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|Title:||STUDIES ON ELECTROCHEMICAL BEHAVIOUR OF SOME ELECTROCHEMICAL BEHAVIOUR AND THEIR INTERACTION WITH CLAYS|
|Authors:||Nautiyal, Ambika Prasad|
|Keywords:||CHEMISTRY;ELECTROCHEMICAL BEHAVIOUR;CLAYS;CYCLIC VOLTAMMETRY|
|Abstract:||Electrochemical investigations of biologically important compounds provide major challenges both from theoretical and mechanistic view point. The turn of the last two decades has seen a continued growth in development of more sensitive and sophisticated electroanalytical techniques. Cyclic voltammetry, which is an advancement of a more general technique polarography provides not only deep insights about the redox mechanism but also probes mechanisms which are coupled with chemical reactions. Thus, the best way to get an interpretation closer to the actual biological process is to study the electrochemical behaviour of such molecules to extend and deepen the understanding of bloe1ectrochemical aspects of the biological transformation undergoing in the living system. Azo compounds as well as amino compounds have long been well known for their analytical and medical potentialities. The determination of these compounds by polarography and spectrophotometry attracted considerable attention. However, very few attempts have been made to study their redox-behavlour. As electrochemical reactions occuring in-vitro at electrode solution interface are superficially similar with that of enzymic reactions at enzyme-solution interface, the electrochemical behaviour of two such compounds, Napthol Red-J(I) an azo dye and 2-aminobenzothiazole (II), a heterocyclic amine has been studied. Na035-^)-N=N-^ ax (I) S^NH2 (ID (ii) The thesis has been divided into five chapters: The first chapter of the thesis is General Introduction and presents a brief review and significant results relevant to the present study on the electrochemical behaviour of various compounds selected as well as their interaction with clays. Studies on the electrochemical behaviour of Napthol Red-J forms the second chapter of the thesis. The studies have been carried out in a wide pH range using polarography, linear and cyclic sweep voltammetry, coulometry and spectral studies. + The reduction of this compound occurred in a single 4e, 4H pH dependent step at d.m.e. as well as at pyrolytic graphite electrode. The El/2 versus pH plot was linear and the dependence of El/2 on pH can be described by the following equat ion: -EU/2) = CO.053 pH + 0.05] V vs SCE The limiting current was linearly dependent on concentration -4 -4 of Napthol Red-J in the range 1x10 to 8x10 N and hence the dye can be safely estimated in this concentration range at d.m.e. (ill) Linear sweep voltammetry of Napthol Red-J exhibited a well defined reduction peak (lie) at a sweep rate of 5mVs In cyclic sweep voltammetry two anodic peaks la and Ilia were also noticed in the reverse Sweep. When the direction of sweep was changed again a cathodic peak lc was observed which formed a quasi-reversible couple with peak. la. The UV-visible spectral studies during reduction indicated that no intermediate capable of absorbing in UV-Vis region is generated. The products of reduction were characterized using various spectroscopic techniques and found as l-amino-2-naptho1 and p-aminobenzenesu1fonic acid. The third chapter of the thesis has been devoted to the electrochemical oxidation of 2-aminobenzothiazole (II), a basic unit of local anaesthetics. In linear and cyclic sweep voltammetry, oxidation of I occurred in a single 2e, 2H pH dependent step to give an azo compound as the product. In phosphate buffers, the peak potential of the peak was linearly dependent on pH according to the relation E = CO.88 - 0.057 pH] volt vs SCE P In cyclic sweep voltammetry, a well defined anodic peak la was observed and in the reverse sweep a cathodic peak lie was noticed. The peak potential of peak lie was also dependent on pH and shifted towards more negative potential with increase in pH. The UV-Vis studies during e1ectroxidation in the range 250-500 mm clearly indicated the increase in absorbance at (iv) longer wavelength (300-400 mm) and with progress of electrolysis a clear maximum at 380 nm was observed. This behaviour indicated the presence of more extensive n-conjugation in the product. The VU-visible spectral changes, mass spectrum and the IR spectrum supported the formation of azo product. The reaction of aromatic amines with clay minerals has been studied by various workers and organic molecules have been found to undergo biotic and abiotic transformations resulting in the formation of coloured clay complexes. Phenols have been found to oxidize to form coloured complexes by clay minerals and the coloured products were shown by gel filteration to be polymers with as high as 3000 molecular weight. Oxidation of benzidine on low-iron smectites has been studied and it was suggested that planar layer silicate surface offers a unique chemical environment for adsorption, promoting oxidation to products. The interaction of purines, pyrimidines and their nucleosides and nucleotides with clays has also been stuied extensively. In most of these studies it was concluded that a variety of interactions of purines and pyrimidines is possible with clays, including protonation of acid clay surfaces, complexation with exchange cations and less energetic interactions such as physical forces or hydrogen bonding. Although, literature abounds with references on clay organic interactions, most of these are limited to the uptake (v) of cationic or non-ionic organic molecules on montmori1 Ionite, kaolinite or illite surface. The adsorption of anionic organic compounds on clay particle surface, has received a lesser attention, since anion adsorption sites are localised on the broken edges and the magnitude of their uptake would be smaller in comparison to that on basal surface. Thus, it was expected that investigations on the adsorption of anionic dyes with clays, as under taken in this dissertation, may provide some understanding of the edge surface chemistry of clay particle, which is still an area of inadequate information. The choice of the mineral nontronite, used in these studies, rests on the fact, that among three layer clays, the surface properties of nontronite minerals have so far not been fully investigated and very few reports appear on the interaction of this clay with organic molecules. Azo dyes are obvious choice as adsorbate in these investigations, since these are readily estimated colorimetrical1y, are sufficiently soluble and usually get strongly adsorbed. Investigations incorporated in the fourth chapter of thesis include the characterization of the clay mineral obtained from Department of Geology, University of Missouri, U.S.A.. The chemical analysis of the clay sample was carried out by standard analytical procedures. Silicon was determined as SiO ; aluminium as AlnO^; iron as FenO and titanium as TiO . The determination of Ca, Mg, Na and K was carried out by flame photometric method. The cation exchange capacity and (vi) anion exchange capacity of the clay sample were also determined and were found to be 70.0 meq per 100 g of clay and 6.22 meq per 100 g of clay respectively. On the basis of chemical analysis, X-ray, thermal analysis, IR etc., it has been concluded that the clay mineral is nontronite. The last chapter of the thesis deals with adsorption studies of two azo dyes, viz., Napthol Red J and Direct orange on Nontronite mineral. The spectral behaviour of the dyes was studied in a wide pH range to elucidate the aggregation possbility. The adsorption studies incidated that the Nontronite mineral followed Frendulich model only in lower adsorbate concentration range. The dye adsorption at the positive edges of the mineral was found to be consistent with the observed change of adsorption with pH. The magnitude of AG and AH values indicate a very weak interaction at the surface whereas the value of AS reflects the feasibility of the process which is also evident by the values obtained for the equilibrium parameter 'R'. Equilibrium parameters for the two dyes were determined and X-ray data of the clay-dye complex has also been presented. (vii) LIST OF PUBLICATIONS 1. Electrochemical Oxidation of 2-Aminobenzothiazole, J. Electroanal. Chem. 200 (1986) 119. 2. Electrochemical Behaviour of Napthol Red-J, an Azo dye, Indian Journal of Chemistry, 26A, (1987) 871. 3. Adsorption of Napthol Red-J on nontronite mineral, clays and clay minerals (communicated). 4. Adsorption behaviour of Direct orange, an azo dye on nontronite mineral, Clays and Clay minerals (communicated) .|
|Research Supervisor/ Guide:||Goyal, R. N.|
Srivastava, S. K.
|Appears in Collections:||DOCTORAL THESES (chemistry)|
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