Fascinating Flexibility in Thermal Analysis

Reduction of manganese oxide
Manganese oxide (MnO₂) is often used in chemistry as an oxidizer but also, for example, as a cathode material in batteries. The STA measurement shows mass steps at approx. 600°C and 950°C which are due to the reduction of MnO₂ into Mn₂O₃ and finally into Mn₃O₄. The values of 9.20% and 3.07% match exactly with the stoichiometrical values thus reflecting the high accuracy of the balance system. Endothermic DSC peaks with enthalpies of 432 J/g and 180 J/g were detected during the reduction steps. The endothermic DSC peak at 1200°C is due to a reversible structural transformation, which is observed at 1148°C upon cooling also as exothermal peak (dashed lines).

Building materials: gypsum and quartz sand
Gypsum and quartz sand are, for example, used in plaster and mortar. The gypsum content of the sample shows a two-step release of H₂O from CaSO₄*2H₂O (dihydrate) into CaSO₄*1/2H₂O (halfhydrate) and finally into CaSO₄ (anhydrite). This requires an entire energy of 122 J/g. Quantitative analysis reveals that the sample contained 23.4% of pure dihydrate. Between approx. 300°C and 450°C, the exothermic formation of β-CaSO₄ with a released energy of 18.3 J/g occurred. The endothermic effect at an extrapolated onset temperature of 573°C is due to the structural α→β transition of quartz (crystalline SiO₂).

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