Formation, characterisation and redox behaviour of water-soluble colloidal manganese dioxide

Abstract

The water-soluble colloidal MnO2 solutions were prepared by the reduction of permanganate by two sulphur-containing reductants (methionine and thiourea) in aqueous neutral media. The nature of the oxidising species present in solution was characterised by coagulation, spectrophotometric and transmission electron microscopic measurements. Upon mixing aqueous solutions of methionine and , a readily distinguishable yellow colour (spherical-shaped; size ca. 4 nm) appears immediately at 25°C. The oxidation rates of ethylenediaminetetraacetic acid (EDTA) with colloidal MnO2 were determined as a function of EDTA (2.0 × 10−4 to 10.0 × 10−4 mol dm−3) and Mn(II) at constant temperature (25°C). The rate constants were calculated by using the second-order kinetic conditions. The effects of added salts, such manganese(II) chloride, sodium fluoride, silver nitrate and sodium perchlorate have been investigated. Order of mixing of Mn(II) has a composite effect on both the redox reactions, i.e. a coloured intermediate (λmax = 500 nm) was formed within the time of mixing of Mn(II), EDTA and colloidal MnO2 which is composed of the EDTA and Mn(III). Mechanisms consistent with the observed kinetic data have been proposed and discussed.