Bismuth Triiodide, BiI₃

Bismuth Triiodide, BiI₃, was probably first prepared by Berthemot by synthesis from the elements; this synthesis has since been effected in a variety of ways. Owing to the small reactivity of the elements, however, combination is effected only with difficulty. The iodide may be purified by sublimation in a slow current of hydrogen.

It may also be obtained by precipitation from a solution of a bismuth salt in acetic acid with potassium iodide, by the action of concentrated hydriodie acid on bismuth trioxide at the ordinary temperature, or on bismuth oxychloride,

3BiOCl + 6HI = 2BiI₃ + BiCl₃ + 3H₂O

or by the action of hydrochloric acid on bismuth oxyiodide,

4BiOI + 9HCl = BiI₃ + 3BiCl₃ + HI + 4H₂O
or
3BiOI + 6HCl = BiI₃ + 2BiCl₃ + 3H₂O

When a mixture of bismuth trisulphide and iodine is heated, bismuth triiodide sublimes, leaving a residue of bismuth thioiodide; according to another authority, bismuth triiodide and sulphur only are produced in accordance with the reaction

Bi₂S₃ + 6I1 = 2BiI₃ + 3S

Bismuth triiodide is also formed by the action of ethyl iodide on bismuth trichloride in the presence of ethyl chloride.

Crystalline bismuth triiodide may be obtained by saturating a solution of Bettendorff's reagent (a solution of stannous chloride in hydrochloric acid) with iodine and adding a solution of bismuth trioxide or bismuth oxychloride in hydrochloric acid. The size of the crystals depends on the concentration of the solution; they can be purified by drying and heating carefully in an evacuated tube to below the melting point, finally subliming in carbon dioxide or hydrogen.

Bismuth triiodide crystallises in the hexagonal system:

a = 7.498 A., c = 20.676 A.

The unit cell contains six molecules. Its colour is variously described as dark green, black and grey-black; in the powdered form it is dark brown. Its density, D₄²⁰, is 5.7, and the molecular volume at -273° C., calculated from the density at -194° C. and the coefficient of expansion, is 98.6. Its melting point is 410° to 439° C. It volatilises at slightly higher temperatures, forming a red-brown vapour. It sublimes unchanged when heated in an atmosphere of carbon dioxide or hydrogen, but evidence of thermal dissociation has been obtained spectroscopically.

It is stable in air, but volatilises with partial decomposition when heated in air, leaving a non-volatile residue of oxyiodide or trioxide. It is not hygroscopic, but is decomposed by water.

It is soluble in hydrochloric acid and in hydriodic acid, slightly soluble in absolute alcohol, and more soluble in benzene, toluene and xylene. It is also soluble in arsenic tribromide. The solubility in organic solvents is greatly increased by the presence of arsenic tribromide.

Bismuth triiodide is hydrolysed slowly by cold water, slightly more rapidly by hot water. The rate of hydrolysis, however, at 25° C. and at 50° C. is so slow that the conditions of equilibrium have not been determined. Two products of hydrolysis have been described: a black compound, which is formed when water is first added to the triiodide, and a brick-red substance which is obtained when the concentration of bismuth in the liquid phase falls below 0.002 gram-atom per litre and which has the composition of bismuth oxyiodide, BiOI. The black substance has not been obtained pure, but analysis indicates a composition corresponding to Bi₂O₃.5HI or 2BiOI.3HI.H₂O.

Bismuth triiodide does not appear to react with hydrogen sulphide when heated in a current of that gas.

When the triiodide is heated with dry ammonia, a brick-red compound, BiI₃.3NH₃, is formed. This substance is decomposed by water with the separation of ammonium iodide.

Bismuth triiodide is decomposed by nitric acid, with the liberation of iodine. Muir stated that it reacted on heating with nitrogen peroxide and was partially converted into oxyiodide; this action was, however, much less complete than the corresponding reaction with either bismuth trichloride or tribromide. It has since been shown that at the ordinary temperature the triiodide is converted to trioxide by reaction with nitrogen peroxide, no oxyiodide being formed unless air is present.

By reaction with caustic alkalis (and less readily with alkali carbonates) it is converted to trioxide, admixed with a little bismuth iodate. The reaction proceeds by stages. Taking the action of a solution of potassium hydroxide as typical, when the concentration of the alkali is less than 0.375 mole per litre the main product is the oxyiodide, BiOI. When the concentration is greater than this, this oxyiodide by a further reaction is transformed into the white compound BiOI.2Bi₂O₃; with excess of alkali the main product is bismuth trioxide. The triiodide reacts with alkali sulphides to form bismuth trisulphide. When heated with excess of mercuric oxide, or of mercuric sulphide, it is converted to the trioxide or the trisulphide, respectively.

The molecular weight of bismuth triiodide, as determined from a solution in fenchone, is in accordance with the formula BiI₃.

The triiodide resembles the trichloride and the tribromide in forming a number of complex compounds and double salts. An iodobismuthous acid, HBiI₄.xH₂O or BiI₃.HI.xH₂O (where x = 3 or 4), is reported to have been obtained in the form of rhombic, pyramidal crystals from a solution of the triiodide in concentrated hydriodic acid by evaporation over sulphuric acid. This dissolves in a solution of potassium iodide, but is decomposed by water forming bismuth oxyiodide.

An examination of cerebrospinal fluid and brain containing bismuth indicated that the bismuth was present in the anion. From a study of ionic migration in the compound Na₂BiI₅ it is found that the bismuth occurs in the complex anion BiI₅^-2. Compounds of the form RBiI₄ and R₃BiI₆, in which R represents an organic radical, have been obtained.

Among inorganic complex or double salts which have been reported are the following:

The sodium salt NaBiI₄.H₂O, prepared by the action of iodine on bismuth in a saturated solution of sodium chloride; it forms brownish- black crystals belonging to the monoclinic system: a:b:c = 0.864:1:0.717; β = 102°21'. Na₂BiI₅.4H₂O, prepared by the action of anhydrous sodium iodide upon bismuth chloride in ethyl acetate; it has a density of 3.33, melts at 93° to 94° C. and is soluble in water and various organic solvents, but undergoes hydrolysis with excess of the former solvent. Na₃Bi₂I₉.12H₂O, which separates out from a concentrated solution of sodium iodide saturated with bismuth triiodide.

Numerous potassium double salts have been reported, including KI.2BiI₃, KI.BiI₃.H₂O, 2KI.BiI₃, 2KI.BiI₃.4H₂O, 3KI.BiI₃, 3KI.2BiI₃.2H₂O, 4KI.BiI₃, 4KI.BiI₃.HI, 4KI.2BiI₃ and 6KI.2BiI₃. Investigation of the condition of equilibrium at 15°, 35° and 55° C. within the system BiI₃-KI-H₂O revealed the existence of two compounds only, KI.BiI₃.H₂O or KBiI₄.H₂O and 2KI.BiI₃.H₂O or K₂BiI₅.H₂O; the former of these crystallises as bright red monoclinic prisms and the latter as deep red flat quadratic prisms with pyramidal ends. Both compounds can be dehydrated by treatment in vacuo over sulphuric acid.

The caesium salt, Cs₃Bi₂I₉, is prepared by the action of a solution of bismuth hydroxide, bismuthyl carbonate or bismuth oxyiodide in hydriodic acid upon caesium nitrate, or by the interaction of caesium iodide with bismuth triiodide. It forms hexagonal crystals, hydrolysed slowly in cold water, rapidly in hot water. It is fairly stable when heated and its use has been suggested for the quantitative determination of caesium.

Somewhat similar compounds of barium, calcium, magnesium, beryllium and aluminium have also been reported.

Most of the foregoing compounds form red or dark red crystals; they are all hydrolysed by water, yielding bismuthyl iodide.

Two ammonium compounds have also been obtained and examined. The salt NH₄BiI₄.H₂O is obtained as a precipitate when iodine reacts on bismuth in a concentrated aqueous solution of ammonium iodide; it may also be obtained by the action of iodine on bismuth in alcohol in the presence of ammonium iodide. It forms black, needle-shaped crystals of the rhombic system. Another compound, (NH₄)₄BiI₇.3H₂O, is obtained when a warm, concentrated solution of ammonium iodide is saturated with bismuth triiodide. On evaporation, large, dark reddish-brown, rectangular prismatic crystals separate. They appear to be isomorphous with the corresponding double salt of antimony; they are hygroscopic and are decomposed by water.

A solution made by adding excess of a solution of potassium iodide to a solution of bismuth nitrate is known as Dragendorff's reagent. It contains a complex iodide of bismuth and potassium and is used in testing for alkaloids.

Complex compounds of bismuth iodide and organic bases have also been described.