A method was needed for preparing stoichiometric U308 for precise determination of uranium by gravimetric methods and for the preparation of uranium isotopic standards by the isotopic dilution method. Voss AND GREENE I have determined the uranium in U308 prepared from uranyl nitrate, uranyl fluoride, and uranium trioxide and found that some of these compounds do not ignite to a stoichiometric U~08 under the usual ignition conditions (800-85 oยฐ for 1-3 h). BROUNS AND MILLS 2 have reported that uranium trioxide was not completely converted to U308 at 800 ยฐ, "even after prolonged heating."
Non-stoichiometry is common with the uranium-oxygen compounds3; however, under properly controlled conditions, stoichiometric UaOs, which can be used as such in precise laboratory work, is produced. The ignition time and temperature necessary for the preparation of stoichiometric U308 are dependent upon the starting chemical compound (parent material) and the sample size (bed depth). Chemical alteration of the parent material, such as treatment with hydrofluoric acid, can change the required ignition conditions. Parent materials studied were uranium metal, uranyl fluoride, uranium peroxide, uranium trioxide, and uranyl nitrate. The ignition conditions for each will be discussed separately.
EXPERIMENTAL
Apparatus
The electric furnace had a safe operating temperature of 136o ยฐ and the muffle was heated uniformly by radiant heat from four bar-type resistance elements. The temperature was controlled to tlO ยฐ by a potentiometer and superheated steam was introduced in the rear of the furnace when needed. The top and sides of the muffle were lined with platinum. A calibrated potentiometer was used in checking the furnace temperature to assure accurate temperature control. 2o-ml platinum dishes were used for all Io-g samples to standardize the bed depth.
A digital voltmeter with platinum-tungsten and platinum-saturated calomel electrode systems was used to determine the U+a/U +4 and U+4/U +6 end-points, respectively.