Aims: Reactions of manganese(III)-porphyrins have been reported as possible models
for closely related and biologically significant systems.
Method: In this study, a stock solution of manganese(III) sulfate was prepared using a standard
method of anodic oxidation of manganese(II) in H2SO4. The Indigo carmine (IC)-Mn(III) reaction
catalyzed by Ru(III), under pseudo-first-order conditions, has been spectrophotometrically
monitored at the IC λmax IC (610 nm) at constant temperature. The experimental rate law for the
reaction is: rate = k' [IC][H+]x[Ru(III)]y, where x and y are fractional orders.
Objective: The reaction rate shows a zero-order dependence on the concentration of the oxidant,
Mn(III), indicating its involvement in fast steps following the slow step. Additionally, the effect on
the rate of adding the reduction product, Mn(II), has been found to be negligible. Variations of the
ionic strength and the dielectric constant of the reaction medium have negligible effect on the rate.
Result: Based on the effect of temperature, activation parameters have been evaluated using
Arrhenius and Erying plots. A suitable mechanism has been presented. The rate law derived
is consistent with experimental data.