Uranyl metaborate

Uranyl metaborate, U02(B02)2, is prepared by heating UsOs with an excess of boric oxide in air at 900°-1100°C. In the presence of excess B2O3 the salt is thermally stable to llOO C. in air and to —950°C. in a nitrogen atmosphere. Without excess B2O3, decomposition begins at 750°C. in air. The greenish-yellow crystals of are insoluble in... 

Preparation of Uranyl Metaborate. An anhydrous boric oxide glass is prepared by thermal decomposition of boric acid in a platinum dish. The uranium oxide powder is then spread on the surface of the boric oxide, and the reactants are heated in air at — 1000°C. until Reaction 1... 

Thermal Stability. Uranyl metaborate is one of the most stable (thermally) uranyl salts the pure salt begins to show evidence of decomposition only when heated above 750°C. Even at 800°C., only 3% decomposition is observed after two days. The reverse reaction occurs to 1100°C. in air when excess B2O3 is present, thus showing that this instability is occasioned by loss of B2O3 from the crystal. The equilibrium pressure of boric oxide over the metaborate between 800° and 1100°C. must be less than that of pure B2O3. An equation derived by Nesmeya-nov and Firsova (16) from effusion data on boric oxide gives a vapor pressure of approximately 0.0002 mm. at 1000°C. and 0.002 mm. at 1100 °C. these values must constitute the upper limit for the uranyl metaborate equilibrium decomposition pressure. No measurable decomposition of U02(B02)2 was detected after two hours at 925°C. in a nitrogen atmosphere, but substantial decomposition occurred after two hours at 1000 °C. 

Reaction with Water. Uranyl metaborate is virtually insoluble in water at 25°C., but the salt hydrolyzes slowly over several days to form uranyl hydroxide. 

Infrared Bands in Sodium, Calcium and Uranyl Metaborates (cm. )... 

After complete conversion to U02(B02)2 is achieved, the uranyl salt is freed from B2O3 by dissolving the excess sesquioxide in absolute methanol. Reasonable care should be taken to minimize access of moisture to the metaborate during the purification step, but completely anhydrous conditions are not required. A sample of IJO2B2O4 prepared as described above gave the following analytical results uranium found 66.80%, theoretical 66.93%, and boron found 6.02%, theoretical 6.08%. 

Attempts to prepare uranyl borate from UO3 instead of U3O8 were only partially successful. The trioxide is more reactive than U3O8 and eliminates the necessity for an oxidation step in the metaborate synthesis reaction, but the low thermal stability of UO3 restricts the reaction temperature to <650°C. Infrared and x-ray analyses of products obtained after heating UO3 and B2O3 mixtures for several days at 650°C. indicated that only a minor conversion to the metaborate had been effected. 

Ur any 1, Sodium and Calcium Metaborate. The spectra of uranyl, sodium, and calcium metaborates are illustrated in Figure 1, and Table II lists the absorption maxima for three compounds. The bands in the 92% B compounds are also located. Data on the sodium and calcium salts, as representative of the ring and chain metaborate structures, are in agreement with results reported by Goubeau and Hummel (6). The spectra given by Wier and Schroeder (20) are more complex and may not represent pure phases. 

Zachariasen WH (1963) The crystal structure of monoclinic metaboric acid. Acta Cryst 16 385-389 Zachariasen WH, Plettinger HA (1954) Crystal chemical studies of the 5f-series of elements XXV The crystal structure of sodium uranyl acetate. Acta Cryst 12 526-530... 

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