Antimony Trichloride, SbCl₃

Antimony Trichloride, SbCl₃, has been known for a long time, having been prepared by Basil Valentine by distilling a mixture of antimony trisulphide and mercuric chloride; it was at one time thought that the substance was a compound of mercury, but Glauber disproved this in 1648. That the trichloride is decomposed by water was known to Basil Valentine, while the oxychloride produced was called by Paracelsus mercurius vitae, in the belief that it was related to mercury. At the end of the sixteenth century the trichloride (probably admixed with oxychloride) was introduced into medicinal preparations by Victor Algarotus of Verona, under the name of pulvis angelicus.

Antimony trichloride is most conveniently prepared by the action of concentrated hydrochloric acid upon antimony trisulphide; when all the hydrogen sulphide has been evolved, the residue is distilled; after rejecting the first distillate, which contains most of the volatile impurities, the final distillate is collected as a white, pasty mass of antimony trichloride.

Numerous chemical reactions resulting in the formation of antimony trichloride have been described. It may be obtained from metallic antimony by the action of chlorine, acid chlorides, magnesium chloride and other metallic chlorides. Hydrochloric acid, free from air, does not attack antimony, but in the presence of air, antimony trichloride is formed slowly; the action is accelerated by the presence of a little nitric acid.

Antimony trioxide is converted into the trichloride by the action of chlorine, chlorides of non-metals and by dissolution in hydrochloric acid. Antimony pentoxide reacts in a somewhat similar manner.

From antimony trisulphide the trichloride may be obtained by the action of chlorine, hydrogen chloride (either in the gaseous form or in solution as described above), thionyl chloride, sulphuryl chloride and ammonium chloride. Concentrated hydrochloric acid reacts with antimony pentasulphide with formation of the trichloride. It is of interest to note that the trichloride has also been obtained by the distillation of a mixture of antimony sulphate and sodium chloride.

Antimony pentachloride is reduced -to the trichloride by heating with antimony.

Stibine may be converted to antimony trichloride by the action of chlorine or phosphorus pentachloride.

A pure solution of antimony trichloride may be prepared by dissolving antimony oxychloride in hydrochloric acid; crystals may be obtained by evaporation from a solution of the trichloride in carbon disulphide or in sulphuryl chloride, by sublimation of the trichloride in a current of carbon dioxide, and by solidification after fusion.

Antimony trichloride can exist in three distinct crystalline modifications with transition points at 65° C. and 69.5° C. respectively. The modification stable at the ordinary temperature crystallises in the rhombic system, forming colourless, transparent, prismatic or octahedral crystals. Its density at 25° C. is 3.14. Its molecular volume (calculated from the density at -194° C. and the coefficient of expansion) is 68.5. It melts at 73.4° C., forming a colourless or yellowish oil. The existence of the first transition point slightly below the melting point is indicated when the substance is heated gradually, for, in the neighbourhood of the melting point the crystals change to a heavy powder. The latent heat of fusion is 3,030 gram-calories per mole. The density of the liquid, and the surface tension (σ), at various temperatures, are as follows:

Temperature, ° C. Density	109.5	127.5	148.5	166.5
(Dt4)	2.599	2.558	2.511	2.471
σ (dynes per cm.)	44.51	41.84	39.43	37.38

The variation of the density of the liquid with temperature (t) between 109° C. and 166° C. is thus given by the expression

D^t₄ = 2.844 – 0.00224t

The calculated mean value for the parachor is 227.4.

Liquid antimony trichloride boils at 220.2° C., its latent heat of vaporisation being 10,700 calories per mole. The critical temperature is 524° C., the ratio of the boiling point to the critical temperature (on the absolute scale) thus being 0.619.

Antimony trichloride may be distilled from solutions containing sulphuric acid provided that hydrobromic acid is dropped slowly into the solution, which latter should be kept at 180° C. On distilling a solution of antimony trichloride in hydrochloric acid, hydrogen chloride distils over first, followed by a mixture of hydrogen chloride and antimony chloride, and finally antimony chloride alone. The volatility of antimony trichloride is reduced by the addition of ferrous chloride. The trichloride does not volatilise when a current of hydrogen chloride is passed through a hot, dilute solution.

The solubility of antimony trichloride in water and the effect of temperature upon solubility is as follows:

Temperature, ° C	0	15	20	25	30	35	40	50	60	72
Solubility (Grams SbCl3 in 100 grams water)	601.1	815.8	920.8	988.1	1068	1152	1368	1917	4531	∞

When excess of water is employed, hydrolysis sets in rapidly, antimony trichloride thus resembling the other trichlorides of this group. The solubilities of the trichloride in aqueous solutions of hydrochloric acid at various concentrations at 20° C. are as follows (the concentration of each solute is given in grams per 100 grams of water):

HCl	0	4.86	12.34	16.80	18.41	23.68	58.08
SbCl3	910.1	895.4	879.0	857.6	866.4	856.3	789.8

The trichloride is soluble in many other solvents including ether, carbon disulphide, absolute alcohol, acetone, chloroform, and to some extent in liquid cyanogen. It is insoluble in carbon tetrachloride.

The molal conductivity of solutions of antimony trichloride in bromine suggests that double molecules of Sb₂Cl₆ exist, which dissociate into Sb₂Cl₃⁺⁺⁺ and 3Cl^- ions. This view is supported by electrolysis experiments.

The dielectric constant has been determined for a series of dilute solutions of antimony trichloride in benzene. At 25° C. the value varies with dilution from 2.377 to 2.588, and at 40° C. from 2.340 to 2.534. From this the calculated value of the dipole moment is 2.75×10^-18 e.s.u.

The chlorides of potassium, rubidium, ammonium and thallium will dissolve in purified liquid antimony trichloride. When the concentration of the solution is above 0.1N, the conductivity is less than that of a corresponding aqueous solution; for more dilute solutions the conductivity is greater. The conductivity increases regularly with rise of temperature from 70° C. to 200° C. It is probable that the degree of ionisation is less for solutions in antimony trichloride than for solutions in water, but that for the dilute solutions the ionic velocity is greater. Solutions of the bromides of potassium, ammonium and thallium in antimony trichloride have also been examined and their conductivities determined.

The viscosity of antimony trichloride from 80° to 200° C. has been determined, and the curve obtained by plotting the fluidity against the temperature appears to show a break at 120° C.

The Raman spectrum, which consists of four lines, suggests the existence of co-valent linkages.

The refractive index for the sodium D line is 1.460.

The vapour pressure of antimony trichloride is as follows:

t° C.	120	130	140	150	160
p (mm.)	29	43	64	92	127

Determinations of the vapour density of antimony trichloride are in agreement with the molecular formula, SbCl₃; this formula is also confirmed by a number of determinations of the molecular weight in solution by the freezing point method.

The heat of formation of antimony trichloride is 91,390 gram-calories per mole.

The trichloride can be distilled without decomposition in a current of hydrogen.

The reaction between antimony trichloride and hydrogen has been investigated by passing a current of hydrogen at the rate of two litres per hour over the chloride. By means of a transformer a pressure of 15,000 volts was applied to the reaction chamber, and an antimony mirror was obtained.

The trichloride is stated to absorb oxygen when exposed to the air; certainly it deliquesces rapidly in air, forming a cloudy liquid, while a solution of antimony trichloride in hydrochloric acid readily absorbs oxygen. Oxidation occurs under the influence of sunlight, and it is suggested that a peroxide is formed at an intermediate stage.

As already stated, the trichloride will dissolve in a minimum quantity of water without decomposition, but if the molecular proportion of H₂O to SbCl₃ exceeds 2 to 1, oxychlorides are formed, the compositions of which depend upon the temperature and the concentration.

Fluorine displaces chlorine from antimony trichloride, the action being vigorous.

Thermal examination of the system SbCl₃-Cl₂ indicates the presence of a compound, SbCl₃.2Cl₂, which freezes at -81.5° C. Antimony pentachloride is also formed. With nitrosyl chloride the compound SbCl₅.NOCl has been obtained. No compound of antimony trichloride with bromine is indicated by thermal examination.

Antimony trichloride is decomposed when heated with sulphur, grey antimony trisulphide, Sb₂S₃, being formed; a solution of the trichloride in hydrochloric acid is not affected, however, when heated with sulphur.

Hydrogen sulphide reacts with the vapour of antimony trichloride to form antimony trisulphide; reddish-brown crystals of antimony thiochloride, SbSCl, are also formed by this reaction at a slightly lower temperature. Amorphous antimony trisulphide is precipitated when hydrogen sulphide is passed through a solution of antimony trichloride in hydrochloric acid to which tartaric acid has been added; in the absence of tartaric acid a thiochloride is formed. Precipitation may also be effected from an ammoniacal solution containing tartaric acid, but not from a solution in acetone. Antimony trichloride dissolves in liquid hydrogen sulphide, the solution possessing appreciable conductivity; the properties of this solution suggest that complex compounds of antimony trichloride and hydrogen sulphide are formed.

Concentrated sulphuric acid has very little action in the cold, but on warming, antimony sulphate is formed with evolution of hydrogen chloride. When a mixture of sulphuric acid and a solution of antimony trichloride in hydrochloric acid is distilled, hydrochloric acid distils over first, followed by antimony trichloride; when finally sulphuric acid distils over, no antimony is found in the distillate, there being a residue of antimony sulphate. By repeated distillations, however, it is possible to obtain all the antimony as trichloride.

When a solution of antimony trisulphide in boiling antimony trichloride is allowed to cool, a double compound, SbSCl.7SbCl₃, separates out in the form of yellow, transparent, rhombic prisms. This substance is deliquescent, and is readily decomposed by heating or by the addition of water.

A number of compounds of antimony trichloride with alkyl sulphides and with alkyl sulphides and halides together have been prepared either by heating the components in a sealed tube at 90° to 120° C., or by heating them in calculated proportions gently under a reflux condenser. Ammonia reacts with molten antimony trichloride to form two ammoniates, namely SbCl₃.NH₃, from which all the ammonia can be removed by heating, and SbCl₃.2NH₃, a yellowish-white, crystalline substance which is stable and volatile. The former of these is much less deliquescent than the trichloride. From these compounds the corresponding double salts SbCl₃.NH₄Cl and SbCl₃.2NH₄Cl may be formed by the action of hydrochloric acid. A tri-ammoniate, SbCl₃.3NH₃, has been prepared by the action of ammonia on a solution of antimony trichloride in acetone. It is a white solid, stable in air; on heating it loses ammonia. More recently, however, it has been shown that when treated with liquid ammonia, one molecule of antimony trichloride absorbs rather more than three molecules of ammonia without appearing to form a definite ammoniate. With liquid ammonia, in fact, antimony trichloride yields a yellow compound to which the formula Sb(NH)Cl has been ascribed. On further treatment an orange nitride, SbN, is obtained.

When potassium cyanate is added to an aqueous solution of antimony trichloride, a crystalline precipitate of antimonic acid is obtained. This is ascribed to the ammonia arising from the reaction

KCNO + 2H₂O = KHCO₃ + NH₃

Nitric oxide, free from nitrogen peroxide, is without action upon a solution of antimony trichloride in chloroform, but in the presence of a trace of the peroxide, a complex crystalline precipitate is obtained. Solid antimony trichloride absorbs nitric oxide, the mixture becoming liquid. From this liquid all the nitric oxide can be removed by treatment in a vacuum.

Antimony trichloride is oxidised by nitric acid, antimonic acid being formed. When a solution of antimony trichloride in nitric acid is distilled, the first distillate consists of a mixture of hydrochloric acid and nitric acid; towards the end some antimony pentachloride distils with partial decomposition; antimony pentoxide remains.

When phosphine is passed through molten antimony trichloride, a black complex substance of indefinite composition is obtained.

Phosphorus pentachloride reacts with formation of the double compound PCl₅.SbCl₅. A reaction also occurs with phosphorus triiodide.

When antimony trichloride is heated with alcohol under pressure, oxychlorides are formed; the reactions with ether, acetone, dimethylamine, benzene, aniline and many other organic compounds have also been described.

Antimony trichloride reacts with stannic iodide, double decomposition taking place. Double decomposition occurs when antimony trichloride is heated with germanium tetraiodide.

A violet compound of copper, Cu₂Sb, is obtained by the action of a hydrochloric acid solution of antimony trichloride containing cuprous chloride upon metallic copper. The reactants should be protected from the air. A similar reaction occurs between antimony trichloride and metallic tin, the compound Sb₂Sn being formed. No reaction has been observed with bismuth.

Reactions also occur with nickel and cobalt, a mono-antimonide being produced in each case.

Among the uses for which antimony trichloride has been suggested is that of an addition to a hydrocarbon fuel to act as an "antiknock." About 18 grains per gallon of petrol are recommended.

When a current of hydrogen chloride is passed into a saturated aqueous solution of antimony trichloride at 0° C., until no more of the gas can be absorbed, a compound 2SbCl₃.HCl.2H₂O is obtained in the form of deliquescent crystals melting at 16° C. It loses hydrogen chloride on heating.

Many double salts with chlorides of the metals of Groups I, II and III have been described, among them being the following:

2LiCl.SbCl₃.5H₂O and 2LiCl.SbCl₃.6H₂O.

NaCl.SbCl₃. It is possible that complexes, yielding ions of H₃SbCl₆, are also formed when antimony trichloride is dissolved in solutions of sodium chloride.

2KCl.SbCl₃, dimorphic, crystallising in the hexagonal5 and the monoclinic systems. Crystals of the latter system have the following elements: a:b:c = 0.7241:1:0.7222; β = 111°3'. An hydrated form, 2KCl.SbCl₃.2H₂O, has also been described. 2KCl.SbCl₃.SbOCl, prismatic crystals of the monoclinic system.

KBr.SbCl₃.H₂O, bright yellow octahedra; 3KBr.2SbCl₃.2H₂O, bright yellow rhombic crystals; 3KBr.SbCl₃.1.5H₂O, yellow crystals of the tetragonal system: a:c = 1:0.7629.

RbCl.2SbCl₃.H₂O, long, colourless crystals (M.pt. 77° C.) of the monoclinic system: a:b:c = 1.699:1:0.820; β = 90°31½. RbCl.SbCl₃, colourless crystals (no definite M.pt.), monoclinic system: a:b:c = 1.732:1:1.085; β = 114°26'. 3RbCl.2SbCl₃, pale yellow crystals, trigonal rhombohedral system: a:c = 1:0.5625; α = 110°54'. 2RbCl.SbCl₃.SbOCl.

3CsCl.2SbCl₃, white or pale yellow prismatic crystals. BaCl₂.SbCl₃.2.5H₂O, fine, star-shaped crystals. CaCl₂.SbCl₃.8H₂O, large, colourless, tabular crystals, probably of the triclinic system.

MgCl₂.2SbCl₃.10H₂O and MgCl₂.SbCl₃.5H₂O.

BeCl₂.SbCl₃.3H₂O and BeCl₂.SbCl₃.4H₂O.

3TlCl.SbCl₃, light yellow scales.

17CdCl₂.SbCl₃.18H₂O, relatively stable, colourless; 17CoCl₂.SbCl₃.32H₂O, violet; AlCl₃.3SbCl₃.6H₂O, very hygroscopic, unstable.

Thermal examination of a number of binary systems, involving antimony trichloride as one of the components, has been made. Among them the following may be mentioned:

SbCl₃-LiCl, SbCl₃-NaCl, SbCl₃-KCl, SbCl₃-KBr, SbCl₃-NH₄Cl, SbCl₃-CuCl, SbCl₃-AgCl; SbCl₃-BaCl₂, SbCl₃-HgCl₂, SbCl₃-HgBr₂; SbCl₃-AlCl₃; SbCl₃-SnCl₂, SbCl₃-SnCl₄, SbCl₃-SnBr₄, SbCl₃-SnI₄; SbCl₃-AsCl₃, SbCl₃-AsBr₃, SbCl₃-SbCl₅, SbCl₃-SbBr₃, SbCl₃-SbI₃, SbCl₃-BiCl₃.