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07 Chapter2.Pdf 22 METHODOLOGY 2.1 INTRODUCTION TO ELECTROCHEMICAL TECHNIQUES Electrochemical techniques of analysis involve the measurement of voltage or current. Such methods are concerned with the interplay between solution/electrode interfaces. The methods involve the changes of current, potential and charge as a function of chemical reactions. One or more of the four parameters i.e. potential, current, charge and time can be measured in these techniques and by plotting the graphs of these different parameters in various ways, one can get the desired information. Sensitivity, short analysis time, wide range of temperature, simplicity, use of many solvents are some of the advantages of these methods over the others which makes them useful in kinetic and thermodynamic studies1-3. In general, three electrodes viz., working electrode, the reference electrode, and the counter or auxiliary electrode are used for the measurement in electrochemical techniques. Depending on the combinations of parameters and types of electrodes there are various electrochemical techniques. These include potentiometry, polarography, voltammetry, cyclic voltammetry, chronopotentiometry, linear sweep techniques, amperometry, pulsed techniques etc. These techniques are mainly classified into static and dynamic methods. Static methods are those in which no current passes through the electrode-solution interface and the concentration of analyte species remains constant as in potentiometry. In dynamic methods, a current flows across the electrode-solution interface and the concentration of species changes such as in voltammetry and coulometry4. 2.2 VOLTAMMETRY The field of voltammetry was developed from polarography, which was invented by the Czechoslovakian Chemist Jaroslav Heyrovsky in the early 1920s5. Voltammetry is an electrochemical technique of analysis which includes the measurement of current as a function of applied potential under the conditions that promote polarization of working electrode6. The technique is mainly useful to study the redox reaction processes in various media, adsorption processes on surfaces and electron transfer mechanisms at chemically modified electrode surfaces. Voltammetry 23 techniques are observed to be superb methods in diverse areas of chemistry, biochemistry, material science, engineering and the environmental sciences for studying oxidation, reduction and adsorption processes. 2.2.1 Electrochemical cell The voltammetry techniques involve the study of the cell reactivity of an analyte. The electrochemical cell mainly consists of an ionic electrolyte i.e. sample dissolved in solvent, and electrodes. A cell made up of different materials and of variable sizes and shapes can be used depending on the type of sample to be analyzed, its amount and the technique. In some cells, reference electrode and working electrode are kept as close as possible whereas in some cases these are placed in separate compartments to avoid the contamination. Using two electrodes, electrochemical cells with low resistance can be investigated successfully. However, for electrochemical analysis especially at high cell resistance, the three electrode cell is preferred. 2.2.2 Electrodes in voltammetry In voltammetry at least two electrodes are required, namely working electrode and the other reference electrode. In modern techniques three electrode systems are used in which third electrode called auxiliary or counter electrode along with these two electrodes. Working electrode is also called as an indicator electrode where the reaction between electroactive species in solution and electrode surface occurs. The potential of this electrode is varied with time and is monitored with respect to the reference electrode. These are solid electrodes usually constructed from platinum, mercury, gold or some form of carbon. To enhance its tendency to become polarized, the dimension of the electrode is kept small. The use of a solid electrode also enhances the range of positive working potential. The second electrode is the reference electrode. This electrode should be reversible and its potential must remains constant throughout the experiment even though current is passed across it. In aqueous solutions commonly used reference electrodes are saturated calomel electrode and silver-silver chloride electrode. The third electrode called as auxiliary or counter electrode is used in voltammetric techniques to minimize the current flow between 24 working and reference electrode. This electrode is generally constructed from inert metal such as platinum with relatively large surface area7. The relation between potential and concentrations of electroactive species can be expressed by the Nernst equation as E = E0 + {(RT/ nF) × ln ([O] / [R])} Where E is cell potential (V), E0 is standard electrode potential (V), R is gas constant (J mol-1 K-1), F is Faraday’s constant (C mol-1), [O] is the activity of oxidized species and [R] is the activity of reduced species. Figure 2.1: Schematic diagram for electrochemical cell with two-electrode (left) and three-electrode (right) systems used for voltammetry 2. 2.2.3 Current in voltammetry Application of potential sufficient to cause oxidation or reduction of redox species of an analyte solution produces current which is the measure of electron transfer. This current which is due to the redox reactions at the working and auxiliary electrode is called faradaic current. The current due to reduction at the working electrode is termed as cathodic current and its sign is positive. Oxidation at the working electrode produces current is termed as anodic current and its sign is negative. The current is plotted as a function of applied potential which is called as voltammogram the shape of which depends on the type of indicator electrode and the potential ramp that are used8. 25 A typical voltammogram is shown below: Figure 2.2: General voltammogram In voltammetric techniques it is observed that, a small amount of current flows through the cell at most potential even in the absence of an electroactive species known as the residual current (ir). The residual current is observed as a combination of current as a result of redox reactions of impurities present in solution and the capacitive or charging current. Diffusion current (id) is the current flowing through the cell which is limited by the rate of diffusion of analyte species to the working electrode. This current is produced due to the concentration gradient produced between an electrode surface and the bulk of the solution. It is a difference between total limiting current flowing through the cell and the residual current. The limiting current is the constant current ahead of the steep rise. It is limited by the rate at which the reactant can be brought to the surface of electrode by mass- transport processes. Other than residual and diffusion currents, a migration current can also flow in an electrochemical cell. Migration current is observed due to the attraction or repulsion of electroactive species by the indicator electrode. This attraction or repulsion of a species can cause the increase or decrease in the rate at which an electroactive species arrives at the electrode surface which generates an additional 26 positive or negative current. This migration current is eliminated by the addition of an excess of inert electrolyte to voltammetric solution called as supporting electrolyte. The presence of excess of supporting electrolyte nullify the electrical force on the reducible ion since the ions of added salt carry practically all the current and the migration current is eliminated. In aqueous solutions electro inactive ionic species such as potassium chloride or potassium nitrate are commonly used as supporting electrolytes. In non aqueous solvents sodium perchlorate, tetrabutyl ammonium perchlorate are used as supporting electrolytes to eliminate the migration current. Sometimes in voltammogram the current peak is observed in the plateau region called current maxima. This is caused due to the adsorption of an electroactive solution species on the electrode surface when indicator electrode is mostly dropping mercury electrode. For quantitative analysis it is necessary to remove the current maxima. Addition of small amount of electro inactive surface active substance known as maxima suppressor can be used for this purpose. This suppressor like detergent which gets adsorbed on the electrode surface and prevents the adsorption of electroactive species causing maxima in voltammogram is used. Gelatins, Triton X- 100 in small amounts are often used maxima suppressors. 2.3 DIFFERENT VOLTAMMETRY TECHNIQUES The variation in different parameters such as use of different types of working electrodes, the rate of change of the applied potential and the current measurements, and the stirring of the solution, gives the different types of voltammetric techniques. Some of these are discussed in this section. 2.3.1 Polarography Polarography is the first important voltammetric technique which differs from other voltammetric techniques because of the use of unique working electrode i.e. dropping mercury electrode (DME) and the solution is not stirred in this technique. The voltage ramp used is linear with a slope of about 1 to 5 mV/s with drop time between 2 and 8 sec. The scan rate can be increased to 200 to 300 mV/s using mechanical device to control the drop time at a smaller value between 0.01 to 1s. The technique is known as rapid dc polarography 9. By comparing the values of half wave 27 potential
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