Arsenic Triiodide, AsI₃

Arsenic Triiodide, AsI₃, may be prepared by gently heating a mixture of the elements. The combination is accompanied by evolution of heat and, on cooling, the triiodide can be obtained by crystallisation from carbon disulphide or xylol. The elements may be heated under a reflux condenser in the presence of a solvent such as carbon disulphide, ether or chloroform, until the colour due to free iodine has disappeared (the removal of the latter is accelerated by having a small excess of arsenic present); the solution is then decanted and cooled, and the triiodide which separates is recrystallised. The product, which is a pharmaceutical preparation, should contain not less than 99 per cent. AsI₃.

Many other methods of preparation have been employed. For example, the triiodide is formed when arsenious oxide, or a mixture of this oxide with sulphur, is heated in iodine vapour; or when arsenious oxide is heated with iodine, hydriodic acid, a mixture of potassium iodide and acetic acid, or a mixture of potassium iodide and potassium hydrogen sulphate. When arsenic disulphide and iodine, in the proportions IAs₂S₂:3I₂, are heated together, arsenic triiodide is formed. When arsenic trisulphide is fused with an excess of iodine, the product is soluble in carbon disulphide and the solution on evaporation deposits arsenic triiodide, then a sulphiodide and finally sulphur; with excess of sulphide the product is the sulphiodide, AsS₂I. If a solution of iodine in carbon disulphide is added to arsenic di- or tri-sulphide, the triiodide and sulphur are formed. The triiodide is also produced when a mixture of the trisulphide and mercuric iodide is heated; when hydriodic acid reacts with arsenic trichloride; and when arsine acts upon iodine, either dry or in alcoholic solution.

Commercial arsenic triiodide frequently contains considerable impurity, and samples prepared from arsenic and ether solutions of iodine have been found to contain up to 30 per cent, of free arsenic owing to the premature stopping of the reaction. This may be avoided by preparing the iodide by the action of potassium iodide on arsenious oxide in hydrochloric acid solution. Free iodine may also be present as an impurity. The following procedure, recommended by Paternosto, yields a product of 99.8 per cent, purity: 2 g. of arsenious oxide are treated with 30 c.c. of hydrochloric acid (dens. 1.19) and to the product is added a solution made by heating together 10 g. of potassium iodide and 10 c.c. of water. The mixture is allowed to react for about 5 minutes and the resulting precipitate is collected on asbestos and dried in a vacuum. The arsenic triiodide is extracted by repeated washing with carbon disulphide; when the latter is removed by evaporation in a vacuum, fine hexagonal plates of the pure triiodide remain. The pure product may also be obtained by refluxing a mixture containing 26.5 g. of arsenious oxide, 5.27 g. of sulphur and 102 g. of iodine at 200° C. for 14 hours, subsequently extracting at 90° C. with mustard oil and recrystallising rapidly from carbon disulphide by cooling in liquid air.

Arsenic triiodide crystallises in orange-red hexagonal plates. The structure is a layer lattice and there are six molecules per unit cell. The iodine atoms are in hexagonal close packing, each arsenic atom being surrounded by six iodine atoms. The distances of closest approach of iodine atoms are 4.28 and 4.13 A., and of arsenic and iodine about 2.97 A. The dimensions of a and c are, respectively, 7.187 and 21.394 A. When fused or sublimed, the colour is brick-red. The crystals are odourless and of density 4.374. The melting point, according to Madson and Krauskopf, is 138.6° C.; the boiling point is 394° to 414° C.

There are indications that the triiodide exists in two polymeric forms. The ordinary red form, when sublimed in a vacuum, gives a yellow sublimate which slowly changes to orange-red at ordinary temperatures; the yellow form is also obtained by cooling to a low temperature by means of solid carbon dioxide and alcohol, the product changing to the orange form as the temperature rises. The two forms possess different vapour pressures, and for this reason the results obtained in determinations of the vapour pressure of the solid are irregular and discordant. The following values for the vapour pressure of the orange form have been obtained:

Temp., ° C.	11.85	34.73	85.60	130.98	145.79	160.39	182.41
Vap. press., mm.	0.000	0.015	0.203	0.706	1.158	2.777	7.891

The T/log p curve shows the melting point of this variety to be about 144° C. The molecular heat of vaporisation is 19,200 calories.

The vapour density is 16.1 (air = 1) corresponding to the molecular formula AsI₃ (15.8), but the vapour, which is yellow, generally contains the products of thermal decomposition. The heat of formation, according to Berthelot, is (As, 3I_gas) 28,800 calories and (As, 3I_solud) 12,600 calories. From measurements of density and coefficient of expansion at low temperatures the molecular volume at 0° Abs. has been calculated to be 93.2, a value which corresponds with that similarly derived for the molecular volume of phosphorus triiodide.

Arsenic triiodide is soluble in carbon disulphide, alcohol, ether, chloroform, benzene, toluene and the xylenes. The solution in carbon disulphide gradually darkens owing to absorption of oxygen and liberation of iodine. With alcohol at 150° C. ethyl iodide is formed. In methylene iodide the triiodide dissolves to the extent of 17.4 parts of AsI₃ in 100 parts of solvent at 12° C. The dipole moment in various solvents has been determined.

The triiodide dissolves freely in water to give a yellow solution with an acid taste and reaction. The concentrated solution is fairly stable and the triiodide may be recovered by distilling off the solvent.

In the solid and liquid forms, arsenic triiodide is a non-conductor of electricity, but some of its solutions are weak conductors; thus, a saturated solution in allyl isothiocyanate at 60° C. has a conductivity of 1.4×10^-4 mho.

When heated above 100° C. arsenic triiodide dissociates slowly into its elements; above its melting point this decomposition becomes more rapid. In air, the products are arsenic, arsenious oxide and iodine, and the action proceeds slowly even below 100° C. and is rapid at 200° C.; at higher temperatures the triiodide burns with a pale blue flame. Heated in an atmosphere of nitrogen in a sealed tube, it dissociates appreciably at 165° C. Although the aqueous solution appears to be stable and does not darken on exposure to air, if allowed to evaporate slowly in an open vessel crystals of an oxyiodide, 2AsOI.3As₂O₃.12H₂O, remain. It has been shown, however, that a very dilute aqueous solution of arsenic triiodide is almost completely hydrolysed and is essentially a solution of arsenious acid and hydriodic acid in equilibrium with a small quantity of the triiodide. In freshly prepared solutions this equilibrium is reached within a few minutes, and for an approximately 0.1N-solution the pH value is 1.1, which is the same as that of a 0.1N-solution of hydriodic acid. In the presence of an excess of hydriodic acid spontaneous evaporation leaves only the triiodide. Solutions in many organic solvents, for example acetone, benzene, glacial acetic acid, are unstable, decomposing with liberation of iodine, as in the case of carbon disulphide solutions mentioned above; the decomposition is accelerated by the presence of water or oxygen, but is apparently unaffected by light. When sulphur is added to a solution of arsenic triiodide in carbon disulphide, orange-coloured crystals of a sulphiodide of melting point 104° C. and of composition AsI₃.3S₈ separate; this compound crystallises in the trigonal system, the dimensions of the unit cell, which contains one molecule, being a = 14.2, b = 24.6 and c = 4.48 A.

Dry hydrogen sulphide in the cold does not react with arsenic triiodide. At 200° C. some of the latter volatilises and is carried away by the gas stream, while the remainder is converted to a crystalline iodosulphide, As₂S₅I. The aqueous solution with hydrogen sulphide yields a precipitate of arsenic trisulphide.

Ammonia is slowly absorbed by the triiodide and a voluminous yellow substance is produced which, if kept over sulphuric acid, loses ammonia until the composition corresponds with the tetrammino-triiodide, AsI₃.4NH₃. At 0° C. more ammonia can be absorbed to yield the dodecammino-compound, AsI₃.12NH₃. If ammonia is passed into a solution of the triiodide in benzene or ether, a voluminous white precipitate, of composition 2AsI₃.9NH₃, is formed. The existence of these ammines as definite compounds has not been established. With phosphine, arsenic phosphide is produced:

AsI₃ + PH₃ = AsP + 3HI

Cryoscopic and ebullioscopic measurements indicate that the halides of phosphorus, arsenic, antimony, titanium and tin readily undergo reciprocal interaction with interchange of halogen atoms and that mixed halides can be formed. Melting and freezing point curves are, however, generally of the eutectic or mixed crystal types, without maxima corresponding with compound formation, and the mixed halides have not been isolated. The molecular weight of the triiodide calculated from the lowering of the freezing point of arsenic tribromide agrees with the formula AsI₃, but in antimony trichloride the result is abnormal, the molecular weight being about half the normal value. This has been attributed to the reaction

3SbCl₃ + AsI₃ = AsCl₃ + 3SbCl₂I

resulting in an increased number of solute molecules. The system AsI₃-PI₃ has been investigated; an isodimorphous series of crystals is formed with a transition point at 73.5° C. According to Karantassis, double decomposition occurs between arsenic triiodide and stannic chloride, but there is no reaction between the triiodide and phosphorus trichloride. With stannic iodide, arsenic triiodide forms a eutectic at 106.2° C. the composition of which is 1SnI₄:0.6936AsI₃. By mixing together a saturated solution of lead iodide and a boiling saturated solution of arsenic triiodide in hydriodic acid, the compound 3PbI₂. AsI₃.12H₂O has been obtained. This is decomposed by water, alcohol or ether. The anhydrous salt may be obtained by heating at 45° C.

Donovan's solution, Liquor Arseni et Hydrargyri Iodidi, a well-known pharmaceutical preparation, is an aqueous solution containing about 1 per cent, each of mercuric iodide and arsenic triiodide. Owing to hydrolysis of the latter the solution, which is colourless or very pale yellow, consists essentially of arsenious acid, mercuric hydrogen iodide, HHgI₃, and hydriodic acid, the pH of the freshly prepared solution being 1.2 to 1.3. On keeping, the solution deteriorates owing to oxidation to arsenic acid, both by the air in contact with the solution and by the oxidising action of iodine and water; free iodine appears in the solution only after a long time. The oxidation is accelerated by light, that of wavelengths between 3200 and 4600 A. being most active. Therefore, in dispensing, the solution should be freshly prepared or, if not used immediately, kept in a well-filled amber bottle, preferably in a refrigerator. The stability of the solution may be increased by replacement of the air in the bottle by an inert gas, replacement of 25 per cent, of the water by honey or syrup, the addition of calcium carbonate or of sodium bicarbonate to ensure the most favourable pH (6.5 to 7.5), or by the addition of 0.4 per cent, of terpin hydrate or of an excess of potassium iodide.

Arsenic triiodide forms unstable complexes with the alkali halides. The rubidium and caesium compounds, 3RbI.2AsI₃ (pseudo-hexagonal, a:c = 1:2.486) and 3CsI.2AsI₃ (hexagonal bipyramids, a:c = 1:2.488), are formed in an analogous manner to the corresponding chlorides. Sodium azide reacts with arsenic triiodide in methyl alcohol or acetone solution to form the compound Na₈[AsI₃(N₃)₈].

Boron tribromide dissolves in arsenic triiodide.