Effects of three different organic acids including racemic
malic
acid, succinic acid, and citric acid, which act as an impurity on
the solubility and metastable zone width of zinc lactate (ZnL2), have been studied. The results show that the presence of
all examined impurities increases the solution solubility and the
values of solubility increase with increasing impurity concentration.
The introduction of impurity also leads to a reduction on the metastable
zone width, and the reductions are pronounced when the impurity concentration
increased. Further, experimental data of metastable zone width were
analyzed using the expression of the Nývlt’s approach
and self-consistent Nývlt-like approach, which can be expressed
in the form: ln(ΔT
max/T
0) = Φ + β ln b, with intercept
Φ = {(1 – m)/m}ln(ΔH
d/RT
lim) + (1/m)ln(f/KT
0)
and slope β = 1/m. Here T
0 and T
lim are the saturation and
nucleation temperature, respectively. m is the apparent
nucleation order, and K is a new nucleation constant
related to the factor f defined as the number of
stable nuclei per unit volume, ΔH
d, the heat of dissolution and R the gas constant.
Comparing to the former one, the latter approach provides a more satisfactory
estimation for the metastable zone width at varying saturation temperature T
0. The constant β for specific system
reveals independence of the temperature, while the constant Φ
increases with increasing saturation temperature. In addition, both
constants are proportional to the impurity concentration. Crystal
habits of final products are also influenced in the presence of impurities,
but the crystal structures are barely changed.