THERMAL-DECOMPOSITION OF SOLID SODIUM SESQUICARBONATE, NA2CO3.NAHCO3.2H2O

The thermal decomposition of solid sodium sesquicarbonate has been studied at temperatures between 350 and 487 K in nitrogen and carbon dioxide atmospheres. In nitrogen, a single-stage decomposition to sodium carbonate occurs, following Avrami-Erofeyev kinetics (n = 2), with an inflexion at ca. 390 K. The activation energies are 24 kJ mol-1 far the high-temperature region and 58 kJ mol-1 for the low-temperature region. In carbon dioxide above 435 K, the single-stage reaction follows contracting disc kinetics with an activation energy of 29 kJ mol-1. In carbon dioxide at low temperatures, wegscheiderite (Na2CO3 . 3NaHCO3) and sodium carbonate monohydrate (Na2CO3 . H2O) are formed, and this reaction follows first-order kinetics, with an activation energy of 67 kJ mol-1. Microscopic evidence is also presented. Relationships between the decomposition of sesquicarbonate and other related compounds are discussed.