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THE KINETICS OF PHOTOCHEMICAL REACTION OF METHYLENE GREEN WITH REDUCTANTS

Saeed, Rehana (2001) THE KINETICS OF PHOTOCHEMICAL REACTION OF METHYLENE GREEN WITH REDUCTANTS. PhD thesis, University of Karachi, Karachi.

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Abstract

Thiazine dyes provide a vast field for the studies of photochemical kinetics in the presence of reductants. Methylene green is a member of thiazine dye which undergoes reduction by the visible light with reductant inpresence of buffer solutions. Acidity plays a significant role in the reduction of the dye. The photochemical reduction of methylene green with organic reductant such as methyl amine, dimethy1 atnine, trimethyl amine, ethylamine, diethylamine and triethylamine in 50% aqueous isopropanol was studied. The photochemical reduction was carried out in deoxygenated system with known intensities of typical light in a special type of optical processor. The optical processor is the combination of several components especially designed apparatus arrangements associated with a temperature controlled unit and a magnetically stirring unit. A monochromatic light of 657 nm mercury red line wavelength was used for irradiation of oxygen free reaction solutions in a double walled temperature controlled reaction cell. The transmitted light was measured and recorded in terms of electrical signals with the help of calibrated galvanometer. Acidities of the solutions were maintained b using acetate buffers which was measured with the help of spectrophotometer based on Hammet acidity function [Ho]. The quantum yield (φ) for photochemical reaction of methylene green with reductants such as methylamine, dimethylamine, trimethylamine, ethylamine, diethylamine and triethylamine were determined as a function of various factors such as: (i) effect of concentratuions of methylene green, (ii) effect of concentrations of reductants (AH2), (iii) effect of acidity (Ho) (iv) effect of temperature, (v) effect of nitrogen passing time, (vi) effect of alkyl group associated with amine or the nature of reductant. It was observed that the quantum yield (φ) at constant concentration of methylene green and reductants was found to be a function of acidity (Ho) and temperature. It was also observed that the quantum yield is a function of reductant concentration (AH2) and independent of the concentration of methylene green at constant acidity and temperature. The effect of nitrogen passing through the solution was studied for making complete oxygen free solution system. The results have been interpreted in terms of reaction mechanism. With the help of the proposed reaction mechanism, specific rate constants and the ratios of certain rate constants were determined. The values of specific rate constants for ethylamines are found relatively higher than methylamines, it was concluded that ethylamines are more reactive than methylamines. The protonated tripet excited state of methylene green(MGH++2T) which was quickly associated with hydrogen ion and form complex (MGH++2TAH2). The triplet excited state of methylene green played a major contribution for the determination of quantum yield (φ). The ratios of certain rate constants for the suggested mechanism have been evaluated. It was found that an equilibrium is established between associated complex (MGH++2TAH2) and one form of triplet state of methylene green with reductant. The complex [MGH++2T AH2) is the rate controlling species and quantum yield is controlled by the equilibrium between one form of the triplet state of methylene green with reductant.It was also observed that reactivity of amines increased as the alkyl group increased in amines, the reactivity in terms of quantum yield was observed that. Trimethylamine > Dimethylamine>Methylamine Triethylamine>Diethylamine>Ethylamine And it was also found that ethyl grouped amines are more reactive than methyl grouped amines.

Item Type:Thesis (PhD)
Uncontrolled Keywords:methylene green, thiazine dyes, methyl amine, dimethy1 atnine, trimethylamine, ethylamine, diethylamine, triethylamine, photoreduction
Subjects:Physical Sciences (f) > Chemistry(f2)
ID Code:905
Deposited By:Mr. Muhammad Asif
Deposited On:10 Nov 2006
Last Modified:04 Oct 2007 21:03

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