Sublimation line

Figure 2.31. Schematic representation of the P/T equilibria in a simple two-component system (forming continuous solid and liquid solutions). In (a) a perspective view of the P-T-X diagram is shown in (b) its projection on the P/T plane. Notice the two single-component systems represented, for instance, for the component B by the three lines SB/G (sublimation line of B representing the gas/so lid equilibrium), SB/LB (melting equilibrium of B) and the boiling line LB/G. The solid solution is indicated by a. Notice in (a) the isobaric and isothermal sections of the diagrams (compare with Fig. 2.1).

The sublimation curve for iodine in Figure 12-27 appears to be a continuation of the vapor pressure curve, but if the data are plotted to scale, a discontinuity is seen at the triple point O. Moreover, this must always be the case. If these two curves were continuous, then the lines representing the variation of In P with 1/T (Fig. 12-20) would have the same slope—but this is not possible. The value of AvapH determines the slope of the vapor pressure line (recall equation 12.1), whereas determines the slope of the sublimation line. However, these two enthalpy changes can never be the same, because = AvapH + AfusW-... 

W-Metbylpbenotbiazine [1207-72-3] M 213.2, a-form m 99.3" and b 360-365", 0-form m 78-79". Recrystn (three times) from EtOH gave a-form (prisms). Recrystn from EtOH/ benzene gave the B-form (needles). Also purified by vacuum sublimation and carefully dried in a vacuum line. Also crystd from toluene and stored in the dark [Guarr et al. J Am Chem Soc 107 5104 1985 Olmsted et al. J Am Chem Soc 109 3297 1987.]... 

Crystd from EtOH or from benzene/abs EtOH, diethyl ether and pet ether. It was sublimed under vacuum and carefully dried in a vacuum line. Stored in the dark under nitrogen. 

Actually, the temperature does not change as heat is added to change the solid to gas at the equilibrium sublimation temperature. Hence, the heat capacity becomes infinite at this temperature, and the dotted line shown in Figure 8.12 should extend vertically to infinity. The compressibility and coefficient of expansion would show a similar behavior. 

FIGURE 7.26 For some substances and at certain pressures, the molar Gibbs free energy of the liquid phase might never lie lower than those of the other two phases. For such substances, the liquid is never the stable phase and, at constant pressure, the solid sublimes when the temperature is raised to the point of intersection of the solid and vapor lines. 

The relative positions of the three lines shown in Fig. 7.25 are different for each substance. One possibility—which depends on the strength of intermolecular interactions in the condensed phases—is for the liquid line to lie in the position shown in Fig. 7.26. In this case, the liquid line is never the lowest line, at any temperature. As soon as the temperature has been raised above the point corresponding to the intersection of the solid and gas lines, the direct transition of the solid to the vapor becomes spontaneous. This plot is the type that we would expect for carbon dioxide, which sublimes at room temperature. 

Movement across a boundary line corresponds to a phase change. The arrows on the figure show six different phase changes sublimation and its reverse, deposition melting and its reverse, freezing and vaporization and its reverse, condensation. 

Another characteristic of this solution is its proneness to crystallization and polymerization. When parts of the exhaust system are constantly welted by Adblue on the same spot, undesired urea crystals or polymers may form if the exhaust line temperature is lower than 300°C. This phenomenon will result in uncontrolled ammonia production when the crystals or polymers melt or sublimate after being heated at significantly higher temperatures (T > 350°C). This may result in ammonia release. Furthermore, the crystals or polymers can also have an impact on the SCR catalyst cells by reducing the catalyst surface and thus reducing the catalyst performances. 

Fig. 1.56.1. Course of main drying observed by a cryomicroscope, in which the freeze drying is carried out. The hydroxy ethyl starch solution is optimally frozen (see Fig. 1.37). The dark lines show the form of the sublimated ice crystals (Fig. 9 from [1.106]).

By drawing a horizontal line across the figure at p = we see how the line cuts the solid-gas phase boundary at —78.2°C. Below this temperature, the stable form of CO2 is solid dry ice, and C02(g) is the stable form above it. Liquid CO2 is never the stable form at in fact, Figure 5.5 shows that CCfyi) will not form at pressures below 5.1 x In other words, liquid CO2 is never seen naturally on Earth which explains why dry ice sublimes rather than melts under s.t.p. conditions. 

E) Normal means 1 atm (760 mm Hg) pressure. Boiling occurs at a temperature at which the substance s vapor pressure becomes equal to the pressure above its surface. On this phase diagram, at 1 atm pressure, there is no intercept on a line separating the liquid phase from the gas phase. In other words, carbon dioxide cannot be liquefied at 1 atm pressure. It is in the liquid form only under very high pressures. At 1.0 atm pressure, solid C02 will sublime — that is, go directly to the gas phase. 

Lines 21 -40. Physical data. The usual crystalline shape, density (note two values reported.), sublimation notation, boiling point data, and so on. K at 25° is the ionization constant of the acid the pH of the saturated solution (2.8 at 25°C) is given. The solubility data (Soly) is very complete, including water solutions at various temperatures, a bit about the phase diagram of the compound, and solubility in other solvents. Note that numerical data is given where possible. 

A better method involves evaporating a solution of LnX3 containing NH4X (in a mole ratio of 1 6 for chlorides and 1 12 for iodides) to dryness. The product is transferred to a vacuum line and is slowly heated to 200°C to drive off all the water. The temperature is then raised to 300°C to sublime off the ammonium halide. This method has been used extensively to produce pure trihalides and has proved to give very good results for trichlorides (52, 78, 79, 98-105), tribromides (100-103, 106-108), and triiodides (39,40,49,100,102,103,106,107,109-111). 

Note that the diagram has three general areas corresponding to the three states of matter solid, liquid, and gas. The line from A to C represents the change in vapor pressure of the solid with temperature for the sublimation (going directly from a solid to a gas without first becoming a liquid) equilibrium. The A to... 

It is observed in figure 9.6 that the calibration thermogram has two peaks associated with endothermic effects. The first (C) reflects the heating and some sublimation of I2. The second (D), recorded after connecting both the sample and reference cells to an auxiliary vacuum line, accounts for the sublimation of the remaining sample and its removal from the cell. The calibration constant, s, is simply the ratio between ArH°(9.11) and the total area (C + D). 

Quenching the vapour with cold air in the chamber may increase the rate of heat removal although excessive nucleation is likely and the product crystals will be very small. Condenser walls may be kept free of solid by using internal scrapers, brushes, and other devices, and all vapour lines in sublimation units should be of large diameter, be adequately insulated, and if necessary, be provided with supplementary heating to minimise blockage due to the buildup of sublimate. One of the main hazards of air-entrainment sublimation is the risk of explosion since many solids that are considered safe in their normal state can form explosive mixtures with air. All electrical equipment should therefore be flame-proof, and all parts of the plant should be efficiently earthed to avoid build-up of static electricity. 

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