The spontaneous crystallisation of binary mixtures.— Experiments on Salol and Betol

Abstract

In a previous paper by the present authors, it was shown that, for the aqueous solutions of certain salts, a definite curve may be traced which gives the temperature of spontaneous crystallisation for a supersaturated solution of any given strength. “Spontaneous crystallisation” is to be understood to signify that, at and below these temperatures, crystallisation may he started by purely mechanical means. This curve, to which they have given the name of “supersolubility curve” (SH or s H of fig. 1), is, for the salts examined by them, nearly parallel to the solubility curve (ME or m E of fig. 1), and generally separated from it by an interval of about 10°. To use Ostwald’s phraseology, it indicates, on the temperature-concentration diagram, the boundary between the area representing the “metastable” condition, in which crystallisation can only take place in contact with a solid crystal, and the area representing the “labile” condition, in which crystallisation may take place spontaneously. Previous investigators have usually endeavoured to trace the process of crystallisation in alloys, fusion mixtures, and solutions, by determining the constitution of the solid which separates. We have endeavoured rather to determine the changing constitution of the liquid, making use of the refractive index for this purpose. The curve was traced, not only by means of the refractive index, which attains a maximum value at the temperature of spontaneous crystallisation, but also by the development of a dense cloud of crystals, which makes its appearance at the same temperature when the solution is stirred. This we term a “labile” shower, in order to distinguish it from the much thinner “metastable” showers, which may occur before this temperature is reached. Solutions enclosed in sealed tubes, so as to be preserved from chance inoculation by dust of the crystalline solid, cannot be made to crystallise by shaking at any temperature higher than that given by the supersolubility curve.