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OSMIUM
Natural Abundance, Stable Isobars Os184, 0.018%, W184 Os186, 1.59%, W186 Os187, 1.64%, Re187 Os188, 13.3%, Pt190 Os189, 16.1%, Pt192 Os190, 26.4% Os192, 41.0% Osmium tetroxide is the only charge material which has been used in the separation of osmium isotopes. The average charge consists of 150 g OsO4 in a style SE stainless steel charge bottle located outside the calutron unit. Osmium tetroxide is prepared by heating osmium metal powder in a stream of oxygen. The extremely volatile osmium tetroxide is swept from the heated zone of the reactor into the cooled charge bottle. An electric heating tape wrapped around the inlet and outlet tubes of the charge bottle eliminates plugging of the openings. A typical synthesis consists of placing 100 g osmium metal powder in a quartz boat which is inserted in a Pyrex reactor 70 cm in length by 8 cm in diameter. An inlet gas-washing bottle containing concentrated sulfuric acid is connected to a 7 mm glass tube having a Teflon stopcock which is attached to the reactor with a tapered glass joint. The exhaust end of the reactor is drawn down to a diameter of 12 mm and terminates in a tapered joint which is connected to a short length of fluorothene tubing and a metal tube with a Swagelok fitting. A cylindrical stainless steel charge bottle, 10.2 cm in diameter by 10.2 cm in length with valved stainless steel inlet and outlet tubes, is connected by the Swagelok fitting to the reactor and five gas-washing bottles. An electric tube furnace heats the reactor while cylinder nitrogen is passed through the inlet-gas scrubber, reactor, charge bottle, and exhaust train of five gas-washing bottles. When a temperature of 500° C is reached, the nitrogen flow is gradually replaced with oxygen. Osmium tetroxide is swept from the reactor and condenses in the charge bottle, which is externally cooled by a mixture of dry ice and carbon tetrachloride. Exhaust gases pass through the first gas-washing bottle, which is empty to prevent a backflow into the charge bottle; the second one, which contains 20% sodium hydroxide and an equal volume of ethanol; the third one, which is also empty; the fourth one, which contains concentrated hydrobromic acid; and the fifth one, which contains oil. A length of Tygon tubing conducts the exhaust gas from the last bottle to a fume hood where it is exhausted from the laboratory. Approximately two hours are required for the conversion and about 96% of the OsO4 condenses in the charge bottle; approximately 2% is trapped in the sodium hydroxide and hydrobromic acid wash bottles. When the conversion is complete, heating is discontinued and nitrogen is passed through the system for ten minutes. The nitrogen is then turned off, both valves on the charge bottle closed, and the container is removed from the system. Blank fittings and clamps are used to close all openings. Due to the initial expense of the material and potential hazards associated with osmium, all unresolved charge material was recovered from calutron components at the end of the separation series at ORNL. In addition to the usual washing of calutron components, the mechanical and diffusion pumps were drained and flushed with Varsol (a hydrocarbon solvent). A large quantity of carbon from the source and receiver was also processed for osmium recovery. All solutions were filtered and sampled for spectrographic analysis. The following tabulation gives the type and quantity of each solution and the osmium content.
The accumulated solids from all solutions are combined with crushed carbon salvage and ignited in oxygen. A Vycor reactor 6.4 cm in diameter and 76 cm in length having the exhaust side drawn down and fitted with a ball joint is used for this procedure. The reactor is placed in a 7.6 cm tube furnace and connected to a train of five one-piece traps which are half filled with 50% hydrobromic acid. The salvage material is heated to 400° C under nitrogen at which time oxygen replaces the nitrogen, and the temperature is raised to 800° C. After two reactors of carbon have been ignited, the first trap in the system is replaced with one containing fresh hydrobromic acid. Trap solutions are combined and granular zinc, 20 mesh, is cautiously added. The hydrogen reduces the osmium out of solution as the element. This reduction is continued until the supernatant is colorless. Hydrochloric acid is added when a deficiency of acid occurs. If periodic spectrographic analyses of the supernatant indicate the osmium content to be < 10 ppm, filtrates are discarded. After washing the reduced osmium with dilute hydrochloric acid and several water washes, the dried osmium is placed in the reactor, oxidized, and collected in hydrobromic acid. Excess hydrobromic acid is boiled out of the solution, the concentrate is diluted with twice its volume of water, and the pH is adjusted to 6 using first sodium hydroxide and finally sodium bicarbonate. Digestion causes the resulting osmium dioxide to agglomerate and settle leaving a clear supernate. The supernate is decanted, acidified with dilute HCl, and treated with granular zinc. If no osmium is apparent, the solution is discarded. The osmium dioxide is washed with 10% solution of ammonium chloride until a test of the wash solution for sodium is negative. Osmium dioxide is dried and reduced to the metal using hydrogen. Ammonium chloride is added to the dried osmium dioxide, the temperature is slowly increased to 900° C, and this temperature is held for three hours or until the reduction appears complete. Ammonium chloride is added to eliminate the possibility of conflagration during hydrogen reduction. Heat is discontinued, the reactor is allowed to cool to 300° C, at which temperature the hydrogen flow is replaced by carbon dioxide, and the reactor cooled to room temperature. Cooling in carbon dioxide keeps hydrogen adsorption to a minimum in the osmium metal powder; otherwise, exposure of osmium containing adsorbed hydrogen to air would produce a catalyzed oxidation of hydrogen causing an explosion. Metallic osmium is innocuous but irritating effects of OsO4 have long been recognized as dangerous. One fatal case has been reported resulting from inhalation of OsO4 which caused capillary bronchitis. However, the principal effects of exposure are those of ocular disturbances, including either temporary or permanent loss of sight, and the halo effect around lights. Mucous membranes of the nose, throat, and bronchi are also attacked by OsO4 vapors, and dermatitis and ulceration can result from skin contact. A well exhausted hood, gloves, goggles, and use of copious quantities of soap and water should eliminate the hazards of OsO4. A good canister type gas mask should be available for use in case of emergency. |
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