THE THERMAL-DECOMPOSITION OF NAHCO3 POWDERS AND SINGLE-CRYSTALS - A STUDY BY DSC AND OPTICAL MICROSCOPY

The thermal decomposition of four commercial powders and of differentl y stored single crystals of sodium hydrogen carbonate is studied by po wer compensation DSC and by optical and FT-IR microscopy. Independentl y of manufacturer, specified purity and price, the thermal curves of a ll the commercial powders show a more or less pronounced low temperatu re peak preceding the one due to the main decomposition. Such small pe ak is not observed when samples of laboratory recrystallized material are used. However the thermal behaviour of the latter preparation diff ers remarkably depending on storage conditions: the material kept in c losed glass containers decomposes at temperatures higher than those of the material stored in a dessiccator in the presence of concentrated H2SO4.The observation by optical microscopy of the behaviour of the su rfaces of single crystals coming from different storage conditions whe n the temperature is raised in a Kofler heater helps the interpretatio n of the data collected. The mechanism of the decomposition is discuss ed and the relevant kinetic parameters reported.