2H3AsO4 + 5H2S = As2S5 + 8H2O.
In this respect arsenic resembles the metallic elements, many of which produce sulphides under similar conditions. The sulphides of arsenic, both those produced artificially and those found in nature, are used as yellow pigments.
ANTIMONY
Occurrence. Antimony occurs in nature chiefly as the sulphide (Sb2S3), called stibnite, though it is also found as oxide and as a constituent of many complex minerals.
Preparation. Antimony is prepared from the sulphide in a very simple manner. The sulphide is melted with scrap iron in a furnace, when the iron combines with the sulphur to form a slag, or liquid layer of melted iron sulphide, while the heavier liquid, antimony, settles to the bottom and is drawn off from time to time. The reaction involved is represented by the equation
Sb2S3 + 3Fe = 2Sb + 3FeS.
Physical properties. Antimony is a bluish-white, metallic-looking substance whose density is 6.7. It is highly crystalline, hard, and very brittle. It has a rather low melting point (432°) and expands very noticeably on solidifying.
Chemical properties. In chemical properties antimony resembles arsenic in many particulars. It forms the oxides Sb2O3 and Sb2O5, and in addition Sb2O4. It combines with the halogen elements with great energy, burning brilliantly in chlorine to form antimony trichloride (SbCl3). When heated on charcoal with the blowpipe it is oxidized and forms a coating of antimony oxide on the charcoal which has a characteristic bluish-white color.
Stibine (SbH3). The gas stibine (SbH3) is formed under conditions which are very similar to those which produce arsine, and it closely resembles the latter compound, though it is still less stable. It is very poisonous.
Acids of antimony. The oxides Sb_{2}O_{3} and Sb_{2}O_{5} are weak acid anhydrides and are capable of forming two series of acids corresponding in formulas to the acids of phosphorus and arsenic. They are much weaker, however, and are of little practical importance.