Rare vapor pressures

Figure 4 shows vapor pressure curves of rare-earth metals[24], clearly showing that there is a wide gap between Tm and Dy in the vapor pressure-temperature curves and that the rare-earth elements are classified into two groups according to their volatility (viz.. Sc, Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, and Lu, non-volatile elements, and Sm, Eu, Tm, and Yb, volatile elements). Good correlation between the volatility and the encapsulation of metals was recently... 

Figure 3. Saturation vapor pressures of rare-earth and alkaline-earth metals at 1800 K.

When the boiling points of metallic impurities are much lower than the boiling point of the main metal, they can simply be distilled away in most cases. The rate and the extent of the removal by distillation of these impurity elements depend upon their partial pressures over the main metal/melt. As an example, let the feasibility of distilling magnesium and magnesium chloride from titanium and calcium from the rare earths be considered. In the firstcase, at 900 °C, the pertinent vapor pressure values are P = 4 10-11 torr, PMg = 105 torr... 

Thulium is a naturally occurring rare metal that exists is very small amounts mixed with other rare-earths. It is a bright silvery metal that is malleable and ductile and can be cut easily with a knife. Its melting point is so high that it is difficult to force it into a melted state. Its vapor pressure is also high, and thus, much of the molten thulium evaporates into the atmosphere. Its melting point is 1,545°C, its boiling point is 2,950°C, and its density is 9.32g/cm. ... 

There is no evidence that inhalation exposures in occupational settings cause the rapid progressive pulmonary fibrosis and injury to the heart, liver, and kidneys that occur after ingestion. Because of the low vapor pressure, there is little inhalation hazard. Spray droplets are usually too large to reach the alveoli. If exposure is excessive, droplets may be inhaled into the upper respiratory tract and cause nosebleed, sore throat, headache, and coughing from local irritant action. Rarely, dermal exposure to paraquat has resulted in systemic poisonings and deaths with renal and pulmonary... 

A striking feature of the ILs is their low vapor pressure. This, on the other hand, is a factor hampering their investigation by MS. For example, a technique like electron impact (El) MS, based on thermal evaporation of the sample prior to ionization of the vaporized analyte by collision with an electron beam, has only rarely been applied for the analysis of this class of compounds. In contrast, nonthermal ionization methods, like fast atom bombardment (FAB), secondary ion mass spectrometry (SIMS), atmospheric pressure chemical ionization (APCI), ESI, and MALDI suit better for this purpose. Measurement on the atomic level after burning the sample in a hot plasma (up to 8000°C), as realized in inductively coupled plasma (ICP) MS, has up to now only rarely been applied in the field of IE (characterization of gold particles dissolved in IE [1]). This method will potentially attract more interest in the future, especially, when the coupling of this method with chromatographic separations becomes a routine method. 

Since the petroleum engineer is rarely concerned with solid hydrocarbons, in the following discussion we will consider only the vapor-pressure line and the liquid and gas portions of the phase diagram. 

Suppose we take a sample of bone-dry air at some temperature, Ti, and directly contact it with water until it becomes saturated at the same temperature. The water vapor that enters into the air contains with it its latent heat of vaporization. The vapor pressure of water out of the liquid will be greater than it is in the saturated air, causing vaporization to occur and subsequently increasing the humidity of the air-water-vapor mixture. The process of vaporization ends when the vapor pressure of the water in the air becomes equal to that of the liquid. At this condition the air is saturated. During the air saturation process, isothermal conditions for the water can be maintained if heat is supplied to replace the heat lost from it to the gas as latent heat of vaporization. Thus, heat transfer during the saturation of a gas with a liquid can be accomplished without a temperature differential (although this is rarely encountered). This type of heat transfer phenomenon, better known as diffusional heat transfer, is different from conduction, convection or radiation. 

Additional factors used to screen candidate iodine fixation compounds were thermal and chemical stability and volatility. Of the low solubility iodides, those of Ag, Cu(I), Pb, Pd, and T1 meet the arbitrary 250°C stability requirement. Several, such as those of Bi and Hg, have excessive vapor pressures. Many of the iodates show excellent thermal stability including those of the alkaline earths, rare earths, Ag, Cu, Pb, Zn, Hg, Th, and U. Several, including AglOo and Hg(I03)2 convert to the iodide on heating(19). 

Environmental Fate. Tetryl released to the environment partitions mainly to water and soil (Army 1987d Lyman et al. 1982 Navy 1984b). Tetryl is transported in soil, surface water, and, rarely, in groundwater (Army 1987d Swann et al. 1983). Because of its very low vapor pressure, it is unlikely to partition to air (Army 1987d). No data were located regarding atmospheric transport of tetryl. 

It is interesting to note that the vapor and liquid compositions are usually different for ideal mixtures. We can see this from Eq. (6.6), since different pure component vapor pressures are rarely equal at the same temperature. This picture changes when nonideal mixtures are considered. As we see from Eq. (6.55, the vapor and liquid mole fractions can become equal when the fugacity and activity coefficients alter the pressure ratio enough to cause the K value to become unity. We then have an azeotrope. 

The majority of pyrethroid insecticides have low volatilities. The heavily used synthetic pyrethroid permethrin is classified as nonvolatile on the basis of its vapor pressure (1.3 x 10 kPa at 20 °C) and is rarely found in indoor air. However, it has recently been reported to be the major pesticide residue found in house dust (USEPA, 2000d). Cyper-methrin [( )-a-cyano-3-phenoxybenzyl-( )-cA,frani -3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate] and cyfluthrin [cyano(4-fluoro-3-phenoxy-phenyl)methyl 3-(2,2-dichloro-ethenyl)-2,2-dimethylcyclopropanecarboxylate] are two other low-volatility pyrethroids commonly used for indoor flea and cockroach control. 

A search for new efficient techniques of rare earth element separation and purification from calcium is a current problem, as production of high-purity rare earths is of great importance in advanced technology and material science. This problem may be solved by vacuum sublimation of volatile compounds when the difference in vapor pressure of the components present is used. This technique of purification was tested for Nd and Ca separation in vacuum. The well-known volatile and thermally stable dipivaloylmethanates were taken as starting substances. It was found that the addition of pivalic acid to the Nd(dpm)3 + Ca(dpm)2 mixture caused an increase in the separation efficiency and led to pure Nd(dpm)3 in the sublimate . ... 

Vapor pressure vs. temperature data are presented graphically for the rare earths by Beavis (7). 

The enthalpy of formation of YF3 was determined by Rudzitis, Feder, and Hubbard 164) using fluorine bomb calorimetry. NdCla was done by solution methods (179), and the enthalpies of formation of LaFs and PrFa were determined by Polyachenok 161) who employed an indirect equilibration technique. A recent torsion-effusion study of the vapor pressure of CeFs 115) yields second and third law values for the enthalpy of sublimation. The thermodynamics of the chlorination of rare earths with gaseous chlorine have also been investigated 144). Gibbs energies of formation were determined for CeClg by solid-state electromotive force techniques 41). 

Since no accurate vapor pressure data are available for the erbium and thulium hahdes, the molar absorptivities were determined directly from a weighed amount of the respective rare-earth halides. Good results could be obtained from this method if the respective halogen, bromine, or iodine were added to the cell such that its pressure at 1000°C. was 1 atm. This procedure greatly reduced the reaction of the rare-earth... 

The chelates can be purified, and mixtures of the complexes can be separated by fractional sublimation and distillation. In the gas phase, in solution, and in the solid state, Tb(thd)3 emits a brilliant green fluorescence when irradiated at 3660 A. with an ultraviolet lamp. Fluorescence is also exhibited by Eu(thd)3, Dy(thd)3, and Sm(thd)3. The praseodymium complex is thermally stable in the gas phase when heated for prolonged periods of time. Vapor pressure measurements on this complex showed no increase in pressure when the sample was heated at 250° for 6 hours. Thermogravimetric analyses and discussions of trends in volatility of the rare-earth-thd chelates have been published. 

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