Mechanistic Approach to the Oxidation of Poly(ethylene glycol) by Potassium Ferricyanide in Basic Media with the Orientation on the Type of Transfer of Electrons in the Absence and Presence of a Ru(III) Catalyst

Abstract

A conventional spectrophotometer was used for the kinetic investigation of catalyzed oxidation of water-soluble alcoholic poly(ethylene glycol) (PEG) by hexacyanoferrate(III) in alkaline media at a constant ionic strength of 2.0 mol dm −3 using ruthenium(III) as a catalyzing agent. However, the redox reaction was relatively slow, but the presence of Ru(III) was found to catalyze the reaction to remarkable rates. The pseudo-first-order kinetics indicated a first-order reaction with respect to concentrations of both reactants and second-order overall reaction kinetics without using the catalyzing agent; however, a second-order reaction with respect to the oxidant and a fractional first-order reaction with respect to both poly(ethylene glycol) and alkali concentrations were detected in the presence of the catalyst. When the alkali concentration increased, the oxidation rates increased, which means that the present redox system was of a base-catalyzed nature. Kinetic evidence for the formation of a 1:1 intermediate complex with interference of free radicals in the redox system mechanism was revealed. Pseudo-firstorder rate constant values of 7.85 × 10 −4 s −1 in the absence of the catalyst and 2.95 × 10 −3 in the presence of the catalyst (5 × 10 −5 mol dm −3 ) were evaluated under reaction conditions of [PEG] = 0.2, [Fe(CN) 6 ] 3− = 7 × 10 −4 , [OH − ] = 1.0, and I = 2.0 mol dm −3 at 30 °C. A possible redox system mechanism in the studied oxidation−reduction reaction is proposed and discussed in view of the kinetic results obtained.