Cryoscopy

Cryoscopy (from Greek kryos-cold and scopeo-I look), a method of studying a substance based on the observation of the freezing point of solutions. As was known for a long time, the freezing point of solutions lies below the freezing point of the corresponding pure solvent. Blagden (1788) established that the lowering of the freezing point, the so-called depression, which represents the difference between the freezing point of the pure solvent and the freezing point of the solution, is proportional to the amount of dissolved substance. Later, Ruedorff (1861-62) independently came to the same results, and Coppet (1871-72) confirmed and supplemented previous studies, discovering that substances of similar properties dissolved in the same volume of water in amounts proportional to their molecular weights cause approximately the same lowering of the freezing temperature. The main work was performed by Raoult (1882-84), who extended the studies to a whole series of organic compounds, using them both as solutes and as solvents; for all such solutions taken in weak concentration, Raoult finally established the following two positions: 1) the lowering of the freezing point of a solution of any substance in a given solvent is directly proportional to the concentration of the solution; 2) the lowering of the freezing point observed in solutions containing 1 g of various substances per 100 g of the given solvent is inversely proportional to the molecular weights of the dissolved substances. From this it follows that the molecular lowering of the freezing point, i.e., that lowering which would be observed in a solution containing 1 gram-molecule of substance in 100 g of solvent, is a constant value characteristic for a given solvent. The determination of the molecular lowering cannot be measured directly, since the above-mentioned regularities are valid only for dilute solutions; it can always be calculated on the basis of observations on dilute solutions and the proportionality between the lowering of the freezing point and concentration by the following formula: where A is the molecular lowering of the freezing point, t is the freezing point of the pure solvent, t is the freezing point of the pure solution, G is the number of g of solvent, g is the number of g of dissolved substance, M is the molecular weight of the dissolved substance. In addition to the method given, the value of the molecular lowering can also be calculated using thermodynamic equations. Recently, the molecular lowering is more often referred not to 100, but to 1.000 g of solvent, in connection with which the values are found to be correspondingly ten times smaller. Below are the freezing points and values of molecular lowering (referred to 1.000 g of solvent) for some of the most commonly used solvents. Solvent Water............ Benzene ........... Acetic acid ("ice") Phenol ........... Naphthalene......... Camphor.......... t° of freezing [Mol. lowering 3.858 5.07 3.8 7.3 6.9 40 Aqueous solutions of substances belonging to the class of electrolytes, namely solutions of salts, acids, and bases, show significant deviations from the above-mentioned regularities. The molecular lowering of the freezing point for water, calculated on the basis of cryoscopic observations on the named solutions, is always significantly greater than the lowering established on aqueous solutions of neutral organic substances, and increases with dilution of the solution, approaching small whole multiples of the latter. In other words, aqueous solutions of electrolytes behave as if they contained twice, three times, or four times as many molecules as correspond to the taken weight. This phenomenon is explained by the electrolytic dissociation of molecules (see Electrolytic dissociation). Sometimes cases (especially in strong solutions) of too great a lowering of the freezing point are observed, which can be explained by the combination of molecules of the dissolved substance with molecules of the solvent, which leads to a decrease in the number of the latter and an increase in the concentration of the solution. In individual rare cases, an abnormally small lowering of the freezing point is noted; for example, for a solution of acetic acid in benzene, a lowering half as great as for benzene solutions of other substances is found. This is explained by the association of molecules of the dissolved substance; in the given example this indicates the doubling of acetic acid molecules. The cryoscopic method finds wide application for determining the molecular weights of substances, mainly those whose vapor density cannot be measured directly. For this, a certain amount of a suitable solvent, for which the molecular lowering is known, is taken, and its freezing point is determined; then a weight of the substance under study is dissolved in it and the freezing point of the resulting solution is measured. Practically, the determination is conveniently performed in a Beckmann apparatus (see Beckmann apparatus). The calculation of the molecular weight is performed by the formula given above. For solutions of electrolytes, the molecular weight of which is known, the method makes it possible to measure the degree of electrolytic dissociation. In the case where both the molecular lowering of the solvent and the molecular weight of the substance are known, the method allows, as can be seen from the formula, to determine the ratio -^, i.e., the concentration of the solution. However, in determining the concentration of electrolyte solutions it is necessary to take into account the dissociation of the latter and instead of the usual characteristic value A for a given solvent, substitute its correspondingly increased value in the formula. In physiological fluids containing both molecules and ions in solution, the method of C. determines not the molecular, but the so-called osmotic concentration of molecules and ions, expressed in gram-molecules and gram-ions. The determination of osmotic concentration of body fluids is especially important in the study of the processes of the intake of nutrient material from body fluids into cells and the processes of excretion of products of reverse metamorphosis from cells and from the body. Great clinical significance, especially for the diagnosis of kidney diseases, has the study of urine and blood. Jones applied the method of C. to the study of the composition of hydrates and solvates (see Hydration, Hydrates). The C.-method, which thanks to its simplicity and speed of technical execution became a very valuable tool of scientific research and, besides the above-mentioned special applications, allowed science to advance greatly in clarifying the nature of solutions in general. A. Dementiev. C. of blood and urine was used quite widely for determining kidney function, resp. their ability to excrete dense components with urine. As a physicochemical method, C. makes it possible only to judge the total molecular concentration of the studied liquid (blood and urine), whereas modern functional diagnosis of kidneys differentiates the disturbances of the excretion function of individual components of urine in connection with views on urination as a sum of several kidney functions (see Diuresis). With the help of C., therefore, it is possible to quickly detect the loss or weakening of the kidneys' ability to concentrate urine and the lowering of the kidneys' accommodative ability. In pathological changes in the kidneys, in particular in their shrinkage, there is a lowering of the "molecular diuresis" (hypostenuria), resp. an increase in the freezing point of urine (A decreases), and in particular a decrease in the amplitude between the maximum and minimum freezing points, equal in the norm to 3°. In the blood, on the contrary, in these cases a lowering of the freezing point is observed (A rises from 0.56° to 0.70° and more). In connection with the above and the successes of modern functional diagnosis of kidneys, C. is currently applied rarely, since it has given way to simpler or more subtle methods (see also Urine, Kidneys, functional diagnosis).