Introduction
1.13 Mass transfer process in voltammetry
The movement of the electro-active substance through solution is called mass transfer at the electrode surface. There are different types of mass transport by which a substance may be carried to the electrode surface from bulk solution. Depending on the experimental conditions, any of these, or more than one might be operating in a given experiment.
In general, there are three types of mass transfer processes by which a reacting species may be brought to an electrode surface [83]. These are,
a) Migration of charged ions in an electric field
b) Diffusion under the influence of a concentration gradient c) Convection due to the motion of the solution or the electrode
1.13.1 Migration
Migration refers to movement of a charged particle in a potential field. In most voltammetric experiments, migration is undesirable but can be eliminated by the addition of a large excess of supporting electrolytes.Ions in the electrolysis solution, being charged them, will move towards the charged electrons, i.e. cation to the cathode and anions to the anode. This motion of charged particle through solution, induced by the charges on the electrodes is called migration [84]. The fraction of the current carried by a given cation
19 and anion is known as its transference number. Migration of charged electro-active substances and intermediates may be effectively eliminated by addition to the solution of an electrochemically inactive salt, called supporting electrolyte which does not undergo an electrochemical reaction at the cathode. When the potential is applied, supporting electrolyte remains unchanged and gives diffusion current.
Generally metal ion (cation) of the sample is migrated towards the cathode due to the electrostatic attraction, where they are reduced. When the cations are migrated towards the cathode a special type of current is produced. This current is called migration current. In cyclic voltammetry the effect of migration is usually eliminated by adding a 50 or 100 fold excess of an inert supporting electrolyte such as KC1, KNO3.
Figure 1.7: Movement of charged particles in a potential field.
1.13.2 Diffusion
The movement of a substance through solution by random thermal motion is known as diffusion. Whereas a concentration gradient exists in a solution, that is the concentration of a substance, is not uniform throughout the solution. There is a driving force for diffusion of the substance from regions of high concentration to regions of lower concentration. In any experiment in which the electrode potential is such that the electron transfer rate is very high, the region adjacent to the electrode surface will become depleted of the electro- active species, setting up a concentration in which this species will constantly be arriving at the electrode surface by diffusion from points further away.
An electrochemical cell in cyclic voltammetry in which the only mode of mass transfer is diffusion to an electrode surface. Since 50-100 fold excess supporting electrolyte is
20 present in the solution, therefore the electrical force on the reducible in is nullified. When the potential is applied, the metal ions are reduced at the cathode and the concentration of the investigated substance is decrease at the cathode region. Hence a concentration gradient is produced. Under this condition, the reducible ions are diffused from the bulk of the solution and diffusion current is produced.
Figure 1.8: Spontaneous movement of particles.
1.13.3 Convection
Convection or hydrodynamic transport is the movement of a substance through solution by stirring. When the electrolyte is stirred or heated the ions are transported from one space to another. At the same time a type of current is produced. This current is called convection current. Removing the stirring and heating can eliminate this current. Convection is a far more efficient means of mass transport than diffusion. Therefore mass transport limited electrolysis currents are much higher for stirred solution than unstirred solution, where diffusion is the only means of mass transport. So, for minimizing convection unstirred vibration free solutions are required. Under such condition, the current is controlled unequally by diffusion of the reacting species through the concentration gradient adjacent to the electrode
Figure 1.9: Movement of particles by stirring.
21 1.14 Cyclic Voltammetry
There are several well established electrochemical techniques for the study of electrochemical reactions. We chose the CV technique to study and analyze the redox reactions occurring at the polarizable electrode surface. This technique helps us to understand the mechanism of electron transfer reaction of the compounds as well as the nature of adsorption of reactants or products on the electrode surface.
CV is perhaps the most effective and versatile electroanalytical technique available for the mechanistic study of redox systems. It enables the electrode potential to be rapidly scanned in search of redox couples. Once located, a couple can then be characterized from the potentials of peaks on the cyclic voltammogram and from changes caused by variation of the scan rate. CV is often the first experiment performed in an electrochemical study. CV consists of imposing an excitation potential nature on an electrode immersed in an unstirred solution and measuring the current and its potential ranges varies from a few millivolts to hundreds of millivolts per second in a cycle. This variation of anodic and cathodic current with imposed potential is termed as voltammogram.
The technique involves under the diffusion controlled mass transfer condition at a stationary electrode utilizing symmetrical triangular scan rate ranging from 1 mV/s to hundreds millivolts per second.
The cyclic voltammetric technique reveals information about the following important phenomena:
i) reversibility of a reaction and a very rapid means of analysis of various systems ii) direct investigation of reactive intermediate
iii) investigation of stepwise electrochemical and chemical reaction iv) redox characteristic of oxidizing-reducing couple
v)investigation of charge transfer reaction at the electrode-solution interface and determination of charge transfer rate constant