Antimony Triiodide, SbI₃

Antimony Triiodide, SbI₃, is the only compound of antimony and iodine that is known to exist with certainty; a pentaiodide has been reported but subsequent investigations have failed to confirm its existence.

Antimony triiodide may be obtained by synthesis; the combination of antimony with iodine is effected by triturating the two elements together in a mortar; the heat generated by the reaction is usually sufficient to volatilise the triiodide produced, and the reaction may even become violently explosive if large quantities of materials are employed. The synthesis is more conveniently carried out by adding an excess of finely divided antimony gradually to iodine maintained at a gentle heat. Combination of the two elements has also been effected by triturating with alcohol, and by adding finely divided antimony to a solution of iodine in carbon disulphide. In the latter method the mixture is warmed until the colour of the iodine disappears and the triiodide is obtained by crystallisation from the solution.

The compound has also been obtained by heating together equivalent proportions of antimony trisulphide and iodine, some thioiodide also being formed, and by adding potassium iodide either to a solution of antimony trichloride in acetone, or to an aqueous solution in the presence of dilute sulphuric acid.

Four modifications of antimony triiodide have been described. Three of these are crystalline and one amorphous. The crystalline forms belong to the trigonal, rhombic and monoclinic systems respectively. The trigonal form appears to be the most stable at ordinary temperatures, but the conditions of equilibrium of the system are not fully understood. Transition points occur at 114° and 125° C., the former indicating an enantiomorphic change from trigonal form to the rhombic form, and the latter the transition from monoclinic to trigonal. Cohen, however, states that antimony triiodide is monotropic and the transition point at 114° C. is purely fortuitous. According to this view the rhombic form, which is obtained by sublimation, is a metastable phase which may exist unchanged for long periods even at low temperatures.

Trigonal antimony triiodide is obtained as reddish, hexagonal crystals whose colour varies with the method of preparation:

a = 7.466 A., c = 20.892 A.

There are six molecules in the unit cell. Its density is 4.848; it melts at 167° C., and boils between 414° and 427° C. It will dissolve in cold, concentrated hydrochloric acid; the solution on hydrolysis yields an oxyiodide. If, however, the solution in hydrochloric acid is boiled for a few seconds, the triiodide is converted into the trichloride and the solution on hydrolysis then yields an oxychloride. The triiodide is readily soluble in hydriodic acid, and from this solution a yellow oxyiodide is precipitated by hydrolysis. It is also soluble in boiling benzene, in carbon disulphide, in methylene iodide, in an aqueous solution of tartaric acid, and in arsenic tribromide; the solution in the last solvent is, however, unstable. With hexachlorethane it forms a dark brown solution which on cooling deposits first a lemon-yellow mass which ultimately changes to a red, crystalline substance. The triiodide is almost insoluble in chloroform, carbon tetrachloride and turpentine.

The dielectric constant of solid antimony triiodide at 20° C. is 9.1; that of the liquid at 175° C. is 13.9. The dipole moment (in organic solvents) is 1.58×10^-18 e.s.u.

The variation of the vapour pressure of liquid antimony triiodide with temperature is as follows:

Temperature, ° C	250	265	280	295	310	325
Vapour pressure (mm.)	23	35	53	80	115	166

Trigonal antimony triiodide is stable in air at ordinary temperatures. It sublimes quite readily; if the sublimation is carried out in the presence of air some decomposition occurs, iodine being liberated and some oxyiodide formed. It can be sublimed without decomposition in an atmosphere of hydrogen or carbon dioxide. It burns when heated in an atmosphere of oxygen, antimony trioxide being formed. In common with the other halides of antimony it is readily hydrolysed by water, yielding an insoluble, yellow oxyiodide; the reddish liquid which is also formed is stated to be a solution of antimony triiodide in hydriodic acid.

The triiodide reacts readily with chlorine with the formation of antimony trichloride and iodine monochloride; with bromine, antimony tribromide and iodine monobromide are similarly formed. By treatment with molten iodine monochloride the triiodide is converted into antimony trichloride and free iodine.

Cold, dilute sulphuric acid has very little action upon antimony triiodide, but if the acid is gently warmed a reaction takes place which results in the liberation of iodine and the formation of antimony sulphate. With hydrogen sulphide a reaction occurs at 150° C. and antimony thioiodide is formed.

Antimony triiodide is readily attacked by concentrated nitric acid and is converted to antimony trioxide; free iodine and oxides of nitrogen are evolved. Dilute acid acts similarly, but more slowly. By reaction with ammonium hydroxide the triiodide is converted into a yellowish-white powder. Nitric oxide appears to have no action upon a mixture of the triiodide and chloroform,6 but nitrogen tetroxide attacks a mixture of the triiodide and ether with the formation of the compound 2Sb₂O₅.N₂O₅. The triiodide is completely converted into trioxide by the action of alkali hydroxides and carbonates.

Antimony triiodide is partially soluble in both alcohol and ether, but the chief effect of both reagents is to convert it into an oxyiodide.

The molecular weight of antimony triiodide, determined by the elevation of the boiling point of various solvents, is abnormally low. The solvents employed were the trichlorides of phosphorus, arsenic and antimony, and tin tetrachloride. Cryoscopic measurements using solutions in methylene iodide also yielded a low result. This may be due to the formation of chemical compounds between the solvents and the solute, or to ionisation of the latter. It is interesting to note that a solution of antimony triiodide in arsenic triiodide has an appreciable conductivity, suggesting that ionisation has taken place.

The heat of formation of trigonal antimony triiodide is 44,205 gram-calories.

When trigonal antimony triiodide is sublimed at a temperature of 114° C. or above, it is converted into the rhombic variety, which is obtained as small, greenish-yellow lamellae, isomorphous with the corresponding trichloride and tribromide.

When a solution of the trigonal modification in carbon disulphide is exposed to bright sunlight for several hours, monoclinic antimony triiodide is obtained. Some oxyiodide and free iodine are formed at the same time. The monoclinic modification forms greenish-yellow, prismatic crystals:

a:b:c = 1.6408:1:0.6682; β = 109°44'.

Its density (D₄²²) is 4.768. When heated to 125° C. it reverts to the trigonal modification.

Amorphous antimony triiodide is obtained by cooling a hot concentrated solution of the trigonal modification in glycerol. It may also be prepared by warming the trigonal modification with a small quantity of potassium acetate and excess of acetic acid; or by heating a mixture of antimony trioxide and potassium iodide with excess of acetic acid at 100° C. It is a yellow powder which melts at 172° C.

Double compounds of antimony triiodide and ammonium iodide have been prepared. The compounds 3NH₄I.4SbI₃.9H₂O (bright red, rectangular prisms), 3NH₄I.2SbI₃.3H₂O (thin rectangular or tetragonal leaves or plates of a dark reddish-brown or red colour), 4NH₄I.SbI₃.3H₂O (large, black, rectangular prisms), are formed by crystallisation from a mixed aqueous solution of the two iodides; in addition, the compound NH₄I.SbI₃.2H₂O (red, tetragonal prisms) is obtained by the action of iodine on a saturated solution of ammonium chloride in contact with metallic antimony.

Antimony triiodide also forms double salts with iodides of the alkali metals, the alkaline earth metals, and aluminium. They are usually prepared by dissolving the triiodide in a solution of the second iodide and crystallising out from the mixture, or by the action of antimony upon a solution of iodine in aleohol in the presence of the second iodide. For example, the compound SbI₃.2KI.H₂O has been obtained by the former method. They are obtained as reddish-black, transparent crystals, which are decomposed by heat, by water and by concentrated sulphuric acid; they will dissolve in hydrochloric and acetic acids.

Salts of a complex antimony iodohydrobromic acid, HSbBrI₃, have been obtained by triturating equimolecular proportions of antimony triiodide and metallic bromide with a non-aqueous substance such as acetic acid or xylene. In this way orange-yellow crystals of sodium antimoniodobromide, NaSbBrI₃, and potassium antimoniodobromide, KSbBrI₃, have been obtained. The corresponding salts of ammonium and lithium, NH₄SbBrI₃ and LiSbBrI₃, are darker in colour, while the zinc salt, Zn(SbBrI₃)₂, is obtained as brown, tabular crystals which are only slowly decomposed by water. The free acid has not been isolated.

Complex compounds of the type R₃S.SbI₄ (where R represents an alkyl radical) have been obtained by the interaction of alkyl iodides, alkyl sulphides and antimony iodide. They are rather unstable; they dissolve in acetone, giving coloured solutions. Conductivity experiments suggest the presence in their solutions of complex ions, which, however, readily undergo dissociation.

The following binary systems have been examined by thermal analysis: antimony triiodide-phosphorus triiodide, antimony tri-iodide-arsenic triiodide, antimony triiodide-arsenic tribromide, antimony triiodide-antimony trichloride, antimony triiodide-antimony pentachloride.