Arsenic Trifluoride, AsF₃

Arsenic Trifluoride, AsF₃, is formed when fluorine reacts with arsenic trichloride or with the arsenides of the alkali or alkaline earth metals; by the action of anhydrous hydrofluoric acid or of acid fluorides on arsenious oxide; by the action of certain metallic fluorides, for example silver or lead fluoride on arsenic trichloride, or of ammonium fluoride on arsenic tribromide; and by the action of iodine pentafluoride on arsenic.

The usual method of preparation is that employed by Dumas, who first discovered the compound in 1826, namely, by the addition of concentrated sulphuric acid (2 parts) to a mixture (1 part) containing equal weights of calcium fluoride and arsenious oxide. The trifluoride is collected in a lead receiver immersed in ice water and rectified by distillation on a water-bath at 65° C. It is best kept in a platinum bottle.

Arsenic trifluoride is a colourless mobile liquid with an odour resembling that of silicon tetrafluoride. It fumes in air owing to its rapid reaction with water vapour (see below). A drop on the skin evaporates rapidly, leaving a painful burn similar to that produced by hydrofluoric acid. According to Thorpe, the density at 0° to 4° C. is 2.6659 and at the boiling point 2-4497, the latter temperature being 60.4° C. Earlier workers gave b.pt. 63° to 64° C. The liquid solidifies to a crystalline mass at -8.5° C. The volume change on heating is expressed by the equation

v_θ = v₀(1 + 0.001443θ + 0.000000297θ²)

The heat of formation of liquid arsenic trifluoride has been obtained indirectly by determining the heats of dissolution of the trifluoride and of a mixture of arsenious oxide and sodium fluoride in about one litre of normal sodium hydroxide solution. The value obtained was 198,300 calories. Assuming Trouton's constant to be 21, the heat of vaporisation is about -7000 calories, so that the heat of formation of gaseous arsenic trifluoride is 191,300 calories.

The molecular volume in the liquid and gaseous states has been measured and from the results the radii of the atoms constituting the molecule have been calculated.

The liquid is a poor conductor of electricity, but some decomposition due to electrolysis occurs, a gas which attacks platinum being liberated and arsenic deposited on the cathode.

The Raman spectrum of arsenic trifluoride consists of four lines with the following frequencies: w₁(1)707, W₂(1)341, w₃(2)644, and w₄(2)274 cm.^-1; a pyramidal molecule is indicated.

The trifluoride readily reacts with water, and a trace of the latter is sufficient to cause the liquid to show an acid reaction. If more water be added there is a slight heating effect and, according to Dumas, hydrofluoric and arsenious acids are formed. Berzelius concluded that hydrofluoarsenic acid was the product. The solution reacts with tin and zinc and also attacks glass. The liquid trifluoride itself when heated in a glass vessel readily attacks the latter, thus:

4 AsF₃ + 3SiO₂ = 2As₂O₃ + 3SiF₄

Admixture of the trifluoride with alcohol or ether, or with fixed or volatile oils, causes some decomposition. Ammonia readily combines to form a white powder, arsenic triamminotrifluoride, As(NH₃)₃F₃; this compound is decomposed by water. Phosphine reacts differently, yielding hydrogen fluoride:

AsF₃ + PH₃ = AsP + 3HF

With the chlorides of phosphorus there is an interchange of halogens, thus -

AsF₃ + PCl₃ = AsCl₃ + PF₃
5 AsF₃ + 3PCl₅ = 5AsCl₃ + 3PF₅

and this is also the case with silicon tetrachloride, thionyl chloride and thiophosphoryl chloride, thus:

4AsF₃ + 3SiCl₄ = 4AsCl₃ + 3SiF₄
2AsF₃ + 3SOCl₂ = 2AsCl₃ + 3SOF₂
AsF₃ + PSCl₃ = AsCl₃ + PSF₃

Sulphur chloride and carbon tetrachloride do not react in the cold. Silico-chloroform at ordinary temperature reacts vigorously, two simultaneous reactions apparently taking place:

(i) AsF₃ + SiHCl₃ = AsCl₃ + SiHF₃
(ii) 4AsF₃ + 3SiHCl₃ = 2AsCl₃ + 3SiF₃ + 3HCl + 2As

Arsenic trifluoride reacts with bromine to form a crystalline substance; iodine dissolves in the liquid yielding a purple-red solution; the nature of these products has not been elucidated.