INTRODUCTION:

The electro-chemical characteristics of carbonyl compounds are historically of interest. Therefore, ketones¹ and many other carbonyl compounds have been frequently studied. One of the workhorses of electrochemical measurements is cyclic voltammetry, an electrochemical technique that is capable of providing a wealth of information about an electrochemical system. It is one of the electro analytical techniques utilized by analytical and synthetic chemists alike and is important in investigating the kinetics and mechanisms of redox reactions^2,3. The real power of the technique lies in its ability to investigate mechanisms and potential of electrode reactions. In view of the foresaid factors the present work is aimed at utilizing cyclic voltammetry in studying the behaviour of MVQ.

materials and methods:

Reagents:

All reagents employed were of analytical grade and doubly distilled water was used throughout the experiment. The compound 4-methyl-3-vinyl quinoline-2(1H)-one is hitherto referred to as MVQ. The compound MVQ was synthesized as reported earlier⁴. Tetra butyl ammonium bromide was obtained from Lancaster (U.K) and used as such.

The Buffer Solutions were prepared according to the procedure reported earlier⁵. The stock solution of MVQ was prepared by dissolving appropriate amounts of the compound in THF. From the stock solution appropriate amounts were taken to get the working solution.

Strength of MVQ stock solution (mM)	Concentration	Strength of working solution(mM)
0.5081	C1	0.05081
	C2	0.06775
	C3	0.08469

Instrumentation:

The experimental set-up for CV measurement consisted of a Solartron model number 1280 ZT electrochemical system (1280 B+ USB 128087S) – CIF analyzer controlled by a personal computer with the Corrware program.

Analytical Measurements:

Electrochemical Cell:

Cyclic voltammetric experiments were performed using a three electrode system consisting of a 3 mm diameter glassy carbon (MF 2012) as working electrode, saturated calomel as reference electrode and a platinum counter electrode immersed in a small glass cell with provision for inserting electrodes and nitrogen purging. All potentials are referred to the reference electrode.

Electrode Preparation:

Platinum electrode wire was treated with 5% HNO₃ to maintain a clear reproducible surface and washed with dilute liquid soap solution. The electrode was then washed thoroughly with triply distilled water and dried with acetone to remove any adhering moisture before introducing into the cell. The polishing of Glassy Carbon Working Electrode was carried out as in literature⁶.

Electrochemical Measurement:

General Procedure:

In each study a blank run of the solvent and quaternary salts were recorded to ascertain the wave due to the organic compounds and also to ascertain background current before every experiment. A low value of background current (10^-6 / 10^-7) was found to be satisfactory for further studies. About 20ml of 0.01M supporting electrolyte solution was dispensed into an electrochemical cell. To it was added appropriate volumes of Britton Robinson buffer and compound (~10^-5M) in a suitable solvent. The total volume was maintained to be 30ml using THF. A blanketing layer of inert gas over the solution was maintained during the experiment. The working electrode (GCE), reference electrode (Calomel electrode) and auxiliary electrode (Pt wire electrode) were placed in the deoxygenated solution. A cable with 3 alligator clamps was used to make the connection. The electrodes were connected to the potentiostat and cyclic voltammograms were obtained at sweep rates: 0.01, 0.02, 0.05, 0.1, 0.15 and 0.2 V s^–1.

Details of the different parameters varied in the study follows:

Variation of Scan rate

For each of the CV runs made the, scan rate was varied as 0.01, 0.02, 0.05, 0.1, 0.15, 0.2 V s^–1.

Variation of Concentration:

Three different concentrations were prepared by pipetting out 3, 4 and 5 ml of the stock solution to get working solutions of ~10^-5M. For each of the concentrations variation in pH and scan rate was carried out.

Variation of pH:

The pH of the BR buffer was varied (~, 24, 6, 8,10) and after each addition the pH of the test solution was recorded. The total volume of the solution was maintained to be 30ml, inclusive of 20ml supporting electrolyte solution.

Calculations:

The computer interface controlled by a properly developed Windows compatible program Corrview allows for the execution of most conventional electrochemistry techniques and provides necessary facilities for data treatment. From the cyclic voltammograms the following data were collected: Epc- cathodic potential; Ipc- cathodic current.

RESULTS AND DISCUSSION:

Cyclic voltammograms of the compound MVQ were acquired by scanning in a negative direction from 0 V to -2.5 V and back to 0 V. As the potential is scanned in a negative direction, the potential becomes sufficiently negative enough to force electron reduction of reducible groups. During the second half of the potential cycle, the reoxidation of the reduced species can take place as the electrode surface becomes a better oxidant. In the present work cathodic cyclic voltammograms of irreversible nature (Fig 1) were obtained under all conditions with MVQ in presence of supporting electrolytes. From these studies the electroactive nature of the compound in the potential range of interest is proved. The voltammogram recorded for blank solutions with solvent AN and supporting electrolyte TBAB showed no specific reaction is indicated here in the range of potential under study. The parameters of greatest interest for a cathodic irreversible cyclic voltammogram are the cathodic peak potential (E_pc) and the cathodic peak current (i_pc); these parameters were measured automatically by the Solartron 1284 model Electrochemical system. The best-fit line for each data set was determined by linear regression analysis.

Scan Rate Dependence:

The effect of scan rate on the cyclic voltammetric behaviour of the MVQ has been investigated at different pH of the test solution (~2,6,10) (Table 1), in the presence of supporting electrolyte TBAB. The peak potential (E_pc) value shifts towards more negative side as the sweep rate u increases (Fig 1). It is well evident from the results that the cathodic peak current (i_pc) increases with increasing sweep rate. The response observed during a voltammetry experiment depends strongly on the rate at which the material approaches the electrode surface. The peak current increases with increasing scan rate. This is because the current is directly proportional to the rate of electrolysis at the electrode surface. Electrolysis occurs at the electrode surface in response to a change in potential in order to maintain the surface concentrations of the oxidized and reduced species at the values required by the Nernst equation. Therefore, the faster the rate of change of potential (i.e., the scan rate), the faster the rate of electrolysis, and hence the larger the current.

The cathodic shift of peak potential with sweep rate⁷, absence of anodic wave on reverse scan and fairly constant value of i_p/u^½(Table 1) at higher scan rates confirm the reduction process to be a diffusion controlled irreversible one.

Plots of current peak intensity (i_pc) against J^1/2were linear (R>0.9) and passed through the origin, which also proved that the reduction process was diffusion-controlled. Similar observations are available in the literature. The authors⁸ have acquired a linear relationship with zero intercept in the plots of cathodic peak current (i_pc) vs the square root of the sweep rate (J^1/2) accounting for the lack of detectable chemical kinetic complications. The current function (ip/Cu^½) has been found to be fairly constant with respect to high sweep rates indicating that the electrode process is diffusion controlled.

The product of i_pcand u^-½increases with potential scan rate, but for a simple irreversible electron transfer reaction, it should remain constant. The variation at low scan rate suggests a complex set of mechanisms for the reaction. At high scan rates the product remains almost constant indicating a simple irreversible electron transfer reaction. Plots of Ep vs logJ and plots of log ip vs log J were constructed and the curve equations obtained (Table 1). ip increased with increasing of J and the slope of log ip-log J curve equation is close to 1 for almost all the compounds studies, which indicated that the adsorption of the compounds at the mercury electrode is ideally adsorptive characteristic^9,10. Representative graphs of ip vs J^1/2, Ep vs logJ and log ip vs log J are given in figures 2, 3 and 4 respectively.

Concentration Dependence:

The concentration of electro active species present in a solution also plays a major role in determining the response observed in a voltammetric experiment. The effect of variation in concentration in the cyclic voltammetric behaviour (Figure 5) of the compounds under study has been investigated at three different concentrations at varying scan rate and pH in THF in the presence of supporting electrolyte TBAB (Table 2).

TABLE 2: EFFECT OF CONCENTRATION ON THE REDUCTION OF MVQ- TBAB SYSTEM WITH BRITTON ROBINSON BUFFER AT pH ~6

Conc.(mM)	u (V/s)	-E_p (V)	I_p(mA)
0.05081	20	0.910	3.0
0.05081	100	1.067	8.4
0.06775	20	0.93	3.9
0.06775	100	1.098	12.2
0.08469	20	0.97	4.9
0.08469	100	1.10	12.8

TABLE 3: EFFECT OF pH ON THE REDUCTION OF MVQ AT DIFFERENT SCAN RATES IN THF

pH	~2			~6			~10
Scan rate u (V/s)	-E_p1 -E_p2(V)	I_p (mA)	i_pu^-1/2	-Ep (V)	I_p (mA)	i_pu^-1/2	-E_p1 -E_p2(V)	I_p (mA)	i_pu^-1/2
0.01	0.76 2.19	1.50 33.0	15 330	0.840	1.40	14.00	0.89 2.3	1.50 56.0	15 560
0.02	0.81 2.2	1.65 40.0	011.6 283.0	0.910	3.00	21.20	0.94 2.35	1.60 61.0	11.3 431
0.05	0.87	1.80	8	1.020	7.50	33.54	0.97	1.80	8
0.10	0.91	3.40	10.8	1.067	8.40	26.50	0.10	2.02	6.4
0.15	0.94	2.60	06.7	1.087	10.2	26.33	1.02	2.20	5.7
0.20	0.96	4.20	09.4	1.107	13.3	29.70	1.03	2.50	5.6

The Randles–Sevcik equation is given by

ip = (2.695)n^3/2AD^1/2Cu^1/2

Where, i_p is the peak current in amperes, A is the electrode area in cm², D is the diffusion coefficient in cm² s^–1, C is the concentration in mol cm^–3, and u is the sweep rate in V s^–1. It is apparent from the equation that the peak current is proportional to concentration, and a linear calibration curve may be constructed from voltammograms at different concentrations. A representative calibration curve constructed from the data obtained in the present study is shown in Figure 6.The plots of i_pc vs concentration (at two different scan rates ) (Fig.6) and ip vs u^-1/2 at different concentrations (Fig.7) fulfill the criteria of the diffusion controlled nature of the electrode process^11,12.

The current corresponding to the reduction process increased as the concentration of the electroactive active species in the solution indicating the increase in the availability of active species at the electrode surface¹³.

Effect of pH:

The effect of pH on the cyclic voltammetric behaviour of MVQ has been investigated at different pH of the test solution (~2,4,6,8,10), in the presence of supporting electrolyte TBAB (Table 3).

Effect of pH 8 and 10 at two different scan rates

pH	u(V/s)	-E_p(E_p2) (V)	I_p(mA)
8	20	0.94	4.2
8	100	1.02	7.3
10	20	0.94(2.35)	1.6(61.0)
10	100	1.0	2.02

Two cathodic peaks were obtained in THF-TBAB system by varying scan rates at pH 2 and 10. The reason for lower reduction potential values in acidic medium many is explained because of the formation of more easily reducible protonated radical intermediates. The higher (negative) E_pc value of the compounds under study in alkaline medium may be ascribed to the greater degree of anionic character.

Peak potentials vary linearly with the pH at all scan rates and concentrations. The E _1/2 values were also found to become more negative with increase in pH for all the compounds under study. This clearly shows the participation of protons in the reduction process.

TMVQ

DMVQ

MVQ

DMVQ

CMVQ

TMVQ

MVQ

CONCLUSION:

Cathodic cyclic voltammograms were obtained under all conditions in presence of the supporting electrolyte TBAB. The peak potential (E_p) value shifted towards more negative side as the sweep rate u increased. The cathodic shift of peak potential with sweep rate, absence of anodic wave on reverse scan and fairly constant value of i_p/u_1/2 at higher scan rates confirm the reduction process to be diffusion controlled irreversible one.

The effect of scan rate on the reduction of MVQ in different pH revealed that the reduction is pH dependent. The plots of i_pc vs concentration and i_p vs u^-1/2 at different concentrations fulfilled the criteria of the diffusion controlled nature of the electrode process. Peak potentials vary linearly with the pH at all scan rates and concentrations. The E_1/2 values were also found to become more negative with increase in pH. Compatibility of glassy carbon electrode in electrochemical studies in THF is quite obvious. This electrode can be used for the quantitative determination of compounds like MVQ based on their cathodic reduction using CV with a limit of determination around 10^-5M.

ACKNOWLEDGEMENT:

The authors thank the authorities of Avinashilingam Deemed University for Women, for having provided facilities to carry out this work.

REFERENCES:

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2. Kissinger PT, Heineman W R. Cyclic voltammetry. J. Chem. Educ. 1983; 60: 702–706.

3. Mabbott GA..An Introduction to Cyclic voltammetry. J. Chem. Educ. 1983; 60:697–701.

4. Lalitha P, Sivakamasundari S and Shanmugam,P., March 5 and 6,2004, Microwave assisted synthesis of organic compounds -An attempt with few sulphur heterocycles -SO20, Abs. Proceedings - National seminar on Isolation, characterization, synthesis and biological studies of organic compounds, organized by Department of chemistry, Gandhigram Deemed University, South India.

5. Prabavathi, N., 1993, Electrochemical Studies of Some Flavonoid Compounds, Ph.D Thesis, Bharadhidasan University, Tiruchirapalli, South India.

6. Harsha Vaswani and Jessica Watkens, Cyclic Voltammetry: A Tutorial. 2000, Colvin Group Rice University

7. Zhang WF, Schmidt-Zhang P, Kossemhi G. Study of the electrosynthesis of poly (3-buteyl thiophene) and its semiconductor properties. Journal of solid-state electrochemistry. 2001; 5(1): 74-79.

8. Weiss H, Friedrich T, Hofhaus G, Preis D. The respiratory-chain NADH dehydrogenase (complex I) of mitochondria. Eur.J.Biochem. 1991; 197: 563.

9. Fdez S, Moreda J M, Arranz A and Arranz J F. Study of the adsorptive stripping voltammetric behaviour of the antihypertensive drug Doxazosin.Anal. Chim. Acta. 1996; 329: 25-31.

10. De Boer H S, Van Oort WJ and Zuman P. Polarographic analysis of corticosteroids : Part 6. Mechanism of polarographic electroreduction of some Δ⁴-3-ketosteroids and Δ^1,4-3-ketosteroids. Anal. Chim. Acta. 1981; 130, 11.

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Received on 04.06.2010 Modified on 20.06.2010

Asian J. Research Chem. 3(4): Oct. - Dec. 2010; Page 1015-1019

Scan rate u (V/s)	Peak potential -E_p (V)		Peak current I_p (mA)	i_p/u_1/2mAV^-1/2s^-1/2	i_p/Cu_1/2
0.01	0.840		1.40	14.00	275.54
0.02	0.910		3.00	21.20	417.5
0.05	1.020		7.50	33.54	660.1
0.10	1.067		8.40	26.50	523.0
0.15	1.087		10.2	26.33	518.0
0.20	1.107		13.3	29.70	585.3
Plot of ip vs u^1/2 y = 31.627x - 1.2168;r = 0.98		Plot of Ep vs logJ y = -0.2072x - 1.2649; r = 0.99		Plot of log ip vs log J y = 0.77x– 1.65; r = 0.97