Compounds of Antimony

The compounds of antimony conform, in general, to the types expected from the position of the metal in the Periodic Classification. Antimony exhibits two valencies only, being tervalent in some compounds and quinquevalent in others. Several substances, such as the suboxide, in which antimony shows an apparent valency of less than three, have been described; but either these have been shown not to be true compounds, or their constitutions have not been fully elucidated. In accordance with its position in the Periodic Table, antimony shows electropositive properties more definitely than arsenic, forming true salts such as the halides and the sulphate; the salts, however, undergo hydrolysis, producing ultimately oxides or hydrated oxides. As might be expected, hydrolysis does not take place so readily, or proceed so completely, as with the compounds of arsenic, and several stable intermediate products are formed.

Antimony, like arsenic, forms a hydride; stibine, however, can only be formed by the evolution method in acid solutions. It is more easily decomposed by heat than arsine.

Antimony halides are characteristic. There are two fluorides, the trifluoride and the pentafluoride, both of which are soluble in water and hydrolysed only slowly. Antimony trifluoride is not hydrolysed below 30° C. The corresponding chlorides hydrolyse more rapidly, the trichloride yielding oxyehlorides, and ultimately (by hydrolysis at 150° C.) trioxide, and the pentachloride yielding hydrated antimony pentoxide (the so-called antimonic acid). Antimony pentachloride dissociates on heating. A third chloride, antimony tetrachloride, appears to exist in complex compounds only; it has not been isolated. Antimony tribromide is the only bromide that has as yet been isolated. The pentabromide is unknown, but compounds have been obtained that may be regarded as derived from it. It is also possible that compounds of a hypothetical antimony tetrabromide may exist. Antimony tri- bromide is very readily hydrolysed. The only iodide known is the triiodide, a well-defined salt which readily hydrolyses. Complex compounds of quinquevalent antimony which contain fluorine and iodine have, however, been prepared.

The halides of antimony readily form double compounds, particularly with the halides of alkali and alkaline earth metals. In many cases there is definite evidence of the formation of complex anions of which antimony forms a constituent. Thus compounds are derived from antimony pentachloride which may be regarded as salts of ortho-, pyro- and meta-chloroantimonic acids, H₃SbCl₈, H₂SbCl₇ and HSbCl₆. Of these acids meta-chloroantimonic acid has alone been isolated. From antimony tribromide, salts of the type M₂Sb₃Br₁₁ have been prepared, in which M represents a monovalent metal. From antimony pentabromide salts of the bromantimonic acids have been obtained, and meta-bromantimonic acid, HSbBr₆.3H₂O, has been isolated. Many other complex compounds of antimony halides have been prepared, the constitutions of which have not been fully elucidated. Most of them are decomposed by water, frequently with hydrolysis.

The chlorides and bromides of antimony are soluble in many organic solvents, and in many cases complexes are formed.

Three oxides are known. Of these, antimony trioxide is amphoteric, forming both antimony salts and antimonites; its basic properties, however, predominate. Antimony tetroxide is neutral or only faintly acidic, and may best be considered as a salt, antimony antimonate, SbSbO₄. Antimony pentoxide is acidic, forming antimonates. It is doubtful, however if a true antimonic acid has been isolated, antimony resembling tin in this respect.

The heats of formation of the oxides are:

Antimony trioxide	163,000 calories
Antimony tetroxide	209,800 calories
Antimony pentoxide	229,600 calories

Of the compounds in which antimony is a constituent of the anion, antimonites are known, principally in the form of meta-antimonites, such as sodium meta-antimonite, NaSbO₂. Ortho- and pyro-antimonites may exist, but the latter in particular are doubtful. The free acids have not been isolated.

Meta-hypoantimonic acid, H₂Sb₂O₅, and its salts, derived from Sb₂O₄, are known. They may be regarded as mixed antimonites and antimonates.

Ortho- and meta-antimonates are known, the majority of the salts being either acid ortho-antimonates of the type KH₂SbO₄, or meta- antimonates of the type KSbO₃. It has also been suggested that the formula for antimonic acid is HSb(OH)₆.

It may also be noted that antimony pentoxide does not liberate chlorine from hydrochloric acid; it will, however, liberate iodine from hydriodic acid.

Three sulphides of antimony are known, corresponding to the three oxides; but the pentasulphide is very difficult to prepare in the pure state, most of the preparations formerly regarded as antimony pentasulphide being mixtures, probably of antimony tetrasulphide and sulphur. The presence of tervalent antimony can usually be shown in such preparations. Complex sulphur compounds, notably with the halides, are also known. They correspond roughly with the oxyhalides.

Of the other inorganic compounds of antimony, the sulphate and the nitrate have been reported, but both are more readily obtained as basic salts. The existence of the latter is somewhat doubtful. It is interesting to note, however, that antimony selenate is insoluble in water and is not decomposed by it.

The only compound of antimony with hydrogen that is known with certainty is the gaseous antimony trihydride, or stibine, SbH₃. A solid substance, described as di-antimony dihydride, Sb₂H₂, is stated to have been obtained by various electrolytic methods and by the reduction of antimony compounds by nascent hydrogen. It is a brownish-black substance, soluble in fairly concentrated nitric acid, but not in other mineral acids; it is insoluble in solutions of caustic alkalis. It is decomposed when heated in a current of hydrogen, and reacts vigorously with fused potassium nitrate. On the other hand, this substance may be merely metallic antimony in a fine state of division, containing a trace of adsorbed hydrogen. Some investigators doubt the existence of a solid antimony hydride.

Three oxides of antimony are known with certainty, namely antimony trioxide, Sb₂O₃, antimony tetroxide, Sb₂O₄, and antimony pentoxide, Sb₂O₅; a complex oxide, Sb₆O₁₃, may also exist. It is probable that some of the confusion which existed among earlier workers in connection with certain of the oxides of antimony may have been due to the extreme slowness with which equilibrium is obtained between antimony pentoxide and its decomposition products. Three of the oxides resemble one another very closely in crystal structure, one modification of each of them possessing a cubic lattice of the diamond type. The oxide Sb₆O₁₃ may have a tetragonal lattice.

Lower oxides of antimony have been described by early writers, but the evidence for the existence of these does not appear to have been confirmed.

Antimony-Sulphur System.

Three sulphides of antimony have been described: the trisulphide, Sb₂S₃, the tetrasulphide, Sb₂S₄, and the pentasulphide, Sb₂S₅. Some doubt has been expressed concerning the existence of the tetrasulphide as a true compound, and it is probable that the pentasulphide has not yet been obtained in a pure condition. Early investigators described a subsulphide, Sb₂S₂, obtained by fusion, but this was quickly proved to be a mixture, or solid solution, of antimony and antimony trisulphide.

Thermal examination of the system antimony-sulphur indicates the existence of one compound only - antimony trisulphide, Sb₂S₃ - which gives rise to a maximum on the freezing point curve at 546° C. One eutectic is obtained, containing 57.5 atomic per cent, of sulphur, and melting at 520° C. A second eutectic has been reported, containing 61.3 per cent, sulphur and melting at 519° C. Two liquid layers are obtained at each end of the system, at 530° C. for the sulphur end, and at 615° C. for the antimony end. (See fig.)

Several compounds of antimony and selenium have been reported, but it is probable that the only true compound is antimony triselenide, Sb₂Se₃. Thermal analysis, supported by thermoelectric and microscopical examination, suggests that the so-called compound Sb₂Se₇ is a mixture of antimony triselenide with selenium, and that the existence of the compounds Sb₄Se₅ and Sb₃Se₄ is doubtful. Some evidence in favour of the existence of the compound SbSe has been adduced from microscopic investigation and investigations of electromotive behaviour.

Antimony triselenide is obtained by the action of a saturated solution of hydrogen selenide upon a solution of an antimony salt. It is a brown powder, melting at 572° C. and soluble in hot alkali solutions. It forms compounds with other metallic selenides.

Antimony pentaselenide is said to be formed by the action of hot hydrochloric acid upon a solution of sodium selenoantimonate, Na₃SbSe₄. The pentaselenide has not been obtained, however, by precipitation with hydrogen selenide.

Two antimony selenites, Sb₂Se₂O₇ and Sb₂Se₄O₁₁, have been described; both were obtained by the action of selenium dioxide upon antimony trioxide.

Two compounds of antimony, sulphur and selenium, Sb₂S₂Se and Sb₂S₃Se₂, have also been described.

Complex selenoantimonites and selenoantimonates (corresponding to the thioantimonites and thioantimonates) of sodium, potassium, and possibly of manganese, have been prepared. The selenoantimonites, Na₃SbSe₃.9H₂O, K₃SbSe₃.9H₂O, Na₂Sb₄Se₇, K₂Sb₄Se₇.3H₂O, are obtained by the action of antimony triselenide upon the corresponding alkali selenide. Sodium selenoantimonate, Na₃SbSe₄.9H₂O, is obtained by fusing together antimony triselenide, selenium, sodium carbonate and carbon, extracting the melt with water and treating the solution so obtained with more selenium. Transparent, orange-yellow crystals, isomorphous with those of sodium thioantimonate, are obtained.

Still more complex compounds of sodium and potassium with antimony, sulphur and selenium have been described.

It is doubtful if direct combination occurs between antimony and nitrogen, although an unstable powder is formed when antimony is heated at a dull red heat in a current of nitrogen. Antimony nitride is said to be formed by the action of antimony trichloride on liquid ammonia. It is described as an orange substance, extremely sensitive to moisture; it is decomposed into the elements on heating to 500° C.

A basic nitrate of antimony, possibly 2Sb₂O₃.N₂O₅, is obtained, mixed with the oxides of antimony, by the action of nitric acid upon antimony, the formation of the nitrate being favoured by using cold acid as dilute as possible. The presence of nitrous acid accelerates the action. A somewhat similar compound is obtained by the reduction of a solution of silver nitrate with metallic antimony.

It is stated that normal antimony nitrate, Sb(NO₃)₃, may be obtained by the action of silver nitrate upon a solution of antimony trichloride in acetone. The normal nitrate, however, does not appear to have been isolated.

A compound of quinquevalent antimony, 2Sb₂O₅.N₂O₅, has been obtained by the action of nitrogen tetroxide upon a solution of antimony tribromide in chloroform, or of antimony triiodide in ether. It is a white, crystalline substance, not decomposed by water.

A compound of antimony and arsenic, Sb₂As, has been described, but a more recent study of the system antimony-arsenic has failed to confirm its existence. Arsenic is frequently associated with native antimony, as in the mineral allemontite.