Transcript Polarography and Voltammetry

Polarography and Voltammetry
Lecture 2
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Polarization Effects
Since we have: Eapplied = Ecell - IR, a plot of current in an
electrolytic cell as a function of applied potential
should be a straight line with a slope equal to the
negative resistance. The observed plot is indeed
linear with small currents. However, as the applied
voltage increases, the current ultimately begins to
deviate from linearity.
Cells that exhibit nonlinear behavior at higher currents
are said to be polarized, and the degree of
polarization is given by an overvoltage, or
overpotential.
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Note that polarization requires the application of a
potential greater than the theoretical value to
give a current of the expected magnitude. Thus,
the overpotential required to achieve a current
of 7.00 mA in the electrolytic cell in previous
figure is about - 0.23 V. For an electrolytic cell
affected by overvoltage, the equation:
(Eapplied = Ecell - IR) then becomes:
Eapplied = Ecell - IR + P
Note that Ecell and the overpotential, P, have
negative values
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Polarization is an electrode phenomenon that may
affect either or both electrodes in a cell. The
degree of polarization of an electrode varies
widely. In some instances, it approaches zero (as
in a reference electrode), but in others, it can be
so large (as in a microelectrode like Hg drop)
that the current in the cell becomes
independent of potential. Under this
circumstance, polarization is said to be
complete. Polarization phenomena are
conveniently divided into two categories:
• concentration polarization and
• kinetic polarization.
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Concentration Polarization
Concentration polarization occurs because of the finite rate
of mass transfer of solute from the solution to the
electrode surface. Electron transfer between a reactive
species in a solution and an electrode can take place only
from a thin film of solution located immediately adjacent
to the surface of the electrode; this film is only a fraction
of a nanometer in thickness and contains a limited
number of reactive ions or molecules. For there to be a
steady current in a cell, this film must be continuously
replenished with reactant from the bulk of the solution.
That is, as reactant ions or molecules are consumed by the
electrochemical reaction, more must be transported into
the surface film at a rate that is sufficient to maintain the
current.
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Example of concentration polarization
Cd2+ + 2e D Cd(s)
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The electrode potential depends on [Cd2 +]s, not
[Cd2 +]b, because [Cd2 +]s is the actual
concentration at the electrode surface.
If [Cd2 +]s = [Cd2+]b, the electrode potential will be
that expected from the bulk Cd2+ concentration.
When [Cd2+]s does not equal [Cd2+]b, we say that
concentration polarization exists.
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Concentration polarization occurs when reactant
species do not arrive at the surface of the
electrode or product species do not leave the
surface of the electrode fast enough to maintain
the desired current. When this happens, the
current is limited to values less than that
predicted by the equation:
Eapplied = Ecell - IR
Reactants are transported to the surface of an
electrode by three mechanisms: (1) diffusion, (2)
migration, and (3) convection. Products are
removed from electrode surfaces by the same
mechanisms.
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Diffusion
When there is a concentration difference between
two regions of a solution, ions or molecules
move from the more concentrated region to the
more dilute. This process, called diffusion,
ultimately leads to a disappearance of the
concentration difference. The rate of diffusion is
directly proportional to the concentration
difference. For example, when cadmium ions are
deposited at a cathode by a current, the
concentration of Cd2+ at the electrode surface
[Cd2+]s becomes lower than the bulk
concentration.
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The difference between the concentration at the surface
([Cd2+]s) and the concentration in the bulk solution [Cd2+]b
creates a concentration gradient that causes cadmium ions
to diffuse from the bulk of the solution to the surface film.
The rate of diffusion is given by:
Rate of diffusion to electrode surface = K([Cd2+]b - [Cd2+]s)
where [Cd2+]b is the reactant concentration in the bulk of the
solution, [Cd2+]s is its equilibrium concentration at the
surface of the cathode, and k is a proportionality constant.
The value of [Cd2+]s at any instant is fixed by the potential of
the electrode and can be calculated from the Nernst
equation.
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In the present example, we find the surface cadmium
ion concentration from the relationship
where Ecathode is the potential applied to the cathode. As
the applied potential becomes more and more
negative, [Cd2+]s becomes smaller and smaller. The
result is that the rate of diffusion and thus the current
become correspondingly larger until the surface
concentration falls to zero, and the maximum or
limiting current is reached.
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Migration
The process by which ions move under the
influence of an electric field is called
migration. This process is the primary cause
of mass transfer in the bulk of the solution in
a cell. The rate at which ions migrate to or
away from an electrode surface generally
increases as the electrode potential
increases. This charge movement constitutes
a current, which also increases with
potential.
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Migration
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Migration of analyte species is undesirable in
most types of electrochemistry, and
migration can be minimized by having a high
concentration of an inert electrolyte, called a
supporting electrolyte, present in the cell.
The current in the cell is then primarily due
to charges carried by ions from the
supporting electrolyte. The supporting
electrolyte also serves to reduce the
resistance of the cell, which decreases the IR
drop.
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Convection
Reactants can also be transferred to or from an
electrode by mechanical means. Forced
convection, such as stirring or agitation, will
tend to decrease the thickness of the
diffusion layer at the surface of an electrode
and thus decrease concentration
polarization. Natural convection resulting
from temperature or density differences also
contributes to the transport of molecules to
and from an electrode.
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Kinetic Polarization
In kinetic polarization, the magnitude of the current is
limited by the rate of one or both of the electrode
reactions, that is, by the rate of electron transfer
between the reactants and the electrodes. To offset
kinetic polarization, an additional potential, or
overvoltage, is required to overcome the activation
energy of the half reaction.
Kinetic polarization is most pronounced for electrode
processes that yield gaseous products and is often
negligible for reactions that involve the deposition or
solution of a metal. Kinetic effects usually decrease
with increasing temperature and decreasing current
density.
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These effects also depend on the composition of
the electrode and are most pronounced with
softer metals, such as lead, zinc, and particularly
mercury.
In common with IR drop, overvoltage effects cause
the application of voltages more negative than
calculated to operate an electrolytic cell at a
desired current. Kinetic polarization also causes
the potential of a galvanic cell to be smaller than
the value calculated from the Nernst equation
and the IR drop:
(Eapplied = Ecell - IR).
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Overvoltage or overpotential
The electrochemical cell is polarized if its actual
potential is different from that expected according
to Nernst equation.
The extent of polarization is measured as
overpotential, P . For an electrode, we have:
P = Eactual – Ereversible(equilib)
Eactual is always smaller than Ereversible, therefore P is
always negative.
Overpotential, P, always reduces theoretical
electrode potential when current is flowing.
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For the reaction:
Cd2+ + 2e D Cd(s)
The electrode potential can be calculated from Nernst
equation:
The measured (actual) electrode potential will be the
same as the reversible electrode potential only when
[Cd2+]s = [Cd2+]b which is far from being the case, when
current is flowing. The difference between the actual
and reversible electrode potential is the electrode
overvoltage.
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For an electrolytic cell, the overall equation that
describes the contributions of important types of
overvoltage and IR drop would be:
Eappl = Ec – Ea – IR + (Pcc + Pck) + (Pac + Pak)
Or:
Eappl = Ecell(reversible) – IR + (Pcc +Pck) + (Pac + Pak)
Where Pcc and Pck are cathodic overpotentials
resulting from concentration and kinetic
polarization, while Pac and Pak are the
corresponding anodic overpotentials due to the
same factors.
Always remember that P carries a negative value
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History of Polarography and
Voltammetry
Developed by Czech chemist Jaroslav Heyrovsky in
the year 1922.
He received the noble prize in chemistry for this
work (1959).
In the 1960s and 1970s significant advances were
made in all areas of voltammetry (theory,
methodology, and instrumentation).
This enhanced the sensitivity and expanded the
applications of analytical methods.
The introduction of low-cost operational
amplifiers also facilitated the rapid commercial
development of relatively inexpensive
instrumentation.
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EXAMPLES OF MERCURY ELECTRODES
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• In polarography, mercury is used as a
working electrode. The working
electrode is often a Hg drop
suspended from the end of a
capillary tube.
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examples of electrodes:
1. 1. HMDE (Hanging mercury drop
electrode)
we extrude the drop of Hg by
rotating a micrometer screw that
pushes the mercury from a reservoir
through a narrow capillary.
2. DME (dropping mercury electrode)
mercury drops form at the end of the capillary tube as a result of
gravity. Unlike the HMDE, the mercury drop of a DME grows
continuously—as mercury flows from the reservoir under the influence
of gravity—and has a finite lifetime of several seconds (2-5s). At the end
of its lifetime the mercury drop is dislodged, either manually or on its
own, and replaced by a new drop.
3. SMDE (static mercury drop electrode)
uses a solenoid driven plunger to control the flow of mercury.
Activation of the solenoid (a device that converts electrical energy into
mechanical movement) momentarily lifts the plunger, allowing mercury
to flow through the capillary and forming a single, hanging Hg drop
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Two versus three electrode cells
There are some problems associated with two electrode cells:
• To study the behavior of analyte at the electrode/electrolyte interface
we require both potential and current to be monitored.
• A 2-electrode cell gives the current flowing between the two electrodes
however, none of the electrode potential is fixed and thus cannot know
at which potential (vs a reference) a reaction occurs.
• In the two electrode case, we are measuring the characteristics of the
whole cell including the counter electrode and the electrolyte.
• In a two electrode system we never know where the interfacial potential
difference occurs if the cell voltage is changed; it might be on both
electrodes and not at the one of interest. In organic solvents the IR drop
can be considerable.
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Three electrodes cells
• A 3 electrode cell is necessary, because the reference electrode must not
take part in the redox reaction. Otherwise, the measured potential will
be inaccurate.
• A 3 electrode cell is necessary to measure the current voltage
characteristics of the working/sample electrode only.
• The counter electrode's role is essentially to ensure that current does not
run through the reference electrode, since such a flow would change the
potential of the reference electrode. That is why the CE surface is much
larger than the working electrode or the reference electrode.
• The three-electrode setup is necessary in the case where the correct
value of polarization potential of the working electrode is to be
determined.
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The optimum
interval between
drops for most
analyses is
between 2 and 5
seconds.
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Control Circuit
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Direct Current polarography (DCP)
• The earliest voltammetric experiment was normal
polarography at a dropping mercury electrode. In
normal polarography the potential is linearly
scanned, producing voltammograms (polarograms)
such as that shown in figure below.
• This technique is discussed above and usually called
Direct Current polarography (DCP)
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Classical DC
polarography
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Polarogram
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• Imax is the maximum current and is also called the
limiting current)
• ir (residual current) which is the current obtained
when no electrochemical change takes place.
• iav (average current) is the current obtained by
averaging current values throughout the life time of
the drop
• id (diffusion current) which is the current resulting
from the diffusion of electroactive species to the
drop surface.
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• Analyte is either reduced (most of the cases)
or oxidized at the surface of the mercury
drop.
• The current–carrier auxiliary electrode is
usually a platinum wire.
• SCE or Ag/AgCl reference electrode is used.
• The potential of the mercury drop is
measured with respect to the reference
electrode.
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Polarogram in more details
A graph of current versus potential in a polarographic experiment
involving Cd2+ reduction is shown below:
Cd2+ + 2e D
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Cd(s)
• When the potential is only slightly negative
with respect to the calomel electrode,
essentially no reduction of Cd2+ occurs. Only
a small residual current flows.
• At a sufficiently negative potential, reduction
of Cd2+ commences and the current
increases. The reduced Cd dissolves in the Hg
to form an amalgam.
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• After a steep increase in current,
concentration polarization occurs. The rate of
electron transfer becomes limited by the rate
at which Cd2+ can diffuse from bulk solution
to the surface of the electrode.
• The magnitude of this diffusion current Id is
proportional to Cd2+ concentration and is
used for quantitative analysis. The upper
trace in the figure above is called a
polarographic wave.
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• When the potential is sufficiently negative
around -1.2 V, reduction of H+ begins and the
curve rises steeply.
• At positive potentials (near the left side of the
polarogram), oxidation of the Hg electrode
produces a negative current.
• By convention, a negative current means that
the working electrode is behaving as the anode
with respect to the auxiliary electrode. A
positive current means that the working
electrode is behaving as the cathode.
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• The oscillating current in the previous figure
above is due to the growth and fall of the Hg
drops.
• As the drop grows, its area increases, more
solute can reach the surface in a given time,
and more current flows.
• The current increases as the drop grows until,
finally, the drop falls off and the current
decreases sharply.
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Half-wave Potential, E1/2
• Half wave potential, E1/2 is an important
feature that can be derived from the
plarogram.
• It is the potential corresponding to one
half the diffusion current i.e. id/2.
• El/2 is a characteristic value for each
element and thus used for qualitative
analysis.
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