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Title of Thesis
Effects Of Solvents On The Reduction Of Methylene Green |
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Author(s)
Noshab Qamar |
Institute/University/Department Details
Department Of Chemistry / Jinnah University For Women,
Karachi |
Session
2009 |
Subject
Chemistry |
Number of Pages
247 |
Keywords (Extracted from title, table of contents and
abstract of thesis)
Dielectric, Constant, Activation,
Parameters, Binary Mixture, Single Sphere, Decoloration,
Degradation, Solvents, Methylene, Effects, Green |
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Abstract
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 opticaldensity 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.
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 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 constantof 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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