The decomposition of permanganate in molten alkali-metal nitrates

Abstract

The decomposition of MnO₄- has been studied in molten (K/Li)NO₃ eutectic at temperatures between 150º and 200ºC. The rate of decomposition in a Pyrex vessel increased in the presence of traces of NO₂^-, Br^- and CN^- and of the manganate(1V) precipitate produced in the decomposition. Various transition and heavy metal ions were also found to accelerate the decomposition of MnO₄^- : the rate law appears to be -d[MnO₄^-]/dt = k[Mⁿ⁺]^2/n[MnO₄^-] where n = oxidation state of metal ion M which was added to the melt solution of MnO₄^- (in the presence of a fixed weight of insoluble reaction product). Nickel and cobalt(11) ions are oxidized by MnO₄^- to insoluble nickel(111) and cobalt(111) manganates(1V). Whilst NO₂^- produced by the thermal decomposition of NO₃^- appears to be the active reductant in (K/Li)NO₃ melts, the reaction is complicated by being partly heterogeneous. The method of preparation of the reaction product critically affects the rate constant k.