Antimony Trisulfide

There are only a few examples of antimony sulfide compounds with useful applications. The most important among the limited selection of sulfide compounds is antimony trisulfide. After antimony trisulfide, antimony pentasulfide has the next most practical significance.

Sb₂S₃ has two forms, namely a crystalline form and an amorphous form. The crystalline variety has a grey color with a metallic luster. It is rhombic in structure and generally occurs as stibnite ore in a number of antimony deposits. It has a density of 4.65 g/cm³, a melting point of 550° C, and a boiling point of roughly 1085° C. Finally, its solubility in water is less than two-thousandths of a percent.

The amorphous form can be grey, black, red, yellow, brown, or purple. The final color depends on the material’s grain size, production method, and trace impurities.

One chemical reaction can produce either form. First, you need to pass hydrogen sulfide gas through a solution of antimony halide. This results in antimony sulfide and hydrochloric acid. Now, if you were to quickly melt the product you can arrive at the crystalline material or the amorphous substance depending in the rate of cooling. A slow, controlled cooling allows for large, black crystals to form. On the other hand, rapidly cooling the melt prevents crystal formation and the end product is amorphous antimony sulfide.

Metallic Antimony - Chemical Properties

Metallic antimony is a relatively stable compound. Under standard conditions, antimony will keep its luster because it does not readily oxidize. Even in moist air and elevated temperatures in the range of 100 - 250° C, it resists oxidization. However, at temperatures above its melting point, powered antimony ignites and burns with a white-green flame. This reaction forms the pure metal into antimony trioxide, Sb₂O₃. At 750 - 800° C, molten antimony can decompose steam to liberate hydrogen.

Antimony does not dissolve in water, and is relatively stable in dilute hydrochloric acid and concentrated hydrofluoric acid. It forms SbCl₃ and Sb₂(SO₄)₃ respectively when brought in contact with concentrated hydrochloric acid and hot concentrated sulfuric acid (90 - 95°:).

Concentrated nitric acid will oxidize antimony, forming Sb₂O₃ or Sb₂O₅. It is believed the reaction mechanism centers around the intermediary formation of nitrous acid. A layer of oxide material forms on the surface of the metal and subsequently blocks any further reaction.

To conclude, antimony will dissolve in aqua regia, a fresh mixture of concentrated nitric and hydrochloric acids, or in a mixed solution of nitric acid and tartaric acid. It sparingly dissolves in phosphoric acid and some organic acids.

Introduction to Metallic Antimony

Metallic antimony readily crystallizes. During production, molten antimony is poured into molds. As the liquid antimony cools, a thin coating of easily melt-able material, a couverture, forms on the surface of the cooling metal. This couverture is notable for the fern pattern of crystals that appears on the surface of the solidified antimony. It is said that these crystals sparkle or twinkle like stars. Consequently, the resulting product is called "star antimony". Many people consider the sharpness of the fern pattern to be an indicator of the quality of the material. However, there is no scientific basis for this conclusion as ambient conditions and cooling rate will have a significant impact on crystal size of metallic antimony.

Yellow Antimony and Explosive Antimony

The final two unstable allotropic forms of antimony are yellow and explosive.

Much like its arsenic and phosphorus analogs, yellow antimony exhibits mostly nonmetal properties. The oxidization, either by air or oxygen, of liquid antimony hydride produces yellow antimony and black antimony. Yellow antimony is only stable below -90° C. At temperatures between -90° C and -50° C, the yellow allotrope will degrade into its black counterpart. At temperatures above -50° C, yellow antimony rapidly converts into regular metallic antimony.

Explosive antimony is produced by the electrolysis of the aqueous solution of antimony halide. It is believed that this is because of the presence high concentrations of antimony ions in the solution. Explosive antimony deposits on the cathode at a current density of 200 A/m² in a hydrochloric acid solution containing 17 - 33% SbCl₃. Explosive antimony has a steel-gray color and a smooth, soft surface. Its density is 5.64 -5.97 g/cm³. It will produce a vigorous explosion when it is gently struck, rubbed, treated with thermal radiation, or heated to 125° C. The explosion is the result of instant liberation of crystalline heat when a foreign force is applied and will occur even under water.

Black Antimony Allotrope

Antimony’s three unstable allotropes are black, yellow, and explosive. An allotrope is a variation of the molecular structure of a single element. Differences in chemical structure will give an element different physical properties depending on the allotropic form. For example, carbon has two common allotropes; diamond and graphite. In the diamond form the carbon atoms are arranged in a tetrahedral lattice configuration. In the graphite form, on the other hand, carbon atoms are bonded in layers of a hexagonal lattice.

Black antimony is an amorphous black powder, with a specific gravity of 5.3. It can be obtained when metallic antimony vapor is shock cooled or when liquid antimony hydride is oxidized by air or oxygen at -40° C. Black antimony is more volatile than grey antimony. Chemically active, it oxidizes in the air at atmospheric temperature. When heated to 400° C in the absence of air, black antimony transforms rapidly into ordinary antimony crystals.

Antimony Contracts Upon freezing

There was a debate about whether antimony contracted or expanded during solidification for much of the 1900s. Reports ranged from a 1% expansion to 1.4% contraction. It is thought that these discrepancies were due to two main factors. First, testing was done on antimony samples that lacked sufficient purity. The colligative aspects of the impurities unduly influenced any resulting data. Second, the testing equipment used during these experiments were not reliable. Furthering the false notion that antimony expanded upon solidification was anecdotal evidence gathered from print cast. It was suggested that the fine detail retained from lead-antimony alloy print casts was due to expansion. In 1962, Kirshenbaum and Cahill settled the debate with their careful work. They determined that antimony does, in fact, contract by 0.79 ± 0.14% during solidification.

More Antimony Properties

Antimony is pretty poor heat conductor. For comparison, copper’s thermal conductivity is roughly twenty times larger than antimony’s. Silver thermal conductivity is even bigger at almost twenty-five times that of antimony. In metals and metalloids, thermal conductivity and electrical conductivity track each other. Antimony is typical in this regard as its electrical conductivity is only 1/27 that of copper. Antimony is atypical compared with other metals because it is fragile, readily fractures, is not malleable, and can be easily powdered. Interestingly, antimony loses mechanical strength as its purity increases. It seems that impurities seem to increase its strength. Antimony is an easily meltable nonferrous metal, melting at 630.5° C. Antimony is relatively volatile metal. However, there is considerable disagreement in the literature over vapor pressure data.