Kinetic approach to the mechanism of the redox reaction of malachite green and permanganate ion in aqueous acidic medium

Abstract

The redox kinetic and mechanistic studies of the reactions of malachite green (MG
+
) and MnO4
–
were
carried out in aqueous acidic medium, at a temperature of 32.5 ± 0.5°C; ionic strength,  = 0.50 mol dm
–3
(Na2SO 4), [H
+
] = 5.00 x 10
–3
mol dm
–3
(H2SO4). In the stoichiometry, one mole of malachite green was
consumed by one mole of MnO4
–
. The reaction is first order in both [MG
+
] and the [MnO4
–
]. The rates of
redox reaction showed dependence on acid concentrations (in the acid range used). Rate equation for
the reaction has been proposed as: –d[MG
+
]/ dt = (a + b[H
+
])[MG
+
][MnO4
–
]. At [H
+
] = 5.00 x 10
–3
mol dm
–
3
, the second order rate constant for the malachite green – MnO4
–
reaction was found to be (11.96 ±
0.13) x 10
–3
dm
3
mol
–1
s
–1
. The rates of reaction displayed negative salt effect. Log k1 versus 1/D gave
positive slope for the reaction. Added anions and cations catalysed the malachite green – MnO4
–
reaction. Results of the Michaelis- Menten analysis gave no evidence of intermediate complex
formation. Based on the results obtained experimentally, the outer sphere mechanism is proposed for
the malachite green – MnO4
–
reaction.