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Introduction to electrochemical technique, Cyclic Voltammery.
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Objective

 

Introduction to electrochemical technique, Cyclic Voltammery, i.e., to study the ferri-ferrocyanide system by the Cyclic voltammetry.

 

Introduction

 

The goal of this experiment is to get familiar with:

    * To test the potentiostat using a dummy cell.
    * Using a modern electrochemical potentiostat to observe the reduction- oxidation of iron (III) to iron (II) and vice versa under the given potential range.
    * To measure the diffusion coefficient for iron (III).

It illustrates the effect of concentration and sweep rate on the current measured in a cyclic voltammetry.

 

Theory

 

Cyclic voltammetry is the most widely used technique for acquiring qualitative information about electrochemical reactions. The power of cyclic voltammetry results from its ability to rapidly provide considerable information on the thermodynamics of redox processes and the kinetics of heterogeneous electron-transfer reactions and on coupled chemical reactions or adsorption processes. Cyclic voltammetry is often the first experiment performed in an electroanalytical study. In particular, it offers a rapid location of redox potentials of the electroactive species and convenient evaluation of the effect of media upon the redox process.
 

In typical cyclic voltammetry, a solution component is electrolyzed (oxidized or reduced) by placing the solution in contact with an electrode surface and then making that surface sufficiently positive or negative in voltage to force electron transfer. In simple cases, the surface is started at a particular voltage with respect to a reference half-cell such as calomel or Ag/AgCl, the electrode voltage is changed to a higher or lower voltage at a linear rate and finally, the voltage is changed back to the original value at the same linear rate. When the surface becomes sufficiently negative or positive, a solution species may gain electrons from the surface or transfer electrons to the surface. This results in a measurable current in the electrode circuitry. The result of cyclic voltammetry obtained in form of cycle between current and potential, potential on X axis and voltage on Y axis. 

Figure 1.1 shows a generic voltammogram. A voltammogram explains the reversibility of the redox couple.

 Epc= Peak cathodic potential; Ipc= Peak cathodic current; Epa= Peak anodic potential; Ipa=Peak anodic current

      The peak potential of the anodic sweep, Epa and the peak potential for cathodic peak, Epc, can be directly read from the program, and the difference between them, ∆Epeak, can be calculated. If redox couple is reversible, then the relationship,        

n ∆ Epeak= 59mV       (1.1)

In addition, the ratio of the anodic peak current to the cathodic peak current is given by:

ipa / ipc = 1       (1.2)

The formal potential Eo, for a reversible redox couple is easily determined as the average of the two peak potentials as follows.

Eo=(Epa+Epc) / 2       (1.3)

Quantitative information regarding analyte concentration can be obtained from the voltammogram using Randles- Sevcik equation. This equation specifies the peak current, ip (anodic and cathodic), in terms of analyte concentration ,C.

ip= 0.4463 n FAC (n F v D/ R T)1/2       (1.4)

where, n=no. of electrons appearing in the half-reaction for the redox couple v=rate at which potential is swept

 F=Faradays constant (96485 C/mol),

A=electrode area (cm2)
R=universal gas constant (8.314 J/mol K)
T=absolute temperature (K)

D= analyte’s diffusion coefficient (cm2/sec)

If temperature is assumed to be 250C (298.15 K), the eqn. can be written as:

Ip = (2.687 x 105) n3/2 v1/2 D1/2 A C

where the constant is understood to have units (i.e., 2.687 x 105 C mol-1 V-1/2).

During the forward scan the Fe(III) get reduced as :

 

 

Fe (III) + e-Fe(II)

 

Similarly during the reverse potential the oxidation of the Fe (II) takes place to (III) which can be shown as:

 

Fe (III) -Fe(II) + e-

 

The redox reaction therefore can be used as an indication of major analytical tool for the determination of trace elements which are electro active in nature. 

 

 

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