A Study in the Mechanics of Crystal Growth from a Supersaturated Solution

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Abstract
An experimental investigation of the concentration field during crystallization is made using an improved modification of W. F. Berg's interference technique. Na2S2O3·5H2O is crystallized at 23°C; concentrations between 40 percent and 55 percent. Graphing the positions of the growing faces versus time indicates, in all cases, a uniform rate of growth with occasional abrupt changes. These discontinuities were, in general, correlated with changes in the perfection of the growing face, the disappearance of a region of poor growth being associated with a decrease rate, whereas the appearance of an imperfection led to a higher rate. A minimum of concentration is observed near the face center, corroborating previous investigations with NaClO3. A diffusion coefficient of 1.26×10−5 cm2/sec was calculated for Na2S2O3 corresponding to 0.80×10−5 cm2/sec extrapolated from J. D. R. Scheffer's data. There is no strict correlation between maximum, minimum, or average concentration at a crystal face and its rate of growth. It appears that the variations in concentration revealed by interferometer maps are to be interpreted as a consequence of solution depletion occasioned by the crystal's growth.

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