Oxidation of inorganic compounds by aqueous permanganate: Kinetics and initial electron transfer steps

Abstract

Abstract In this study, the kinetics of the reactions between aqueous permanganate (Mn(VII)) and selected inorganic compounds (e.g. KSCN, NaNO2, Na2SO3, Na2S2O3, KI, N2H4, H2O2, and NH2OH) were investigated in aqueous solutions at pH 5–9 with excess pyrophosphate (L) by detecting the decay of Mn(VII) using a stopped-flow technique. With excess L, soluble Mn(III)L rather than MnO2 was generated, which had negligible interference on examining the change of the absorbance of Mn(VII) at 525 nm (∼45 M−1 cm−1 for Mn(III)L vs 2500 M−1 cm−1 for Mn(VII)). The reactions of Mn(VII) with Na2SO3, Na2S2O3, KSCN, NaNO2, and KI (referred to as SA) exhibited second-order kinetics. Comparatively, the reactions of Mn(VII) with N2H4, H2O2, and NH2OH (referred to as SB) showed two-phase kinetics (i.e., an initial lag phase and a secondary fast phase), and the second-order rate constants (k1, M−1 s−1) were calculated in the initial phase. In the second phase, Mn(III)L reacted with SB producing Mn(II), and then Mn(II) reacted fast with Mn(VII) producing Mn(III)L. The kinetics of the reaction between Mn(II) and Mn(VII) with excess L at pH 4–9 were examined and the loss rate of Mn(VII) increased with increasing pH value. The species-specific second-order rate constants (k1i, M−1 s−1) for the reactions of Mn(VII) with inorganic compounds were determined by the experimental k1 values. Furthermore, a linear free energy relationship (LFER) was found (log(k1i) = 2.86 − 1.90E(2)0) between log k1i and 2−e− reduction potentials for inorganic species (i.e., SO32−, S2O32−, SCN−, H2O2, NH2OH, I−, and N2H4) while only NO2− showed 1−e− transfer process, indicating highly active aqueous manganese intermediates (Mn(V) and Mn(VI)) formed in situ upon Mn(VII) reduction.