Thermoanalytical studies on the non-isothermal dehydration and decomposition of <i>dl</i>-lactates of a series of transition metals

Abstract

The non-isothermal decomposition of a series of <i>dl</i>-lactates hydrates of chromium(III), manganese(II), iron(II),cobalt(II), nickel (II), copper(II) and zinc(II) has been studied with dry air as the purge gas and heating rate programmed at 10 Kmin^-1. The α-<i>T </i>plots obtained from the TG data of none of the purely dehydration steps and the decomposition steps of

cobalt(II) and zinc(II) compounds show any induction period, indicating no physical desorption, nucleation or branching while the α<i>-T </i>plots of the other decomposition phenomena show a period of induction indicating that physical desorption and nucleation are taking place. The TG data of the dehydration steps have also been analyzed using various differential, difference-differential, and integral methods, viz., Freeman-Carroll, Horowitz-Metzger, Coats- Redfern, Zsako, Fuoss-Salyer-Wilson and Karkhanavala-Dharwadkar methods. Values of activation entropy, Arrhenius factor, and order of reaction have been approximated and compared. The values of activation energy (<i>E</i>) for the dehydration steps are less than 60 kJmol^-1 as expected for removal of weakly bound water of crystallization from such compounds. The entropy of activation (Δ<i>S</i>^#) values have high magnitude in each case (˂ -150 JK^-1mol^-l) indicating an appreciable degree of rearrangement.