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Kinetics of sodium fluosilicate precipitation
Author(s) -
Söhnel O.
Publication year - 1976
Publication title -
kristall und technik
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.377
H-Index - 64
eISSN - 1521-4079
pISSN - 0023-4753
DOI - 10.1002/crat.19760111104
Subject(s) - supersaturation , nucleation , precipitation , crystallography , crystal (programming language) , chemistry , thermodynamics , aqueous solution , kinetics , surface tension , dislocation , phase (matter) , materials science , analytical chemistry (journal) , chromatography , physics , organic chemistry , meteorology , computer science , programming language , quantum mechanics
The formation of Na 2 SiF 6 by discontinuous precipitation of dilute H 2 SiF 6 with a 40% excess of an aqueous solution of NaCl under various conditions was studied. The values of induction time, number of crystals formed, their final size and their habit were determined during precipitation from solutions, whose initial supersaturation was 2 < S < 11. At S < 7–8 the crystals were formed by heterogeneous nucleation, whereas at S ≳ 7–8 homogeneous nucleation mechanism began to prevail. Once formed, the Na 2 SiF 6 crystals were growing according to the screw‐dislocation mechanism till they reached visible size; the corresponding values of kinetic order of nucleation and of the growth rate constant were g = 1.35 and k g = 4.32 × 10 −8 cm 2.05 sec −1 g −0.35 , resp. The value of interfacial tension on the phase boundary Na 2 SiF 6 crystal — saturated solution was determined (σ ∼ 52 erg/cm 2 ). The resulting Na 2 SiF 6 crystals conformed to log‐normal distribution irrespective of conditions of precipitation. The dependence of the final size of crystals on supersaturation exhibited a maximum at S ∼ 6. Crystals of Na 2 SiF 6 had a hexagonal habit, which was near to a spherical form at lower supersaturations, while dendritic crystals were formed at higher supersaturations.