Molybdenum vapor pressure

Chemical products are produced from technical-grade oxide in two very different ways. Molybdenum trioxide can be purified by a sublimation process because molybdenum trioxide has an appreciable vapor pressure above 650°C, a temperature at which most impurities have very low volatiUty. The alternative process uses wet chemical methods in which the molybdenum oxide is dissolved in ammonium hydroxide, leaving the gangue impurities behind. An ammonium molybdate is crystallized from the resulting solution. The ammonium molybdate can be used either directly or thermally decomposed to produce the pure oxide, MoO. ... 

Next, let the example of vanadium, which, in the as-reduced condition, may contain a variety of impurities (including aluminum, calcium, chromium, copper, iron, molybdenum, nickel, lead, titanium, and zinc) be considered. Vanadium melts at 1910 °C, and at this temperature it is considerably less volatile than many of the impurity metals present in it. The vapor pressure of pure vanadium at this temperature is 0.02 torr, whereas those of the impurity elements in their pure states are the following aluminum 22 torr calcium 1 atm, chromium 6 torr copper 23 torr iron 2 torr molybdenum 6 1CT6 torr nickel 1 torr lead 1 torr titanium 0.1 torr and zinc 1 atm. However, since most of these impurities form a dilute solution in vanadium, their actual partial pressures over vanadium are considerably lower than the values indicated. Taking this into account, the vaporization rate, mA, of an element A (the evaporating species) can be approximated by the following free evaporation equation (Langmuir equation) ... 

Mo (g). Jones, Langmuir, and Mackay1 reported values for the vapor pressure of solid molybdenum. The energy states of gaseous monatomic molybdenum are evaluated from the data of Catalan1 and Meggers and Kiess.1 See also Bacher and Goudsmit.1... 

The quantity of molybdenum hexafluoride may be measured either in a preweighed vessel or by vapor-pressure measurements in a previously calibrated volume on the line. It was pointed out by the checker that while molybdenum hexafluoride is being handled in the line the stopcock D should normally be kept closed. 

The experimental set-up has been previously described (A). Silicon and germanium metal were vaporized from a vitreous carbon crucible over the temperature range of 1475-1675°C and 1325-1500°C respectively. Tin and lead were vaporized from a molybdenum crucible over a temperature range of 1125-1240°C and 644-742°C respectively. This corresponds to an approximate metal vapor pressure range of 1x10 -1x10 torr for each metal. 

Besides the above-mentioned application for volatile, stable compounds, the technique of matrix isolation is superior to gas-phase investigation of compounds with a low vapor pressure. The vapor pressure of molybdenum hexacarbonyl, Mo(CO)6, for instance, is only 10 mbar at room temperature, which is insufficient for ordinary gas-phase infrared measurements. However, if Mo(CO)6 is flushed with a stream of matrix gas and if the final mixture is deposited as a matrix, then the density of the Mo(CO)6 molecules increases considerably. This integration effect (the deposition time varies from 5 minutes to several hours) is very important for high-temperature species and has interesting analytical applications. 

The hexacarbonyls of chromium, molybdenum, and tungsten are mononuclear the internal bonding parameters are presented in Table 3. The coordination geometry is that of a regular octahedron of idealized Oh syuunetry. The crystal packing is due to weak van der Waals interactions see van der Waals Forces), which explains the volatility of these compounds (the vapor pressures of Cr(CO)6, Mo(CO)6, and W(CO)6 at 30 °C are 0.28, 0.27, and 0.06 mmHg, respectively), and their solubility in organic solvents. 

Koch and Anable (4) have measured the vapor pressure of liquid molybdenum by a Langmuir free-evaporation method. They... 

The sonochemical oxidation of molybdenum carbonyl would not occur in the bubble, due to its low vapor pressure. Therefore, the sonochemical reaction of... 

Molybdenum hexacarbonyl is a highly toxic substance. It is listed in Toxic Substance Control Act. Acute symptoms include headache, dizziness, nausea, vomiting, and fever. Because of its low vapor pressure, the risk of exposure of this compound, however, by inhalation is lower than that from the volatile tetra- or pentacoordinated metal carbonyls. Ingestion of the compound can cause death. Absorption of its solution through skin may cause severe poisoning, manifesting the same effects as those from other routes of exposure. 

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