Antimony Trihydride, SbH₃

Antimony Trihydride, SbH₃. - Antimony Trihydride, or Stibine, SbH₃ was first obtained in 1837. It may be prepared by the action of nascent hydrogen upon a solution of an antimony salt, the gas obtained being mixed with a large excess of hydrogen. The reaction is most conveniently carried out by the addition to a solution of an antimony salt of metallic zinc or aluminium and a mineral acid. When iron is used no antimony trihydride appears to be formed; with tin the yield is very small. If the reaction is carried out in alkaline solution, no hydride is formed. In this manner antimony trihydride differs from the corresponding hydride of arsenic.

Antimony trihydride may also be prepared by the action of dilute mineral acids upon alloys of antimony, convenient alloys being those of antimony with zinc, magnesium, sodium amalgam, potassium and calcium. If, however, the alloys of antimony and calcium are chemically pure, no hydride is obtained. Alloys with strontium and barium act less readily. Alloys of antimony and lead may be used with concentrated hydrochloric acid or hydrobromic acid. Alloys of antimony with lithium, aluminium, thallium and iron are unsuitable. The most satisfactory results are obtained with alloys of zinc or magnesium. The state of division of the alloy, and the temperature at which the reaction takes place, greatly influence the yield, the best results being obtained by allowing small portions of finely-divided alloy to fall gradually into cold dilute, oxygen-free hydrochloric acid.

Attempts have been made to prepare antimony trihydride by electrolytic processes. Newbury found appreciable quantities of this gas in the hydrogen liberated from an antimony cathode in acid solution. Later investigations have been carried out using both acid and alkaline solutions. No stibine is produced at low concentrations, while with increasing concentrations the gas is decomposed almost as soon as it is formed, particularly in alkaline solutions. The yield decreases with rise of temperature. The electrolytic formation of stibine has been studied quantitatively, and equations have been adduced correlating the percentage yield of stibine at an antimony electrode in solutions of caustic alkali with temperature, with hydrogen-ion concentration, and with the voltage between a hydrogen electrode immersed in the experimental solution and a saturated calomel electrode; similar equations for solutions of sodium carbonate or sodium sulphatq have also been obtained. In acid solutions, however, stibine is formed only with difficulty, the required current density being sufficiently high to cause elevation of the temperature of the electrolyte to an inhibitive degree. Alkaline solutions appear to be more favourable to stibine formation, but it is necessary to remove rapidly any stibine produced by blowing a current of hydrogen past the electrode; failing this the stibine is at once decomposed with precipitation of metallic antimony. It has also been suggested that, although the electrolytic method of preparation is not practical, the best conditions are obtained by using a concentrated aqueous solution of sodium acetate containing acetic acid, with an antimony cathode. The best yield is obtained with a current density of 14 amps, per sq. dm., increase in current increasing the yield and increase in voltage diminishing it.

The gas is most conveniently dried by passing over calcium chloride or phosphorus pentoxide, then collecting over mercury; other desiccating agents cause decomposition. It may be separated from hydrogen by liquefaction.

Antimony trihydride is a colourless gas with a very characteristic smell, described as faintly resembling that of hydrogen sulphide; its taste is extremely unpleasant, and it is very poisonous. Its vapour density at 15° C. and 754 mm. Is 4.36 (air = 1), in agreement with the formula SbH₃. It shows appreciable deviation from the gas laws. When cooled in liquid ethylene it solidifies to a snow-white mass, crystals being formed in liquid air. The solid melts at -88° C., forming a colourless liquid which boils at -17° C. The density of the liquid is 2.26 at -25° C. and 2.34 at -50° C. The gas is slightly soluble in water (to the extent of 0.2 volume in 1 volume of water); the solution in water free from air is fairly stable. It is more soluble in alcohol (15 volumes in 1) and is very soluble in carbon disulphide (250 volumes in 1). It is also fairly soluble in other organic solvents, but such solutions are in general less stable than aqueous solutions.

In its physiological effects the gas strongly resembles arsenic trihydride; exposure to an atmosphere containing 1 per cent, is fatal to mice in a few seconds. Although opinions as to the physiological effect of stibine are conflicting, especially among the earlier workers, it is probable that its action upon human blood is similar to that of arsine, in that the oxyhemoglobin is reduced.

Antimony trihydride is an endothermic compound; the heat of formation, determined by decomposing the gas into its elements by means of the electric spark, is -33,980 gram-calories at constant pressure, and -34,270 gram-calories at constant volume. From an investigation of the electrolytic formation of stibine, the free energy of the reaction 2Sb + 3H₂ = 2SbH₃ has been calculated to be 62,100 gram-calories for two moles of the gas in an acid solution, and 62,000 gram-calories for two moles in an alkaline solution.

Stibine is readily decomposed into its elements; if the gas is pure and dry, however, it remains fairly stable when kept in a thoroughly clean glass vessel. Air and aerated water produce some decomposition, but water free from air appears to be without action. Decomposition does not appear to be caused by light. The velocity of the decomposition depends considerably upon the nature of the surface in contact with the gas, an etched surface, or one coated with an antimony mirror, acting catalytically. The presence of hydrogen does not affect the rate of decomposition; oxygen poisons the antimony mirror which, however, recovers its activity after some hours. It is probable that the effect of oxygen is to oxidise the hydride, not the mirror itself. The rate of decomposition is also affected by the nature of the surface of the antimony mirror.

By the action of heat alone stibine is decomposed more readily than arsine, rapid decomposition occurring at temperatures above 150° C. If the reaction is carried out in a clean glass tube heated locally, an antimony mirror is deposited on both sides of the heated part; this reaction is employed in the well-known Marsh's test.

Liquid stibine is partially decomposed even at low temperatures, decomposition beginning between -65° C. and -56° C. Decomposition takes place more rapidly in the liquid than in the gaseous state, but no evidence has been obtained of the formation of a lower hydride as one of the products of decomposition.

Stibine is oxidised by air or oxygen even at low temperatures according to the equation

4SbH₃ + 3O₂ = 4Sb + 6H₂O

Under ordinary conditions black antimony is deposited, but at -90° C. the yellow modification is obtained. Liquid air does not cause oxidation. When burned in air, antimony trioxide is obtained instead of the metal.

Stibine is readily decomposed at the ordinary temperature by the halogens, forming antimony halides and halogen acids.

Sulphur reacts slowly with a mixture of stibine and hydrogen heated to 100° C., antimony trisulphide and hydrogen sulphide being formed; the action is accelerated by light. Pure stibine reacts readily with finely divided sulphur. Hydrogen sulphide appears to be without action at the ordinary temperature. When stibine is passed into concentrated sulphuric acid a black precipitate is obtained which is probably metallic antimony.

Neither nitrogen nor ammonia reacts with stibine. The gas is oxidised by oxides of nitrogen and by nitric acid. Phosphorus trichloride has no action, while the pentachloride reacts only slowly. The iodides of phosphorus and the halides of antimony react with decomposition of the gas.

Stibine is oxidised when an electric spark is passed through a mixture of the gas with carbon dioxide, according to the equation:

2SbH₃ + 3CO₂ = 2Sb + 3H₂O + 3CO

Decomposition is also induced by the action of potassium hydroxide and other alkali and alkaline earth hydroxides.

The action of stibine on a number of aqueous salt solutions has been studied. Stibine resembles arsine in its action on an aqueous solution of potassium permanganate. The precipitated manganese sesquioxide, Mn₂O₃, is more flocculent when stibine is used, and the solution contains potassium antimonate and a trace of manganese. The reaction may be represented by the equation:

2KMnO₄ + SbH₃ = Mn₂O₃ + K₂HSbO₄ + H₂O

The reaction between stibine and an aqueous solution of silver nitrate has received considerable attention. The black precipitate that is formed was formerly thought to be silver antimonide, Ag₃Sb. The reaction was therefore thought to be different from that with arsine, but subsequent investigation has shown that the proportion of antimony in the precipitate does not correspond to that required by silver antimonide. It is now considered that the action of stibine closely resembles that of arsine, and may perhaps be represented by the equations:

SbH₃ + 3AgNO₃ = Ag₃Sb + 3HNO₃
Ag₃Sb + 3AgNO₃ + 3H₂O = 6Ag + Sb(OH)₃ + 3HNO₃

The second reaction occurs with excess of silver nitrate. The hydrated antimony oxide is almost completely insoluble in the resultant liquid and is thus precipitated with the silver. In this way the action differs from that of arsine. The precipitate is also stated to contain a little metallic antimony. The antimony oxide can be dissolved out of the precipitate by treatment with hot concentrated hydrochloric acid or with tartaric acid. By the action of stibine on a concentrated aqueous solution of silver nitrate a greenish-yellow coloration is obtained, but the substance producing this colour has not been isolated. It is suggested that this compound may be Ag₃Sb.3AgNO₃, corresponding to the similar compounds of phosphorus and arsenic produced by the action of phosphine and arsine respectively on solutions of silver nitrate, and the suggestion is supported by the results of approximate analysis of the coloured mixture. On this assumption the action of stibine may be represented by the equations:

SbH₃ + 6AgNO₃ = Ag₃Sb.3AgNO₃ + 3HNO₃
Ag₃Sb.3AgNO₃ + 3H₂O = 6Ag + Sb(OH)₃ + 3HNO₃

thus further emphasising the resemblance between phosphine, arsine and stibine. It has also been suggested that the greenish-yellow coloration may be due to the formation of the compound Ag(SbH₃)NO₃, analogous to Ag(NH₃)Cl, but there appears to be no confirmation of this. The reaction with silver nitrate has been suggested for the detection of traces of stibine.

When stibine acts upon sodium aurichloride, a violet stain is produced; similar stains are obtained with phosphine and arsine, but not with hydrogen. This reaction is suggested as a sensitive test for these hydrides. Organic matter must be destroyed prior to the test; hydrogen sulphide also interferes.

The best absorbents for stibine are solutions of silver salts, iodine and iodic acid.

A number of organic substitution compounds of stibine have been prepared and described. They are for the most part more stable than stibine itself.