The thermal decomposition of potassium permanganate and related substances. Part I. Chemical aspects

Abstract

The thermal decomposition of KMnO₄ in the temperature range 25–900° in air and nitrogen has been studied by thermogravimetric analysis (TGA), differential thermal analysis (DTA), X-ray diffraction, and chemical analysis. The idealised decomposition equation (250°, air) was found to be 10 KMnO₄→ 2·6₅ K₂MnO₄+(2·3₅ K₂O,7·3₅ MnO_2·05)+ 6 O₂.

Decomposition in air at higher temperatures (up to 540°) or in nitrogen gave the same amount of K₂MnO₄ but larger amounts of O₂ owing to changes in composition of the ‘K₄Mn₇O₁₆’ phase (in brackets). This poorly crystalline material rapidly absorbs water vapour from the atmosphere to form a poorly crystalline hydrate ‘K₄Mn₇O₁₆,nH₂O’(n ca. 5); the hydrate dehydrates on heating above 100°. ‘K₄Mn₇O₁₆’ and its hydrate have different (but unknown) crystal structures. The hydration and dehydration reactions are rapid and reversible.

Decomposition of K₂MnO₄ above 540° in air and nitrogen has been studied by the same techniques. The decomposition equation (620°, nitrogen) was found to be 10 K₂MnO₄→ 5·7 K₃MnO₄+ 0·5(2·9 K₂O,8·6 MnO_2·1)+ 3·40 O₂

This reaction was shown to be reversible in air. K₂MnO₄ reacts with atmospheric CO₂ to form unidentified phases at ca. 400° but these give the same decomposition products at higher temperatures as were obtained in a CO₂-free atmosphere. K₃MnO₄ was shown, by high-temperature X-ray diffraction, to undergo a number of phase changes on heating to 900°; there is considerable hysteresis on cooling. K₃MnO₄ is stable in both dry air and nitrogen in the temperature range 25–900° but reacts rapidly with water vapour.

There is good agreement between our experimental results and most of those of earlier workers; there are, however, considerable differences in interpretation. Similar but less detailed studies were made for the isomorphous Rb compounds.