THE SOLID-STATE DEHYDRATION OF D-LITHIUM POTASSIUM TARTRATE MONOHYDRATE IS COMPLETED IN 2 RATE-PROCESSES .1. THE DECELERATORY DIFFUSION-CONTROLLED 1ST REACTION
Ak. Galwey et al., THE SOLID-STATE DEHYDRATION OF D-LITHIUM POTASSIUM TARTRATE MONOHYDRATE IS COMPLETED IN 2 RATE-PROCESSES .1. THE DECELERATORY DIFFUSION-CONTROLLED 1ST REACTION, Philosophical transactions-Royal Society of London. Physical sciences and engineering, 347(1682), 1994, pp. 139-156
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)