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Photokinetic Study of Methylene Green with Dimethylamine
Rehana Saeed
and Fahim Uddin*
Photochem Research Lab, Dept. of Chemistry,
University of Karachi, Karachi-75270,
e-mail:
fahim_uddin01@yahoo.com, Pakistan
Abstract:
The photokinetics of methylene green with dimethylamine in 50% aqueous
isopropanol was studied. The quantum yield (j)
for the photochemical reaction of methylene green with dimethylamine
was determined as a function of concentrations of methylene green,
concentrations of reductant [AH2], acidity [Ho]
and temperature. The photochemical reduction was carried out in a
special type of optical processor associated with a deoxygenating
system, a temperature controlling unit and a magnetic stirring system.
A monochromatic light of 657 nm wavelength was used for irradiation of
oxygen free reaction solutions in a double walled reaction cell. The
transmitted light was measured and recorded in terms of electrical
signals with the help of a calibrated galvanometer. Acidities of the
solutions were measured with the help of a spectrophotometer based on
the Hammett acidity function [Ho]. It was observed that the
quantum yield varies with reductant concentration [AH2] and
is independent of the concentration of methylene green at specific
values of acidity and temperature. The results have been interpreted
in terms of the reaction mechanism. The values of specific rate
constants and ratios of rate constants were determined. It was found
that 2 equilibria exists: i.e. one between the triplet state of
methylene green with the proton and the protonated triplet state of
methylene green [ ], and the other between the protonated triplet
state of methylene green with reductants and associated the complex of
methylene green [ ]. Thermodynamic parameters were also evaluated as a
function of acidity.
Key
words: Photokinetics, reduction, methylene
green, dimethylamine, mechanism, thermodynamic parameters.
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