Synthesis, identification and thermal decomposition of double sulfites like Cu2SO3·MSO3·2H2O (M=Cu, Fe, Mn or Cd)

Double sulfites with empirical formula Cu2SO3·MSO3·2H2O (where M is Cu, Fe, Mn, or Cd) were obtained by saturation with sulfur dioxide gas of an aqueous mixture of MII sulfate and copper sulfate at room temperature. The salts obtained were identified by infrared spectra, X-ray powder diffraction and elemental analysis. The compounds studied are isostructural with the CuII replacement by MnII, FeII, and CdII in Chevreul’s salt (Cu2SO3·CuSO3·2H2O). The thermal behavior of the double sulfites was evaluated by thermogravimetry analysis (nitrogen and air atmospheres) and differential scanning calorimetry. These salts are thermally stable up to 200°C. The structures of sulfite ion coordination influence strongly the course of the thermal decomposition. The sulfite species coordinated to the metal through the oxygen are easier oxidized to sulfate than sulfur-coordinated species, leading to preferential formation of MIISO4 and Cu2O in the first step. The weight gain relative to the second step can be due to oxidation of Cu2O to CuO and/or oxidation of MIISO3 to MIISO4 probably by self-generated atmosphere formed by dense volatile products liberated in the latter step.