Arsenic Nitrosyl Hexafluoride, AsF₅.NOF

Arsenic Nitrosyl Hexafluoride, AsF₅.NOF, was obtained by Ruff by passing nitrosyl fluoride through arsenic trichloride, the mixture being kept cool. When absorption was complete, any nitrosyl chloride was removed by storage over fused sodium hydroxide in a vacuum. The product consists of white crystals, stable in dry air even at high temperature. It is decomposed by water and consequently is unstable in moist air; it is also decomposed by caustic alkali and by concentrated hydrochloric acid. On warming with antimony pentafluoride, arsenic pentafluoride is produced thus:

AsF₅.NOF + SbF₅ = AsF₅ + SbF₅.NOF

The following fluoarsenates have been described by de Marignac. Potassium hexafluoarsenate, 2KF.2AsF₅.H₂O, obtained by addition of hydrofluoric acid to a solution of potassium arsenate, yields small rhombic crystals, stable when dry but decomposed by water. The crystals have axial ratios a:b:c = 0.8401:1:2.5172. When heated, the crystals melt and decompose, giving off water and hydrogen fluoride. The aqueous solution of the salt on crystallisation deposits rhombic plates of potassium oxytetrafluoarsenate, KF.AsOF₃.H₂O. A solution of potassium hexafluoarsenate containing excess of potassium fluoride and hydrofluoric acid, on crystallisation deposits rhombic crystals of potassium heptafluoarsenate, 2KF.AsF₅.H₂O. The axial ratios of these crystals are a:b:c = 0.8847:1:0.6453. The arsenate is stable in dry air. By repeatedly crystallising this salt from its aqueous solution, or by crystallising a solution of the oxytetrafluoarsenate in dilute hydrofluoric acid, a crystalline mass is obtained to which the formula 4KF.AsF₅.AsOF₃.3H₂O has been ascribed.

A crystalline ammonium hexafluoarsenate has not been obtained. A mixture of ammonium arsenate and hydrofluoric acid on evaporation yields a gum-like mass. Arsenious oxide dissolves in a boiling solution of ammonium fluoride, but on cooling the arsenious oxide separates from the solution.