Vapour pressure-composition isotherm

Fig. 6.12 Vapour pressure-composition isotherms for phosgene in organic solvents [1110] (A) aromatic solvents, (B) xylene, (C) chloroaliphatic solvents (facing page), and (D) 1,2-dichloroethane (facing page).

Vapour pressure-composition isotherms for the COClj-HCl system have been investigated between -75 and 0 C up to a total pressure of about 170 kPa, see Fig. 6.15 [ICI109]. A more detailed study has been published for temperatures of -7.2 (266.2 K) and -79.6 "C (193.6 K) [757a] and these are illustrated for both liquid and vapour phases in Table 6.15 and Figs. 6.16(A) and 6.16(B). 

Fig. 6.15 Vapour pressure-composition isotherms for the system COClj-HCl between -75 and 0 C [ICI109].

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. 

Fig. 9.6 Vapour pressure - composition isotherms for the COClj-CCl system [ICI2].

Tetrachloromethane is an important solvent for phosgene, and vapour pressure -composition isotherms for this system are illustrated in Fig. 9.6 [ICI2]. These curves show, as expected, a slight positive deviation from Raoult s law. Earlier, less reliable, data are... 

If the total pressure of the vapour at constant temperature is represented as a function of the compositions of the two phases, the p-liquid and p-vapour curves are obtained. The p-liquid curves—that is, the curves representing the total vapour pressures of liquid binary mixtures as functions of the composition of the liquid phase—are most important they are usually referred to simply as the vapour-pressure curves of the mixture. Each curve is an isotherm. 

Other Metal Sulphides. The equilibrium between stoicheiometric TaSj and a non-stoicheiometric phase Ta g4 has been established by measurement of the electrical properties of compositional isotherms with the vapour pressure of sulphur between 900 and 1200 °C. The rules governing the formation of stable structures in the series of compounds (ZnO, and chalcogenides... 

Gas-Liquid Critical Curves of Binary Systems. In Figure la the pressure-temperature-composition surface is represented schematically for the gas-liquid equilibria of a binary system in a simple case. The dashed lines are the vapour pressure curves of the pure components they end at the critical points CP I and CP II of the pure components I and II. Some pressure-composition cuts for constant temperature are given. The critical point of the binary system is situated at the extreme value of each p x) isotherm or (not shown in Figure la) at the extreme value of each T x) isobar. The line that connects the critical points of all binary mixtures is the critical curve in a pressure-temperature projection the critical curve is the envelope of all p T) curves for constant composition (so-called isopleths). At temperatures and pressures beyond the critical curve the constituents are miscible in all proportions. 

Any deviation of an intcrmetallic compound from the strict stoichiometric composition always means that there is a disturbance in the ideal order of the crystal lattice. That is to say, defects are present. Suppose that the isothermal activity of metal A (i.e. the relative vapour pressure compared to the pure material) is measured in a binary system A-B in which inter-metallic compounds with finite ranges of homogeneity exist. Then, for an arbitrarily chosen phase diagram, a plot of activity versus composition as shown schematically in Fig. 1-4 results. 

Figure 1.3 Phase behaviour of carbon dioxide/water system at temperatures between the critical hydrate temperature and the upper critical solution temperature, (a) Typical pressure/composi-tion diagram for carbon dioxide/water (a Class B2 system) at temperatures below the critical temperature of carbon dioxide but above the critical hydrate formation temperature. Data for arms B and C are shown in (b) and (c) respectively, (b) Solubility of liquid CO2 in water as a function of temperature and pressure (arm C in (a)), (c) Solubility of water in liquid CO2 as a function of temperature and pressure (arm B in (a)), (d) The three phase pressure curve compared with the vapour pressure curve of carbon dioxide showing the critical locus CsU (i.e. locus of points such as C on (e) where vapour properties merge with those of solvent-rich liquid). (Data reference [75].) (e) Detail of the isothermal pressure/composition diagram at 25°C (on left) and at temperature between Tc and Tu (on right). Subscripts 1 and 2 denote water-rich and C02-rich phase. Critical point C is shown as blocked-in circle. (Data reference for (b) and (c) is [81].)...

As an example of more complex systems and descriptions, the Ni-Mg system is shown in Fig. 2.32 (adapted from Levinsky 1997). In (a) an isobaric section of the diagram is shown (a low pressure has been considered in order to have a certain extension of the gas phase which consists essentially of Mg vapour). In Fig 2.32(b) there is an isothermal section of the diagram at 700°C. Notice, for different values of pressure, the change in the sequence of phases stable at different compositions. A value of the pressure close to atmosphere is approached at the top of the figure. In Fig 2.32(c) the usual Tlx diagram is shown. This can be considered an isobaric phase diagram if pressure is relatively low but still higher than the sum of the equilibrium partial pressures of the components. 

It should be noted that if both (18.61) and (18.62) are satisfied as in the case of liquid-vapour equilibrium, the slopes of the coexistence curves at constant pressure, (18.46) and (18.47), are always of the opposite sign to the curves of coexistence at constant temperature, (18.49) and (18.50). Thus if the isobaric curves are increasing at a given composition, the isothermal curves are decreasing, and furthermore if the isobaric curves pass through a maximum, the isothermal curves must pass through a minimum, and conversely. 

PTxy data contain the maximum of information. Isobaric data are preferable in design because incorporate the temperature effect. Indeed, the activity coefficients are much more dependent on temperature than pressure. Isothermal data are preferred in scientific studies. Vapour composition data are less accurate than liquid composition. 

The phase behaviour of water and carbon dioxide has been studied by various authors, so literature data in wide ranges of temperature and pressure is available. Selected data for the relevant pressure and temperature conditions are presented in Fig. 15.9 [15,16]. CO2 has the lowest solubility in water barely exceeding 6 wt% at 313 K and 20.3 MPa. The effect of pressure on solubility weakens above 5 MPa as depicted by the near vertical isotherm progress. The composition of the light phase shows around 4 wt% water vapour at 1.1 MPa and 373 K. 

When molecular ions give the base peaks in mass spectra, one can decrease the electron energy and obtain a mass spectrum which mainly consists of molecular ions. The estimated total ionization cross-section can be utilized and the partial pressures of vapour species calculated. When molecular ions give only a minor peak in the mass spectrum, decreasing the electron energy does not give satisfactory results and therefore some thermodynamic approaches have been developed. These are based on the variation of ion current intensities with the isothermal change of vapour composition. 

The isothermal change of vapour composition is governed by the thermodynamic laws, and their analytical expressions can be included in the equation that relates measured and apportioned ion currents. A variation of partial pressures at constant temperature can be achieved by changing the composition of the solid (liquid) phase equilibrated with the vapour. When an individual compound is under study, Knudsen cells of Figures IB, C and D allow one to compare the saturated and unsaturated vapours at the same temperature and apportion the measured ion current. 

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