Vapour, eutectic point

Fig. 18.1. Sublimation temperatures and eutectic point of a mixture of vapours at constant p.

The constant pressure diagram for this system is shown schematically in fig. 21.14. The boiling point of the mixture is independent of composition as shown by the horizontal dotted line at except when the second component disappears when, of course, the boiling point rises abruptly to that of the pure component T or T ), The line T E gives the composition of the vapour in equilibrium with pure liquid 1 as a function of temperature. The equilibrium temperature is lower than the boiling point of 1 as its partial pressure in the vapour phase is lower than total pressure. Similarly T E gives the composition of T mixed vapour in equilibrium with p liquid 2. At the eutectic point we have co-existence of the two liquid phases and vapour. The lines T E and T E are given by equations like (18.23) and (18.23 ). 

Phosgene and boron(III) chloride are miscible in all proportions [1329], as predicted earlier [738a] and the phase diagram for the COClj-BClj system (Fig. 9.3) reveals a eutectic point at -142.3 C (74.4 mole % COCIj) and no evidence for any complex formation. Moreover, the vapour pressure - composition isotherm (0 C) for this system (Fig. 9.4) shows a positive deviation from Raoult s law (although Henry s law appears to be well obeyed [649]), indicating the presence of unfavourable interactions between phosgene and boron(III) chloride [376]. Thus, the purification of boron(HI) chloride from traces of phosgene will not be complicated by the formation of a thermodynamically stable complex. 

If to the system ice— water at o succinic nitrile is added, the temperature will fall and continued addition of the nitrile will lead at last to the eutectic point b (Fig, 53), at which solid nitrile, ice, solution, and vapour can coexist. The temperature of the eutectic... 

The cryohydric or eutectic point is thus clearly seen to be the point of intersection of the solubility curve of the salt and the freezing-point curve of water. At this point, also, the curves of the univariant systems ice—salt— vapour and ice—salt —solution intersect. The cryohydric point is therefore a quadruple point, and represents an invariant system. 

Concentration-Temperature Diagram.—In this diagram the temperatures are taken as the abscissae, and the composition of the solution, expressed in atoms of chlorine to one atom of iodine, is represented by the ordinates. In the diagram, A represents the melting-point of pure iodine, 114°. If chlorine is added to the system, a solution of chlorine in liquid iodine is obtained, and the temperature at which solid iodine is in equilibrium with the liquid solution will be all the lower the greater the concentration of the chlorine. We therefore obtain the curve ABF, which represents the composition of the solution with which solid iodine is in equilibrium at different temperatures. This curve can be followed down to 0°, but at temperatures below 7 9 (B) it represents metastable equilibria. At B iodine monochloride can be formed, and if present the system becomes invariant B is therefore a quadruple point at which the four phases, iodine, iodine monochloride, solution, and vapour, can co-exist. Continued withdrawal of heat at this point will therefore lead to the complete solidification of the solution to a mixture or conglomerate of iodine and iodine monochloride, while the temperature remains constant during the process. B is the eutectic point for iodine and iodine monochloride. 

Corresponding to the point Q/the melting-point of pure iodine, there is the point C, which represents the vapour pressure of iodine at its melting-point. At this point three curves cut i, the sublimation curve of iodine 2, the vaporisation curve of fused iodine 3, CiB, the vapour-pressure curve of the saturated solutions in equilibrium with solid iodine. Starting, therefore, with the system solid iodine— liquid iodine, addition of chlorine will cause the temperature of equilibrium to fall continuously, while the vapour pressure will first increase, pass through a maximum and then fall continuously until the eutectic point, B (Bjl), is reached. At this point the system is invariant, and the pressure will therefore remain constant until all the iodine has disappeared. As the concentration of the chlorine increases in the manner represented by the curve B/H, the pressure of the vapour also increases as represented by the curve Bj/iHi. At the eutectic point for iodine monochloride and iodine trichloride, the pressure again remains constant until all the monochloridc has disappeared. As the concentration of the solution passes along the curve HF, the pressure... 

Pressure-Temperature Dia am. —If sulphur dioxide is passed into water at 0°, a solution will be formed and the temperature at which ice can exist in equilibrium with this solution will fall more and more as the concentration of the sulphur dioxide increases. At — 2 6°, however, an eutectic point is reached at which solid hydrate separates out, and the system becomes invariant. The curve AB (Fig. 86), therefore, represents the pressure of the system ice—solution II.— vapour, and B represents the temperature and pressure at which the invariant system ice—hydrate—solution II.— vapour can exist. At this point the temperature is — 2-6°, and the pressure 21-2 cm. If heat is withdrawn from this system, the solution will ultimately solidify to a mixture of ice and hydrate, and there wull be obtained the univariant system ice— hydrate— vapour. The vapour pressure of this system has been determined down to a tern- p perature of — 9 5 , at which temperature the pressure amounts to 15 cm. The pressures for this system are represented by the curve BC. 

Orthophosphorio Acid —Preparation—Physical Properties of Solid Hydrates of P2Os—Solubilities, Melting-points and Eutectics of the System HjP04-Ha0—Densities of Aqueous Solutions—Vapour Pressures—Conductivities of Concentrated and Dilute Solutions—Viscosities—Refractive Index—Basicity and Neutralisation of the Phosphoric Acids—Constitution... 

It is extremely important that the temperature at which a product is freeze-dried is balanced between the level that maintains the frozen integrity of the product and that which maximizes its vapour pressure. Such a balance is a key to optimal drying. The typical phase diagram of Fig. 2.1 illustrates this point. Most products are frozen well below their eutectic or glass transition point, and then the temperature is raised to just... 

All commercial electrolyses employ an electrolyte which is close in composition to the eutectic KF2HF (melting point 82°C), because of the low vapour pressure of hydrogen fluoride above such melts. In the electrolysis, the overall cell reaction is... 

The eutectic melts at 362°C, and in the presence of an equal proportion of beryllium chloride, the melting-point is depressed to as low as 238°C. This allows electrolysis to be carried out well below the boiling-point of beryllium chloride (500°C), and even below 400°C, where the vapour pressure is still considerable. 

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