Iodine sublimation pressure

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. 

At normal temperatures and pressures, iodine is a shiny, purplish-black or gray solid. Near room temperature, iodine sublimes (i.e., it does not melt to form a liquid but goes directly from the solid to the gas phase). It is found at a level of about 60 parts per billion (ppb) by weight in seawater but its concentration is enhanced in marine organisms. 

Like liquids, solids have vapor pressures. Figure 16.49 shows iodine vapor in equilibrium with solid iodine. At 25°C and 1 atm iodine sublimes that is, it goes directly from the solid to the gaseous state without passing through the liquid state. Sublimation also occurs with dry ice (solid carbon dioxide) under these conditions. 

Iodine sublimes at room temperature and pressure water does not. Explain the differences you would expect to observe at room temperature if 5.0 grams of iodine crystals were sealed in a 10.-mL container and 5.0 mL of water were sealed in a similar container. 

The sublimation pressure of Thl4(cr) was measured by mass-loss Knudsen effusion (617 to 760 K) and mass spectrometry, in separate studies. The latter confirmed that the sublimation of TI1I4 is accompanied by a small (5 - 10%) dissociation to a lower iodide and iodine. 

Free iodine in solution has a very high vapor pressure. In solutions of pH 11 or greater containing iodine, iodine is always present in the air above the solution. Acidic solutions actually drive the iodine out of solution and into vapor. Elemental iodine sublimes to vapor readily and free iodine is easily taken into the body by inhalation or absorption through the skin. Any chemical technique that produces free iodine as a reaction byproduct is therefore extremely hazardous. Among such techniques are iodine-labeling... 

Two familiar solids wifh significant sublimation pressures are ice and dry ice (solid carbon dioxide). If you live in a cold climate, you are aware that snow may disappear from fhe ground even though the temperature may fail to rise above 0 °C. Under these conditions, the snow does not melt it sublimes. The sublimation pressure of ice at 0 °C is 4.58 mmHg. That is, the solid ice has a vapor pressure of 4.58 mmHg at 0 °C. If the air is not already saturated with water vapor, the ice will sublime. The sublimation and deposition of iodine are pictured in Figure 12-25. 

Even at temperatures well below its melting point of 114 °C, solid iodine exhibits an appreciable sublimation pressure. Here, purple iodine vapor is produced at about 70 °C. Deposition of the vapor to solid iodine occurs on the colder walls of the flask. 

Iodine is a dark-coloured solid which has a glittering crystalline appearance. It is easily sublimed to form a bluish vapour in vacuo. but in air, the vapour is brownish-violet. Since it has a small vapour pressure at ordinary temperatures, iodine slowly sublimes if left in an open vessel for the same reason, iodine is best weighed in a stoppered bottle containing some potassium iodide solution, in which the iodine dissolves to form potassium tri-iodide. The vapour of iodine is composed of I2 molecules up to about 1000 K above this temperature, dissociation into iodine atoms becomes appreciable. 

Many solid substances (camphor, iodine, naphthalene, etc.), are known which are appreciably volatile at ordinary temperatures. Others, such as the metals, are apparently quite fixed, but they probably possess a definite, although very small vapour-pressure, even at ordinary temperatures. Thus, if magnesium is heated to 550° for a few hours in a magnesia boat enclosed in a vacuous tube it sublimes in beautiful crystals on the cool part of the tube. The vaporisation of a solid without previous fusion is called sublimation the vapour-pressure (like the vapour-pressure of a liquid), is definite for each temperature, is independent of i the volume of the vapour space, and increases with rise of temperature. 

Bromine is a dark-red liquid with high specific gravity. Iodine is a black solid which sublimes at atmospheric pressure producing a violet vapor. Both are used in CVD but to a lesser degree than either fluorine or chlorine. 

Simple sublimation is a batch-wise process in which the solid material is vaporised and then diffuses towards a condenser under the action of a driving force attributable to difference in partial pressures at the vaporising and condensing surfaces. The vapour path between the vaporiser and the condenser should be as short as possible in order to reduce mass-transfer resistance. Simple sublimation has been used for centuries, often in very crude equipment, for the commercial production of ammonium chloride, iodine, and flowers of sulphur. 

Vacuum sublimation is a development of simple sublimation, which is particularly useful if the pressure at the triple-point is lower than atmospheric, where the transfer of vapour from the vaporiser to the condenser is enhanced by the increased driving force attributable to the lower pressure in the condenser. Iodine, pyrogallol, and many metals have been purified by vacuum sublimation processes in which the exit gases from the condenser are usually passed through a cyclone or scrubber to protect the vacuum equipment and to minimise product loss. 

The jacketed addition funnel is removed and 1.5 g of cuprous bromide-dimethyl sulfide complex (Note 7) is added through a powder funnel. A 500-mL, pressure-equalizing addition funnel (long-tipped) is attached to the flask and flushed with argon. As the anion solution is cooled in a dry ice-isopropyl alcohol bath, a solution of 530 g (2.08 mol) of sublimed iodine in 500 mL of anhydrous tetrahydrofuran is placed in the addition funnel. This solution is added dropwise to the cooled slurry over approximately 90 min (Note 8). The solution is stirred for about 15 min at low temperature. 

The phase diagram also illustrates why some substances which melt at normal pressure, will sublime at a lower pressure the line p = Pa intersects at Tg the locus OR of the points defining the solid-vapour equilibrium, i.e. at the pressure pj, the substance will sublime at the temperature T. Sometimes the opposite behaviour is observed, namely that a substance which sublimes at normal pressure will melt in a vacuum system under its own vapour pressure This is a non-equilibrium phenomenon and occurs if the substance is heated so rapidly that its vapour pressure rises above that of the triple point this happens quite frequently with aluminium bromide and with iodine. 

Owing to the volatility of cyanogen iodide, the yield is slightly dependent on the speed of operation. By the above method sublimation as a means of purification is avoided. If, however, sublimation is desirable, it can be accomplished with appreciable speed only under reduced pressure and at temperatures at which cyanogen iodide is slowly decomposed into iodine and cyanogen. The vacuum must be constantly renewed during the operation. 

Sublimation differs from ordinary distillation because the vapour condenses to a solid instead of a liquid. Usually, the pressure in the heated system is diminished by pumping, and the vapour is condensed (after travelling a relatively short distance) on to a cold finger or some other cooled surface. This technique, which is applicable to many organic solids, can also be used with inorganic solids such as aluminium chloride, ammonium chloride, arsenious oxide and iodine. In some cases, passage of a stream of inert gas over the heated substance secures adequate vaporisation. 

The greenish-black crystals of Bil3 are composed of a hexagonal close packed array of iodine atoms with bismuth atoms in the interstitial spaces, with six nearest neighbours at 3.07 A. It is prepared by heating the elements in stoichiometric quantities, and purified by subliming under reduced pressure. 

I2 (g). Vapor pressure data on solid iodine were reported by Baxter and Grose,1 Baxter, Hickey, and Holmes,1 and Ramsay and Young.1 Haber and Kerschbaum1 measured the vapor pressure at low temperatures. Giauque2 critically reviewed the data, and deduced for the heat of sublimation, Fs= —16.069+0.0040 ( +273.1), which gives at 18°, —14.91. Earlier calculations were made by Wohl,3 Dewar,1 and Nernst.1... 

I2 (liq.). Vapor pressure data for liquid iodine were reported by Ramsay and Young,1 Stelzner and Niedersehulte,1 and Rassow.1 These data yield for the heat of vaporization, V— —12.75 + 0.0056 ( +273.1). Combination of this equation with that for the heat of sublimation gives for the heat of fusion of solid iodine, F= —3.3+0.0016 ( +273.1), or —4.0 at the melting point. See also Rideal.1... 

Only a few solids have vapor pressures near atmospheric at safe temperatures, among them COz, UF , ZrCL(, and about 30 organics. Ammonium chloride sublimes at 1 atm and 350°C with decomposition into NH3 and HC1, but these recombine into pure NH4C1 upon cooling. Iodine has a triple point 113.5°C and 90.5 Torr it can be sublimed out of aqueous salt solutions at atmospheric pressure because of the entraining effect of vaporized water. 

VOLATILE. Having a low boiling or subliming temperature at ordinary pressure in other words, having a high vapor pressure, as ether, camphor, naphthalene, iodine, chloroform, benzene or methyl chloride. 

There is a AH for the process of sublimation, called the latent heat of sublimation (heat of sublimation). Sublimation is the conversion from the solid state to the gaseous physical state, skipping the liquid state. Elemental iodine, I2, and C02 are substances that sublime at 1 atm pressure. 

Elemental iodine, I2 (solid, sublimes at 184°C), consists of violet-black rhombic crystals with a lustrous metallic appearance. More irritating to the lungs than bromine or chlorine, its general effects are similar to the effects of these elements. Exposure to iodine is limited by its low vapor pressure, compared to liquid bromine or gaseous chlorine or fluorine. 

Sublimation is a process that involves the direct conversion of a solid to a gas without passing through the liquid state. Relatively few solids do this at atmospheric pressure. Some examples are the solid compounds naphthalene (mothballs), caffeine, iodine, and solid carbon dioxide (commercial Dry Ice). Water, on the other hand, sublimes at — 10°C and at 0.001 atm. Sublimation temperatures are not as easily obtained as melting points or boiling points. 

The open end of the reaction tube is attached to an oil pump through a dust trap, and dry nitrogen is admitted at a rate such that the pressure is maintained at or below 25 mm. The bismuth(III) iodide is sublimed into the receiver end of the tube by heating with the spread burner flame and with an auxiliary Tirrill burner adjusted to maximum heat. Satisfactory progress of the sublimation is indicated by the persistence of yellowish-brown bis-muth(III) iodide vapors and by the absence of the violet color of iodine vapor. When the sublimation is complete, the tube is allowed to cool to room temperature in nitrogen at atmospheric pressure. The product is scraped from the receiver section of the tube and stored out of contact with the atmosphere. The yield is 72% based on bismuth. Anal. Calcd. for Bil3 Bi, 35.44. Found Bi, 35.42. The same result is found upon analysis of a resublimed sample of this product. 

Iodine is a black solid with a slight metallic luster. At atmospheric pressure it sublimes giving a violet vapor. Its solubility in water is slight (0.33 g L l at 25°C). It is readily soluble in nonpolar solvents such as CS2 and CC14 to give violet solutions spectroscopic studies indicate that dimerization occurs in solutions to some extent ... 

Other experimental and theoretical methods have been developed for the determination of the heat of sublimation of solid iodine these too are suitable for undergraduate laboratory experiments or variations on this experiment. Henderson and Robarts have employed a photometer incorporating a He-Ne gas laser, the beam from which (attenuated by a CUSO4 solution) has a wavelength of 632.8 nm, in a hot band near the long-wavelength toe of the absorption band shown in Fig. 3. Stafford has proposed a thermodynamic treatment in which a free-energy function ifef), related to entropy, is used in calculations based on the third law of thermodynamics. In this method either heat capacity data or spectroscopic data are used, and as in the present statistical mechanical treatment, the heat of sublimation can be obtained from a measurement of the vapor pressure at only one temperature. 

The halogens are volatile, diatomic elements whose color increases with increasing atomic numbers. Chlorine is a yellow-green gas at room temperature, bromine is a dark red liquid, and iodine is a lustrous, black, crystalline solid with a high vapor pressure that sublimes easily. 

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