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VANADIUM
Natural Abundance, Stable Isobars aV50, 0.25%, Ti50 V51, 99.75%, Cr50 Vanadium oxytrichloride and vanadium trifluoride have been used as charge material in the separation of vanadium isotopes. Vanadium trifluoride proved to be more satisfactory, and the usual charge consists of 200 g VF3 in a style C-18 graphite charge bottle. A method for the preparation of vanadium trifluoride without using anhydrous hydrogen fluoride or fluorine was devised to eliminate the hazards associated with handling these corrosive gases. Anhydrous VF3 is prepared by the fusion of ammonium bifluoride with vanadium trioxide. One mole of vanadium trioxide is mixed thoroughly with 12 moles of ammonium bifluoride. Twice the stoichiometric quantity of ammonium bifluoride is used to ensure completeness of reaction. The mixture is placed in a graphite crucible and gently heated over a gas flame. At a temperature of approximately 100ºC the mixture becomes fluid and is stirred with a graphite rod containing a thermocouple. As the temperature is slowly increased to 250ºC, water and excess ammonium bifluoride are driven off leaving a green solid residue of ammonium hexafluovanadate. Thermal decomposition of ammonium hexafluovanadate is carried out by heating to 550ºC while flushing continuously with nitrogen. The decomposition chamber is a nickel cylinder having a removable cover at one end. The gas inlet line passes through this cover and extends almost to the bottom of the cylinder. The short outlet tube also passes through the cover plate and is electrically heated to prevent condensation of the decomposition products being carried out by the flushing stream of nitrogen. A temperature of 550ºC is maintained until white fumes are no longer observed coming from the outlet tube, and the flow of nitrogen is continued until the container is cooled to room temperature. The VF3 produced in this manner is a fine gray-green powder and exhibits crystallographic properties normally associated with vanadium trifluoride. Contamination from the nickel container is found to be < 0.05%. A more detailed description of the VF3 synthesis, complete with bibliography, has been published as ORNL CF-58-5-95, Preparation o/ Vanadium Trifluoride by the Thermal Decomposition of Ammonium Hexafluovanadate (iii) by B. J. Sturm and C. W. Sheridan. Vanadium and its compounds are unquestionably toxic. Both ingestion and inhalation can be prevented by good hygienic practices and the use of a respirator or fume hood with good exhaust ventilation. aV5O is radioactive with a half-life of 4.8 × 1014 years. |
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