EFFECT OF RELATIVE-HUMIDITY AND TEMPERATURE ON MOISTURE SORPTION AND STABILITY OF SODIUM-BICARBONATE POWDER

The effects of relative humidity and temperature on moisture sorption rate and stability profile of sodium bicarbonate powder exposed to var ious storage conditions were investigated using gravimetrical and acid -titration methods. In the study, samples were placed in desiccators, which contained various saturated salt solutions to generate desired l evels of relative humidity, denoted as RH, ranging from 30-93% RH. The weight of each sample was determined periodically. The results show t hat both the temperature and relative humidity are key factors in mois ture sorption isotherm and decomposition kinetics. The threshold relat ive humidity RHth is defined as the relative humidity above which sodi um bicarbonate powder starts to rapidly gain weight. The critical rela tive humidity RHc is defined as the relative humidity above which sodi um bicarbonate powder either starts to rapidly gain weight or starts t o rapidly hydrolyze (resulting in losing weight). The moisture sorptio n isotherm shows an unusual phenomenon at high RH, where the weight of the sodium bicarbonate powder either increases or decreases with time , depending on the storage temperature. When the powder is exposed to a lower temperature and high humidity level (e.g. at 25 degrees C and 93% RH), the weight increases rapidly, due to the fact that the relati ve humidity exceeds the RHth at this temperature. On the other hand, w hen the powder is exposed to higher temperature and high humidity leve ls (e.g. at 40 degrees C, 89% RH and at 55 degrees C, 82% RH), the wei ght of the powder decreases with time since the relative humidity is b elow the RHth at these temperature levels and the decomposition rate i s higher than the moisture sorption rate. The decomposition of sodium bicarbonate results in the release of carbon dioxide. The values of RH c at 25 and 40 degrees C were determined to be between 76 and 88% RH a nd between 48 and 75% RH, respectively. The accurate moisture content and the stability profiles of sodium bicarbonate were determined using a titration-drying-titration procedure. The results of determination show that sodium bicarbonate powder is stable below 76% RH at 25 degre es C and below 48% RH at 40 degrees C, respectively. (C) 1998 Elsevier Science B.V. All rights reserved.