qualitative analysis
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3. Interference current in classical DC
polarography
? Residual current (1) redox reactions of
impurities in solution (2) charging of Hg
drop (non-faradaic current / non-redox current)
? Migration current The current produced
by static attraction of the electrode to
sought-for ions
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? Polarographic Maximum (or malformed peak )

• Complex artifactual phenomenon
• Less likely at low drop rates, in concentrated
  electrolyte, or low concentration of
  electroactive species
• Lessened by inclusion of surfactants in medium

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? Oxygen wave

• Dissolved oxygen is easily reduced at
  many working electrodes. Thus an aqueous solution
  saturated with air exhibits two distinct oxygen
  waves.
• The first results from the reduction
  of oxygen to hydrogen peroxide
• O2 2H 2e- ? H2O2
• The second wave corresponds to the further
  reduction of hydrogen peroxide
• H2O2 2H 2e- ? 2H2O
• Sparge solutions with high purity N2 or Ar for
  5-20 min

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Factors that affect limiting diffusion current
Characteristics of capillary hight of
Hg Potential of dropping Hg electrode Composition
of solution Temperature
Factors that affect half-wave potential
Type and concentration of supporting
electrolyte Temperature Forming complex Acidic of
solution
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Question
Why a reference electrode with large area
and a dropping mercury electrode with very small
area are used to electrolyze in polarographic
analysis ?
Why large amount of supporting electrolyte
is added to sample solution?
Why does nitrogen gas pass through the
solution before electrolysis ?
In the process of polarographic analysis
whether or not to carry out stirring the
solution? Why?
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4. Polarographically quantitative analytical
methods
(id)avg Kc
?Direct comparison method ?Calibration curve
method ?Standard addition method
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5. Applications
Fundamental studies Inorganic applications Organic
applications Applications in pharmaceutical and
biochem fields