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Arsenic Pentafluoride, AsF5

Arsenic Pentafluoride, AsF5, may be prepared by the interaction of arsenic and fluorine in a platinum vessel, or by the action of bromine and antimony pentafluoride on arsenic trifluoride. The reagents should be dry, and if the reaction in the second case is carried out in a glass vessel, the apparatus should be in one piece and well dried. The bromine is added to the mixture of fluorides at -20° C. and, after cooling in liquid air, the arsenic pentafluoride is obtained by heating on a water-bath at 55° C., the gas passing through a reflux condenser to a receiver surrounded by liquid air. The product contains bromine, which is removed by passing the gas over molten sulphur.

Arsenic pentafluoride is a colourless gas at ordinary temperature. It condenses to a pale yellow liquid of boiling point -52.8° C. and freezes to a white solid of melting point -79.8° C. The vapour density corresponds with a mol. wt. 169.5 (AsF5 = 169.9). The density curve of the liquid is represented by.

D = 3.505 - 0.00534(273 + t)

The density of the solid is 3.02 at -91° C. Vapour pressure determinations for the solid and liquid have been made, the results being as follows:

Solid AsF5: t (° C.).-116.7-106.1-98.4-91.7-87.3-82.7
V.p. (mm.)1.36.518.342.269.8114.7
Liquid -77.2-71.2-65.1-61.1-56.8-53
V.p. (mm.)180.2266.4386.0487.9604.7747.9


In contact with moist air the gas gives dense white fumes. It dissolves in water with evolution of heat. It also dissolves in alcohol, ether and benzene; and in aqueous alkali and arsenic trifluoride, some heat being developed. In contact with paraffin oil or wax, charring gradually occurs; sugar and paper, if moist, are also blackened. Turpentine vapour gives a black cloud on contact with the gas. When heated in contact with silicon, silicon tetrafluoride and arsenic are formed. The gas attacks dry glass when heated, silicon tetrafluoride and probably arsenic pentoxide being produced. With phosphorus, some phosphorus trifluoride is formed. The gas reacts with iodine in the cold. The metals mercury, zinc, lead, bismuth and iron are converted to fluorides; copper is attacked only on heating. Tungsten is not affected by the gas.

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