18b Voltammetry-electrochemical Techniques

Chapter 18b

Voltammetry: Dynamicelectrochemical techniques

Abul Hussam

18b.1 INTRODUCTION

Voltammetry is a part of the repertoire of dynamic electrochemicaltechniques for the study of redox (reduction–oxidation) reactionsthrough current–voltage relationships. Experimentally, the current re-sponse (i, the signal) is obtained by the applied voltage (E, the exci-tation) in a suitable electrochemical cell. Polarography is a special formof voltammetry where redox reactions are studied with a droppingmercury electrode (DME). Polarography was the first dynamic electro-chemical technique developed by J. Heyrovsky in 1922. He was awardedthe Nobel Prize in Chemistry for this discovery.

The characteristic shape of i–E curve depends on the nature of theredox couple in the condensed phase, its thermodynamics, kinetics,mass transfer, and on the voltage–time profile (E�t). In this section wewill discuss various voltammetric techniques and their applications inmodern chemistry.

With the introduction of modern electronics, inexpensive computers,and new materials there is a resurgence of voltammetric techniques invarious branches of science as evident in hundreds of new publications.Now, voltammetry can be performed with a nano-electrode for the de-tection of single molecular events [1], similar electrodes can be used tomonitor the activity of neurotransmitter in a single living cell in sub-nanoliter volume electrochemical cell [2], measurement of fast electrontransfer kinetics, trace metal analysis, etc. Voltammetric sensors arenow commonplace in gas sensors (home CO sensor), biomedical sensors(blood glucose meter), and detectors for liquid chromatography. Volt-ammetric sensors appear to be an ideal candidate for miniaturizationand mass production. This is evident in the development of lab-on-chip

Comprehensive Analytical Chemistry 47S. Ahuja and N. Jespersen (Eds)Volume 47 ISSN: 0166-526X DOI: 10.1016/S0166-526X(06)47028-8r 2006 Elsevier B.V. All rights reserved. 661

A. Hussam

technologies with applications ranging from capillary electrophoresischips with integrated electrochemical detection [3], chemical plumetracking [4], micro-electrophoresis system for explosive analysis [5],and clinical diagnostic devices [6–8]. There are hundreds of chemicaland biochemical electroactive species, which are amenable to the volt-ammetric detection system. Table 18b.1 shows selected applications inthree categories encompassing analytical, environmental, and biomedi-cal sensing applications. In biosensing, the p-PAP probe for immuno-assay is used for clinical diagnostic testing [9]. The redox active DNAtags could be potentially used for genotyping with significant clinicalpotential [10].

18b.2 TOOLS OF THE TRADE

All electrochemical reactions are carried out in a suitable cell withelectrodes connected to a programmable voltage or current source. Foranalytical work the dimension of the electrodes are in the range ofmicrometers to millimeters. Almost in all electrochemical studies, onealso needs an inert supporting electrolyte to carry most of the charges.The electrodes, their properties, and the working principle of the poten-tiostat is described below.

18b.2.1 Cells and electrodes

Any container or a flow system with three electrodes closely placed canbe used for electrochemical studies. Some electrochemical cells areshown in Fig. 18b.1. Most electrochemical cells contain three elec-trodes. These are the working electrode (W), counter electrode (C), andthe reference electrode (R). Table 18b.2 shows the materials and prop-erties of W, R, and C.

The working electrode (W) is the substrate on which the redox re-action takes place. Generally, working electrodes are made of platinum,gold, mercury, and carbon. Solid working electrodes come in two mostcommon shapes—button as planar electrodes and wire as cylindricalelectrodes. Metal and carbon fibers are also used to make dot-shapedultramicroelectrodes with few micrometers in diameter. Mercury is theclassical electrode for polarography. In polarography, a glass capillary isused to deliver the liquid mercury in drops known as the DME. This isthe only electrode where the surface of the electrode is renewed witheach new drop. Hanging mercury drop electrode (HMDE) and staticmercury drop electrode (SMDE) are also used but are less common.

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continued

TABLE 18b.1

Analytical applications of selected reactions studied by voltammetric techniques

Analyte Redox reactions Eo (V vs. ref) Applications andcomments

Probe molecules for general cell characterization (CA and CV, SWV)Fe(CN)6

3�Ferricyanide Fe(CN)63�+e�3Fe(CN)6

4�

0.200 V vs. Ag/AgCl Reversible redox couple to test cellperformance

R2Cp2Fe FerroceneR ¼ �H, �COOH

R2Cp2Fe3R2Cp2Fe++e� Cp:Cyclopentadiene

0.200 mV vs. Ag/AgCl

Reversible test candidatein non-aqueous solventsand a redox mediator

Toxic metals (ASV, SWASV)As H3AsO3+3H++3e�3As(s)+3H2O 0.247 V vs. SHE Trace inorganic arsenic

species in groundwaterand biological samples

Pb Pb (II)+2e�3Pb(s) �0.126 V vs. SHE Trace Pb (II) in water,blood, paint, etc.

Cd Cd (II)+2e�3Cd(s) �0.402 V vs. SHE Trace Cd (II) in water andbiological samples

Hg Hg (II)+2e�3Hg(s) 0.852 V vs. SHE Ultratrace Hg (II) in theenvironment (sub-ppb)

Gas sensing (CA)H2 (g) 2H++2e�3H2 (g) 0.0 Hydrogen gas sensorAsH3 (g) AsH3+4H2O3H3AsO4+8H++8e� 0.247 V vs. SHE Inorganic arsenic in the

environment andbiological sample throughhydride generation

Volta

mm

etry:

Dyn

am

icelectro

chem

ical

techn

iqu

es

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3

Table 18b.1 (continued )

Analyte Redox reactions Eo (V vs. ref) Applications andcomments

CO (g) CO(g)+H2O3CO2+2H++2e� 0.0 vs. Ag/AgCl Basis of CO gas sensor

Molecules for bio-sensing (CA, SWV)H2NC6H4OH p-aminophenol p-PAP

H2NC6H4OH3HN ¼ C6H4O+2H++2e� 0.25 V vs. Ag/AgCl Probe molecule for ELISAimmunoassay sensor forantigen and antibody

Glucose Glucose+O23Gluconic acid+H2O2

(needs glucose oxidase as catalyst)H2O23O2+2H++2e�

0.60 V vs. Ag/AgCl Blood glucose monitor.H2O2 is the redox species.O2 is often replaced by amediator molecule e.g.,Ferrocene

M(bpy)x2+

M ¼ Fe, Co, RuM(bpy)x

2+3M(bpy)x3++e�

X ¼ 2, 3, 5, 60.3–0.7 Vvs. Ag/AgCl

Reversible molecular tagfor DNA detection

CA—chronoamperometry, CV—cyclic voltammetry, SWV—square wave voltammetry, ASV—anodic stripping voltammetry, SHE—standardhydrogen electrode.

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4

(e)

(a)

(c)

(b)

(d)

Fig. 18b.1. Electrochemical cells and representative cell configurations.(a) Schematic diagram of a cell–potentiostat system. (b) Typical laboratorycell with Hg-drop electrode and drop knocker. (c) Voltammetric cell as detectorat the end of a high-performance liquid chromatographic column. (d) A two-electrode (graphite) chip cell for biosensor development. (e) Three-electrodechip cells on a ceramic substrate for bioanalytical work.

Voltammetry: Dynamic electrochemical techniques

Owing to the toxicity of mercury and its disposal problem, solid elec-trodes are now very popular. In particular, electrodes made of carbonsuch as glassy carbon, graphite, carbon paste, and carbon fibers havegained popularity. Mercury, gold, bismuth, and other metals can bedeposited as thin metal films on carbon and serves as thin metal filmelectrodes (TMFE) with excellent analytical advantages in trace metalanalysis. The choice of working electrode is determined by the redox

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TABLE 18b.2

Electrode materials and properties

Type of electrode Material Properties Comments

W: Working orindicator electrode

Pt—Platinum in 1 M sulfuric acid �0.25–1.2 V Ultramicroelectrodes aremade of metal or carbonfibers

Pt—Platinum in 1 M NaOH �1.0–0.6 VHg—Mercury in 1 M sulfuric acid �1.2–0.50 VHg—Mercury in 1 M KCl or NaOH �2.0–0.20 VC—Carbon in 1 M HclO4 �0.2–1.5 VC—Carbon in 0.1 M KCl �1.5–1.0 V(Glassy carbon, carbon paste, graphite,diamond film).

(shows effectivepotential window)

R: Referenceelectrode

Ag/AgCl, satd. KCl (SSC) 0.197 V vs. NHE At 25 1C. NHE—normalhydrogen electrodeHg (l)/Hg2Cl2 (s), satd. KCl (SCE) 0.241 V vs. NHE

Ag wire (Pseudo reference) 0.200 V vs. NHEC: Counter orauxiliary electrode

Platinum, carbon or steel Similar to theworking electrodeexcept for steel

Must be larger than Welectrode

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potential of the analyte and the potential window within which thesolvent and the supporting electrolyte remains electrochemically inert.Table 18b.2 shows the effective potential window for most commonworking electrodes.

Most common reference electrodes are silver–silver chloride (SSC),and saturated calomel electrode (SSC, which contains mercury). Thereference electrode should be placed near the working electrode so thatthe W-potential is accurately referred to the reference electrode. Thesereference electrodes contain concentrated NaCl or KCl solution as theinner electrolyte to maintain a constant composition. Errors in electrodepotentials are due to the loss of electrolytes or the plugging of the porousjunction at the tip of the reference electrode. Most problems in practicalvoltammetry arise from poor reference electrodes. To work with non-aqueous solvents such as acetonitrile, dimethylsulfoxide, propylene car-bonate, etc., the half-cell, Ag (s)/AgClO4 (0.1 M) in solvent//, is used.There are situations where a conventional reference electrode is notusable, then a silver wire can be used as a pseudo-reference electrode.

The counter electrode is the current carrying electrode and it mustbe inert and larger in dimension. Platinum wire or foil is the mostcommon counter electrode. For work with micro- or ultramicroelec-trode where the maximum current demand is of the order of few micro-amperes, the counter electrode is not necessary. At very low current, atwo-electrode system with the reference electrode can function as thecurrent-carrying electrode with very little change in the composition ofthe reference electrode. Many commercial glucose sensors and on-chipmicrocells have such electrode configuration.

18b.2.2 Instrumentation: Potentiostats

The instrumentation for voltammetry is relatively simple. With theadvent of analog operational amplifiers, personal computers, and inex-pensive data acquisition-control system, many computer-controlledelectrochemical systems are commercially available or custom made.Programming complex excitation waveforms and fast data acquisitionhave become a matter of software writing.

The function of a simple three-electrode system can be understoodfrom Fig. 18b.2a. The variable voltage source, Vs, is placed between theworking and the counter electrodes so that the electrochemical reactioncan take place on both electrodes at the applied excitation potential. Thecurrent response flowing during a redox reaction is monitored by thevoltmeter across a standard resistor, Rf. Since we are only interested in

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Fig. 18b.2. (a) A simple three-electrode system with a variable voltage source.(b) A potentiostat made of three operational amplifiers.

A. Hussam

the redox reactions taking place on the working electrode, a referenceelectrode is placed near the working electrode through a high-inputimpedance voltmeter so that accurate electrode potentials can be meas-ured. Note that all current is flowing between W–C, and none through R.

Figure 18b.2b shows a modern potentiostat made of three opera-tional amplifiers. Here, the excitation voltage from a computer-controlled digital-to-analog converter is fed into OP1 through aresistor. The output of OP1 is connected to the counter electrode.The reference electrode is connected through a follower OP2 in a feed-back configuration so that it does not draw any current at the summingpoint, S. The working electrode is connected to OP3, which is a simplecurrent-to-voltage converter. The output voltage, Eo, of OP3 is relatedto cell current, i ¼ Eo/Rf, through a feedback resistor, Rf. Since theworking electrode is at virtual ground, the reference electrode tracksthe W-potential at all times. The current through the cell is controlledby OP1 so that R is always at the applied potential, –Ea. Because W isgrounded, Em (vs. R) equals to Ea despite any change in cell resistance.A data acquisition system controlled by a computer can be used tomeasure Em, i, and time. Since modern high-performance operationalamplifiers are inexpensive the circuit above can be built with couple ofdollars. Such a system can be used to perform any voltammetric ex-periments with software programmed excitation and current sampling.

18b.3 ELECTRODE PROCESSES

There are three major processes responsible for the response of aworking electrode due to voltage excitation. These are electron transfer

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due to redox reaction, charging of the double-layer (an omnipresentcapacitor on the electrode), and mass transfer dynamics. The elec-tron transfer signifies the redox reaction, the rate of which is controlledby the excitation potential. This is the primary source of total cur-rent in the cell and is known as the faradaic current, if. The chargingand discharging of the double layer due to voltage excitation de-mand some current flow, which is known as the charging current, ic.Once the electrode reaction starts, the analyte is transported tothe electrode surface through diffusion, convection, and migration.Diffusion is the transport of species in a concentration gradient createdby the electron transfer reaction. Convection is the forced movement offluid by mechanical means such as stirring, pumped flow, ultra-sound, rotating electrode, etc. When charged species move underan electric field it is called migration. For most electroanalytical work,conditions are maintained to minimize the migration current by usinga large concentration (tenth of a molar or higher) of supporting elec-trolytes such as KCl, KClO4, strong acid or base, tetraethylammoniumperchlorate (in nonaqueous solvents), etc. The supporting elec-trolyte carries all the charges through ion transport in the electricfield. Thus the electroactive species present in small concentra-tions (mM or less) is not affected by migration effect. Therefore, thetotal current flow during an electrochemical reaction is a linearcombination of all the contributions: iTotal ¼ id+ic, where id is thediffusion current. This relation assumes that the faradaic currentis controlled by diffusion because the rate of electron transfer ismuch faster compared to diffusion. Let us look at the diffusion cur-rent, the faradaic current, and the charging current in a quantitativeway.

18b.3.1 Diffusion current and mass transfer

Consider the fast redox reaction on the electrode surface (i.e., fastheterogeneous charge transfer kinetics):

O þ ne� ¼ R (18b.1)

(e.g., O ¼ [Fe(CN)6]3� and R ¼ [Fe(CN)6]4�, Eo0

¼ 200 mV vs. SSC)For a reversible, fast reaction the net reaction rate, Vrxn, is control-

led by the rate at which analyte, O, reaches the surface by mass trans-fer, Vmt.

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Therefore,

V rxn ¼ Vmt ¼i

ðnFAÞðunit : mol s�1 cm�2Þ (18b.2)

where i is the measured current (Ampere ¼ C s�1), n the number ofelectrons involved in the redox reaction, A the area of the electrode(cm2), and F the Faraday constant (96,500 C mol�1). The rate is thesame as that of the flux, J, of species on the electrode surface. Let usnow consider that analyte, O, at mM concentration is reduced on asmall working electrode in a solution containing large concentration ofa supporting electrolyte (e.g., 1.0 M KCl). The electrochemical cell alsohas a stirrer or the working electrode can be rotated mechanically as ina rotating disc electrode. The function of the supporting electrolyte is tominimize the migration current and the stirrer produces a constantdiffusion layer thickness for O and R. We assume that only O is presentin the beginning. When a potential is applied and O is reduced to R onthe electrode surface, a concentration gradient is established near theworking electrode. This is shown in Fig. 18b.3. Knowing that the rate ofmass transfer is proportional to the concentration gradient (slope ofline 1) on the electrode surface and assuming a linear concentrationgradient, one can write

Vmt ¼i

ðnFAÞ¼ Do

C�

o � CoðX ¼ 0Þ� �

d(18b.3)

t=0, C0*

x

C

C0(x=0)

CR(x=0)

0d

Bulk solution

1

2

3CR

*=0

Fig. 18b.3. Ideal concentration gradients of O and R during a redox reaction onthe surface of the working electrode. The x-axis shows the distance from theelectrode surface. Bulk solution is stirred to maintain a constant diffusionlayer thickness, d.

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where Do is the proportionality constant which is same as the diffusioncoefficient (cm2/s) of O, C�

o the bulk concentration of O (mol/cm3), Co

(x ¼ 0) is the surface concentration of O on the electrode surface, and dthe diffusion layer thickness (cm). Assuming d is constant at a constantstirring rate, the term Do/d is a constant mass transfer coefficient,mo ¼ Do/d, in cm/s, which is same as that of the first-order hetero-geneous electron transfer rate constant. A similar equation can also bewritten for the concentration gradient for R (slope of line 3)

i

ðnFAÞ¼ DR

CRðX ¼ 0Þ � C�

R

� �d

(18b.4)

when the electrode potential reaches a value such that Coðx ¼ 0Þ � C�

o;the concentration gradient can be shown by line 2. Therefore,

il ¼ nFAmoC�

o (18b.5)

This is the limiting current where the mass transfer rate reaches amaximum value. The limiting current is achieved at a potential when Ois reduced to R as fast as O reaches the electrode surface. Assumingthere is no R in the beginning (C�

R ¼ 0), and combining Eqs. (18b.3),(18b.4), and (18b.5), the surface concentrations of O and R can be de-rived as

CoðX ¼ 0Þ ¼il � i

ðnFAmoÞ(18b.6)

CRðX ¼ 0Þ ¼i

ðnFAmRÞ(18b.7)

For fast electron transfer kinetics, the surface concentrations of Oand R are at dynamic equilibrium and assumed to obey the Nernst law

E ¼ E0o þ

RT

nF

� �ln

CoðX ¼ 0Þ

CRðX ¼ 0Þ

� �(18b.8)

Substituting Eqs. (18b.6) and (18b.7) in Eq. (18b.8), one gets the re-lation between current and voltage known as the steady-state volt-ammogram (Fig. 18b.4a)

E ¼ E0o þ

RT

nF

� �ln

mR

mo

� �þ

RT

nF

� �ln

ili� 1

� �(18b.9)

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ilcilc

ila

i i

E vs. ref E vs. refE1/2

0

Reduction

Oxidation

0

(a) (b)

Fig. 18b.4. Steady-state voltammograms when initially (a) O is present and(b) both O and R are present.

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In Eq. (18b.9), when i ¼ il/2, the third term becomes zero and Ebecomes independent of concentrations. The corresponding potential,

E ¼ E1=2 ¼ E0o þ

RT

nF

� �ln

mR

mo

� �(18b.10a)

known as the half-wave potential is characteristic of the system. Forsmall molecules the mass transfer coefficients (mo, mR) are similar andE1/2 ¼ Eo

0

, the formal potential. The limiting or plateau current of asteady-state voltammogram is directly proportional to the bulk con-centration of O as shown in Eq. (18b.5). Experimentally, a steady-statevoltammogram can be obtained by scanning the potential starting fromthe nonfaradaic region (region of no electron transfer reaction) to sev-eral hundred millivolts more negative past the formal potential. Steady-state voltammogram can also be obtained under experimental condi-tions where the diffusion layer thickness changes very little over thetime of the experiment i.e., when mass transfer coefficients remainalmost unchanged. Such conditions can be achieved at a very slow scanrate or with ultramicroelectrode, in some pulse techniques, and in flowsystems.

When both O and R are present as soluble electroactive species,Eq. (18b.9) can be rewritten as

E ¼ E0o þ

RT

nF

� �ln

mR

mo

� �þ

RT

nF

� �ln

ðilc � iÞ

ði� ilaÞ

� �(18b.10b)

Here, we distinguish cathodic and anodic limiting currents as ilc and ila,respectively. The cathodic current being positive and the anodic current

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being negative are conventions. Figure 18b.4b shows i–E curve withlimiting currents and the location of the formal potential at zero.

18b.3.2 Faradaic current: Electron transfer kinetics

Consider the same general redox reaction,

O þ e� ¼ R

is taking place on an electrode at a potential E. Assuming this reactionis entirely controlled by the kinetics of electron transfer, the net cur-rent flow is the difference between cathodic and anodic currents

i ¼ if � ia ¼ nFA kfCoðX ¼ 0; tÞ � kbCRðx ¼ 0; tÞ� �

(18b.11)

where, kf and kb are the forward and reverse rate constants (unit: cm/s).Based on activation energies for anodic and cathodic processes and

when the bulk concentrations of O and R are equal, one can write

kf ¼ ko expð�af ðE� Eo0ÞÞ and kb ¼ ko expðð1 � aÞf ðE� Eo0

ÞÞ (18b.12)

Combining Eqs. (18b.11) and (18b.12) one can write the familiarButler–Volmer relation between current, potential, and the kineticparameters

i ¼ FAko CoðX ¼ 0; tÞ expð�af ðE� Eo0ÞÞ

h�CRðX ¼ 0; tÞ expðð1 � aÞf ðE� Eo0

ÞÞ

ið18b:13Þ

where, ko is the standard heterogeneous electron transfer rate constant,Co(x ¼ 0,t) and CR(x ¼ 0,t) are the surface concentrations of O and R atany time t, a is the transfer coefficient which is a measure of the sym-metry of the activation energy barrier, and f ¼ F/RT. Eq. (18b.13)shows that both thermodynamic and kinetic information can be ob-tained from a voltammogram. A large ko indicates the system comes toequilibrium faster and a small ko indicates a slow equilibration. Thevalue of ko can range 10–10�9 cm/s. It is found that for a steady-stateexperiment in an unstirred solution ko40.02 cm/s leads to a reversiblei–E curve and koo0.005 cm/s to an irreversible i–E curve. Eq. (18b.12)shows that kf and kb can be made very large by changing the potentialof the working electrode even if ko is small. When the system reachesequilibrium (i ¼ 0), Eq. (18b.13) reduces to the Nernst equation(Eq. (18b.8)). Note that for fast reaction the current is controlled by

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the nature of the mass transport and diffusion. However, in most realsituations both kinetics and diffusion have to be considered together.

18b.3.3 Charging current: Effect of double layer

The electrode solution interface for an ideal polarizable electrode (IPE)has been shown to behave like a capacitor. The IPE is the metal so-lution interface where no charge transfer can occur when a potential isapplied across the interface. Generally, most working electrode mate-rials work like an IPE within a couple of voltage range. Figure 18b.5ashows the model of such a capacitor. At a fixed potential the metal sidewill assume a charge, which requires the assembly of opposite chargesin the solution side. During this process there flows a current called thecharging current. The transient current flows whenever the potentialacross this capacitor is changed. The origin of the capacitor is the com-pact inner layer of the water dipole as the dielectric medium separatingan outer diffused charged layer. This model is called the double layer.Typical capacity of such a double-layer capacitor, Cd, is in the range of10–100 mF/cm2.

Figure 18b.5b shows the equivalent circuit of the metal solutioninterface composed of Cd and the solution resistance Rs. When a voltagepulse, E, is applied across such a RC circuit, the transient current flow

Fig. 18b.5. (a) The capacitor-like metal solution interface, the double layer.(b) The equivalent circuit with solution resistance and overall double-layercapacitor. (c) Charging current transient resulting from a step-potential atan IPE.

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due to the charging of the capacitor is given by

ic ¼E

Rs

� �exp

�t

RsCd

� �(18b.14)

The charging current decays exponentially with a time constantt ¼ RsCd. Therefore, it is necessary to reduce RsCd as fast as possibleespecially for fast voltammetry. At 3t, the charging current decays 95%of its initial value. For example, if Rs ¼ 10O and Cd ¼ 20mF/cm2,t ¼ 200 mS i.e., the double-layer capacitor is discharged by 95% at600 mS. The time constant can be reduced by increasing the supportingelectrolyte concentration. Cd is also directly proportional to the area ofthe electrode, thus by using an ultramicroelectrode (diameter 50m orless), one can drastically reduce the capacitance background and in-crease the response time of the cell. This is one of the most importantadvantages of ultramicroelectrode. However, some ic is always presentin a voltammetric experiment and regarded as the source of back-ground signal. Since the measured current in a voltammetric experi-ment is the sum of faradaic and charging current, in order to increasethe sensitivity of the technique the ratio of faradaic/charging currenthas to be increased. This is precisely the goal of various pulse voltam-metric techniques. Charging current is also a weak function of electrodepotential that can be reduced by differential current sampling at nearlythe same potential.

18b.4 TECHNIQUES AND APPLICATIONS

The following section describes various voltammetric techniques andapplications. In most cases we assume a reversible fast reaction anddiffusion as the rate-controlling step.

18b.4.1 Chronoamperometry: The simplest experiment

The understanding of the nature of transient current after the impo-sition of a potential pulse is fundamental to the development of volt-ammetry and its analytical applications. Consider the same reaction,O+ne� ¼ R, taking place in a quiet solution at a potential such thatthe reaction is diffusion controlled. Figure 18b.6a shows the pulse andFig. 18b.6b shows the concentration gradient O as a function of timeand distance from the electrode surface.

Since the solution is quiet, and the surface concentration is Co

(x ¼ 0) ¼ 0, the reduction reaction can only continue through expansion

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Diffusion

Mixed

Kinetic

Co*(t=0)

Diffusion

Time Time

iCE

0 Xd0

t1 t2

t3

(a) (b) (c)

M

K

Fig. 18b.6. (a) Shape of the voltage pulses for diffusion control, mixed diffu-sion-kinetic control, and kinetic control, (b) concentration gradient of Oshowing expansion of the diffusion layer with time for complete diffusioncontrolled reaction, and (c) current transients show diffusion controlled,mixed kinetics and diffusion control, and complete kinetics controlled reac-tions corresponding to voltage pulses shown in (a). Note that the equations arederived only for the diffusion controlled case.

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of the diffusion layer thickness, d with time. For a planar electrode withlinear diffusion,

d ¼ pDtð Þ1=2 (18b.15)

Combining Eqs. (18b.3) and (18b.15) one gets

it ¼ðnFAD1=2

o C�

oÞ

ptð Þ1=2

(18b.16)

This is known as the Cottrell equation. It shows that the faradaic tran-sient current, it, decays t�1/2. In contrast, the capacitance current decaysexponentially and much faster. According to Eq. (18b.16) a plot of it vs.t�1/2 is a straight line, the slope of which can be used to calculate the D ofthe analyte if the area of the electrode is known. Eq. (18b.16) is also usedto measure the active area of an electrode by using species with knownD. At a spherical electrode (such as HMDE) of radius, r, the Cottrellequation has an added spherical term

it ¼ ðnFADoC�

oÞ1

Dptð Þ1=2

þ1

r

" #(18b.17)

At short t the first term dominates and at longer t, it becomesindependent of t. This is the limiting current. Also, when ro50m, it

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Fig. 18b.7. (a) Chronoamperogram showing the response due to a triple pulse500–0–500 with a 3 mm diameter glassy carbon working electrode in 2.0 mMPotassium Ferricyanide in 0.1 M KCl. No current was recorded for the initialpotential, 500 mV, where no faradaic reduction took place. (b) The same so-lution, except with a 10 mm diameter Pt working electrode. Current was re-corded for the initial potential at 500 mV for 0–4000 ms where no faradaicreduction took place. Note the magnitude of current scale.


becomes independent of t. This happens with ultramicroelectrodeswhere the steady-state current dominates. Figure 18b.7a shows thechronoamperogram for the reduction (shown as the negative current) of2.0 mM potassium ferricyanide in 0.1 M KCl after application of pulse500–0 mV vs. SSC. The positive going current is the oxidation of theferrocyanide to ferricyanide due to a reverse pulse 0–500 mV. Both showa high-current (200mA) spike at shorter time and the limiting it ¼ 15mAat longer time. With a 3 mm diameter glassy carbon working electrode,the rate of analyte diffusion is slow enough to catch the reduced speciesto be oxidized by the reverse pulse.

Figure 18b.7b shows the response for the same solution, exceptwith a 10mm diameter Pt working electrode. Here, the pulse is500–0–500 mV vs. SSC. The electrode shows a steady-state responseof 1500 pA for the reduction of ferricyanide and near-zero response forthe oxidation of ferrocyanide after the application of the reverse pulse.The loss of reduced species from the vicinity of an ultramicroelectrodeis significant due to the spherical nature of diffusion. The current dur-ing the reverse pulse can therefore be regarded as the charging currentof the double layer. Limiting and steady-state current measurement isthe basis for electrochemical detection of species in flow systems such asflow-injection analysis, electrochemical detector for liquid chromato-graphy, capillary zone electrophoresis (CZE), gas sensors, blood glucosemeasurements, etc.

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A. Hussam

18b.4.2 Linear and cyclic voltammetry

Linear and cyclic voltage sweep techniques are the most popularvoltammetric techniques due to their ease of use for diagnosis of var-ious redox reactions and their chemical complications. In a linear scanvoltammetry (LSV), the electrode potential is changed linearly at afixed scan rate from an initial potential, E1, where no faradaic reactiontakes place to a potential, Ev, where the reaction is diffusion controlledin a quiet solution. In cyclic voltammetry (CV), the potential is scannedback to a final potential (Ef), which is often the initial potential from Ev.Figure 18b.8 shows the process and the resulting voltammogram. Sincemodern instruments use digital-to-analog converters the scan ramp iscomposed of small voltage steps (2–5 mV) to approximate a linear ramp.Current is sampled at fixed intervals on the step-pulse to constructthe voltammogram. Generally, current sampling is delayed to theend of the step to minimize the charging current and increase thefaradaic/charging ratio. This is not possible with conventional analoginstruments.

The basic shape of LSV and CV, a peak-shaped voltammogram canbe explained as follows. For a reduction reaction, O+ne� ¼ R, when thevoltage is made more negative the surface concentration of O starts todecrease and thus increases the concentration gradient and the reduc-tion current starts to rise. Eventually, at more negative voltage thesurface concentration reaches zero and diffusion cannot deliver O tothe surface at the same rate. This results in decreasing current and a

Fig. 18b.8. (a) Potential–time profile for CV, (b) Definition of parameters in aCV, (c) CV of a [Fe(CN)6]3�/[Fe(CN)6]4� redox couple at 10, 50, and 100 mV/sscan rate. Ei ¼ 600 mV, Ev ¼ �300 mV, and Ef ¼ 600 mV vs. SSC. Note thatreduction is shown as negative current.

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peak-shaped i–E curve. In a quiet solution, only at a very slow scan rate(e.g., o5 mV/s), the concentration gradient remains low and unchangedwhich results in a steady-state current plateau. With a reversible sys-tem, when the scan is reversed, R accumulated near the electrode isoxidized back to O. The current on the reverse scan shows a peakedresponse with opposite sign. Note that the charge associated with theanodic process (oxidation current) is lower than the forward reductionprocess, due to the loss of accumulated R from the electrode surface bydiffusion. Figure 18b.8c also shows that peak current increases withincrease in scan rate. This is due to the faster rise in concentrationgradient in a shorter time.

For a reversible system the peak current at 25 1C is given by theRandles–Sevcik equation

ipc ¼ �ð2:69 � 105Þ n3=2A D1=2

o C�

o s1=2 (18b.18)

where, ipc cathodic (reduction) peak current and s the scan rate (V/s).All other terms are defined earlier. Eq. (18b.18) shows that the peakcurrent is directly proportional to the bulk concentration of analyte andincreases with the square-root of scan rate, s. Table 18b.3 lists thediagnostic criteria for reversible quasi-reversible and irreversible reac-tions. These criteria should be tested with CV performed over a widerange of scan rates (at least 10 to 1000 mV/s). One should realize that asthe scan rate increases the charging current starts to dominate over thefaradaic current and distorts the CV from which no meaningful mea-surements can be made. This limitation can be overcome by usingultramicroelectrodes where, at high scan rate, the faradaic current in-creases more than the charging current. Scan rates excess of 100 V/swere used to study fast electron transfer reactions and reactive inter-mediates with ultramicroelectrodes.

An irreversible reaction shows no oxidation (anodic peak) and thekinetic parameters (acn

0 and ko) can be obtained from the shift in peakpotentials as a function of scan rate

Epc ¼ Eo0�

RT

ðacn0FÞ

� �0:780 þ ln

D1=2o

ko

!þ 0:5 ln

ðacn0FsÞ

ðRTÞ

� �" #(18b.19)

In practice, the majority of redox reactions behave more like a quasi-reversible system. It is also common that a reaction that behaves re-versibly at low scan rate becomes irreversible at high scan rate passingthrough a quasi-reversible region.

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TABLE 18b.3

Diagnostic tests and quantitative criteria for cyclic voltammograms ofreversible and irreversible redox reactions at 251C

Reversible Quasi-reversible Irreversible

DEp ¼ |Epa–Epc| ¼ 59/n mV

DEp459/n mV andincreases with s

No reverse peak

|ipa/ipc| ¼ 1 |ipa/ipc| ¼ 1 whentransfercoefficient ¼ 0.5

ipc ¼ �(2.99� 105) n(acn

0)1/2 A Do1/2 Co

* s1/2

ip vs. s1/2 is a straightline

ip vs. s1/2 is a straight line ip vs. s1/2 is a straightline

Epc and Epa- independentof s

Epc shifts negativelywith increasing s

Epc shifted by�30(acna) mV for each10-fold increase in s

i N t�1/2 beyond Ep

O and R must be stableand kinetic of electrontransfer is fast

O and R must be stablewith no follow-upreactions

Definition of symbols: DEp ¼ peak potential difference, Epa ¼ peak potential at cathodic peak cur-rent, Epc ¼ peak potential at anodic peak current, ipa ¼ anodic peak current, ipc ¼ cathodic peakcurrent, s ¼ scan rate, t ¼ time after peak (the Cottrell region), n ¼ number of electrons involvedin redox reaction. Rate parameters (acn

0) and heterogeneous rate constant can be found fromirreversible wave.

A. Hussam

CV is extensively used for the study of multi-electron transfer re-actions, adsorbed species on the electrode surface, coupled chemicalreactions, catalysis, etc. Figure 18b.9 shows some of the examples.

18b.4.3 Polarography

Polarography is the classical name for LSV with a DME. With DME asthe working electrode, the surface area increases until the drop falls off.This process produces an oscillating current synchronized with thegrowth of the Hg-drop. A typical polarogram is shown in Fig. 18b.10a.The plateau current (limiting diffusion current as discussed earlier) isgiven by the Ilkovic equation

id ¼ 708 n D1=2o m2=3 t1=6 C�

o (18b.20)

where, m is the flow rate of Hg-drop (mg/s) and t is the drop time (s).All other parameters have their usual meanings. While the faradaic

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Fig. 18b.9. Example cyclic voltammograms due to (a) multi-electron transferredox reaction: two-step reduction of methyl viologen MV2++e� ¼

MV++e� ¼ MV. (b) ferrocene confined as covalently attached surface-modi-fied electroactive species—peaks show no diffusion tail, (c) follow-up chemicalreaction: A and C are electroactive, C is produced from B through irreversiblechemical conversion of B, and (d) electrocatalysis of hydrogen peroxide de-composition by phosphomolybdic acid adsorbed on a graphite electrode.


current increases with the area of the DME, the charging current in-creases with rate of increase in area (dA/dt) that decreases as the drop-size increases. Therefore, charging current decays with drop growthtime, t.

ic;t ¼ 0:00567 ðEz � EÞ m2=3 t�1=3 Ci (18b.21)

where, Ez is potential of zero charge for Hg (a constant value in a specificmedium), E is the applied potential and Ci is the integral capacity of thedouble layer (in mF/cm2). Figure 18b.10b shows the faradaic and char-ging current transients for the life of a drop. It shows that currentsampling near the end of drop-life can maximize faradaic/charging thus

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Faradaic

Charging

t

i

Fig. 18b.10. (a) Polarogram showing reduction Cd(II) in solution, (b) compar-ison of faradaic and charging current transients at a DME.

A. Hussam

the detection sensitivity. This is precisely done in Tast polarography andother pulse polarographic techniques. Note that, like other techniques,id is proportional to the bulk concentration of analyte.

18b.4.4 Pulse voltammetry

The realization that current sampling on a step pulse can increase thedetection sensitivity by increasing the faradaic/charging ratio is thebasis for the development of various pulse voltammetric (or polaro-graphic) techniques. Also, the pulses can be applied when it is necessaryand can reduce the effect of diffusion on the analyte. Figure 18b.11shows the waveform and response for three commonly used pulse volt-ammetric techniques: normal pulse voltammetry (NPV), differentialpulse voltammetry (DPV), and square-wave voltammetry (SWV).

In NPV, the electrode is held at an initial potential, Ei, where nofaradaic current flows. After a fixed delay, td (100–5000 ms), a potentialpulse, DE (2–20 mV) is applied for pulse duration, tp (20–50 ms). Thefaradaic current flows only during this short time. The current is thensampled near the end of this pulse for few milliseconds and averaged.The pulse returns to the initial potential and the next pulse is increasedby few millivolts to increase DE. This process continues until DEreaches the potential where the reaction is diffusion controlled. Theresult is a sigmoid-shaped current response. In NPV, the pulse timing issuch that the concentration gradient is large and the current is sampledon the pulse when the capacitance background decreases (within1–10 ms) but not eliminated. After each current sampling the initial

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Fig. 18b.11. Figures show the pulse waveform and response for three tech-niques: (a) normal pulse voltammetry (NPV), (b) differential pulse voltam-metry (DPV), and (c) square-wave voltammetry (SWV).


condition of the electrode is re-established by returning to the initialpotential for a longer delay, td. In NPV, the maximum diffusion currentis given by the equation

id;max ¼ðnFA D1=2

o C�

oÞ

ðptsÞ1=2

(18b.22a)

where ts is the sampling time after the application of the pulse. Clearly,the increase in sensitivity is due to the short pulse duration(ts ¼ 20–50 ms) compared to seconds in classical polarographic or volt-ammetric experiments. NPV can be easily used to measure 10�5 Manalyte with a detection limit of 10–6 M. For a reversible redox couple,one can easily record the oxidation current of the reduced species bysampling the current after the pulse returns to the base potential. Thisis a form of reverse pulse voltammetry (RPV).

Detection sensitivity better than that in NPV can be achieved bydifferential current sampling with small pulses. Figure 18b.11b showsthis scheme known as differential pulse voltammetry (DPV). Here, thepotential is changed from an initial potential in small steps (2–5 mV)and a voltage pulse of a short duration (50 ms) is superimposed at theend of a long step (500–5000 ms). The current is sampled beforethe beginning of the pulse and near the end of the pulse as shown inFig. 18b.12b. In differential pulse polarography (DPP), this is near the

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Pb

Cd

Cu

(a)

td

t

Oxidation scanEd

tq-E

(b)

Fig. 18b.12. (a) Voltage–time profile for anodic stripping voltammetry(ASV) and (b) ASV of an unknown solution with two aliquot additions of100 ppb each of Cd and Pb in the final solution. The peak at �190 mV is that ofCu present in the unknown. Experimental conditions: Initial deposition po-tential, Ed ¼ �800 mV, final potential ¼ 0, deposition time, td ¼ 120 s, quitetime, tq ¼ 30 s, step potential ¼ 5 mV, pulse height ¼ 20 mV, pulsedelay ¼ 100 ms, sampling width ¼ 17 ms, and sampling frequency ¼ 6000 Hz.

A. Hussam

end of the drop-time. The signal is the difference between these twocurrent values, which is recorded as a function of the increased steppotentials. The current difference shows a peak-shaped voltammogram,which is easy to recognize as a signal peak. The peak appears only in thepotential region (near formal potential) because a small potentialdifference (pulse height) makes a significant change in current flow.Thus, the sensitivity of DPV can be increased by increasing the voltagepulse, however, with some decrease in resolution. The peak differentialcurrent and the peak potential is given by

Dipeak ¼nFAD1=2

o C�

oZ

ðptsÞ1=2

(18b.22b)

Z ¼1 � exp

nFEpulse

2RT

� �h i1 þ exp

nFEpulse

2RT

� �h i (18b.23)

Epeak ¼ E1=2 �Epulse

2(18b.24)

where, Epulse is the pulse height. As Epulse increases Z approaches unityand Dipeak reaches the id,max of NPV. The advantage of DPV is the

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reduction in capacitance current realized by taking the current differ-ence over a small potential change, while in NPV the charging currentincreases as Epulse increases. The increase in faradaic/charging ratio ismanifested in a lower detection limit of 10–8 M of redox species for areversible couple in a suitable supporting electrolyte.

Square wave voltammetry (SWV) is a variation of DPV, where apure square wave is superimposed on a staircase ramp as shown inFig. 18b.11c. Like DPV, here the current is sampled at the end of theforward and reverse pulses and subtracted. The current differenceplotted as a function of increased ramp potential yields the peak-shapedsquare wave voltammogram. In SWV, the square wave frequency canbe increased to an effective scan rate of 1 V/s (200 Hz at 5 mV incre-ment) compared to 50 mV/s in DPV or NPV, yet retaining similar ad-vantages as DPV. Many slow reactions such as the irreversiblereduction of dissolved oxygen produce very little faradaic responsewith fast SWV scan. Thus, the removal of oxygen is not necessary toobtain good response for many species in the reduction region. This is aclear advantage for the development of field analytical methods wheresolution de-aeration is difficult. Due to the fast response, SWV was usedto measure the entire voltammogram of species on-the-fly in a flowsystem such as LC electrochemical detectors.

Variations of the three-pulse techniques were developed by choosingcurrent sampling points to further minimize the effect of capacitancebackground and to deal with irreversible reactions. These can be foundin modern electrochemical literatures.

18b.4.5 Stripping voltammetry

Stripping voltammetry or stripping analysis has a special place inelectrochemistry because of its extensive application in trace metalanalysis. Stripping voltammetry (SV) is a two-step process as shownschematically in Fig. 18b.12. In the first step, the metal ion is reducedto metal on a mercury electrode (thin mercury film on glassy carbon ora HMDE) as amalgam.

Mnþþ ne�3MðHgÞ (18b.25a)

The deposition is performed at a potential where the current is diffu-sion controlled at a steady state. The steady-state diffusion is main-tained for a deposition time, td, by stirring the solution, usually with amagnetic stirrer. At the end of the deposition time, the stirrer is turnedoff for a quiet time, tq (about 30 s) while the deposition potential is held.

685

A. Hussam

After tq is passed, the second step starts by scanning the potential fromEd to a potential when all the deposited metals are re-oxidized (thereverse of reaction 25). The oxidation current recorded as a function ofpotential is the anodic stripping voltammogram (ASV). A typical ASV ofthree metals (Cd, Pb, and Cu) deposited on a mercury film electrode isshown in Fig. 18b.12b. The sensitivity of ASV can be improved by in-creasing the deposition time and by using the pulse technique to recordthe oxidation current. ASV in Fig. 18b.12b was obtained by using thesquare wave voltammetry. In most cases a simple linear or step ramp issufficient to measure sub-ppm level of metals in aqueous solution. Thepeak current of a linear scan ASV performed on a thin mercury filmelectrode is given by

ipeak

¼ ðn2 F2 slA C�

mÞ

ð2:7 RTÞ(18b.25b)

where, s is the scan rate, l is the mercury film thickness (few microns), Ais the area of the electrode, Cm

* is the concentration of reduced metal(mol/cm3), and n is the number of electrons involved in the oxidationreaction. Eq. (18b.25b) shows that the peak current is proportional tothe concentration of the deposited metal which in turn is proportional tothe bulk concentration of the analyte metal ion when all other para-meters remain unchanged. Amalgam-forming metals such as Pb, Cd, Cu,Zn, Bi, Sb, In, Tl, Ga, Au, As, Se, Te, etc., can be determined at parts-per-billion (ppb) concentration in aqueous solution. By using a gold-filmelectrode, ASV was used to measure ppb As(III) and As(V) in thousandsof groundwater samples in one of the largest environmental applications[11]. Speciation information such as these is difficult to obtain withother analytical techniques. Hand-held instruments employing the ASVprinciple are now available for the routine analysis of Pb in blood, andZn, Pb, Cd, and Cu in drinking water samples. Table 18b.4 shows a list ofASV methods validated and approved by various environmental andanalytical organizations such as EPA, AOAC, and ASTM.

While ASV is used to measure metal cations, cathodic strippingvoltammetry (CSV) can be used to measure some anions. Mercury isknown to dissolve as mercurous ion (Hg2

2+) at a potential more positivethan 220 mV vs. SSC. The Hg2

2+ forms insoluble salts with many anionssuch as halides, S2�, CN�, SCN�, SH�, etc.

2HgðlÞ þ 2A�3Hg2A2ðsÞ þ 2e� (18b.26)

In CSV, the anions are deposited as the Hg2A2 (s) salt on a mercuryelectrode in the deposition step and then reduced to Hg (l) in

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TABLE 18b.4

List of approved stripping voltammetric methods for the measurement ofmetal ions in environmental and test solutions

Approved stripping voltammetric electroanalytical methods

� American Society for Testing Materials: ASTM Method D3557-95: Cadmiumin water

� American Society for Testing Materials: ASTM Method D3559-95: Cadmiumin water

� EPA Method 7198: Hexavalent chromium in water (1986)� EPA Method 1001: Lead in drinking water by ASV (1999)� EPA Method 7063: Arsenic and selenium in sediment samples and extracts

by ASV� EPA Method 7412: Mercury in aqueous samples and extracts by ASV� AOAC Method 986.15: Arsenic, cadmium, lead, selenium, and zinc in human

and pet foods (1988)� AOAC Method 982.23: Cadmium and lead in food (not for fats and oils)

(1988)� AOAC Method 974.13: Lead in evaporated milk (1976)� AOAC Method 979.17: Lead in evaporated milk and fruit juice (1984)� NIOSH Method 7701: Lead in air by ultrasound/ASV (1999)� Method 3130: Metals by anodic stripping voltammetry (proposed by

American Public Health Association, American Water Works Association,and the Water Environment Federation, 1995)


the stripping step. CSV is not limited to Hg electrode. Any electrodeforming insoluble salt with analyte anion that can be reversibly reducedto the metal can be used in CSV.

18b.5 FUTURE OUTLOOK

Topics discussed above are some basic principles and techniques involtammetry. Voltammetry in the frequency domain where i–E re-sponse is obtained at different frequencies from a single experimentknown as AC voltammetry or impedance spectroscopy is well estab-lished. The use of ultramicroelectrodes in scanning electrochemicalmicroscopy to scan surface redox sites is becoming useful in nano-research. There have been extensive efforts made to modify electrodeswith enzymes for biosensor development. Wherever an analyte under-goes a redox reaction, voltammetry can be used as the primary sensingtechnique. Microsensor design and development has recently received

687

A. Hussam

significant attention due to the new capabilities that micro-fabricationtechnology has made possible for the scientific sensor community. Inparticular, electrochemical sensors have attracted considerable atten-tion due to its low cost and less-complex fabrication process when com-pared to optical-based sensors. In a voltammetric sensor, both excitationand response signals are electrical and hence do not require any con-version devices. Whereas, an optical sensor requires an optical source,optical filters, optical detection, and conversion devices that ultimatelyadd to the fabrication complexity and its cost. For example, in CZE, amicrofabricated electrochemical cell (as detector) enables the lowestdetection limit (10 pM) with the highest sensitivity and a large lineardynamic range (41 million) compared to other detection techniques[12]. Electrochemical detection has also been demonstrated to be a vi-able method for identifying biomolecules in lab-on-chip platforms[13,14]. Electrochemical detection based on redox potential is selective,sensitive, and rapid, so the method can be used as a general analytedetection with minimal sample preparation.

REFERENCES

1 R. Schuster, V. Kirchner, X.H. Xia, A.M. Bittner and G. Ertl, Nanoscaleelectrochemistry, Phys. Rev., 80(25) (1998) 5599–5602.

2 C.D.T. Bratten, P.H. Cobbold and J.H. Cooper, Micromachining sensorsfor electrochemical measurements in subnanoliter volumes, Anal. Chem.,69 (1997) 253–256.

3 A.T. Wooley, K. Lao, A.N. Glazer and R.A. Mathias, Capillary elect-rophoresis chip with integrated electrochemical detection, Anal. Chem.,70 (1998) 684–688.

4 T. Kikas, H. Ishida and J. Janata, Chemical plume tracking. 3. Ascorbicacid: A biologically relevant marker, Anal. Chem., 74 (2002) 3605–3610.

5 A. Hilmi and J.H.T. Luong, Micromachined electrophoresis chip withelectrochemical detectors for analysis of explosive compounds in soil andgroundwater, Environ. Sci. Technol., 34 (2000) 3046–3050.

6 M. Eggers and D.A. Ehrlich, A review of microfabricate devices for genebased diagnostics, Hematol. Pathol., 9 (1995) 1–15.

7 P. Selvaganapathy, M.A. Burns, D.T. Burke and C.H. Mastrangelo, Inlineelectrochemical detection for capillary electrophoresis, IEEE, 451–454, 2001.

8 J. Lorraine, Which chip will be in your diagnostic device? R&D Magazine,pg. 33, December 2003.

9 T.T. Hua, C.E. Lunte, H.B. Halsall and W.H. Heineman, p-Aminophenylphosphate: An improved substrate for electrochemical enzyme immuno-assay, Anal. Chim. Acta, 214 (1988) 187–195.

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10 M. Rodriguez and and A.J. Bard, Electrochemical studies of the interac-tions of metal chelates with DNA, Anal. Chem., 62 (1990) 2658–2662.

11 S.B. Rasul, Z. Hossain, A.K.M. Munir, M. Alauddin, A.H. Khan andA. Hussam, Electrochemical measurement and speciation of inorganicarsenic in groundwater of Bangladesh, Talanta, 58(1) (2002) 33–43.

12 C. Vogt and G. L. Klunder, Fresenius J. Anal. Chem., 370 (2001) 316.13 J. Rossier and H. Girault, Enzyme linked immunoabsorbent assay on a

microchip with electrochemical detection, Lab on a Chip, 1 (2001) 153–157.14 A. Frey, M. Jenkner, M. Schienle, C. Paulus and B. Holtzapfl, Design of an

Integrated Potentiostat Circuit for CMOS Bio Sensor Chip, IEEE, 2003,pp. 9–12.

Suggested Reading and Resources

A. J. Bard and L. R. Faulkner, Electrochemical methods: Fundamentals andApplications, 2nd ed., Wiley Sons & Inc., New York, NY, 2002.B. B. Damaskin, The principle of current methods for the study of electro-chemical reactions, McGraw-Hill, NY, 1967.D. Pletcher, R. Greff, R. Peat, L.M. Peter and J. Robinson, Instrumentalmethods in electrochemistry, Horwood Publishing, England, 2001.A.M. Bond, W.N. Duffy, X.J. Guo, X. Jang and D. Elton, Changing the look ofvoltammetry—Can FT revolutionize voltammetric technique as it did for FTNMR?, Anal. Chem., 186A, May 1, 2005.

REVIEW QUESTIONS

1.
Explain the conditions under which a steady-state voltammogramis obtained.
2.
Explain ways to reduce the charging current over the faradaiccurrent.
3.
List the kind of information one can get from cyclic voltammetry. 4. Explain what is done in anodic stripping voltammetry. Why is
stripping the most sensitive voltammetric technique? What are itslimitations?

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18b Voltammetry-electrochemical Techniques

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