Phosphonium Bromide, PH₄Br

Phosphonium Bromide, PH₄Br, was made by Serullas in 1831 from the component gases by direct union. Its dissociation pressure reaches only 1 atmosphere slightly below +38° C. It can also be made by passing phosphine into the most concentrated aqueous hydro- bromic acid, or hydrogen bromide dissolved in phosphoryl chloride, or from phosphine and bromine. It forms colourless cubic crystals which sublime at about +30° C. The heat of formation of the solid from the gases PH₃ and HBr is +23 Cals., while that evolved when the initial materials are bromine (liquid), hydrogen and phosphorus (solid) is +44.1 Cals.