Cyclic Voltammetry: Principle, Instrumentation & Applications

Voltammetry techniques - CV- Analysis

1. Cyclic Voltammetry Dr.J.Anuradha

2. ELECTRO ANALYTICAL TECHNIQUES Electro analytical chemistry, as the name implies, involves the analysis of chemical species through the use of electrochemical methods. Generally, we monitor alterations in the concentration of a chemical species by measuring changes in current in response to an applied voltage with respect to time. According to Faraday's law, the charge is directly proportional to the amount of species undergoing a loss (oxidation) or gain (reduction) of electrons. Q = n F e Current is the change in charge as a function of time. I = dQ / dt  

3. VOLTAMMETRY Voltammetry gives information about an  analyte  is obtained by measuring the current as the potential is varied Linear sweep voltammetry Staircase voltammetry Squarewave voltammetry Anodic stripping voltammetry   Cathodic stripping voltammetry   Adsorptive stripping voltammetry   Alternating current voltammetry Normal pulse voltammetry Differential pulse voltammetry Chronoamperometry Cyclic voltammetry  (CV) Types

4. WHY CV ? A wide range of current over potentials Fast technique (single scans typically recorded in every 100 ms ) Low signal to noise ratio . the product of the electron transfer reaction that occurred in the forward scan can be probed again in the reverse scan Implication: A single CV scan doesn’t tell you much, don’t over interpret it !

5. CYCLIC VOLTAMMETRY Cyclic Voltammetry is a type of potentiodynamic   electrochemical  measurement . In a cyclic voltammetry experiment the working electrode potential is ramped linearly versus time. Cyclic voltammetry takes the three electrode setup; when working electrode reaches a set potential, the ramp is inverted. This inversion can happen multiple times during a single experiment. The current at the  working electrode  is plotted versus the applied voltage to give the cyclic voltammogram trace .

6. Cyclic voltammetry is a very versatile electrochemical technique which allows to probe the mechanics of redox and transport properties of a system in solution. The potential is applied between the reference electrode and the working electrode and the current is measured between the working electrode and the counter electrode . In cyclic voltammetry , the direction of the potential is reversed at the end of the first scan. Thus, the waveform is usually of the form of an isosceles triangle. Gives The potential is scanned back and forth linearly with time between two extreme values – the switching potentials using triangular potential waveform

7. T he black box Reference Electrode: Defines “ ” potential for the cell. Use: Ag/ AgCl Working Electrode: Where the redox reaction action occurs Use : glassy carbon, platinum and gold. Counter / Auxiliary Electrode: Needed to complete circuit. Use a Pt wire

8. For an redox reaction induced at a working electrode, the rate determining step may be any one of the following individual step depending on the system: rate of mass transport of the electroactive species, rate of adsorption or desorption at the electrode surface, rate of the electron transfer between the electroactive species and the electrode, or rates of the individual chemical reactions which are part of the overall reaction scheme. For the oxidation reaction involving n electrons, The Nernst Equation gives the relationship between the potential and the concentrations of the oxidized and reduced form of the redox couple at equilibrium (at 298 K): where E is the applied potential and E ' the formal potential; [Ox] and [Red] represent surface concentrations at the electrode / solution interface, not bulk solution concentrations.

9. During the reducing scan, the surface concentration of species Ox progressively decreases , resulting in an increased concentration gradient and a larger current. As reduction continues , the concentration of Ox at the electrode surface is depleted, and the current peak will decay if fresh Ox from the bulk solution does not have enough time to diffuse to the electrode surface . When the direction of the potential scan is reversed, a peak resulting from the re-oxidation of reduced Red is observed . When a redox reaction is 100% reversible, the oxidation and reduction peak currents are equivalent, and the peak current is given by the relation: i p = 0.4463nF (nF/RT) 1/2 (D a ) 1/2 (V) 1/2 C a A where V is the potential scan rate in V/s and C a is analyte concentration in mol/cm 3 .

10. Cycle could be run again, many times.

11. 1. Number of charge (Q) The integrated area under each wave represents the charge Q associated with the reduction or oxidation of the adsorbed layer Q = n F A Γ n: number of electrons F: Faraday constant A: t he electrode surface area Γ: the surface coverage in moles of adsorbed molecules per surface area 2. Capacitance (C) I = vC The peak current is proportional to scan rate v, I cap : current v: scan rate C d : capacitance Cyclic voltammograms _ quantitative information

12. 3. Number of electrons (n) D E p = │ E pa - E pc │ = 90.6 / n 4. Surface coverage ( Γ ) I peak = n 2 F 2 vA Γ(4 RT ) - For a reversible electrode reaction at 25 °C, the difference in peak potentials, DE p is expected to be When the number of electrons is known, the surface coverage can be calculated by the equation:

13. The Baseline Issue Record i-t over i-V

14. 1. Study of Reaction Mechanisms APPLICATIONS Examples - Ligand exchange reactions as in iron porphyrin complexes - Oxidation of chlorpromazine to produce a radical cation and subsequent reaction with water to produce sulfoxide - Oxidation of dopamine in the presence of ascorbic acid - Electrochemical oxidation of aniline 2 . Quantitative Determination To evaluate electrochemical reversibility by looking at the difference between the peak potentials for the anodic and the cathodic scans. E p =  E p,a −  E p,c  = 0.05916 V /  n For example, for a two-electron reduction, we expect a ∆ E p  of approximately 29.6 mV. For an electrochemically irreversible reaction the value of ∆ E p  will be larger than expected. 3. Electrochemical Reversibility and Determination of  n Concentration of analyte, Number of electrons per molecule in analyte and Diffusion coefficient

15. 4 . Adsorption in cyclic voltammetry - Repetitive voltammograms for micromolar riboflavin at a HMDE note: peak separation is smaller than for solution phase couple Peak area also gives coverage Q = nFA , can be used to determine area occupied by molecule - can give orientational information - Ferrocene was irreversibly adsorbed in the form of a self-assembled monolayer to a Au surface using a long-chain alkylthiol . 

16. Cyclic sweeps are used to measure corrosion that proceeds at about the same rate all over the metals surface (uniform) and corrosion at discrete sites on the surface e.g. pitting crevice and stress corrosion cracking ( localised ). - To determine the degree of localised corrosion the amount of hysteresis between the positive going sweep and the negative going sweep is calculated. 5 . Corrosion studies in CV 1 mM K 3 Fe(CN) 6  in 0.1 M KCl solution red - 20 mV/s, gold - 50 mV/s, green - 100 mV/s, blue - 250 mV/s, black - 500 mV/s

17. CV can provide information about the chemistry of redox couples. This is perhaps the most attractive feature of this technique. For example, if a redox couple undergoes two sequential electron-transfer reactions, you will see two peaks in the voltammogram. If either the reduced or oxidized species is unstable, you might see a voltammogram such as that shown in Figure. Such qualitative information is very useful . Cyclic voltammogram of a system where the reduced species is unstable 6. Study of Reactions Mechanisms

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