Occurrence of colloidal manganese dioxide in permanganate reactions

Abstract It is shown that there exist several forms of colloidal MnO2 stable in both aqueous and dichloromethane solutions. Stabilization of the colloid may occur in two different ways: decreasing the polarity of the particles at the colloid surface and endowing the colloidal particles with a negative electrostatic charge. The manganese species identified in other works as a manganese(IV)—phosphate complex or as a cyclic manganate(V) diester (among others) are actually different forms of colloidal MnO2. A mathematical model for the UV—Vis spectrum of this species, taking into account both the light-absorbing and light-scattering contributions to the total absorbance, is proposed. From both nephelometric and spectrophotometric measurements, it has been estimated that the average size of the colloidal particles of several forms of colloidal MnO2 is in the range N = (0.41–2.93) × 106 (MnO2 units)/particle. The contribution of a single MnO2 unit to the polarizability of a colloidal particle has been estimated to be α 0 = 26.7 ± 1.4 A 3 . The ways for reduction and coagulation of colloidal MnO2 to occur are also discussed.