Uranyl Sulphide, UO₂S

Uranyl Sulphide, UO₂S, is obtained by adding ammonium sulphide to an alcoholic solution of a uranyl salt, washing the precipitate with alcohol and drying in a vacuum. Precipitation from aqueous solutions yields a brown amorphous hydrated form, which is soluble in dilute acids, including acetic acid, and also in ammonium carbonate. Complete precipitation is obtained only after prolonged warming of the mixture containing a slight excess of ammonium sulphide; the precipitate then becomes olive green, crystalline, and can be easily filtered, but it consists largely of hydrated uranous oxide and sulphur, due to hydrolytic decomposition by the warm water. Uranyl sulphide is obtained as black, tetragonal, needle-shaped crystals by fusing together pure dry potassium thiocyanate (12 parts), urano-uranic oxide (3 parts), and flowers of sulphur (5 parts). It rapidly decomposes on heating, leaving a residue of the green oxide.

If uranyl sulphide, freshly precipitated from aqueous solution, is digested for two days with excess of ammonium sulphide, the mixture being in contact with air, a deep red solid of complex composition, and known as uranium red is produced. It has been shown by Kohlschutter that a series of similar red compounds may be obtained by reducing solutions of alkali uranates, containing excess of uranyl sulphate or nitrate and a little free alkali, by means of hydrogen sulphide; in each case an orange-yellow compound is first produced, which on treatment with alkali yields a blood-red uranium red. This is decomposed by acids, with liberation of hydrogen sulphide and sulphur. The composition of the orange-yellow substance appears to be 5UO₃.2K₂O.H₂S₂, while that of the potassium uranium-red corresponds to 5UO₃.2K₂O.HKS₂. Thus all the uranium is in the hexavalent condition, and from a study of the two compounds on decomposition, Kohlschutter suggests the following constitutional formulae:

1. Orange-red parent compound.	2. Uranium red.