Carbon dioxide sublimation pressure

Carbon dioxide (C02, melting point -56.6°C at 76 psi - 527 kPa, density 1.9769 gL at 0°C) is a colorless, odorless gas. Solid carbon dioxide sublimes at -79°C (critical pressure 1073 psi, 7397 kPa, critical temperature 31°C). High concentrations of the gas do cause stupefaction and suffocation because of the displacement of ample oxygen for breathing. Carbon dioxide is soluble in water (approximately 1 volume carbon dioxide in 1 volume water at 15°C), soluble in alcohol, and is rapidly absorbed by most alkaline solutions. 

The low-pressure phase diagram of carbon dioxide, shown in Fig. 10, is different from that of water in a number of respects. Carbon dioxide sublimates at l.Oatm, leading to solid C02 being called dry ice. The triple point is at 5.1 atm... 

The horizontal line at 101.3 kPa intersects the vapor pressure curve for the solid at -78.5°C. Therefore, solid carbon dioxide sublimes at this temperature. This sublimation point is equivalent to the normal boiling point of a liquid such as water. Because dry ice is at equilibrium with carbon dioxide gas at -78.5°C, it is frequently used to provide this low temperature in the laboratory. 

The phase diagram for carbon dioxide (Fig. 16.58) differs significantly from that for water. The solid/liquid line has a positive slope, since solid carbon dioxide is more dense than liquid carbon dioxide. The triple point for carbon dioxide occurs at 5.1 atm and -56.6°C, and the critical point occurs at 72.8 atm and 31°C. At a pressure of 1 atm, solid carbon dioxide sublimes at -78°C, a property that leads to its common name, dry ice. No liquid phase occurs under normal atmospheric conditions, making dry ice a convenient coolant. 

At temperatures and pressures below the triple point, carbon dioxide may be either a solid (dry ice) or a gas, depending upon temperature conditions. Solid carbon dioxide at a temperature of -109.3°F ( 78.5°C) and 1 atmosphere transforms directly to a gas (sublimes without passing through the liquid phase). Lower temperatures will result if solid carbon dioxide sublimes at pressures less than atmospheric. Thermodynamic properties of saturated carbon dioxide in the solid, liquid, and vapor phases are given in Tables 1A and IB. 

The triple point of carbon dioxide is at -57°C and 5.1 atm (Figure 11.12 top). Therefore, the solid sublimes if warmed at any pressure below 5.1 atm. This is why solid carbon dioxide sublimes at normal atmospheric pressure (1 atm). Above 5.1 atm, however, the solid melts if warmed. Sulfur has a more complicated phase diagram (Figure 11.12 bottom). It displays three triple points, one of them involving two different solid forms of sulfur (called rhombic sulfur and monoclinic sulfur), as well as the vapor. 

The solid-gas equilibrium line represents those conditions of pressure and temperature where sublimation occurs. For HjO, obvious sublimation occurs at pressures lower than those that are normally experienced. (Sublimation of ice does occur slowly at normal pressures, which is why ice cubes get smaller over time in your freezer. The so-called freezer burn of frozen foods is caused by sublimation of ice from the food. This is why it s important to wrap frozen food tightly.) Flowever, for carbon dioxide, normal pressures are low enough for sublimation. Figure 6.4 shows a phase diagram for CO2, with the 1-atm position marked. Liquid CO2 is stable only under pressure. Some gas cylinders of carbon dioxide are high enough in pressure that they actually contain liquid CO2. 

We can also start to understand why some substances, such as soHd carbon dioxide, sublime to a vapor without first forming a liquid. There is no fundamental requirement for the three lines to lie exactly in the positions we have drawn them in Fig. 3.3 the Hquid line, for instance, could lie where we have drawn it in Fig. 3.4. Now we see that at no temperature (at the given pressure) does the liquid phase have the lowest molar Gibbs energy. Such a substance converts spontaneously directly from the solid to the vapor. That is, the substance sublimes. 

Recall from Section 11.8 that CO2 has a triple point at —57 °C and 5.1 atm. At atmospheric pressure, therefore, the liquid phase of CO2 does not exist. Solid carbon dioxide sublimes directly to the gas phase when heated, which is why solid CO2 is called dry ice. ... 

GaUic acid is heated with about half its weight of water in a copper autoclave until the pressure reaches 1.2 MPa (12 atm) and the temperature is 175°C. Steam and carbon dioxide are released but sufficient water is retained to maintain the pyrogaHol as a Hquid. The cooled solution is decolorized with animal charcoal and is then evaporated until the volatile pyrogaHol distills into iron receivers. The sohdified material is purified by repeated distillation, sublimation, or vacuum distillation at 200°C in the presence of diaLkyl phthalates (8). 

Vapor pressure data for soHd carbon dioxide are given in Table 2 (10). The sublimation temperature of soHd carbon dioxide, 194.5 K at 101 kPa (1 atm), was selected as one of the secondary fixed points for the International Temperature Scale of 1948. 

If the pump is a filter pump off a high-pressure water supply, its performance will be limited by the temperature of the water because the vapour pressure of water at 10°, 15°, 20° and 25° is 9.2, 12.8, 17.5 and 23.8 mm Hg respectively. The pressure can be measured with an ordinary manometer. For vacuums in the range lO" mm Hg to 10 mm Hg, rotary mechanical pumps (oil pumps) are used and the pressure can be measured with a Vacustat McLeod type gauge. If still higher vacuums are required, for example for high vacuum sublimations, a mercury diffusion pump is suitable. Such a pump can provide a vacuum up to 10" mm Hg. For better efficiencies, the pump can be backed up by a mechanical pump. In all cases, the mercury pump is connected to the distillation apparatus through several traps to remove mercury vapours. These traps may operate by chemical action, for example the use of sodium hydroxide pellets to react with acids, or by condensation, in which case empty tubes cooled in solid carbon dioxide-ethanol or liquid nitrogen (contained in wide-mouthed Dewar flasks) are used. 

B. Di-tert-butyl dicarbonate. A solution of 20.0 g. (0.076 mole) of di-i-butyl tricarbonate in 75 ml. of carbon tetrachloride is placed in a 600-ml. beaker fitted with a magnetic stirrer, and 0.10 g. (0.0009 mole) of freshly sublimed l,4-diazabicyclo[2.2.2]octane (DABCO) is added (Note 9). Rapid evolution of carbon dioxide begins at once. The reaction mixture is stirred at 25° for 45 minutes to complete the loss of carbon dioxide (Note 10), and then 35 ml. of water, containing sufficient citric acid to make the aqueous layer slightly acidic, is added. The layers are separated and the organic layer is dried over anhydrous magnesium sulfate and then concentrated at 25° with a rotary evaporator. The residual liquid is distilled under reduced pressure to separate 13.3-15.1 g. (80-91%) of di-butyl dicarbonate as a colorless liquid, b.p. 55-56° (0.15 mm.) or 62-65° (0.4 mm.) n T> 1.4071-1.4072 (Note 11). 

At a pressure below the triple point pressure, the solid can change directly to a gas (sublimation) and the gas can change directly to a solid, as in the formation of carbon dioxide snow from the released gas. 

Iodine sublimes more readily than ice because its triple-point pressure, 90 mm Hg, is much higher. Sublimation occurs on heating (Figure 9.6) below the triple-point temperature, 114°C. If the triple point is exceeded, the solid melts. Solid carbon dioxide (dry ice) has a triple-point pressure above 1 atm (5.2 atm at — 57°C). Liquid carbon dioxide cannot exist at 1 atm pressure regardless of temperature. Solid C02 always passes directly to vapor if allowed to warm up in an open cantainer. 

B Carbon dioxide is liquid at 60 atm and 25° C. When it is released into a room at 1 atm and 25° C, as the pressure lowers, the system reaches the liquid-vapor boundary, at which pressure the liquid is changed to vapor. The vaporization absorbs sufficient heat to cool the C02 to below its sublimation temperature at 1 atm. As a result, fine particles of solid C02 snow are produced. 

Liquid carbon dioxide is usually stored under 20 bar pressure at — 18°C. Compression and cooling of the gas between the temperature limits at the triple point and the critical point will cause it to liquefy. The triple point is the pressure temperature combination at which carbon dioxide can exist simultaneously as gas, liquid and solid. Above the critical temperature point of 31°C it is impossible to liquefy the gas by increasing the pressure above the critical pressure of 73 bar. Reduction in the temperature and pressure of liquid below the triple point causes the liquid to disappear, leaving only gas and solid. (Solid carbon dioxide is also available for cryogenic work and at —78°C the solid sublimes at atmospheric pressure.)... 

We call solid carbon dioxide (CO2) dry ice . To the eye, it looks just like normal ice, although it sometimes appears to smoke see below. Carbon dioxide is a gas at room temperature and only solidifies (at atmospheric pressure) if the temperature drops to about —78 °C or less, so we make dry ice by cooling gaseous CO2 below its freezing temperature. We call it dry ice because, unlike normal ice made with water, warming it above its melting temperature leaves no puddle of liquid, because the CO2 converts directly to a gas. We say it sublimes. 

Dry ice is carbon dioxide (CO2) in its solid phase. We call it dry because it is wholly liquid-free at such solid CO2 looks similar to normal ice (solid water), but it melts without leaving a puddle. We say it sublimes, i.e. undergoes a phase change involving direct conversion from solid to gas, without liquid forming as an intermediate phase. CC>2(i) can only be formed at extreme pressures. 

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. 

For some purposes solid carbon dioxide ( dry-ice ), sublimation temperature —78.5 °C or mixtures of dry-ice and acetone (temperature —78 to —95 °C) are used as coolants. These are obviously not as efficient as liquid nitrogen and they should not be used with chemicals which have an appreciable vapour pressure at the appropriate temperatures. 

At the right combination of pressure and temperature, matter can move directly from a solid to a gas, or vapor. This type of phase change is called sublimation, and it s the kind of phase change responsible for the white mist that emanates from dry ice, the common name for solid carbon dioxide. Movement in the opposite direction, from gas directly into solid phase, is called deposition. 

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