Effect of relative humidity and temperature on moisture sorption and stability of sodium bicarbonate powder

Abstract

The effects of relative humidity and temperature on moisture sorption rate and stability profile of sodium bicarbonate powder exposed to various 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 levels of relative humidity, denoted as RH, ranging from 30–93% RH. The weight of each sample was determined periodically. The results show that both the temperature and relative humidity are key factors in moisture sorption isotherm and decomposition kinetics. The threshold relative humidity RH th is defined as the relative humidity above which sodium bicarbonate powder starts to rapidly gain weight. The critical relative humidity RH c is defined as the relative humidity above which sodium bicarbonate powder either starts to rapidly gain weight or starts to rapidly hydrolyze (resulting in losing weight). The moisture sorption 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°C and 93% RH), the weight increases rapidly, due to the fact that the relative humidity exceeds the RH th at this temperature. On the other hand, when the powder is exposed to higher temperature and high humidity levels (e.g. at 40°C, 89% RH and at 55 oC, 82% RH), the weight of the powder decreases with time since the relative humidity is below the RH th at these temperature levels and the decomposition rate is 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°C were determined to be between 76 and 88% RH and 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°C and below 48% RH at 40°C, respectively.