THE SOLID-STATE DEHYDRATION OF D-LITHIUM POTASSIUM TARTRATE MONOHYDRATE IS COMPLETED IN 2 RATE-PROCESSES .1. THE DECELERATORY DIFFUSION-CONTROLLED 1ST REACTION

A kinetic and mechanistic study of the dehydration of d lithium potass ium tartrate monohydrate has been undertaken. Water evolution is compl eted through two separate rate processes. The first reaction is the de celeratory, 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 mu m. About 4 % of the constituent water was evolved from the single crys tals studied, rising to 50 % from crushed powder reactants. The second reaction, reported in Part II, is a nucleation and growth process yie lding the crystalline anhydrous salt. Gravimetric measurements for the first reaction identified three distinct dehydration processes. The f irst step was the rapid release of loosely bonded superficial water. T he subsequent two deceleratory stages are characterized as diffusive l oss of H2O molecules from a crystal zone that is at first ordered but later becomes disordered as the water-site vacancy concentration incre ases. Rate measurements based on water evolution measured the activati on 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 v apour. Electron microscope observations identified a structural discon tinuity ca. 1 mum below reacted crystal faces, evidence of superficial retexturing of the reactant. Rates of powder dehydrations were more r eproducible than those of crystals but the kinetic behaviour was simil ar. The same rate equations were obeyed and the activation energy was unaltered. Water loss during the first reaction of this crystalline hy drate gives a comprehensive layer of extensively dehydrated material a cross all surfaces. Subsequently, in or under this water depleted laye r, salt is recrystallized and dehydration continues as a nucleation an d growth reaction (part II. following paper)