Bismuthyl Fluoride, BiOF

Bismuthyl Fluoride or Bismuth Oxyfluoride, BiOF, can be obtained by the decomposition of BiF₃.3HF or of BiOF.2HF as described above; or by adding freshly precipitated bismuth hydroxide to hydrofluoric acid until the acid is just neutralised. It is described as a heavy, white powder, of density 7-5 at 20° C., not deliquescent, decomposed when heated to bright redness.

Bismuth trifluoride does not combine with fluorine except perhaps in traces at -80° C. If so-called "bismuthic acid" or potassium bismuthate is added to 40 per cent, hydrofluoric acid at -10° C., a colourless, very unstable solution is obtained which probably contains a compound of quinquevalent bismuth. The compound has not been isolated, as it decomposes when the solution is evaporated. It may be bismuth pentafluoride, BiF₅, but it appears more probable that it is mainly bismuth oxytrifluoride, BiOF₃. The solution has strong oxidising properties, as shown by its action on hydrochloric acid, potassium iodide and alcohol (the latter being oxidised to aldehyde). If potassium fluoride is added to the solution before evaporation, the substance Bi₃O₄F₇.3KF separates out as small, yellow crystals; with excess of potassium fluoride, a double compound of bismuth oxytrifluoride and potassium fluoride, BiOF₃.3KF, is obtained as well-formed, colourless, prismatic crystals which decompose in moist air, becoming yellow.