Antimony Trisulphide, Sb₂S₃

Antimony Trisulphide, Sb₂S₃, has been known from very early times, under a variety of names, such as antimonium crudum, grey antimony ore, antimony glance, antimony glass, stibnite. Until the eighteenth century it was frequently confused with antimony metal. It exists in at least three forms, a crystalline form and two amorphous forms, each of which may be obtained in a variety of ways.

Crystalline antimony trisulphide may be formed by fusing together the elements; by subjecting a mixture of the elements to high pressure or by heating the elements with water under pressure. It may also be formed from antimony by the action of sulphur dioxide, and from antimony trioxide, or from antimonates, by fusion with sulphur. It is also obtained by the action of hydrogen sulphide on the vapour of antimony trichloride or other antimony compounds, and by the prolonged heating at a high temperature of potassium antimonyl tartrate with a solution of ammonium thioeyanate, or with potassium thiocyanate in the presence of tartaric acid. In the latter case the amorphous variety is obtained at lower temperatures, the crystalline at higher.

From antimony pentasulphide the crystalline trisulphide may be obtained by heating at 200° to 300° C. in a current of carbon dioxide; by the prolonged action of sunlight on a dilute solution in hydrochloric acid containing hydrogen sulphide; or by heating in a tube at 250° C. with a solution of sodium bicarbonate.

From antimonic solutions the trisulphide is obtained by the action of hydrogen sulphide at 70° C. in the presence of chromic chloride; the presence of the latter is essential for the formation of the black modification of antimony trisulphide in a pure condition. Preparation may also be effected by alternating current electrolysis of sodium thio- sulphate solutions using antimony electrodes.

The amorphous variety may be transformed into the crystalline by heating in a neutral atmosphere; by heating with water at 200° to 300° C. in a closed tube, or by heating with hydrogen sulphide. The transformation is also effected by the action of dilute acids, especially hydrochloric acid.

Amorphous antimony trisulphide is said to be formed by quenching the molten substance rapidly. A much purer product is obtained by distilling the trisulphide in a stream of nitrogen and condensing the vapour rapidly; admixed sulphur may be removed by treatment with carbon disulphide.

The amorphous form is also obtained by the action of sodium thio- sulphate upon solutions of antimony salts. The technical product is obtained by this method. A solution of sodium thiosulphate containing sodium hydroxide is added to one of antimony trichloride; the colour of the resulting red product is influenced by the proportion of sodium hydroxide, becoming yellower with increase of this reagent. The reaction may be represented by the equations:

SbCl₃ + 3Na₂S₂O₃ = Na₃Sb(S₂O₃)₃ + 3NaCl
2Na₃Sb(S₂O₃)₃ = Sb₂S₃ + 3Na₂S₃O₆

The product is generally contaminated with oxide.

A more usual laboratory method of preparation, however, is by precipitation from solutions of antimony salts by hydrogen sulphide. Tartaric acid should be present to prevent the formation of thio-salts. An investigation into the separation of antimony and tin by hydrogen sulphide in hot hydrochloric acid solution indicated that, with a concentration of 30 c.c. concentrated acid in 100 c.c. solution precipitation of antimony trisulphide began at 95° C., that of antimony pentasulphide at 80° C.; the presence of ammonium chloride in the solution lowered the temperature at which precipitation began in each case.1 It is probable that the precipitate of antimony sulphide obtained from hydrochloric acid solution by hydrogen sulphide is seldom pure, being contaminated with antimony oxychloride. The precipitate should be heated in an atmosphere of carbon dioxide at 250° C. in order to convert it to the black variety.

For commercial purposes crude antimony trisulphide is usually obtained by liquation from antimony ores. The chief impurities are arsenic, lead, iron, copper and other metals. Purer products are obtained by melting together a mixture of finely divided refined antimony and sulphur, or by saturating a solution of antimony trioxide in dilute hydrochloric acid with hydrogen sulphide and passing carbon dioxide through the boiling liquid.

Polymorphic forms of antimony trisulphide that have been investigated are the red precipitated form, the black crystalline form and the natural stibnite. The two latter appear to differ only in density. The densities of the three varieties are:

Red precipitated form	4.1205 to 4.421
Greyish-black form	4.2906
Stibnite	4.6353

Stibnite crystallises in the rhombic system

a:b:c = 0.9926:1:1.0179

From X-ray examination, however, the lengths of the edges of the rhombic cell are given as

a = 11.39 A., b = 11.48 A. and c = 3.89 A. giving as axial ratios

a:b:c = 0.992:1:0.338

There are four molecules of Sb₂S₃ in the unit cell.

Transformation from the orange-red to the black variety takes place on heating at just over 200° C., with evolution of heat. The influence of a number of substances on the transformation point, including carbon dioxide, hydrogen sulphide, ammonium chloride, antimony trichloride, metallic silver, potassium nitrate, sodium chloride, ammonium sulphate, and acids of different concentrations, has been investigated. In the presence of water and various ions, raising the temperature to 75° C. hastens the change, the order of effectiveness of various ions and water being (in diminishing order): anions, S⁼, water, NO₃^-, Cl^-, SO₄⁼, C₂H₃O₂^-; cations, H⁺, water, Na⁺, NH₄⁺. In aqueous hydrochloric acid the times taken for complete transformation in solutions of concentrations 12N, 7N and N were 0.5 day, 1 day and 10.5 days respectively. The effect of temperature on the time taken for the transformation (using a 20 per cent, solution of hydrogen chloride) was as follows:

Temp., ° C.	26.5°	30°	35°	40°	68.5°	75°
Time required	44 hrs.	29 hrs.	16 hrs.	9 hrs.	62 mins.	32 mins.

In a corresponding solution of hydrogen bromide no change occurred after 20 hours at 75° C.

The hardness of stibnite on Mohs' scale is 2 to 2.5. Its density and that of the prepared variety have already been given. Its melting point is about 550° C. It can be distilled, the boiling point being 1090° to 1150° C.; some decomposition takes place. Antimony trisulphide begins to volatilise at 650° C.; volatilisation is rapid at 800° to 850° C. and is complete at 917° C. The electrical conductivity of single crystals is not purely electronic, electrolytic decomposition accompanying prolonged passage of current.

Amorphous antimony trisulphide varies in properties according to the method of preparation. In colour it ranges from red to greyish- black. Most specimens contain water, and it has been suggested that a hydrate is formed; also that the black form is anhydrous and the red form hydrated. It has been observed, however, that the loss in weight which occurs when the red variety is heated to 100° C. is not due so much to loss of water as to secondary reactions involving absorption of oxygen and loss of sulphur dioxide. When the sulphide is precipitated from a solution free from chlorides it does not suffer any loss in weight on prolonged heating at 250° C. in a current of carbon dioxide. The densities of different specimens vary from 4.120 to 4.421. An investigation of the heat of formation of different specimens reveals only very slight differences. The calculated heat of formation (from sulphur vapour and solid antimony) is 86,490 gram-calories, and that from rhombic sulphur and solid antimony 38,300 gram-calories (both calculations refer to black antimony-trisulphide).

The chemical properties of all varieties of the trisulphide are similar, but the amorphous form is the most active. All varieties are decomposed when heated, metallic antimony being formed.

Antimony trisulphide is reduced by hydrogen, reduction beginning at 360° C. The reaction

Sb₂S₃ + 3H₂ ⇔ 2Sb + 3H₂S

is reversible. From a study of the equilibrium between antimony trisulphide and hydrogen the dissociation pressures of the trisulphide have been calculated, and it is found that log p is a linear function of T, where p is the partial pressure of the sulphur vapour and T the absolute temperature. At temperatures up to the melting point, the partial pressure of hydrogen sulphide is proportional to the temperature; above the melting point the equilibrium is disturbed owing to the solubility of metallic antimony in molten antimony sulphide, the composition of the gas then depending not only on the temperature, but also on the concentration of the solution of antimony in antimony trisulphide. If antimony is present in excess, the solution is, of course, always saturated, and again the equilibrium is determined by temperature alone. Antimony trisulphide reacts with hydrogen under the influence of the silent electric discharge (15,000 volts), the products being an antimony mirror and hydrogen sulphide.

Antimony trisulphide burns in oxygen with the formation of sulphur dioxide and a mixture of antimony trioxide and tetroxide. The heat of reaction has been calculated and is given, in gram-calories, by the equation

Sb₂S₃ + 4.5O₂ = Sb₂O₃ + 3SO₂ + 328,400

The conditions of roasting antimony trisulphide have also been studied, together with the conditions of oxidation to antimony trioxide and antimony tetroxide, and the dissociation of the latter. At 190° C. oxidation to antimony trioxide begins, the action becoming rapid at 340° C. and complete at 445° C. Above this temperature oxidation to antimony tetroxide takes place, and continues up to 900° C., at which temperature dissociation of the latter into antimony trioxide begins and is complete at 1130° C. By the action of ozone, antimony trisulphide is converted into antimony sulphate.

By prolonged digestion with water the amorphous trisulphide decomposes, yielding antimony trioxide and hydrogen sulphide. It reacts with steam to form an oxysulphide. It is oxidised by hydrogen peroxide to sulphate, and even to pentoxide; ammoniacal solutions of hydrogen peroxide and sodium peroxide react to form antimonates.

Antimony trisulphide reacts vigorously with fluorine in the cold, yielding the trifluoride. Chlorine reacts less vigorously with the heated sulphide, which, however, is decomposed by hydrogen chloride. The trisulphide will dissolve in aqueous hydrochloric acid, the solubility depending upon the concentration of the acid and of the hydrogen sulphide in the solution. If the pressure of hydrogen sulphide over the solution is increased, the action is reversed and antimony trisulphide is precipitated. Complete dissolution can be obtained by removal of hydrogen sulphide. Antimony trisulphide is also decomposed by bromine and by iodine.

Thermal examination of a number of binary sulphide systems in which antimony trisulphide forms one component has been made. This includes the systems with bismuth trisulphide, lead sulphide, tin sulphide, cuprous sulphide, silver sulphide. The ternary systems copper-antimony-sulphur and nickel-antimony-sulphur have been examined.

Sulphur dioxide has very little action on antimony trisulphide. The trisulphide is dissolved slowly by concentrated sulphuric acid, yielding an acid antimony sulphate, sulphur dioxide and sulphur. Dilute sulphuric acid assists the transformation from the amorphous variety to the crystalline. By heating with potassium sulphate, potassium antimonate is obtained. The trisulphide reacts with both sulphuryl chloride and thionyl chloride, antimony trichloride being formed in each case.

Antimony trisulphide is appreciably soluble in aqueous ammonia, the solubility increasing with rise of temperature. In solutions of ammonium carbonate, however, it is practically insoluble. Treatment with concentrated nitric acid yields a mixture of antimony nitrate and sulphate, and with fuming nitric acid a mixture of antimony pentoxide and sulphuric acid. Hydrogen sulphide is evolved by the action of slightly diluted nitric acid (6N); this action is retarded by the presence of hydrazine. In the presence of the latter a number of secondary reactions also occurs. The trisulphide is completely converted to trichloride by heating with a mixture of ammonium chloride and ammonium nitrate.

A complex reaction takes place when antimony trisulphide is heated in a current of phosphine, the products being phosphorus, antimony and hydrogen sulphide.

Antimony trisulphide is reduced to metallic antimony when heated with carbon, carbon disulphide also being formed; it is partially reduced by heating in a current of carbon monoxide at red heat. In the reaction

Sb₂S₃ + 3CO ⇔ 2Sb + 3COS

equilibrium moves to the right with rise of temperature. Antimony trisulphide also reacts with carbon dioxide at a dull red heat, the products including sulphur dioxide, carbon monoxide and carbonyl sulphide.

When antimony trisulphide is fused with excess of alkali, a mixture of alkali antimonite and thioantimonite is produced; but if excess of the trisulphide is used, antimony oxysulphide is obtained instead of alkali antimonite; some metallic antimony may be precipitated if the fusion is carried out at a high temperature. Amorphous antimony trisulphide is soluble in excess of an aqueous solution of potassium hydroxide, but is reprecipitated on the addition of hydrochloric acid; the crystalline form behaves similarly on warming. When heated with potassium cyanide, partial reduction takes place; on heating with a mixture of potassium cyanide and sodium carbonate a mirror is obtained when the reaction is conducted in a current of hydrogen, but not when carbon dioxide is substituted for hydrogen.

Antimony trisulphide is reduced by heating with many metals, metallic antimony being formed, which combines with excess of the metal to form antimonides.

Sols of antimony trisulphide have been obtained by the action of hydrogen sulphide upon water saturated with antimony trioxide; or upon a dilute solution of potassium antimonyl tartrate; or by the addition of a few drops of a solution of potassium sulphide to a suspension of amorphous antimony trisulphide in water. These sols vary in colour from blood-red to yellow. They may be purified by dialysis, tartaric acid, however, being difficult to separate. Sols free from foreign matter are stable even on warming. Many acids and their salts cause precipitation of the trisulphide, the efficiency of a salt in this respect increasing with the valency of the cation. (Iron, however, appears to act exceptionally.)

Two hydrates of antimony trisulphide, namely Sb₂S₃.2H₂O and Sb₂S₃.H₂O, have been described, but their existence does not appear to have been established definitely.

From an examination of the precipitates obtained by the action of hydrogen sulphide upon potassium antimonyl tartrate it has been assumed that compounds of antimony trisulphide with hydrogen sulphide may exist. The precipitates, however, are of variable composition.

Compounds of antimony trisulphide with metallic sulphides have been described. These have generally been assumed to be complex thioantimonites, related to a number of hypothetical, and complex, thio-antimonious acids. Compounds of the types Na₃SbS₃, Na₂Sb₄S₇, Na₆Sb₄S₉, NaSbS₂ and NaHSb₄S₇ have been described. Many of them occur naturally. They may be prepared by fusion, or by the action of antimony trisulphide upon solutions of the metallic sulphides. Alkali thioantimonites may be prepared by the action of alkali hydroxide, carbonate or sulphide upon antimony trisulphide; or by the action of alkali sulphide upon antimony trichloride. Corresponding compounds of the heavy metals may be obtained by double decomposition. Thio-antimonites of alkali and alkaline earth metals (formerly known as "liver of antimony") vary in colour from yellow to reddish-brown, those containing the greater proportion of antimony trisulphide having the darker colour. Crystalline specimens can be prepared. They melt at a low temperature, and are fairly stable when gently heated out of contact with air; they decompose when heated strongly, being converted into thioantimonates. When heated in air they burn. Thio-antimonites of the heavy metals are grey to black in colour, the natural products being crystalline, synthetic products amorphous. Some of them can be melted without decomposition when heated in the absence of air; but for the most part they decompose readily, forming a sublimate of antimony trisulphide. Heated in air, they are converted to oxides, with evolution of sulphur dioxide.

Only the thioantimonites of the alkali and alkaline earth metals are soluble in water. They are for the most part hygroscopic, and many are decomposed, as are also their solutions, on exposure to air. The natural thioantimonites are decomposed by the action of nitric acid and other oxidising agents, and in some cases by hydrochloric acid. They are completely decomposed by solutions of alkali sulphides.

The existence of antimony tetrasulphide, Sb₂S₄, which has been described as a reddish-yellow or red powder, does not appear to have been fully established. Several methods for the preparation of this powder have been described, including the action of hydrogen sulphide upon a hydrochloric acid solution of antimony tetroxide, or potassium meta-hypoantimonate, K₂Sb₂O₅, or upon a solution of the complex compound 3KCl.2SbCl₄; and the action of carbon disulphide upon antimony pentasulphide. On heating it is converted to antimony trisulphide. It dissolves in hydrochloric acid yielding hydrogen sulphide; it also dissolves in ammonia forming a yellow solution. More recently it has been suggested that golden antimony sulphide, generally regarded as impure antimony pentasulphide, is really a mixture of antimony tetrasulphide, antimony trisulphide and free sulphur; it is further claimed that pure antimony tetrasulphide may be obtained by decomposing zinc thioantimonate with dilute hydrochloric acid, according to the equation.

Zn₃Sb₂S₈ + 6HCl = Sb₂S₄ + 3ZnCl₂ + H₂S₂ + 2H₂S

The tetrasulphide is regarded as being of the type M_x(SbS₄)_y, in the special case when M = Sb and x = y.