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Chapter II Solution Growth….
dissolved state and in the form of solids. If the crystallizing substance is an
electrolyte, an excess of composite ions of any sign may serve as impurities.
In crystallization from solution, the solvent itself represents a massive
impurity in the system [111]. It is not merely a separate phase through which
the solute must diffuse towards the growing crystal face; but it can also
interact with the solute. It can be involved in the salvation of ions or
molecules in solution, in the adsorbed layer and at the active growth sites.
The solvent, therefore, can have an important influence on the crystallization
process.
Crystal nucleation can occur either homogeneously or
heterogeneously or by secondary nucleation. Homogeneous nucleation
occurs either spontaneously or as induced. On careful examination, many
reports of spontaneous nucleation are found to have been induced in some
way. It is generally accepted that true examples of spontaneous nucleations
are rarely encountered. A super-cooled system can be seeded unknowingly
by the presence of atmospheric dust. Aqueous solutions prepared in the
laboratory normally may contain 10 6 to 10 8 solid particles per cm 3 acting as
heteronuclei. Large volumes of a given system nucleate spontaneously at
smaller degrees of super-cooling than small volumes and, therefore, the
larger samples stand a greater chance of being contaminated with active
heteronuclei. The size of the solid foreign bodies is important and there is an
evidence to suggest that the most active heteronuclei in liquid solution lie in
the rage 0.1 to 1 m.
Heterogeneous nucleation occurs due to materials in
contact with the solution. Minute cracks and crevice in the surface of the
nucleation cell may act as a catalyst for nucleation.
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