Chapter 23 Voltammetry 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 and his Polarograph 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 died in 1967. http://nobelprize.org/chemistry/laureates/1959/heyrovsky-bio.html Jaroslav Heyrovsky and his Polarograph http://www.cas.cz/aa/foto/heyrovs.htm

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 A + ne P → ← iI = kCA 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 (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 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 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 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 Chromatographic detection Determination of oxygen and glucose Detection of end point in volumetric titration 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 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  [C]o Ilkovic equation

A, a 1 M solution of HCl that is 5 ×10–4 M in Cd2+ 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) amalgams Xa+ + ne  X(a–n)+ 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 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 Standard addition Concentration Id Id Eapplied Current due to unknown plus standard addition Current due to unknown Concentration Eapplied Ilkovic equation Id = 607 n D1/2 C m2/3 t1/6 Id (unknown) = k C Id (unknown + standard ) = k Cx{Vx / (Vx+Vs)} + k Cs{Vs / (Vx+Vs)} Cx = Cs Vs / {R(Vx + Vs ) – Vx}

Internal standard Id Eapplied 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 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 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 (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 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 t1 → t2 : anodic 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 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. 64

Potential vs time waveform and 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 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