K. C. Patel and David E. Goldberg Brooklyn College of C U N Y Brooklyn, New York 11210
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Pre~arationand Structural Characterization of Ammonium Hexaquovanadium(II) Sulfate
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A n advanced inorganic chemistry experiment
Since students in general chemistry lahoratory are doing preparations of simple coordination compounds and their characterization by quantitative determination of the elements present, it is possible in advanced courses to have them prepare and characterize the structures of more difficult compounds. Students enjoy the opportunity t o use modern instrumentation for this type of relatively difficult task. Interest has grown to a very high level in the last decade in the preparation and structural studies of unstable compounds of the transition metal ions. In dealing with this type of problem, students develop insight, decisiveness, and patience. If the metal ion is unstable in air, the following techniques can he used, depending upon the facilities available: 1) use of highly effective glove boxes containing a nitrogen atmosphere, 2) use of a layer of petroleum ether, toluene, kerosene, or ligroin on the surface of the solution or 3) use of all-glass apparatus (that is, a vacuum line) which has been flushed out with COz, NZor coal-gas. The specific compound t o be described here, ( N H ~ ) Z V ( H Z O ) O ( S O was ~ ) Z ,prepared by students in the senior level inorganic lahoratory a t this college. Most salts and solutions of vanadium(I1) ion are unstable in air, and hence commerciallv unavailable. Thev anaerobic . reauire . conditions for their preparation and preservation. Preparation of the above compound then involved reduction of vanadium(V) to vanadiui(11) and stabilization of vanadium(I1) by crystallizing it in (NH&SOn a s a Tutton salt. Electrolytic reduction described by Larkworthy e t al.,' is the best method to obtain pure aqueous vanadium(I1) solutions; however, the use of zinc metal and mineral acid does serve the purpose in the present experiment. Structural characterization was done using group theoretical arguments2 to interpret the infrared spectra and visible near infrared reflectance spectra. Experimental The Tutton salt is prepared as fallows: To a 1-1 suction flask. 9.1 g of Vz05 (0.050 mole) and 300 ml of water are added. (It is possible to substitute 0.10 mole of NHIVOB tor the VzOs.) To this solution, 50 ml of concentrated H2SOn is added slowly and carefully, with stirring. The resulting solution is reduced to a violet vanadium(l1) solution, with intermediate blue vanadium(lV) and green vanadium(II1) solutions, by addition of excess granular zinc metal. A solution of 13.2 g of (NH&SOn (0.100 mole) in 50 ml of w&r in a 1-1 Ehrlenmeyer flask is deoxygenated by bubbling nitrogen through it for 15 min. The apparatus shown in Figure 1 is assembled after passing nitrogen gas through the filter and other parts to remove all traces of air. Then the violet vanadium(1I) solution solution by use of nitrogen gas, as is filtered into the (NH1)~SOn shown inPigure 1. (Only a moderate pressure of nitrogen is used; high pressure will cause the apparatus to disassemble.) Cubic crystals of violet-colored Tutton salt, (NH4)2V(H20)sare usually formed immediately. If the solution should be too dilute, however, addition of methanol is required to induce crystallization. The solid is filtered and dried by suction in a glove box under nitrogen atmosphere. Alternatively, filtration is done in a nitrogen-purged filter flask with an open Buchner funnel under a stream of nitrogen which is supplied through a large inverted funnel almost completely covering the Buchner funnel. After complete drying, the crystals are removed from the funnel 868 /Journal of Chemical Education
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