Cyclic Voltammetry

Cyclic voltammetry is the most extensively used technique for attaining qualitative information regarding electrochemical reactions. It suggests a swift location of redox potentials of the electroactive species [44]. In Cyclic voltammetry, the working electrode ?s potential is varied by time. The starting potential is set such that where no electrode reaction takes place and dragged to a potential where oxidation or reduction of the analyte takes place. When the potential has passed through one or more electrochemical reaction, the linear sweep of the voltammogram reversed its direction and in this way we can detect the products and intermediate species of the forward electrochemical reaction. A supporting electrolyte is used to inhibit movement of charged species of reactant and products. Figure1.14: Cyclicvoltammogram showing electrochemical parameters Where, Epa = anodic peak potential Ef = final potential Ipa = anodic peak current Ei = initial potential Ipc = cathodic peak current Epc = cathodic peak potential

Cyclic voltammetry has gained recognition due to very simple experimentation but a large number of data can be obtained by interpreting the voltammogram obtained from cyclic voltammetry [44]. It is a powerful tool for the determination of redox potentials, assessment of kinetics of electron transfer, revealing chemical reactions that pursue or precede the electrochemical reaction. 1.5.1Electrochemical reactions In electrochemical reaction, different electrodes with particular functions are placed simultaneously in an electrolyte solution that has an electroactive species. Different electrodes are arranged as given below ? Working electrode ? Reference electrode ? Auxiliary electrode Figure1.15: Electrochemical cell use in cyclovoltammetric measurements . Electrode Reactions In a characteristic electrode reaction, the transfer of charge (like electron) between an electrode and a species in solution takes place. The electrode reaction usually involves a series of steps which are given below: 1. There is movement of reactant species to the interface, termed as mass transport. 2. Now transfer of electron takes place by means of quantum mechanical tunneling between the reactant species and the electrodes. 3. The product leaves the electrode to permit fresh reactant for the reaction. Figure1.16: Diffusion of reactants to electrode

Electrochemically reactions are of three types depending on their electrochemical behavior . Reversible electrochemical reactions . Irreversible electrochemical reactions . Quasi-reversible electrochemical reactions 1.5.1.1 Reversible electrochemical reaction A redox reaction can be termed as reversible electrochemical reaction, if both species swiftly exchange electrons with the working electrode. This type of reaction can be recognized from a cyclic voltammogram by calculating the potential difference among the two peaks potential [45]. Figure1.17: Cyclicvoltammogram of reversible electrochemical reaction The given equation concerns to an electrochemically reversible system: Where n = No. of electrons involved in the electrochemical reaction Epa = potential of anodic peak Epc = potential of cathodic peak

The criteria which determine the reversibility of a reaction is given as under; The scan rate does not change the separation positions between the cathodic and anodic peak potentials, but somewhat dependent on the cycle number. Figure1.18: Cyclicvoltammogram of reversible electrochemicalreactionat different scan rate The difference between the anodic peak potential and cathodic peak potential is equals to or about to 0.058/n at 298 K. where n represents the transferred electrons during the electrochemical reaction. The peak current ratio (Ipa/Ipc ) is independent of scan rate and always equal to unity [46]. 1.5.1.2 Irreversible electrochemical reaction If there take place a very slow electron exchange between the working electrode and the redox specie then it is termed as irreversible electrochemical reaction. The separation of peak is greater than 0.058/n V and peak position depends on the scan rate. Figure1.19: Cyclicvoltammogram of irreversible reaction Following general reaction shows the process of irreversibility. O + ne ? R The criteria for the determination of the reversibility of a reaction are given as under: . As we change the scan rate, there is a shift in peak potential. Figure1.20: Cyclicvoltammogram of irreversible reaction at different scan rate . The ratio between anodic peak current and cathodic peak current is not equal to unity. . The value Epc-Epa/n is greater than 59/n mV. Peak width can be findby following equation. [Epc-Epa/2]=1.857(RT/anF) In this equation n represents the number of electrons transferred and ? represents the charge transfer coefficient[47]. 1.5.1.3 Quasi-reversible process Quasi-reversible process exhibits intermediate behavior between irreversible and reversible process. In this process, both mass transfer and current transfer control the process. Following figure [1.12 ] shows quasi-reversible process. Figure1.21: Cyclicvoltammogram of quasi-reversible electrochemical reaction Nernst equation is not applicable to quasi-reversible process because there is very slow electron transfer. Following equation is used to find out diffusion coefficient [48]. Ip= 2.99x105n(?n)1/2AC?D?1/2V1/2 Where A = the surface area of working electrode n = the number of electrons transfer during the electrochemical process D?= is the diffusion coefficient C? = is the concentration of analyte in mole dm-3 V = is the scan rate in volt per second.