MEASUREMENT OF CRYSTALLIZATION OF POTAS-SIUM BROMATE FROM ITS QUIESCENT AQUEOUS SOLUTION BY DIFFERENTIAL SCANNING CALORIMETER

GROWTH RATE OF CRYSTALLITE

Abstract

Heat evolution rate curves were recorded with KBrO₃-quiet supersaturated aqueous solution at constant cooling temperature, T, by using a differential scanning calorimeter and were analyzed on the basis of the growth rate of all crystallites pre-formed in the sample solution. Taking microscopic observation into account, the expression of the overall growth rate, R, was derived, on the assumption that the crystallite surface is Kossel''s and that its growth rate is determined by two-dimensional nucleation rate, J''=k₁''S^m''.
R=βhAJ''≈k₁"(C₀-C)^2/3S^m'' (1)
where C, C₀ and S are concentration at t=t and =0 and supersaturation ratio, respectively. m'' is number of solute in a critical nucleus and is not constant, but depends on S and T.
The values of two-dimensional nucleation parameters (m'' radius, free edge energy σ'' and surface energy estimated from σ'' and activation energy) were of reasonable order of magnitude. Comparison of these values with those of the three-dimensional parameters obtained previously is discussed. It may be concluded that Eq. (1) and hence the expression for J'' proposed in this paper are adequate for the growth rate and two-dimensional nucleation rate, respectively.