Oxychlorides of Tervalent Antimony

Many substances of a complex nature, obtained by different investigators, have been described as oxychlorides of tervalent antimony; a study of the hydrolysis of antimony trichloride, however, suggests that only two of these, namely SbOCl and Sb₄O₅Cl₂, are true compounds, although unstable intermediate products may be formed. The hydrolysis does not appear to be progressive, as was thought by early workers. It is probable that by the addition of sufficient water antimony trichloride is completely hydrolysed to a hydrated form of antimony trioxide and hydrochloric acid, the former of which forms a bulky, white, amorphous precipitate. On standing, this precipitate tends to adsorb hydrochloric acid, the composition of the resulting product depending upon the concentration of the supernatant solution. This adsorption product tends to change on standing for a considerable time into a very finely crystalline substance, the needle-shaped crystals of which may have the composition Sb₄O₃(OH)₃Cl₃; these in turn are transformed to the oxychloride SbOCl. From the table it will be seen that the solid phase in equilibrium with concentrated mixtures of antimony trichloride and water is the oxychloride SbOCl, while that in equilibrium with more dilute mixtures is the oxychloride Sb₄O₅Cl₂. The two oxychlorides can be readily distinguished from one another by their crystalline form. It is suggested that the intermediate substance Sb₄O₃(OH)₃Cl₃ reacts in accordance with the equations

Sb₄O₃(OH)₃Cl₃ + HCl = 4SbOCl + 2H₂O
Sb₄O₃(OH)₃Cl₃ = Sb₄O₅Cl₂ + H₂O + HCl

It is possible that another intermediate unstable crystalline product, Sb₄O₃(OH)₅Cl, may also be formed. The transition from SbOCl to Sb₄O₅Cl₂ takes place when the concentration of chlorine in the solution is approximately 8-0iV; and the transition from antimony trioxide to an oxychloride when the chlorine concentration is about 0.1N. Examination of the equilibrium of the system Sb₂O₃-HCl-H₂O at 25° C. revealed no transition points corresponding to the formation of oxychlorides.

Equilibrium in the system Sb₂O₃-HCl-H₂O at 25° C

Chlorine, gram-atoms per litre.	Antimony, gram-atoms per litre.	Chlorine, gram-atoms per litre.	Antimony, gram-atoms per litre.
0.198	0.00012	2.10	0.0522
0.402	0.000172	2.29	0.0755
0.601	0.000298	2.54	0.120
0.799	0.000610	2.81	0.177
1.02	0.00124	3.36	0.309
1.32	0.00565	3.85	0.440
1.56	0.0116	4.79	0.745
1.74	0.0194	5.93	1.13
1.90	0.0299

Equilibrium in the system SbCl₃-H₂O at 25° C
Solid and Liquid Phases in Equilibrium at Different Concentrations.

Liquid Phase.		Solid Phase.
Antimony, per cent.	Chlorine, per cent.	Antimony, per cent.	Chlorine, per cent.
44.0	41.9	69.9	20.7
39.4	38.8	70.0	20.6
33.0	34.5	69.8	20.7
28.7	31.6	70.0	20.6
27.1	30.4	69.9	20.6
23.8	27.9	70.1	20.7
21.2	25.8	70.0	20.7
18.7	23.7	. . .	20.7
16.2	21.4	Mixture of Crystals
14.8	20.7	. . .	11.8
11.3	17.5	76.1	11.2
7.5	14.3	76.1	11.2
4.5	11.7	76.1	11.3
1.9	9.0	76.0	11.1
1.7	8.9	76.2	11.2
Note. - SbOCl contains Sb 70.29 per cent.; Cl 20.47 per cent. Sb4O5Cl2 contains Sb 76.34 per cent.; Cl 11.15 per cent.

As the conditions of equilibrium within the system SbCl₃-H₂O-HCl (or of the more complete system Sb₂O₃-H₂O-HCl) do not appear to have been fully elucidated, it may be desirable to include here the results of investigations by previous workers. In the table below data are included for three different temperatures, namely 15° C., 20° C. and 50° C. For each temperature the formulae of the stable solid phases are stated, together with the composition, or range of composition, of the liquid phase in equilibrium therewith. It will be noted that the existence of an oxychloride Sb₃O₂Cl₅ or SbCl₃.(SbOCl)₂, indicated by these investigations, has not been confirmed by the more recent work of Lea and Wood.

Hydrolytic dissociation is retarded and may be inhibited by the presence of hydrogen chloride, the chlorides of alkali and alkaline earth metals, and tartaric acid.

Antimony Oxychloride, or Antimonyl Chloride, SbOCl, is obtained as an amorphous white powder, or in the form of small rhombohedral crystals, by the hydrolysis of antimony trichloride at the ordinary temperature as already described, care being taken to avoid excess of water. By heating antimony trichloride with alcohol in a sealed tube at 160° C., antimony oxychloride is also obtained in the form of colourless crystals of the monoclinic system: a:b:c = 0.8934:1:0.7577; β = 103°29'.

The heat of formation from the elements is 89,700 gram-calories per mole.

When heated this oxychloride decomposes, yielding first Sb₄O₅Cl₂ and SbCl₃, ultimately, with strong heating, passing into a mixture of antimony trioxide and antimony trichloride. Volatilisation of antimony trichloride begins at 170° C. Hot water converts antimonyl chloride into Sb₄O₅Cl₂.

Antimonyl chloride is insoluble in alcohol and ether, but soluble in carbon disulphide, chloroform and benzene, as well as in hydrochloric acid and in a solution of tartaric acid.

The oxychloride Sb₄O₅Cl₂ may conveniently be prepared by methods similar to those adopted for SbOCl, using a moderate excess of water for hydrolysis, or by heating with alcohol at 140° to 150° C.

This second oxychloride is obtained in the form of minute prismatic crystals of the monoclinic system: a:b:c = 1.234:1:3.081; β = 121°2'. Its density is 5.014. It melts without decomposition; but at a higher temperature it is decomposed into a mixture of the trichloride and the trioxide. It is not affected by cold water, but it loses chlorine when treated repeatedly with boiling water, while water at 150° C. decomposes it completely, antimony trioxide being formed. It is also converted into the oxide by treatment with alkali solutions. Mineral acids convert it into the corresponding normal salts, oxalic acid into a basic oxalate. When heated with sulphur it is converted into black antimony trisulphide, with evolution of sulphur dioxide; intermediate products may also be formed. The heat of formation from the elements is 328,800 gram-calories.

The following compounds of tervalent antimony, chlorine and sulphur, in addition to those previously mentioned, have been described. Some of them may perhaps be thiochlorides: SbSCl.SbCl₃, Sb₄S₅Cl₂, Sb₅S₆Cl₃ and Sb₈S₁₁Cl₂.