16 Cyclic voltammetry

Dr. Heena Rekhi Rekhi and Neha Sharma

  1. Description

 

Cyclic voltammetry is an analytical technique which is highly efficient for obtaining qualitative information of electrochemical reactions. Cyclic voltammetry is a very popular technique for electrochemical studies, for obtaining information about fairly complicated electrode reactions. CV is infrequently used for quantitative analysis, it find wide applicability in study of oxidation/reduction reactions, the detection of reaction intermediates, and the observation of follow-up reactions of products formed at electrodes. In CV the applied potential is swept in first one direction and then the other while the current is measured. A CV experiment may use one full cycle, a partial cycle, or several cycles.

  1. Principle of Cyclic Voltammetry

 

A cyclic voltammogram is obtained by applying a linear potential sweep (that is, a potential that increases or decreases linearly with time) to the working electrode. As potential is applied below and above the formal potential, E°, of an analyte, current flows through the electrode as a result of either oxidation or reduction of analyte. The magnitude of this current is proportional to the concentration of the analyte in solution, which allows cyclic voltammetry to be used in an analytical determination of concentration. The instrumentation pertaining to process cyclic voltammetry comprises three-electrode assembly constituting working, reference and auxiliary electrode immersed in a test solution. The potentiostat applies and maintains the potential between the working and reference electrode while at the same time measuring the current at the working electrode due toflow of charge between auxillary electrode and working electrode. A plotter or a computer serves as a recording device for resulting cyclic voltammogram which is a plot of current versus potential.

  1. Fundamentals of Cyclic Voltammetry

 

Cyclic Voltammetry constitutes an electroanalytical techniques where measuring current as a function ofapplied potential gives information about in the analyte. Moreover cyclic voltammetry is used for various non-analytical techniques such as surface adsorption, fundamental studies on redox processes and electrode kinetics, electron transfer mechanism as well. CV is useful to obtain information concerning its physical and chemical properties. However, electroanalytical methods serve as a quantitative tool for the determination of diffusion coefficients, transfer numbers and oxidation potential. Moreover, CV can be combined with spectroscopy in situ to provide information concerning molecular structures and reaction mechanisms of transient electroactive species. Measurement of current while cycling the potential of an electrode, which is immersed in an unstirred solution is involved in cyclic voltammetry. The potential of this working electrode is controlled versus a reference electrode.

The potential axis is also a time axis that is related to scan rate. The excitation signal for CV is a linear potential scan with triangular waveform. On sweeping the potential of an electrode between two values often called switching potential is observed on triangular potential excitation signal. The current density is referred to as normalisation of measured current to the electrode surface. Plot of applied potential versus current density is referred to as cyclic voltammogram. Corresponding to every electrode reaction there exist unique value of potential for which a peak in the measure current can be observed.The electrode material, sweep rate and concentration of electrolyte affects the peak width and height of a particular electrochemical process.

 

To bring out an oxidation process, a positive potential is applied giving rise to an anodic peak current (ipa) which frequently gives an oxidation peak at known potential (Epa). To bring out a reduction process, a negative potential is applied giving rise to an cathodic peak current (ipc) which frequently gives a reduction peak at known potential (Epc).

 

Instrumentation

 

The most common planning to deal with electrochemical cell mainly consists of three different electrodes:

 

  • Working electrode (WE)
  • Reference electrode (RE)
  • Counter electrode (CE)

 

4.1.1. Working Electrode.

 

The ideal working electrode is very clean metal surface with a well defined geometry that is in direct contact with an electrochemical test solution. Working electrodes commonly employed are made from electrochemically inert metal. The most widely used metals are mercury, platinum, gold, and various forms of carbon. Solid metals are typically fashioned into disks surrounded by a chemically inert layer made up of teflon, glass, or epoxy. Mercury, being a liquid, tends to be used as a spherical droplet in contact with the solution. Voltammetric response of the electrode is affected by shape and size of the electrode. Surface area is directly proportional to the overall current observed at an electrode so disk shaped electrodes with diameters greater than 100 mm, or macro electrodes, generally produce easily measured currents in the microampere to milliampere range. Microelectrodes with dimensions less than 100 mm are typically produce currents in the pico to nanoampere range. However there are no specific electrochemical equipment are required to observe overall currents at microelectrodes. Following electrodes are used as working electrodes:

 

a) Mercury Electrode

b) Platinum Electrode

c) Gold Electrodes

d) Carbon Electrodes

e) Rotated Electrodes

 

4.1.2.   Reference Electrode

 

In the experiment concerning voltammetry the measurement of potential of a working electrode is always done with respect to standard electrode that is reference electrode. To build an electrochemical cell the reference electrode is the simplest cell used as a half cell. Half cell allows the potential of the to be determined alternate to standard hydrogen electrode. For this reason, a number of other reference electrodes have been developed. However in dealing with aqueous solution saturated calomel electrode (SCE) is used as a reference electrodes.

 

4.1.3. The Counter Electrode

 

Whenever the measurement is made the current is necessarily forced to flow through the reference electrode. Internal composition of a reference electrode is greatly affected on the flow of large amount of current through it resulting in potential drift from expected standard value. For this reasons, it is enviable to make electrochemical measurements without current flowing through the reference electrode. Modem three and four electrode potentiostat use a feedback circuit to prevent this from happening, but this feedback circuit requires that an additional auxiliary electrode be introduced into the electrochemical cell. The function of the auxillary electrode is to make sure that large amount of current flows through it so that only a small current flows through reference electrode. The auxiliary electrode can be prepared from any material using the desired geometry of an electrode.

 

Material constituting electrode should be chemically inert to the test solution and auxillary electrode should have large surface area because large amount of current can then flow through it. Mostly platinum wire is used, but stainless steel, aluminium or copper wire may work in non-corrosive solutions where metal cation interference is not a concern. However electrochemical cell can itself serve the purpose of auxillary electrode if it is made of metal. Because current flows at the auxiliary electrode, electrochemical processes willalso occur there. The auxiliary electrode must oxidize if the working electrode is reducing. Diffusion of products generated at auxillary electrode to the working electrodemay cause interference with the experimental measurement. When this trouble arises, the auxiliary electrode is positioned in a separate section containing an electrolyte solution that is in ionic contact with the main test solution via a glass frit.

Theory

 

In theory, any atom or molecule can be oxidized or reduced if enough energy canbe provided. However, the range of energies that can be applied is limited by theexperimental conditions. So we need only consider molecules that are said to beelectrochemically active. Application of reverse scan results in reoxidation of a species that underwent reduction during cathodicpolarization of working electrode in unstirred solution. Voltage scan rates has been correlated to cathodic and anodic peak currents and differences incathodic and anodic peak potentials rates has been donemathematically for various electrochemical reactions. However, no much differences in sweep rates are observed in cyclic and single sweep voltammetry.

 

The important parameters of a cyclic voltammogram are the magnitudes of anodic peak current, the cathodic peak current, the anodic peak potential andcathodic peak potential. Initially, the bulk solution contains only the reduced form (R) so that at potentials lower than the redox potential, i.e. the initial potential, there is no net conversion of R into O, the oxidised form (point A). Exponential increase in anodic current is observed on varying potential.As R is converted into O, concentration gradients are set up for both R and O, anddiffusion occurs down these concentration gradients. Instantaneous oxidation of R to O is observed at anodic peak. Therefore, the current now depends upon the rate of masstransfer to the electrode surface and so the time dependence is resulting in anasymmetric peak shape.

 

Applications of CV

 

Cyclic voltammogram is widely employed in organic and inorganic chemistry. CV has become a very popular technique for electrochemical studies and has proved as a sensitive tool for obtaining information about fairlycomplicated electrode reactions.

 

For the assessment of various kinetic and thermodynamic parameters such as entropy (S), change in number of electrons (n), heterogeneous rate constant (ko), Gibb’s free energy (G) and diffusion coefficient (Do) of a number of redox reactions and associated chemical reactions cyclic voltammetry has found an extensive applications. These methods are chiefly valuable in redox process and to study the multiple electron transfer in an electrochemical reaction.

 

CV techniques can be used for the estimation of a number of inorganic, organic andorganometallic compounds.

 

CV studies in rat brain, in vivo studies in animals, bacteriaand even plants are picking up.

 

With the introduction of newer electrode material of very small size, these methods of chemical analysis in living systems might grow even faster.

 

CV studies of solid and molten electrolyte might prove useful for trace analysis.

 

Background-subtracted cyclic voltammetry can be employed for measuring lowerconcentration.

 

Cyclic voltammetry finds applications in in-vivo monitoring in neurotransmitters. Rapid voltammetric measurements makes it suitable technique forthe detection of dynamic concentration changes in the micromolar range that occurs in the extracellular environment of the brain.

 

The good temporal and chemical resolution of such in-vivo cyclic voltammetric experiment offers improved understanding of the chemistry of the brain.

 

Cyclic Voltammetric detectors have large applications in chromatography. Industrial corrosion processes are being monitored using CV technique.

 

Voltammetric amazingly low detection limits are being used to monitor lead levels in thebloodstreams.

 

Electrodes coated with special polymers find use as glucose detectors fordiabetics.

 

Bibilography

  • R.L.McCreery, K.K.Kline “Laboratory Techniques in Electroanalytical Chemistry” 2nd Edition (P.T.Kissinger, and W.R.Heineman, New York Eds), 1995.
  • R.L.McCreery, “Electroanalytical Chemistry” Vol.17 (Dekker, New York Eds),1991.
  • Journal of Chemical Education, 702-706 (1983)
  • Journal of Chemical Education, 697-702 (1983)
  • M. Gomez, F. J. Gonzalez, and I. Gonzalez, Electroanalysis 15, 635 (2003).