The solid state dehydration of d lithium potassium tartrate monohydrate is complete in two rate processes I. The deceleratory diffusion-controlled first reaction

Abstract

A kinetic and mechanistic study of the dehydration of d lithium potassium tartrate monohydrate has been undertaken. Water evolution is completed through two separate rate processes. The first reaction is the deceleratory, diffusion-controlled release of water from the superficial zones of the reactant crystals. The yield of this process corresponds to the dehydration of a superficial layer of crystal, thickness 10 µm. About 4% of the constituent water was evolved from the single crystals studied, rising to 50% from crushed powder reactants. The second reaction, reported in Part II, is a nucleation and growth process yielding the crystalline anhydrous salt. Gravimetric measurements for the first reaction identified three distinct dehydration processes. The first step was the rapid release of loosely bonded superficial water. The subsequent two deceleratory stages are characterized as diffusive loss of H 2 O molecules from a crystal zone that is at first ordered but later becomes disordered as the water-site vacancy concentration increases. Rate measurements based on water evolution measured the activation energy of this third step as 153 + 4 kJ mol -1 . Irreproducibility of rate data is ascribed to variations in numbers and distributions of imperfections between individual crystals. The extent and rate of the first reaction increased when initiated in small pressures of water vapour. Electron microscope observations identified a structural discontinuity ca. 1 µm below reacted crystal faces, evidence of superficial retexturing of the reactant. Rates of powder dehydrations were more reproducible than those of crystals but the kinetic behaviour was similar. The same rate equations were obeyed and the activation energy was unaltered. Water loss during the first reaction of this crystalline hydrate gives a comprehensive layer of extensively dehydrated material across all surfaces. Subsequently, in or under this water depleted layer, salt is recrystallized and dehydration continues as a nucleation and growth reaction (part II, following paper).