Transcript voltammetry / 전압전류법 분석

Dong-Sun Lee / cat - lab / SWU
2012-Fall version
Chapter 23
Voltammetry
Invention of the battery by Alessandro
Volta
In 1799 Volta constructed a battery from a pile of alternating
silver and zinc disks, with an absorbant material soaked in brine
between each disk. This apparatus, know as the voltaic pile,
produced an electric current, thereby disproving the old theory
that animal matter had to be present for electricity to be
produced.
Almost immediately William Nicholson used this apparatus to
decomposed water by electrolysis and later, in 1807, Humphrey
Davy discovered potassium and sodium using the same process.
Volta was awarded the Legion of Honour by Napolean in
recognition of his work, and the unit of electric potential was
named the volt in his honour.
Volta's discovery provided scientists with a reliable source of
reasonably large electric currents, thereby revolutionising the
science of elelctricity and facilitating the research into
electrolysis that made the likes of Michael Faraday and
Humphrey Davy famous.
http://www.chemsoc.org/timeline/pages/1799_01.html
Volta's experimentations at the French National Institute
in November of 1800 in which Napoleon Bonaparte was
present.
http://www.batteryuniversity.com/partone-2.htm
Polarography
Polarography is based on measuring the current of an electrolysis cell in which the potential
of a working electrode is varied continuously.
Discovery : Jaroslav Heyrovsky(1922, Czechoslovakian chemist, 1959 Nobel Prize)
Detection limit : near 10–9 M
Precision : around 5%
Apparatus :
Working electrode : dropping-mercury electrode(DME: reproducible and polarizable
microelectrode) suspended from the bottom of a glass capillary tube(0.05mm).
Analyte is either reduced or oxidized at the surface of the mercury drop.
Auxiliary electrode : Pt wire for the current carrying
SCE reference electrode : The potential of the mercury drop is measured with SCE.
Applied potential : about 0.01 V ( 0 ~ 3.0 V)
Cell current : 0.01 ~ 100 A ( accuracy : 0.01 A)
Both qualitative and quantitative information is obtained from plots of the current generated
in the cell as a function of applied potential.
Jaroslav Heyrovsky was born in Prague on 20th December, 1890, the fifth child of Leopold Heyrovsky, Professor of Roman Law at
the Czech University of Prague, and his wife Clara, née Hanl.
He obtained his early education at secondary school till 1909 when he began his study of chemistry, physics and mathematics at the
Czech University, Prague. From 1910 to 1914 he continued his studies at University College, London, under Professors Sir William
Ramsay, W.C.Mc.C. Lewis and F.G. Donnan, taking his B.Sc. degree in 1913. He was particularly interested in working with Professor
Donnan, on electrochemistry.
During the First World War Heyrovsky did his war service in a military hospital as dispensing chemist and radiologist, which enabled
him to continue his studies and to take his Ph.D. degree in Prague in 1918 and D.Sc. in London in 1921.
Heyrovsky started his university career as assistant to Professor B. Brauner in the Institute of Analytical Chemistry of the Charles
University, Prague; he was promoted to Associate Professor in 1922 and in 1926 he became the first Professor of Physical Chemistry at
this University.
Heyrovsky's invention of the polarographic method dates from 1922 and he concentrated his whole further scientific activity on the
development of this new branch of electrochemistry. He formed a school of Czech polarographers in the University, and was himself in
the forefront of polarographic research.
In 1950 the Professor was appointed Director of the newly established Polarographic Institute which has been incorporated into the
Czechoslovak Academy of Sciences since 1952.
Many universities and seats of learning have honoured Professor Heyrovsky. He was elected Fellow of University College, London, in
1927, and received honorary doctorates of the Technical University, Dresden, in 1955, the University of Warsaw in 1956, the
University Aix-Marseille in 1959, and the University of Paris in 1960. He was granted honorary membership of the American
Academy of Arts and Sciences, Boston, Mass., in 1933; of the Hungarian Academy of Sciences in 1955; the Indian Academy of
Sciences, Bangalore, in 1955; the Polish Academy of Sciences, Warsaw, in 1962; was elected Corresponding Member of the German
Academy of Sciences, Berlin, in 1955; member of the German Academy of Natural Scientists, Leopoldina (Halle-Saale) in 1956;
Foreign Member of the Royal Danish Academy of Sciences, Copenhagen, in 1962; Vice-President of the International Union of
Physics from 1951 to 1957; President and first honorary member of the Polarographic Society, London; honorary member of the
Polarographic Society of Japan; honorary member of the Chemical Societies of Czechoslovakia, Austria, Poland, England and India.
In Czechoslovakia he was awarded the State Prize, First Grade, in 1951, and in 1955 the Order of the Czechoslovak Republic.
Heyrovsky has lectured on polarography in the United States of America in 1933, the USSR in 1934, England in 1946, Sweden in
1947, the People's Republic of China in 1958, and in U.A.R. (Egypt) in 1960 and 1961.
In 1926 Professor Heyrovsky married Marie Koranová, and there are two children of the marriage, a daughter, Judith, and a son,
Michael.
From Nobel Lectures, Chemistry 1942-1962, Elsevier Publishing Company, Amsterdam, 1964
This autobiography/biography was written at the time of the award and later published in the book series Les Prix Nobel/Nobel
Lectures. The information is sometimes updated with an addendum submitted by the Laureate. To cite this document, always state the
source as shown above.
Jaroslav Heyrovsky and his Polarograph
http://www.cas.cz/aa/foto/heyrovs.htm
Jaroslav Heyrovsky died in 1967.
http://nobelprize.org/chemistry/laureates/1959/heyrovsky-bio.html
Voltammetric Methods
Voltammetry are based on measurement of current as a function of the potential
applied to a small electrode. Unlike potentiometry measurements, which employ
only two electrodes, voltammetric measurements utilize a three electrode
electrochemical cell. The use of the three electrodes (working, auxiliary, and
reference) along with the potentiostat instrument allow accurate application of
potential functions and the measurement of the resultant current.
A Three electrode cell used
for anodic stripping
voltammetry.
The working electrode is a
glassy carbon electrode on
which a thin mercury film has
been deposited.
An electrolysis step is used to
deposit lead into the mercury
film as an amalgam.
After the electrolysis step, the
potential is scanned
anodically toward positive
values to oxidize (strip) the
metal from the film.
Excitation signals
In voltammetry, the voltage of the working electrode is varied systematically while the
current response is measured. Several different voltage-time functions, called excitation
signals, can be applied to the electrode.
wave form
Square wave
http://chem.ch.huji.ac.il/~eugeniik/polarography.htm
Voltage versus time excitation signals used in voltammetry.
Linear Sweep Voltammetry
Linear sweep voltammetry is a general term applied to any voltammetric
method in which the potential applied to the working electrode is varied
linearly in time. These methods would include polarography, cyclic
voltammetry (CV), and rotating disk voltammetry. The slope of this ramp has
units of volts per unit time, and is generally called the scan rate of the
experiment.
The value of the scan rate may be varied from as low as mV/sec (typical for
polarography experiments) to as high as 1,000,000V/sec (attainable when
ultramicroelectrodes are used as the working electrode). With a linear
potential ramp, the faradaic current is found to increase at higher scan rates.
This is due to the increased flux of electroactive material to the electrode at
the higher scan rates The amount of increase in the faradaic current is found to
scale with the square root of the scan rate. This seems to suggest that
increasing the scan rate of a linear sweep voltammetric experiment could lead
to increased analytical signal to noise. However, the capacitive contribution to
the total measured current scales directly with the scan rate. As a result, the
signal to noise of a linear sweep voltammetric experiment decreases with
increasing scan rate.
A manual potentiostat for voltammetry.
The working electrode is the electrode at which the analyte is oxidized or reduced. The
potential between the working electrode and the reference electrode is controlled.
Electrolysis current passes between the working electrode and a counter electrode.
A supporting electrolyte is a salt added in excess to the analyte solution. Most
commonly, it is an alkali metal salt that does not react at the working electrode at the
potentials being used. The salt reduces the effects of migration and lowers the resistance
of the solution.
An operational amplifier circuit for
measuring voltammetric current.
An operational amplifier potentiostat.
The three electrode cell has a working
electrode(WE), reference electrode(RE),
and a counter electrode (CE).
Voltammetric electrodes
Hanging mercury drop electrode : an electrode in which a drop
of mercury is suspended from a capillary tube.
Dropping mercury electrode : an electrode in which successive
drops of mercury form at the end of a capillary tube as a result of
gravity, with each drop providing a fresh electrode surface.
Static mercury drop electrode : an electrode in which successive
drops of mercury form at the end of a capillary tube as a result of
mechanical plunger, with each drop providing a fresh electrode
surface.
Some common types of
volammetric electrodes.
(a) A disk electrode
(b) a mercury hanging
drop electrode
(c ) a dropping mercury
electrode
(d) a static mercury
dropping electrode.
Potential ranges for three types of electrodes in various supporting electrolytes.
Voltammogram
Linear sweep voltammogram for the reduction of a hypothetical species A to give a
product P. The limiting current il is proportional to the analyte concentration and is
used for quantitative analysis. The half-wave potential E1/2 is related to the standard
potential for the half reaction and is often used for qualitative identification of species.
A + ne →
←P
iI = kCA
Hydrodynamic voltammetry
Hydrodynamic voltammetry is a type of voltammetry in which the analyte
solution is kept in continuous motion.
Mass transport: the movement of material toward or away from the
electrode surface. Mass transport processes include diffusion, migration, and
convection.
Diffusion: the movement of material in response to a a concentration
gradient.
Convection: the movement of material in response to a mechanical force,
such as stirring a solution.
Schematic showing transport of
Fe(CN)63– toward the electrode and
Fe(CN)64– away from the electrode
following the reduction of Fe(CN)63–.
Concentration gradient for the analyte
showing the effects of diffusion and
convection as methods of mass
transport.
(a) Rotating disk electrode. Only the
polished bottom surface of the electrode,
which is typically 5 mm in diameter,
contacts the solution.
(b) Schematic concentration profile of
analyte near the surface of the rotating
disk electrode when the potential is great
enough to reduce the concentration of
analyte to 0 at the electrode surface.
Visualization of flow patterns in a flowing
stream. Laminar flow, shown in the left ,
becomes turbulent flow as the average velocity
increases. In turbulent flow, the molecules
move in an irregular, zigzag fashion, and there
are swirls and eddies in the movement. In
laminar flow, the streamlines are steady as
layers of liquid slide by each other in a regular
manner.
Flow patterns and regions of interest near the
working electrode in hydrodynamic
voltammetry.
Concentration profiles at an
electrode/solution interface during the
electrolysis
A + ne = P
from a stirred solution of A.
Voltammograms for two-component mixtures.
Half-wave potentials differ by 0.1 V in curve A
and 0.2 V in curve B.
Voltammetric behavior of iron(II) and iron(III) in a citrate medium. Curve A: anodic wave for a
solution in which c(Fe2+) = 1 ×10–4 M. Curve B: anodic/cathodic wave for a solution in which
c(Fe2+) = c(Fe3+) = 1 ×10–4 M. Curve C: cathodic wave for a solution in which c(Fe3+) = 1 ×10–4
M.
Dissolved oxygen
Oxygen is electroactive at a mercury cathode giving rise to two waves. The first
wave is due to its reduction to H2O2, and the second is from further reduction to
H2O.
O2 + 2H+ + 2e  H2O2
E1/2 = – 0.15 V (vs SCE)
H2O2 + 2H+ + 2e  2H2O
E1/2 = – 0.9 V (vs SCE)
In certain instances, these waves
may be used for the determination
of dissolved oxygen. In other times,
they may interfere with the
accurate measurement of other
polarographic waves.
Bubbling an inert gas such as
nitrogen or argon through the
solution for 5 to 10 minutes will
reduce the oxygen concentration
below the normal detection limit.
Voltammogram for the reduction of oxygen in an air-saturated
0.1 M KCl solution. The lower curve is for a 0.1 M KCl solution
in which the oxygen is removed by bubbling nitrogen through
the solution.
Application of hydrodynamic voltammetry
1) Chromatographic detection
2) Determination of oxygen and glucose
3) Detection of end point in volumetric titration
4) Fundamental studies of electrochemical processes
A voltammetric system for detecting
electroactive species as they elute from a
column. The cell volume is 1 l.
Three-dimensional square wave
polarograms used in HPLC detection.
Amperometry:
a form of voltammetry in which we measure current as a function of time
while maintaining a constant potential to the working electrode. Since the
potential is not scanned, amperometry does not lead to a voltammogram.
Typical cell arrangement for amperometric titrations with a rotating platinum disk electrode.
Typical amperometric titration curves:
(left) analyte is reduced, reagent is not
(center) reagent is reduced, analyte is not
(right) both reagent and analyte are reduced.
The Clark voltammetric (left) or amperometric (right) oxygen sensor. Cathodic reaction:
O2 + 4H+ + 4e = 2 H2O
Anodic reaction:
Ag(s) + Cl– = AgCl(s) + e
The Clark oxygen sensor is widely used in clinical lab. For the determination of dissolved
oxygen in blood and other body fluids.
Enzyme based glucose sensor
Amperometric or pulsed voltammetry applied to glucose oxidase-coated carbon fibre
electrodes (glucose sensor) was used for glucose determination
The outer layer is polycarbonate film that is
permeable to glucose but impermeable to
proteins and other constituents of blood.
The middle layer is an an immobilized enzyme.
(glucose oxidase)
Glucose + Oxygen  glucuronic acid + H2O2
The inner layer is a cellulose acetate membrane,
which is permeable small molecule such as
H2O2.
H2O2 + 2OH–  O2 + H2O + 2e
Polarography
Polarography is an voltammetric measurement whose
response is determined by combined
diffusion/convection mass transport. Polarography is a
specific type of measurement that falls into the general
category of linear-sweep voltammetry where the
electrode potential is altered in a linear fashion from the
initial potential to the final potential. As a linear sweep
method controlled by convection / diffusion mass
transport, the current vs. potential response of a
polarographic experiment has the typical sigmoidal
shape.
What makes polarography different from other linear
sweep voltammetry measurements is that polarography
makes use of the dropping mercury electrode (DME).
Picture of a DME
http://www.chem.vt.edu/chem-dept/tissue/4114/
Polarography apparatus featuring a dropping- mercury working electrode.
A plot of the current vs. potential in a polarography experiment shows the
current oscillations corresponding to the drops of Hg falling from the capillary.
If one connected the maximum current of each drop, a sigmoidal shape would
result. The limiting current (the plateau on the sigmoid), called the diffusion
current because diffusion is the principal contribution to the flux of
electroactive material at this point of the Hg drop life, is related to analyte
concentration by the Ilkovic equation:
id = 708nD1/2m2/3t1/6c
Where D is the diffusion coefficient of the analyte in the medium (cm2/s), n is
the number of electrons transferred per mole of analyte, m is the mass flow
rate of Hg through the capillary (mg/sec), and t is the drop lifetime is seconds,
and c is analyte concentration in mol/cm3.
http://www.chem.vt.edu/chem-ed/echem/polarogr.html
Polarograms
A polarogram is a plot of current as a function of the potential applied to a
polarographic cell.
1. Residual current : small, slightly increasing current flowing even in the absence of
any electroactive analyte.
(1) Faradaic current : The residual current arises primarily from the presence of
impurities in the supporting electrolyte (ex. 0.1M KCl) and the solvent.
(2) Condensor current : charging current : A capacitance effect caused by the
presence of a double layer at the surface of the mercury drop.
2. Decomposition potential
3. Diffusion current : Id is directly proportional to the concentration of the analyte.
Id
= limiting current – residual current
Ilkovic equation
 [C]o
Polarograms for
A, a 1 M solution of HCl that is 5 ×10–4 M in Cd2+
and B, a 1 M solution of HCl.
Residual current for a 0.1 M
solution of HCl.
The current we seek to measure in
voltammetry is faradiac current due to
reduction or oxidation of analyte at the
working electrode.
Faradaic current : any current in an electrochemical
cell due to an oxidation or reduction reaction.
Cathodic current: a faradaic current due to a
reduction reaction.
Anodic current: a faradaic current due to an
oxidation reaction.
Charging current: due to electrostatic attraction or
repulsion of ions in solution and electrons in the
electrode.
Current maxima
A distortion of the polarographic wave appears to be due to absorption
phenomena at the surface of the mercury drop.
The maxima may be removed by
the addition of surface active
agent( maxima suppressors)
such as gelatine, methyl
cellulose or Triton X-100.
Half-wave potential
The two most common types of reactions at a dropping mercury electrode are :
Mn+ + Hg + ne
 M(Hg)
Xa+ + ne
 X(a–n)+
amalgams
If these half reactions are reversible, the following relationship ( called the Heyrovsky
equation) can be derived.
I = k ([Mn+]o – [Mn+]s)
Id = k [Mn+]o
[Mn +]o = ( Id – I) k
[M]o = [Mn+]o – [Mn+]s
I = kR [M]o
E = Eo – (0.05916 / n) log ([M]o / [Mn+]o)
= Eo – (0.05916 / n) log {I / (Id – I )}(k / kR)
If I = Id /2,
E1/2 = Eo – (0.05916 / n) log (k / kR)
E = E1/2 – (0.05916 / n) log {I / (Id – I )}
Graph of equation E = E1/2 – (0.05916 / n) log {I / (Id – I )}.
There are a number of limitations to the polarography experiment for quantitative analytical
measurements. Because the current is continuously measured during the growth of the Hg
drop, there is a substantial contribution from capacitive current. As the Hg flows from the
capillary end, there is initially a large increase in the surface area. As a consequence, the initial
current is dominated by capacitive effects as charging of the rapidly increasing interface
occurs. Toward the end of the drop life, there is little change in the surface area which
diminishes the contribution of capacitance changes to the total current. At the same time, any
redox process which occurs will result in faradaic current that decays approximately as the
square root of time (due to the increasing dimensions of the Nernst diffusion layer). The
exponential decay of the capacitive current is much more rapid than the decay of the faradaic
current; hence, the faradaic current is proportionally larger at the end of the drop life.
Unfortunately, this process is complicated by the continuously changing potential that is
applied to the working electrode (the Hg drop) throughout the experiment. Because the
potential is changing during the drop lifetime (assuming typical experimental parameters of a
2mV/sec scan rate and a 4 sec drop time, the potential can change by 8 mV from the beginning
to the end of the drop), the charging of the interface (capacitive current) has a continuous
contribution to the total current, even at the end of the drop when the surface area is not rapidly
changing. As such, the typical signal to noise of a polarographic experiment allows detection
limits of only approximately 10-5 or 10-6 M. Better discrimination against the capacitive current
can be obtained using the pulse polarographic techniques.
Qualitative information can also be determined from the half-wave potential of the polarogram
(the current vs. potential plot in a polarographic experiment). The value of the half-wave
potential is related to the standard potential for the redox reaction being studied.
Applications of polarography
1)
Qualitative identification of an unknown :
Since the half-wave potential is characteristic of the substance being reduced or
oxidized at a polarographic electrode, E1/2 for an unknown can be compared with
known values to try to identify the species by polarography.
2) Quantitative analysis :
Since the magnitude of the diffusion current is proportional to the concentration of
analyte, the height of polarographic wave tells how much analyte is present.
A. Standard curves
B. Standard addition method
C. Internal standard method
3) Polarographic study of chemical equilibrium
4) Polarographic study of chemical kinetics
Standard curve
Id
Standard addition
Id
Current due to unknown plus
standard addition
Current due to unknown
Concentration
Eapplied
Ilkovic equation
Id (unknown) = k C
Id = 607 n D1/2 C m2/3 t1/6
Id (unknown + standard ) = k Cx{Vx / (Vx+Vs)} + k Cs{Vs / (Vx+Vs)}
Cx = Cs Vs / {R(Vx + Vs ) – Vx}
Internal standard
Id
Current due to I.S.
Current due to unknown
Eapplied
Fig. 11.
Id (analyte) / Id (I.S.) = k [Analyte] / [I.S.]
[Analyte] = Id (analyte) [I.S.] / k Id (I.S.)
Polarograms. a) 1.4mM Fe(III), b) 0.7mM
Fe(III)+0.7mM Fe(II), c) 1.4 mM Fe(II)
Pulse polarography
DC polarography :
The voltage applied to the working electrode increases linearly with time
Differential pulse polarography :
Pulses are superimposed on the linear voltage ramp. The height of pulse is
called its modulation amplitude.
Enhanced sensitivity of pulsed polarography is due mainly to an increase in
the faradaic current and a decrease in the condensor current
(Left) Staircase voltage profile used in sampled current polarography.
Current is measured only during the intervals shown by heavy, colored lines.
Potential is scanned toward more negative values as the experiment progresses.
Lower graph shows that charging current decays more rapidly than faradaic
current after each voltage step.
(Right) Sampled current polarograms of (a) 5 mM Cd2+ in 1 M HCl and (b) 1 M
HCl alone.
Normal-Pulse Polarography (NPP)
Pulse polarographic techniques are voltammetric measurements which are variants of the
polarographic measurement which try to minimize the background capacitive
contribution to the current by eliminating the continuously varying potential ramp, and
replacing it with a series of potential steps of short duration. In Normal-pulse
polarography (NPP), each potential step begins at the same value (a potential at which no
faradaic electrochemistry occurs), and the amplitude of each subsequent step increases in
small increments. When the Hg drop is dislodged from the capillary (by a drop knocker
at accurately timed intervals), the potential is returned to the initial value in preparation
for a new step.
For this experiment, the polarogram is obtained by plotting the measured current vs. the
potential to which the step occurs. As a result, the current is not followed during Hg drop
growth, and normal pulse polarogram has the typical shape of a sigmoid. By using
discrete potential steps at the end of the drop lifetime (usually during the last 50-100 ms
of the drop life which is typically 2-4 s), the experiment has a constant potential applied
to an electrode with nearly constant surface area. After the initial potential step, the
capacitive current decays exponentially while the faradaic current decays as the square
root of time. The diffusion current is measured just before the drop is dislodged, allowing
excellent discrimination against the background capacitive current. In many respects, this
experiment is like conducting a series of chronoamperometry experiments in sequence on
the same analyte solution. The normal pulse polarography method increases the
analytical sensitivity by 1 - 3 orders of magnitude (limits of detection 10-7 to 10-8 M,
relative to normal dc polarography.
Potential wave form for normal pulse voltammetry.
http://www.chem.vt.edu/chem-ed/echem/npp.html
Differential Pulse Polarography (DPP)
Differential Pulse Polarography is a polarographic technique that uses a series of discrete
potential steps rather than a linear potential ramp to obtain the experimental polarogram.
Many of the experimental parameters for differential pulse polarography are the same as
with normal pulse polarography (for example accurately timed drop lifetimes, potential step
duration of 50 - 100 ms at the end of the drop lifetime). Unlike Normal Pulse Polarography,
however, each potential step has the same amplitude, and the return potential after each
pulse is slightly negative of the potential prior to the step.
Differential pulse polarography
In this manner, the total waveform applied to the DME is very much like a combination of a
linear ramp with a superimposed square wave. The differential pulse polarogram is obtained
by measuring the current immediately before the potential step, and then again just before
the end of the drop lifetime. The analytical current in this case is the difference between the
current at the end of the step and the current before the step (the differential current). This
differential current is then plotted vs. the average potential (average of the potential before
the step and the step potential) to obtain the differential pulse polarogram. Because this is a
differential current, the polarogram in many respects is like the differential of the sigmoidal
normal pulse polarogram. As a result, the differential pulse polarogram is peak shaped.
Differential pulse polarography has even better ability to discriminate against capacitive
current because it measures a difference current (helping to subtract any residual capacitive
current that remains prior to each step). Limits of detection with Differential Pulse
Polarography are 10-8 - 10-9 M.
Excitation signals for differential pulse polarography.
Voltammogram for a differential pulse polarography experiment. Here i = is2 – is1.
The peak potential, Epeak, is colsely related to the polarographic half-wave potential.
(left) Differential pulse polarogram: 0.36 ppm tetracycline. HCl in 0.1 M acetate
buffer, pH 4.
(right) DC polarogram: 180 ppm tetracycline. HCl in 0.1 M acetate buffer, pH 4.
Potential wave form for differential
pulse voltammetry.
A typical differential pulse voltammogram.
Comparison of DC and differential pulsed polarography of chlordiazepoxide.
The example above shows the simultaneous
determination of Zn , Cd, Pb and Cu using
standard addition
http://www.topac.com/polarography.html
Square wave polarography :
Square wave polarography is more sensitive and much faster than
differential pulse polarography. The square wave is also better at rejecting
background signals such as those generated by reduction of oxygen.
Waveform for square wave polarography.
Generation of a square-wave voltammetry excitation signal. The staircase signal in (a) is
added to the pulse train in (b) to give the square-wave excitation signal in (c ).
Current response for a reversible reaction to excitation signal.
Potential wave form for square wave
voltammetry.
A typical square wave voltammogram.
http://www.epsilon-web.net/Ec/manual/Techniques/Pulse/pulse.html
Cyclic Voltammetry (CV)
Cyclic voltammetry (CV) is an electrolytic method that uses microelectrodes and an
unstirred solution so that the measured current is limited by analyte diffusion at the
electrode surface. The electrode potential is ramped linearly to a more negative potential,
and then ramped in reverse back to the starting voltage. The forward scan produces a
current peak for any analytes that can be reduced through the range of the potential scan.
The current will increase as the potential reaches the reduction potential of the analyte, but
then falls off as the concentration of the analyte is depleted close to the electrode surface.
As the applied potential is reversed, it will reach a potential that will reoxidize the product
formed in the first reduction reaction, and produce a current of reverse polarity from the
forward scan. This oxidation peak will usually have a similar shape to the reduction peak.
The peak current, ip, is described by the Randles-Sevcik equation:
ip = (2.69x105) n3/2 A C D1/2 v1/2
where n is the number of moles of electrons transferred in the reaction, A is the area of the
electrode, C is the analyte concentration (in moles/cm3), D is
The potential difference between the reduction and oxidation peaks is theoretically 59 mV
for a reversible reaction. In practice, the difference is typically 70-100 mV. Larger
differences, or nonsymmetric reduction and oxidation peaks are an indication of a
nonreversible reaction. These parameters of cyclic voltammograms make CV most suitable
for characterization and mechanistic studies of redox reactions at electrodes.
Cyclic voltammetry
In cyclic voltammetry, a periodic, triangular wave form is applied to the working
electrode. The portion between times to and t1 is a linear voltage ramp. In CV, the time
is on the order of seconds. The ramp is then reversed to bring the potential back to its
initial value at time t2.
Cyclic voltammetry is used principally to characterize the redox properties of
compounds and to study the mechanisms of redox reactions.
Waveform used in cyclic voltammetry.
Cyclic voltammetric excitation signal.
Cyclic Voltammetry
• t0 → t1 : cathodic wave
– Instead of leaving off at the top of the
wave, current decreases at more
negative potential
← diffusion is too slow to replenish
analyte near the electrode
• t1 → t2 : anodic wave
– The potential is reversed and, reduced
product near the electrode is oxidized
Epa  Epc 
2.22 RT 57.0

(mV)
nF
n
(at 25 C)
Cyclic voltammograms are recorded either with
an osciloscope or with a fast X-Y recorder.
The current decreases after the cathodic peak
because of concentration polarization.
For a reversible reaction, half-wave potential
lies midway between the cathodic and anodic
peaks.
(a) Potential vs time waveform and
(b) cyclic voltammogram for a solution that is
6.0 mM in K3Fe(CN)6 and 1.0M in KNO3.
Fe(C5H5)2
5.375mM (left) and 0.5375 (right) mM Ferrocene in Acetonitrile
Cyclic voltammogram of the insecticide
parathion in 0.5 M pH 5 sodium acetate
buffer in 50% ethanol.
a) Structure of C60 (buckminsterfullerene),
b) Cyclic voltammetry
c) Differential pulse polarography
Stripping analysis
A small fraction of analyte from a dilute solution is first electrically deposited
in a single drop of mercury by electroreduction. This amalgam forming
preconcentration step requires approximately 60 seconds. The electroactive
species is then stripped from the mercury drop by making the potential more
positive and oxidizing the species back into solution. The current measured
during the oxidation is related to the quantity of analyte that was initially
deposited.
The customary setup for stripping analysis involves a hanging-drop electrode.
Stripping analysis is the most sensitive of polarographic techniques ( detection
below nM)
M+n + ne + Hg
→
← M(Hg)
Apparatus for stripping analysis.
accumulation
Cd
http://www.siue.edu/~michsha/c549/sld010.htm
(a) Excitation signal for stripping determination of Cd2+ and Cu2+,
(b) Voltammogram.
Differential pulse anodic stripping voltammogram of 25 ppm
zinc, cadmium, lead, and copper.
Differential pulse voltammogram for 5 ×10–10 M riboflavine.
Adsorptive preconcentration for 5 min (A), and 30 min (B) at –0.2 V.
Q
&
A
Thanks
Dong-Sun Lee / CAT / SWU