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STUDIES OF THE PHOTOCHEMICAL KINETICS OF METHYLENE BLUE WITH REDUCTANTS

Khalid, Zahida (2001) STUDIES OF THE PHOTOCHEMICAL KINETICS OF METHYLENE BLUE WITH REDUCTANTS. PhD thesis, University of Karachi, Karachi.

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Abstract

The present investigation reveals the study of photochemical reduction of methylene blue by some organic reductants. For this purpose urea, allylurea, phenyhurea, thiourea and allylthiourea were chosen as suitable reducing materials, because their homologues and benzene ring substituted derivatives are well characterized and are prepared easily in a fairly good crystalline state. The reduction was carried out in 50% v/v aqueous 2-propanol. The photochemical reactions were characterized by using special optical arrangements for the irradiation of deaerated test solutions. As a starting point, the photoreduction of methylene blue was carried out in presence of various reducing agents at different acidities. Acidities of the solutions were determined by Hammett acidity function (Ho). The best selected acidities range found to be from 4.06 to 7.59. The quantum yield calculated at various acidities, decreases as the acidity values increases. The quantum yield of the reaction varies directly with the increase in the concentration of the reductants and temperatures. Morover, it was found that quantum yield for the reaction was independent of the concentration of the methylene blue. The results and experimental findings obtained were also interpreted in terms of comparative reaction mechanism associated with reported work, in terms of structure and concentration of reducing agents. Ratios of the rate constants were calculated by using various relations in proposed mechanism. However tests were also put forwarded to verify the values of the rate constants of various steps. From the present investigation it has become more certain that the quantum yield is essentially controlled by two equilibria. i. The triplet state of methylene blue with the proton and the protonated triplet state of methylene blue. MBH+T + H+ _______ MBH++ 2T H ii. The protonated triplet state of methylene blue with reductants, and associated complex of methylene blue. MBH++2T + AH2 _______ MBH++2T + AH2 Since the quantum yield is directly proportional to the rate constant and varies with temperature, Arrhenius and Eyring relationships were used to calculate activation parameters such as energy of activation (Ea), free energy change of activation (∆G#), enthalpy change (∆H#), and entropy change (∆S#), of activation

Item Type:Thesis (PhD)
Uncontrolled Keywords:methylene blue, photochemical reduction, urea, allylurea, phenyhurea, thiourea, allylthiourea, photo-polymerisation, quantum yield
Subjects:Physical Sciences (f) > Chemistry(f2)
ID Code:794
Deposited By:Mr. Muhammad Asif
Deposited On:19 Oct 2006
Last Modified:04 Oct 2007 21:02

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