DETERMINATION OF KINETIC-PARAMETERS FOR THE DEHYDRATION OF CALCIUM-OXALATE MONOHYDRATE BY DIFFUSE-REFLECTANCE FT-IR SPECTROSCOPY

Variable-temperature diffuse reflectance infrared spectroscopy was use d to study the reaction kinetics of the dehydration of calcium oxalate monohydrate. The kinetic analysis of the dehydration process indicate s that there are two reactions taking place, as well as the presence o f two different equimole water molecular environments in the crystal s tructure of calcium oxalate monohydrate. Partial least-squares calibra tion of the dehydration profiles was carried out to identify the evolu tion sequence of the two different types of water molecules in the cry stal structure of calcium oxalate monohydrate. The results indicate th at the water molecules in the crystal structure which are responsible for the stretching absorptions at 3428 and 3336 cm-1 are eliminated fi rst at a higher rate, during the first part of the reaction, and the o thers responsible for the absorptions at 3486 and 3058 cm-1 are elimin ated at a later stage. The unexpected and unidentified band arising fr om bending modes of one of the types of water molecules is shown to ar ise from the same water molecules that are responsible for the stretch ing absorptions at 3486 and 3058 cm-1. Kinetic parameters were determi ned for the dehydration step, and the overall activation energy calcul ated for the dehydration step is in agreement with values reported in the literature. However, the clear presence of two distinct reactions invalidates all previous determinations of reaction order.