DEVICE FOR DETERMINING SOLUBILITY OVER A WIDE TEMPERATURE RANGE

I. Ya. Medik-Gaikazyan, V. I. Medik-Gaikazyan

(Presented by the scientific seminar of the Radio Engineering Faculty)

This communication describes a device that makes it possible, over a relatively wide temperature range, to determine the solubility of various salts from measurements of the density of a solution, with the aim of establishing the boundaries of the region of metastability of saturated solutions and the influence exerted on this region by impurities. As is known, the presence of a wide region of metastability in a number of cases favors the rapid growth of large single crystals of certain salts from supersaturated aqueous solutions during their cooling [1, 2]. Therefore, the study of solubility and of the influence of various factors on it opens up ways toward the rational selection of effective impurities and their dosages. In connection with the importance of the problem of crystal formation [3], the method for plotting solubility curves acquires special significance.

Fig. 1. Diagram of the device for determining solubility.

The principal part of the device described (Fig. 1) is a vessel consisting of two tubes connected to one another symmetrically

connected to them by tubes of smaller diameter, through which the solution, for the purpose of mixing, is continuously transferred from one half of the vessel to the other (Fig. 2). Each time this operation is accompanied by the division of the liquid into two streams, squeezed one onto the other in a position opposite to their natural gravitational distribution by density, i.e., in such a way that layers which in the adjacent part of the apparatus occupied a lower position each time come out onto the surface. Owing to this, even a few transfers of the liquid ensure a completely uniform distribution of the substance throughout the entire volume; this is facilitated to a considerable extent by the twisting of the liquid at the points where it exits from the narrow tubes into the wide ones. The intensity of the mixing, which is carried out without any splashing, is easy to observe if a supersaturated salt solution containing small crystals in suspension is mixed in the apparatus. Without this it is almost imperceptible.

Fig. 2. Vessel for splash-free mixing of liquid.

The transfer of the liquid is effected by connecting both parts of the apparatus with two bottles, from which air is supplied to and withdrawn from the apparatus by means of the reversible flow of water from one bottle, fixed on a crank and making a slow rotary motion (1–2 rpm) in a vertical plane, into another bottle located at the level corresponding to the mean position of the first bottle. The length of the crank arm must be such as to provide the required air pressure in the apparatus.

The apparatus can easily be thermostated; moreover, in order to eliminate losses of liquid, if mixing is carried out at elevated temperature, reflux condensers are installed.

To obtain solubility curves, a known amount of water is poured into the apparatus and a weighed portion of salt is added. After mixing, carried out until complete dissolution of the salt, the density of the solution corresponding to the given concentration of the solution and the specified temperature is determined with an areometer. The length of the part of the areometer stem that protrudes from the solution is determined with a horizontal microscope to an accuracy of up to 0.1 mm, which corresponds to a change in the density of the solution of the order of \(10^{-4}\ \text{g/cm}^3\). The load on the areometer consists of ba-

pieces of iron wire. This makes it possible, before measuring the density, to move the hydrometer away from the walls of the instrument by means of the electromagnet, or to immerse it in the solution in order to wash from the surface of its stem particles of salt and droplets of solution that have accidentally fallen on it. After repeated density values corresponding to the given solution concentration have been obtained, a new weighed portion of salt is introduced into the instrument and, after mixing, density measurements are made at the new, higher concentration of the solution. The saturation concentration is determined from the point of inflection on the solubility curve (Fig. 3), and

Fig. 3. Curves of the dependence of density on the concentration of an aqueous solution of NH$_4$Cl at different temperatures.

Fig. 4. Saturation curves 1 and supersaturation curves 2 for pure ammonium chloride in water (circles). Saturation curves 1 and supersaturation curves 2 for ammonium chloride in a 0.5% aqueous pectin solution (points).

this point can be approached several times, alternately adding known quantities of water and salt. After this, the temperature is raised and the saturation concentration at the next

temperature. From the inflection points of the curves in Fig. 3, the saturation curve 1 in Fig. 4 is constructed; it has an almost straight course. Moreover, for a pure solution of ammonium chloride (circles) and for a solution containing pectin in an amount of 0.5% (dots), very close values are obtained. The points of the supersaturation curve for a solution of known concentration are determined with continuous stirring and slow cooling of the solution to the temperature at which grains of salt appear in it. The next point of this curve, corresponding to a lower salt concentration, is obtained after adding to the apparatus a known small amount of water, sufficient to dissolve the precipitated sediment, and after further lowering the temperature. In Fig. 4, straight line 2 is drawn through the points obtained in pure ammonium chloride, and straight line 3 through the points obtained in the presence of pectin in an amount of 0.5%. It follows from the graph that pectin considerably broadens the metastability region of the \(NH_4Cl\) solution, which apparently also makes it possible to grow large single crystals of this salt from aqueous solutions of ammonium chloride [4].

References

1. Buckley H., Crystal Growth, IL, 1954.
2. Crystal Growth, Collection of Articles, Publishing House of the Academy of Sciences of the USSR, 1959.
3. Formation of Crystals, Publishing House of the Academy of Sciences of the USSR, Moscow, 1958.
4. Erlich F., Zeitschrift fur anorganische Chemie, B. 203, 1, 31, 1931.