22

METHODOLOGY 2.1 INTRODUCTION TO ELECTROCHEMICAL TECHNIQUES Electrochemical techniques of analysis involve the measurement of or current. Such methods are concerned with the interplay between solution/ 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 viz., , the , and the counter or 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, , , , 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 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 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, , 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 as E = E0 + {(RT/ nF) × ln ([O] / [R])} Where E is cell potential (V), E0 is standard (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 . 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 of the analyte to standard substance qualitative analysis is done using polarography. Quantitative polarographic analysis is carried out using Ilkovic 1/2 2/3 1/6 equation, id = knD m t c or by using working curve / standard addition method. Mechanism and kinetics of various reactions in aqueous solutions can also be studied using polarography. The various forms of polarography such as ac polarography, pulse polarography, differential pulse polarography, and square wave polarography are also observed to be used to study the reaction mechanism and the kinetics 10-12. 2.3.2 Cyclic Voltammetry In voltammetric technique generally the potential is scanned in one direction i.e. either positive or negative potentials. In cyclic voltammetry (CV) the potential is scanned in both directions. A applies a potential ramp to indicator electrode to progressively change the potential and then reverses the scan, returning to the initial potential. In one CV cycle if potential is scanned to more positive value the oxidation of electroactive species takes place. When the potential reaches the predetermined switching potential the direction of the scan is changed to negative potential and the resulting current is monitored. The voltammogram gives peak current instead of limiting current. Separate peaks are observed for the oxidation and the reduction reactions characterized by peak potentials and peak currents. Cyclic voltammetry is observed to be widely used technique in kinetics and thermodynamics to study the electron transfer, presence of intermediates in redox reactions and the reversibility of the reactions13-15. It is also useful in the assessment of redox mechanisms16. 2.3.3 Linear sweep voltammetry (LSV) is the voltammetric technique in which electrode potential is varied at a constant rate throughout the scan and the resulting current is measured. The potential between the working electrode and the reference electrode is swept linearly with time. In this method the auxiliary electrode is used to maintain the reference electrode at a definite balancing the potential of the working electrode17. The voltammogram shows a peak or trough in the current signal at the potential at which the analyte species starts to get oxidized or reduced. It is a very useful technique to study the irreversible reactions. It is used to

28

examine the direct methane production via biocathode which is an irreversible reaction18. Kinetics of the solid state reactions can also be studied using linear sweep voltammetry19. 2.3.4 Stripping Voltammetry There are three types of stripping voltammetric techniques - anodic stripping voltammetry (ASV), cathodic stripping voltammetry (CSV), and adsorptive stripping voltammetry (AdSV). Each one of the techniques has its own distinctive features, but the methods have two steps in common. In the first step, the analyte species in the sample solution is concentrated onto or into a working electrode. It is known as preconcentration step that results in the exceptional sensitivity that can be achieved. The preconcentrated analyte is measured or stripped from the electrode in the second step by the application of a potential scan. Peak shape voltammogram is obtained when monitored current is plotted against the potential scan during the stripping step. The peak current obtained is proportional to the concentration of analyte reduced or oxidized at the working electrode. Due to the preconcentration step the stripping techniques have very low detection limits and the sensitivity and selectivity of the analysis are excellent. The required conditions are that the small electrode surface, high rate of stirring, and constant deposition time during analysis. Stripping voltammetry techniques are widely used for the determination of trace and ultratrace metals and their distribution in a sample 20. 2.3.5 The limiting current is measured in polarography using dropping mercury electrode (DME). By replacing DME with the solid electrode which mechanically stir the solution, current can be measured. The electroactive species is brought to the electrode by convention when electrode is rotated. The approach is called as hydrodynamic voltammetry since the theory of flow of electroactive species to a rotated electrode is identical with the theory of flowing fluids or hydrodynamics7. The rotating electrode can be a disk or a wire. The (rde) consists of a conductive disk generally made of noble metal like platinum or glassy carbon. It is embedded in an inert non-conductive polymer or resin and is attached to an electric motor which can control the electrode's rotation rate. Using such electrode

29

only uni directional flow of the electroactive species to the electrode surface is obtained. The qualitative and quantitative data can be obtained using the ring disk electrode. The rate and mechanism of an electrochemical reaction can also be determined using another type of electrode namely rotated ring-disk electrode (rrde). This type of electrode consists of a ring around the central disk which acts as a second working electrode. The ring and disk i.e. two electrodes are separated by an insulating material and connected to the potentiostat through separate electrical connections. In such a case the electroactive species formed at the disk is travels outward over the ring on the rotation of the electrode. The amount of electroactive species reaches the ring as the function of electrode rotation rate or as a function of disk potential is measured which helps to determine the rate constant and the reaction mechanism21. A wire can also be used as rotated electrode. It can be rotated along its axis, perpendicular to axis or at some angle relative to its axis. The same potential ramp is used for hydrodynamic voltammetry as used in classical polarography. The resulting voltammogram has a similar shape as obtained in polarography, except for the lack of current oscillations from the growth of the mercury drops. In hydrodynamic voltammetry the current obtained on the plateau of the wave is controlled by convection rather than diffusion as in polarography. The limiting current ilim is directly proportional to the concentration of the electroactive species in the solution. When a rotating disk electrode is in use the limiting current is proportional to the square root of the rotation rate of the electrode when the flow of electroactive species is one directional7.

The limiting current ilim is given by the equation 2/3 -1/6 1/2 1/2 ilim = 1.55 nFA D v п N C Where n is number of electrons transferred in the half reactions, F is Faraday constant, A is the electrode area (cm2), D is the diffusion coefficient (cm2/s), v is the kinematic viscosity of the solvent (cm2/s), N is the rotation rate of the electrode (r/s), and C is the concentration (mM) of electroactive species.

30

Applications of hydrodynamic voltammetry  To study the fundamentals of an electrochemical processes.  Detection of end point in coulometric and volumetric titrations  The technique is useful for wide range of analytes that undergo oxidation or reduction at more positive potentials since the method is not limited to mercury electrodes.  The limiting current value can be used for the quantitative analysis with working curve or standard addition method.  The qualitative analysis can be done using half wave potential.  The limiting current values can also be used for the study of mass transfer rate at solid electrodes in flowing solutions22.  Other than quantitative and qualitative analysis hydrodynamic voltammetry technique has been also used for determination of soil enzyme activity 23.  The chemical species which may exit from chromatographic columns or a continuous-flow apparatus can be determined using hydrodynamic voltammetry.  Measurement of the amount of reaction product or unreacted reactants reaching at the rotated electrode as a function of electrode rotation rate or as a function of the electrode potential is possible which can be used to determine the specific reaction rate or the mechanism of a reaction 24-26. In consequence it has been observed that these various voltammetric techniques are useful for the study of reaction kinetics and mechanism. In the present work we have used the hydrodynamic voltammetry technique to study the rapid kinetics of halogenations of five membered aromatic heterocycles. Kinetics data of such reactions have been lacking due to the rapidity of these reactions. The conventional techniques are unsuccessful to study such reactions. Hydrodynamic voltammetry is observed to be a simple yet efficient technique to study the rapid reaction kinetics. The nano diffusion current due the halogens is measured at the rotating platinum electrode. The details of the experimental set up used in the present work are discussed in the further section.

31

2.4 EXPERIMENTAL SET UP FOR HYDRODYNAMIC VOLTAMMETRY 2.4.1 Construction of rotating platinum electrode The Rotating Platinum Electrode constructed from pyrex tubing which consist of glass tube with a small well at the bottom which looks like a inverted ‘ T ’shaped. Platinum wire having diameter of 0.5 mm is sealed in the bottom of electrode in such a way that it protrudes from the wall of a length of 6 mm inverted ‘T’ shaped glass tubing. Platinum wire thus extends to the outer edge of the small well at the bottom of the glass tube. The tube is filled with mercury to height of 10 mm. The copper wire is dipped inside the tube for external electrical contact. A wooden pulley and the pair of ball bearing are joined to this glass tube, having the total length of 32cm. The ball bearings are fixed tightly to a stand with the help of corks. The wooden pulley is then connected to a synchronous motor using stiff thread. The distance between the motor and pulley consisting of electrode is adjusted such a way that the electrode rotates at a speed of 600 rpm. With the rotation of electrode the lower 4cm portion of the glass tube affects a stirring action in the solution. Rotation speed is kept constant during the experiment in order to obtain the linear relation between current and amount of reagents added. 2.4.2 Electrochemical cell The rotating platinum electrode is connected to the positive terminal of a potentiometer with the help of a copper wire which is inserted inside the glass tube for external connection. The rotating platinum electrode acts as a working electrode in an electrochemical cell. The saturated calomel electrode connected to the negative terminal of potentiometer acts as a reference electrode. A constant potential of +0.1V applied versus the saturated calomel electrode at the rotating platinum electrode, using a potentiometer. A galvanometer with the sensitivity of 5.0 X 10-9 amp mm-1 provided with a lamp and scale arrangement connects to the potentiometer via shunt to control the current flowing through the galvanometer. A battery providing 2.0 V is connected to the potentiometer. The current produced due to the reduction of the halogens in the reaction under study is measured in terms of deflection of the galvanometer light spot with the help of a lamp and scale arrangement. A shunt is used to control the galvanometer current within the scale limits. The assembled

32 electrochemical cell is arranged in a thermostat to attain a required constant temperature throughout the kinetic study. 2.4.3 The electrochemical processes involved at the two electrodes are as follows:

X2 + 2e X + X At the positive electrode, RPE

- - 2 Hg + 2 Cl Hg2Cl2 + 2e At the negative electrode, SCE - + - X2 +2 Hg + 2 Cl Hg2Cl2 + X + X Overall reaction The experimental set up of the electrochemical cell with the RPE and the SCE is shown in Figure 2.3

Figure 2.3:- Experimental set up of electrochemical cell with rotating platinum electrode and saturated calomel electrode

1-Rotating platinum electrode, 2- Saturated calomel electrode, 3- KNO3 Agar Bridge, 4- Thermostat, 5- Wooden pulley, 6- Pair of ball bearing, 7- A.C. motor, 8- Potentiometer.

33

2.5 REFERENCES 1. Bard A.J. and Faulkner L.R., Electrochemical Methods: Fundamentals and Applications, 2nd Ed., John Wiley, 2001, 833. 2. Zoski, C.G., Handbook of , 1st Ed., Elsevier, 2007, 891. 3. Wang J., Analytical Electrochemistry, 3rd Ed., New Jersey by John Wiley, 2006, 250. 4. Harvey David, 2.0, Electronic Version, 2009, 729. 5. Heyrovsky J., Chem. Listy., 16, 1922, 256. 6. Skoog D. A., Holler F. J. and Crouch S. R., Principles of Instrumental Analysis, 6th Ed., Thomson Brooks / Cole, Canada, 1998, 716. 7. Braun R. D., Introduction to Instrumental Analysis, 2nd Ed. McGraw-Hill Book Co., New York, 1987, 762-790. 8. Harvey David, Modern Analytical Chemistry, 1st Ed., McGraw-Hill Companies, 2000, 510. 9. Bond A. M., Modern Polarographic Methods in Analytical Chemistry, Dekker, INC, New York, 1980, 1-15. 10. Oldham K. B. and Parry E.P., Anal. Chem., 40 (1), 1968, 65–69. 11. Olafson R. W., The Journal of Biological Chemistry, 256, 1981, 1263- 1268. 12. Jindal H.L., Matsuda Kiyoshi, Tamamushi Reita, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 90(2), 1978, 185-196. 13. Nicholson R.S., Anal. Chem., 37, 1965, 1351–1355. 14. DuVall, Stacy DuVall, McCreery, Richard, Anal. Chem., 71, 1999, 4594– 4602. 15. Bond Alan M. and Feldberg Stephen, J. Phys. Chem., 102, 1998, 9966– 9974. 16. Geiger W. E., Organometallics, 30, 2011, 28-31. 17. Kounaves Samuel P., Voltammetric Techniques, Handbook of Instrumental Techniques for Analytical Chemistry, 709–725.

34

18. Cheng Shaoan, Xing Defeng, Call Douglas F., and Logan Bruce E., Environ. Sci. Technol., 43(10), 2009, 3953–3958. 19. Stevanovic J. S., Jovic V. D. and Despic A. R., Journal of Electroanalytical Chemistry, 349(1–2), 1993, 365-374. 20. Achterberg E. P., Braungardt C., Analytica Chimica Acta,400(1–3), 1999, 381–397. 21. Adams R. N., Electrochemistry at solid electrodes, N. Dekker Publisher, New York, 1969, 94-101. 22. Matsuda Hiroaki , Yamada Joseph, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 30(2), 1971, 261-270. 23. Sazawa Kazuto and Kuramitz Hideki, Sensors, 15, 2015, 5331-5343. 24. Rao T. S., Mali S. I. and Dangat V. T., J. Univ. Poona Sci. Tech., 52, 1979, 111-114. 25. Rao T. S., Mali S. I. and Dangat V. T., Tetrahedron, 34, 1978, 205. 26. Dangat V. T., Bonde S. L., Gayakhe A. S. and Ghorpade B. S., Ind. J. Chem., 28A, 1989, 321-322.

35