Cryohydrate

A liquid solution may be separated into its constituents by crystallising out either pure solvent or pure solute, the latter process occurring only with saturated solutions. (At one special temperature, called the cryohydric temperature, both solvent and solute crystallise out side by side in unchanging proportions.) We now consider what happens when a small quantity of solute is separated from or taken up by the saturated solution by reversible processes. Let the saturated solution, with excess of solute, be placed in a cylinder closed below by a semipermeable septum, and the w7hole immersed in pure solvent. The system is in equilibrium if a pressure P, equal to the osmotic pressure of the saturated solution when the free surface of the pure solvent is under atmospheric pressure, is applied to the solution. Dissolution or precipitation of solute can now be brought about by an infinitesimal decrease or increase of the external pressure, and the processes are therefore reversible. If the infinitesimal pressure difference is maintained, and the process conducted so slowly that all changes are isothermal, the heat absorbed when a mol of solute passes into a solution kept always infinitely... 

Dimethyoxymethane, Nitrate cryohydrate, Liquid hydrogen cyanide... 

Anhydrous hydrazine, Cyanogen bromide, Isopropyl alcohol, Sodium nitrite, Sodium bicarbonate, Copper nitrate cryohydrate Sodium nitrate, Sodium chloride. Sugar, Charcoal powder Potassium nitrate. Sodium chlorate. Sugar, Charcoal powder Potassium nitrate. Potassium chlorate. Sugar, Charcoal powder Sodium nitrate. Potassium chlorate. Sugar, Charcoal powder 3-Pyridol, Ethylmethylamine, Formaldehyde, Pyridine, Dimethylcarbamoyl chloride. Sodium carbonate. Chloroform, Sodium sulfate, 1,10-Dibromodecane, Acetone, Acetonitrile, Charcoal, Ethyl acetate... 

The equilibrium conditions with soln. of magnesium and potassium sulphates, at different temp., are illustrated by the diagram, Fig. 5. The point A represents the f.p. of water, 0° B, the cryohydric temp., —2 9° for magnesium sulphate C, the cryohydric temp., —L50, for potassium sulphate D, the cryohydric temp., —4 5°, for the mixed potassium sulphate and schonite E, the cryohydric temp., —5°, for the mixture of schonite and MgS04.12H20 F, the transition point, 0 7°, for the... 

The solubility of ammonium iodide 6 in water is very great. At —27 5°, the cryohydric temp., water dissolves 55 per cent, of the salt, and at 15°, 62 5 per cent. A contraction occurs when this salt dissolves in water. Thus, H. Schiff and U. Monsacchi 7 calculate from the specific gravity determinations of W. H. Perkin and W. W. J. Nicol ... 

The solubility of sodium sulphide in water, studied by N. Parravano and M. Fornaini, shows a cryohydrate temp.—10° when 9 34 per cent, of anhydrous sulphide is present in soln. The enneahydrate exists in aq. soln. up to 48 9°, where it is transformed into the hemihenahydrated sodium sulphide, Na2S.5 H20, which exists in a labile condition between 48 9° and 91 5°, and is stable between 91 5° and 94°. The hexahydrated sodium Sulphide, Na S.eH O, is stable between 48° and 91 5° but the enneahydrate does not change into the hexahydrate at 48°, rather... 

The cryohydric or eutectic temp, of the trihydrated salt is —17 8°, and the observed results are plotted in Fig. 82. The trihydrated salt melts at B, Fig. 82, the congruent m.p. 29 88°, at which temp, the crystalline trihydrated salt can be in equilibrium with two different sat. soln., one containing a larger and the other a smaller proportion of water than corresponds with LiN03.3H20. The transition point C of the trihydrated to the hemihydrated salt, 29 6°, is a little below the congruent m.p. of the trihydrated salt. 

The terminal numbers with potassium, rubidium, and csesium nitrates represent the b.p. of sat. soln. at nearly normal press. for sodium nitrate the corresponding value is 67 6 (119°). Determinations of the solubility of sodium nitrate have been made by G. J. Mulder, Earl of Berkeley, A. Ditte, L. Maumene, A. fitard, etc.30 The solubility curve of sodium nitrate has been carried upwards 781 (180°), 83 5 (220°), 915 (225°), and 100 (313°), the last-named temp, represents the m.p. of the salt. According to L. C. de Coppet, the eutectic or cryohydric temp, of sodium nitrate is —18"5°, and the eutectic mixture is not a definite hydrate, NaN03.7H20, as A. Ditte once supposed. A. fitard represents the solubility S... 

Modern inorganic chemistry is a quantitative science. Consequently, when performing experimental work, students must determine the yield of the substances obtained and certain constants such as the boiling points, solubility, and cryohydrate points, and also perform the required calculations with the use of the fundamentals of thermodynamics. 

How can the shape of the temperature-time curve be explained Define the eutectic point (cryohydrate point). 

TEMPERATURE OF COOLING MIXTURE CONSISTING OP ICE AND SELECTED SALTS (CRYOHYDRATE POINT)... 

Salt Formula Mass of salt per 100 g of ice, g Cryohydrate point t, C... 

Table 19. COMPOSITION AND CRYOHYDRATE POINT OF COOLING MIXTURES ...

Next would come the hydrates containing columns of water molecules, such as are found in the ettringite group, Cag[Al(0H)8]a(S04)s-26HaO then sheets, such as those of the clay minerals or of gypsum and finally the cryohydrates with complete networks of linked waters, as in the case of the hydrates of the gases already mentioned. 

CRYOHYDRATE. An eutectic system consisting of a salt and water, having a concentration at which complete fusion or solidification occurs al a definite temperature (eutectic temperature) as if only one substance were present. 

The point C is known as the eutectic point or cryohydric point of the system. It gives the lowest temperature which can be attained in the system, i.e., -23°. At the cryohydric point the solution freezes at constant temperature without change of composition. The eutectic composition is 52% KI and 48% ice. 

The eutectic point in KI - H20 system is also known as cryohydric point. 

A study of the curves in fig. 5 is particularly interesting from the point of view of the Phase Rule. AB represents the various states of equilibrium between ice and ferric chloride solutions, a minimum temperature being reached at the cryohydric point B, which is —55° C. At this point ice, solution, and the dodecahydrate of ferric chloride are in equilibrium. The number of degrees of freedom is nil—in other words, the system is invariant, and if heat be subtracted the liquid phase will solidify without change of temperature until the whole has become a solid mass of ice and dodecahydrate. Further abstraction of heat merely lowers the temperature of the system as a whole. 

Cryohydrates wore formerly (as by Guthrie) treated as chemical compounds that they are mixtures may, however, foe seen with the microscope in coloured salts (KaCr Oi) moreover the composition of these so-called hydrates may alter if the freezing occurs under different pressure (Roloff, Zeitschr. f. Phys. Chem. 17. 325). 

The lines correspoial to the following states a v salt, vapotir, ancryohydric point, only with the reservation that it is the cryohydric point, not for atmosph( .ric priisstire, hut for the maximum pressure of the saturated solution. 

Fig. 4 did, on the left of EiD to ice, right of that to water. Di corresponds to the pressure of pure water at the freezing point, and consequently lies above a m, which refers to the saturated solution also lies in the continuation of the curve OAj, since that, like a d, gives the pressure of ice from T)- to the right lies the vapour pressure curve for water. Further, the boundary between water and ice, DjEi, may be drawn from Dj upwards, and from A the line A a of cryohydric pressure, which is given on the horizontal plane by a line corresponding to the composition of the cryohydric solutions for difierent pressures. The new areas, given only by the projection, relate to conditions not previously taken into account ... 

GiADBi, unsaturated solution and ice, bounded by g a for the cryohydric state, Ej d for water, and ad for the melting point. 

Whilst the various types of physical mixture have been dealt with above, from complete immiscibility to complete miscibility, two special cases may now be taken, the first of which is that of benzoic acid and water. Here we are essentially concerned with the fact that in the same pair of bodies, immiscibility may be gradually transformed into simple, then mutual solubility, and finally into complete miscibility. In the previous cases the transformation is effected by rise of temperature below the cryohydric temperature ice and benzoic acid are practically without action on one another on fusion of the ice, one-sided solution of the benzoic acid begins later, on fusion of the acid, mutual solubility occurs, changing eventually to complete miscibility. AlexejefTs investigations on this point have settled that benzoic acid, after an increase of solubility with rise of temperature has shown itself, melts at 90°, i.e. 31-4° under the usual melting point This is, therefore,... 

AL, saturated solution ending in the cryohydric ])(>int where vapour, ice, salt, and solution can coexist, and whicdi is consequently a quadruple point. 

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