Thermally decarboxylated sodium bicarbonate: Interactions with water vapour, calorimetric study

摘要

Isothermal titration calorimetry (ITC) was used to study interactions between water vapour and the surface of thermally converted sodium bicarbonate (NaHCO 3 ). The decarboxylation degree of the samples was varied from 3% to 35% and the humidity range was 54–100%. The obtained enthalpy values were all exothermic and showed a positive linear correlation with decarboxylation degrees for each humidity studied. The critical humidity, 75% (RH o ), was determined as the inflection point on a plot of the mean?ΔHkJ/mole Na 2 CO 3 against RH. Humidities above the critical humidity lead to complete surface dissolution. The water uptake ( m ) was determined after each calorimetric experiment, complementing the enthalpy data. A mechanism of water vapour interaction with decarboxylated samples, including the formation of trona and Wegscheider’s salt on the bicarbonate surface is proposed for humidities below RH o .