Nitrosylselenic Acid, NO.O.SeO₂.OH

An attempt to prepare Nitrosylselenic Acid, NO.O.SeO₂.OH, by the action of liquid nitrogen tetroxide on concentrated selenic acid, cooled with solid carbon dioxide, resulted in the formation of a dark blue solid, the composition of which suggested it to be the compound SeO₂(O.NO)₂. It melted at -13° C. and above this temperature decomposed into its components. The blue colour may, however, be attributed to the decomposition of the nitrogen tetroxide by the action of water to form nitrogen trioxide and nitric acid,3 for when the reagents are mixed there is a separation into two layers, the upper containing nitrogen trioxide, nitric acid and a little selenic acid, whilst the lower is a mixture of nitrogen trioxide and selenic acid. The lower layer solidifies when sufficiently cooled, and on gently warming then decomposes into nitric oxide and nitrosylselenic acid, which is colourless.

The acid is best prepared by the action of a large excess of liquid nitrogen trioxide on ice-cold anhydrous selenic acid. The excess of liquid trioxide is subsequently allowed to evaporate away.

Nitrosylselenic acid is a colourless crystalline snow-like mass, which melts at 80° C. with decomposition. It is unstable even at atmospheric temperatures and is immediately decomposed by water.

A blue compound, apparently analogous to sulphonitronic acid, is formed on the addition of mercury to a solution of nitrosylselenic acid in selenic acid monohydrate, or by reducing a solution of sodium nitrite in concentrated selenic acid by means of mercury. The presence of copper selenate facilitates the reaction. The selenonitronic acid is very readily oxidised and appears to be stable only in the presence of concentrated selenic acid.