Evidence of quasi-intramolecular redox reactions during thermal decomposition of ammonium hydroxodisulfitoferriate(III), (NH4)2[Fe(OH)(SO3)2]·H2O

Abstract

Synthesis of ammonium hydroxodisulfitoferriate(III), (diammonium catena-{bis(μ 2-sulfito-κO,κO)-μ 2-hydroxo-κ2O}ferrate(III) monohydrate) (NH4)2[Fe(OH)(SO3)2]·H2O (compound 1) and its thermal behavior is reported. The compound is stable in air. Its thermal decomposition proceeds without the expected quasi-intramolecular oxidation of sulfite ion with ferric ions. The disproportionation reaction of the ammonium sulfite, formed from the evolved NH3, SO2 and H2O in the main decomposition stage of 1, results in the formation of ammonium sulfate and ammonium sulfide. The ammonium sulfide is unstable at the decomposition temperature of 1 (150 °C) and transforms into NH3 and H2S which immediately forms elementary sulfur by reaction with SO2. The formation and decomposition of other intermediate compounds like (NH4)2SnOx (n = 2, x = 3 and n = 3, x = 6) results in the same decomposition products (S, SO2 and NH3). Two basic iron sulfates, formed in different ratios during synthesizing experiments performed under N2 or in the presence of air, have been detected as solid intermediates which contain ammonium ions. The final decomposition product was proved to be α-Fe2O3 (mineral name hematite).