Uranyl Iodate, UO₂(IO₃)₂

Uranyl Iodate, UO₂(IO₃)₂, is obtained by precipitation of a solution of uranyl nitrate by means of sodium iodate in presence of a large excess of nitric acid. According to Ditte, the anhydrous salt is obtained as yellow crystals by mixing the boiling solutions and crystallising at 60° C. A pentahydrats, UO₂(IO₃)₂.5H₂O, is obtained when iodic acid or sodium iodate is added to a cold solution of uranyl acetate. When heated to 250° C. the iodate decomposes, yielding iodine and urano-uranic oxide. According to Artmann, however, the iodate does not exist in the anhydrous condition, but only as the mono- and di-hydrates, the nature of the product in the precipitation of uranyl nitrate by means of sodium iodate depending upon the conditions. From hot and acid solutions the monohydrate, UO₂(IO₃)₂.H₂O, is obtained, while from cold solutions the dihydrate, UO₂(IO₃)₂.2H₂O, is precipitated. The monohydrate exists in two modifications: a rhombic prismatic form, stable at ordinary temperatures, of density 5.220 at 18° C., and a pyramidal form of density 5.052 at 18° C. The former is converted into the latter by boiling with water, while the reverse change takes place slowly at ordinary temperatures. The two varieties differ in solubility, 100 c.c. of water dissolving at 18° C. 0.1049 gram of the prismatic form and 0.1214 gram of the pyramidal form. The di-hydrate has a lighter yellow colour and crystallises in irregular aggregates. It is slightly soluble in water, the solution being strongly acid. When heated, uranyl iodate retains one molecule of water and does not become anhydrous. Artmann, therefore, suggests that the molecule of the compound should be represented by the formula (OH)(UO₂)IO₃.IO₃H., i.e.



The potassium salt, K(UO₂)(IO₃)₃.3H₂O, is precipitated when potassium iodate is added in large excess to uranyl iodate solution as a light yellow powder of density 3.706. It is sparingly soluble in and is hydrolysed by water; it is converted into uranyl iodate by excess of uranyl nitrate.