Phosphorus Oxytrifluoride, POF₃

The production of this compound by the partial hydrolysis of PF₅ or PF₃Cl₂ has already been mentioned. It may also be made by passing electric sparks through a mixture of PF₃ and oxygen. The first method of preparation consisted in heating phosphorus pentoxide with a fluoride, such as cryolite. It is also produced by the action of hydrogen fluoride on phosphorus pentoxide.

It is best prepared by the action of silver, zinc or lead fluoride on POCl₃. The oxychloride is allowed to drop gradually on to anhydrous zinc fluoride in a brass tube at 40° to 50° C. The gas passes out through a lead tube, then through a condenser at -20° C. to retain POCl₃, through a tube of zinc chloride to absorb the last traces of oxychloride, and finally is collected over mercury.

It is colourless, with a pungent odour, and fumes slightly in the air. The vapour density is 3.68 to 3.71 (air = 1) and 52.0 (H = l), the theoretical densities for POF₃ being 3.69 and 52.0 respectively. The gas condenses to a colourless liquid which boils at -40° C. and freezes to a white solid at -68° C. When dry it does not attack glass in the cold, but does so when heated, although not so strongly as phosphorus trifluoride. On the other hand, whilst the trifluoride was absorbed very slowly by water, the oxyfluoride was absorbed very quickly. Neither gas is completely hydrolysed, appearing to form fluooxyacids.

Fluorine is pre-eminent among the halogens in its power of replacing oxygen of oxyacids to form fluoacids, as is exemplified by such well-known compounds as H₂SiF₆. Examples of fluoacids are also found among the more strongly electronegative elements of Group VI B, i.e. sulphur and its congeners.

In the case of phosphorus such compounds as P(OH)₃OK.KF may perhaps be regarded as belonging to this type.

It has already been noted that the hydrolysis of phosphoryl fluoride is not at first complete. It reaches a stage at which the addition of "nitron" gives a salt, C₂₀H₁₆N₄HPO₂F₂ (m.pt. 230.5—232.5° C.), of difiuophosphoric acid, POF₂(OH). The ammonium salt of this acid has been prepared. The acid is also produced when phosphorus pentoxide is fused with ammonium fluoride, or when the pentoxide is dissolved in aqueous hydrofluoric acid. The last-mentioned solution, when kept at the ordinary temperature, undergoes hydrolysis, and with "nitron" yields the "nitron" salt of hexafluo phosphoric acid, HPF₆, the ions of which have also been prepared by dissolving phosphorus pentafluoride in cold water. The ions, PF₆^-, are stable towards boiling water. Solutions of the potassium salt do not give precipitates with salts of the alkaline earth metals.