Chemical Properties of Antimony

Antimony does not combine directly with hydrogen, and can be volatilised by heating in a stream of that gas. Antimony compounds are reduced by nascent hydrogen in acid (but not in alkaline) solution forming antimony trihydride or stibine. Metallic antimony is precipitated from solutions by hydrogen at high temperature and pressure. With solutions of antimony trichloride of concentrations up to 50 per cent., and hydrogen at a pressure up to 150 atmospheres, the quantity of precipitated material is proportional to the pressure. The reaction is of the first order with pressures between 15 and 150 atmospheres and concentrations less than normal. It is calculated that from a normal solution of antimony chloride at 20° C., with hydrogen at a pressure of 100 atmospheres, the precipitation of 1 per cent, of metal would require 160 years. Increase of hydrogen ion retards the precipitation of antimony. From the behaviour of antimony as a catalyst in the silent reaction between hydrogen and oxygen, the metal does not appear to absorb hydrogen.

Antimony is not appreciably affected by exposure to dry air, but in the presence of light and moisture oxidation takes place. It burns brilliantly even in very dry oxygen. It is oxidised to antimony pentoxide by ozone.

It decomposes steam only at very high temperatures, hydrogen and antimony trioxide being formed; precipitated antimony reacts more readily. Neutral hydrogen peroxide is without action, but in the presence of an alkali antimonates are formed.

Antimony burns vigorously in fluorine; it also combines directly with chlorine, bromine and iodine, with the first even when very dry. Hydrofluoric acid is without action, while hydrochloric acid attacks antimony only in the presence of air. The solvent action of hydrochloric acid is increased by the addition of a little nitric acid; aqua regia converts the metal completely to antimony trichloride and antimony pentachloride. With nitrosyl chloride the compound SbCl₅.NOCl is obtained.

When antimony is fused with sulphur, combination takes place with the formation of antimony trisulphide. (It is doubtful whether either a higher or a lower sulphide is also formed.) The elements will also combine when heated together with water in a sealed tube to 200° C.; and when a powdered mixture is subjected to high pressures. Dry hydrogen sulphide attacks antimony at 360° C. and above, antimony trisulphide being formed; when the metal is heated moderately in a stream of sulphur dioxide, a mixture of antimony trioxide and antimony trisulphide is obtained. Warm concentrated sulphuric acid attacks antimony forming antimony sulphate, but both dilute and cold acid are without action.

When antimony is heated with the vapour of thionyl chloride, the latter is decomposed with formation of antimony trichloride. Sulphur monoxide is also formed and it is suggested that the reaction proceeds according to the equation:

3SOCl₂ + 2Sb = 2SbCl₃ + 3SO

When antimony is heated in a stream of sulphuryl chloride (diluted with carbon dioxide) the metal is converted to the trichloride.

Antimony does not combine with nitrogen. It is attacked by nitric acid with evolution of nitric oxide, the remaining products depending upon the concentration and temperature of the acid. The action takes place only slowly in the absence of nitrous acid. Antimony nitrate may be formed when cold, dilute acid is used; but more generally a mixture of oxides of antimony is obtained. The metal will not burn in the vapour of nitric acid. It will dissolve in nitric acid to which has been added tartaric acid or certain other organic acids, and the solution remains clear on warming.

Molten antimony combines readily with phosphorus. It reduces both phosphorus trioxide and phosphorus trisulphide when heated for a long time with those substances; it reacts quantitatively with phosphorus trichloride. When heated with phosphorus pentachloride, a mixture of antimony trichloride and phosphorus trichloride is obtained. The trichloride, trioxide and trisulphide of arsenic react similarly with antimony.

When antimony is heated in a current of carbon dioxide a reaction takes place, beginning at 830° C., and which, at 1100° C., may be represented by the equation

2Sb + 3CO₂ = Sb₂O₃ + 3CO

Antimony will react with the alkali metals forming antimonides. It is attacked by solutions of alkalis and of alkali salts. As has been stated above, antimony acts as a reducing agent under certain conditions; it will reduce the following salts, at least partially: ferric chloride, ferric sulphate, potassium ferricyanide, potassium nitrate, and potassium permanganate (forming manganese dioxide). The reduction of silver nitrate depends upon the concentration of the solution: from a 0.5N or 0.25N solution silver may be precipitated quantitatively, but with a weaker solution (0.05N) the reduction is incomplete, and a compound, 2Sb₂O₃.N₂O₅, is formed. Gold chloride is completely reduced by antimony. Antimony compounds are reduced to the metal by the action of Bettendorf's reagent (a mixture of stannous chloride and hydrochloric acid). Metallic antimony reacts with iron at the melting point of the former.

The atomicity of antimony does not appear to be known with certainty. From a calculation of the heat of evaporation it has been deduced that antimony, between the boiling point and melting point, is monatomic, remaining in that condition when cooled to 357° C. At lower temperatures, polymerisation takes place. On the other hand, a theoretical calculation based upon van der Waal's equation indicated that the molecule of antimony may contain about twelve atoms.

The normal electrode potential, Sb/Sb⁺⁺⁺, is +0.244 volt measured on the hydrogen scale at summer temperature. In a 10N solution of potassium hydroxide at 20° C. the electrode potential is given by



the process of solution being represented by



reaction proceeds according to the equation



the potential is given by



Passivity of the antimony anode is not attained unless the current density exceeds 7.5 amperes per sq. dm.

The antimony electrode has recently received considerable attention. The electrode consists of a rod of metal connected by a copper wire to a calomel cell. The rod should be clean, and should dip into a solution containing suspended purified antimony trioxide, which should be stirred continuously. Cast antimony appears to be preferable to electrolytic metal. The value of E is given as

E = + 0.030 + 0.05915pH

The temperature coefficient is stated to be 0.0013 volt per degree in soils of varying pH value, while the pH value against a standard calomel electrode varies according to the following:



The electrode gives a linear relation between the observed e.m.f. and the pH values with a probable error of 0.01 to 0.08 pH. It compares satisfactorily with the hydrogen and the quinhydrone electrodes, and has been recommended for use in connection with the examination of acids and alkalis, soils, blood, sugar liquors, etc. It does not appear to be suitable for use in connection with the leather industry.

The e.m.f. of the cell:

Explosive Sb | SbCl₃ solution | Rhombohedral Sb

is 0.014 volt, the temperature coefficient dE/dT being 6.8×10^-5 volt per degree. If the current exceeds a certain limiting value the cell explodes.

Antimony exhibits the valve effect in nearly all electrolytes even at 600 to 700 volts. This is due to the formation of a layer of oxide on the surface of the anode.

The potential difference between antimony and air is 0.16 volt.