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|EFFECTS OF SOLVENTS ON THE REDUCTION OF METHYLENE GREEN
|JINNAH UNIVERSITY FOR WOMEN KARACHI PAKISTAN
|The photo reduction of methylene green (MG+) with ethylene diammine tetra acetic acid (EDTA) in aqueous alkaline and aqueous alkaline miscible organic solvent was carried out spectrophotometrically by visible light. Reaction was followed by change in optical density of methylene green with time course. It was observed that change in solvent showed no significant change in wavelength but change in optical density showed the effect of solvent on dye reduction process. Therefore reaction was followed at λmax 658 nm. The influence of operational parameters, such as dye concentration, reductant concentration, change in pH, change in solvent composition, and change in ionic strength at different temperature revealed that these parameters strongly influence the decoloration of the dye. The apparent rate of decoloration was calculated from observed absorption data on the kinetics of bleaching of dye, showed that reaction follows pseudo first order kinetics. EDTA was found to be an effective reducing agent for the photo reduction of methylene green in aerobic condition. Consumption of EDTA in the reduction of methylene green leads to conclude that it is oxidized. This is an unexpected result since EDTA does not normally function as a reducing agent. The nitrogen containing chelating agents with secondary or tertiary nitrogen behaved, as electron donors in photochemical reduction of dye. H+/ 2e- + MG + EDTA MGH alkali Results showed that dye reduction process depend upon the concentration of EDTA and alkali where as it was independent upon the dye concentration. The salting agent KNO3 has been found to decline overall rate of reduction of methylene green with EDTA in aqueous medium as well as in mixed solvent system. Detailed kinetics and thermodynamics aspects have been discussed in relevant section of discussion to realize the interaction between methylene green and EDTA. Regeneration of color was observed upon turning off source of light. Effect of increasing concentration (10-30%) of water 1 miscible organic solvent (methanol and ethanol) on dye decoloration showed that rate of reaction increase as the dielectric constant of water decreases. Spectral intensities of dye in binary solvent mixture of different composition showed the dominating character of aqueous medium with no remarkable difference in values of λmax. This may be attributed to the similar reaction of H+ ion abstraction from reductant in mixed solvent system as in aqueous medium. During the course of study of the kinetics of reduction of methylene green, it was found that mixed solvent medium results in pronounced enhancement of rate in EDTA and at alkaline pH while in case of methylene green the following order was observed H2O > Methanol > Ethanol. An attempt has been made to give an explanation of imperative role of dielectric constant of the medium taking into consideration of single sphere and double sphere complex in reaction mixture. Our study revealed that single sphere complex was found to be most suitable complex existed in the mixed organic solvent with dye and reductant. In presence of alkali and nitrate ion concentration the values of single sphere complex model were not coinciding with the values obtained theoretically in mixed solvent system. Therefore HPLC analysis was carried out to check the dye molecule degraded or not. HPLC analysis suggested that a significant amount of the dye degrade in presence of nitrate ion and alkali and additional peaks which may be off by product were produced. This leads to confirm the non identical values of single sphere and double sphere model in presence of nitrate and alkali. Rate of removal of color showed a linear relationship with respect to water content below 30% and temperature between 20- 40 oC where as increase in concentration of organic solvent showed the inhibition of dye decoloration at given optimum condition. Therefore study was restricted up to 30% of methanol/ ethanol binary mixtures.
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