EVALUATION OF HEAT-CONDUCTION MICROCALORIMETRY IN PHARMACEUTICAL STABILITY STUDIES .5. A NEW APPROACH FOR CONTINUOUS MEASUREMENTS IN ABUNDANT WATER-VAPOR

A technique which allows continuous microcalorimetric measurement in abundant water vapour at 100% relative humidity (RH) has been designed and evaluated at 25.0-degrees-C. The incorporation of water of hydration in various types of anhydrous lactose powder was studied. In some experiments, lactose samples were prestored at humidities ranging from 58 to 94% RH and thereafter characterized with the new technique at 100% RH. The incorporation process is the main contributor of heat. The heat contributions from other sources in the sample vessel were also investigated. Heats of adsorption and vaporization were negligible, since they cancelled each other out. However, the heat from the incorporation process was somewhat reduced by the heat of mutarotation from beta- to alpha-lactose and possibly also by the heat from a minute amount of dissolution of the lactose. The enthalpy change for the bonding of water of hydration to roller-dried beta-lactose was calculated as approx. 16 kJ/mol at 25-degrees-C.