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

Variable-temperature diffuse reflectance infrared spectroscopy was used to study the reaction kinetics of the dehydration of calcium oxalate monohydrate. The kinetic analysis of the dehydration process indicates that there are two reactions taking place, as well as the presence of two different equimole water molecular environments in the crystal structure of calcium oxalate monohydrate. Partial least-squares calibration of the dehydration profiles was carried out to identify the evolution sequence of the two different types of water molecules in the crystal structure of calcium oxalate monohydrate. The results indicate that the water molecules in the crystal structure which are responsible for the stretching absorptions at 3428 and 3336 cm-1 are eliminated first at a higher rate, during the first part of the reaction, and the others responsible for the absorptions at 3486 and 3058 cm-1 are eliminated at a later stage. The unexpected and unidentified band arising from bending modes of one of the types of water molecules is shown to arise from the same water molecules that are responsible for the stretching absorptions at 3486 and 3058 cm-1. Kinetic parameters were determined for the dehydration step, and the overall activation energy calculated 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.