Chemical Properties of Selenium

In its general chemical behaviour, selenium occupies an intermediate position with respect to sulphur and tellurium. It combines directly with many elements, e.g. oxygen, hydrogen, fluorine, chlorine, bromine and most metals. Details of some of its compounds will be given later, whilst descriptions of others will be found under the heading of the companion element in other volumes of this series. In the molten condition selenium is partially or completely miscible with many metals, e.g. antimony, lead, copper, bismuth, silver and gold, the fused mass constituting a mixture of the metallic selenide and the element present in excess. Antimony thus yields selenides of compositions Sb₂Se₃ and SbSe, bismuth similarly gives the selenides Bi₂Se₃ and BiSe, copper forms cuprous selenide, Cu₂Se, and silver the selenide Ag₂Se. By completely fusing selenium with sodium in an atmosphere of hydrogen the existence of a series of selenides Na₂Se, Na₂Se₂, Na₂Se₃, Na₂Se₄ and Na₂Se₆, analogous to the sulphides and polysulphides, has been shown. Selenium combines with sodium or potassium dissolved in liquid ammonia, producing the monoselenide Na₂Se or K₂Se, or the tetraselenide Na₂Se₄ or K₂Se₄, according to the relative proportions of the two elements present.

Selenium reduces hot aqueous solutions of silver or gold salts with the formation of silver selenide or metallic gold, respectively. In the case of silver salts the reaction corresponds with the equation:

4AgNO₃ + 3Se + 3H₂O = 2Ag₂Se + H₂SeO₃ + 4HNO₃.

The element is unaffected by water, but in a fine state of division hydrogen peroxide oxidises it to selenic acid. Ozone in the presence of water gives a similar result.

Selenium is soluble in sulphuric acid, forming a green solution which, in the case of the "metallic" form, probably contains a compound of composition SO₃Se, and in the case of the red amorphous variety, a polymeric form of this compound. The presence of selenium does not affect the electrical conductivity of sulphuric acid. Dilute aqueous potassium hydroxide dissolves the red variety, producing a solution which probably contains polyselenides; in the presence of sodium hydrosulphite, however, only sodium selenide, Na₂Se, is obtained.

When selenium is heated with a metallic oxide or carbonate, a mixture of selenide and selenite is commonly obtained. At high temperatures selenium is able to displace sulphur partially from sulphides such as copper sulphide and silver sulphide, the effect probably being due to selenium being less volatile than sulphur. At ordinary temperatures selenium has practically no action on thionyl chloride, but when heated in thionyl chloride vapour selenium tetrachloride is formed according to the equation:

Se + 2SOCl₂ = SeCl₄ + S + SO₂.

Sulphuryl chloride, on the other hand, is attacked by selenium at ordinary temperatures, selenium tetrachloride again being formed:

Se + 2SO₂Cl₂ = 2SO₂ + SeCl₄.

Pyrosulphuryl chloride, S₂O₅Cl₂, converts selenium into a colourless crystalline compound, SeSO₃Cl₄ or SeCl₄.SO₃, generally known as sulphur-selenium oxytetrachloride, which melts at 165° C. and boils at 185° C., and which can also be obtained by the combination of its constituents, selenium tetrachloride and sulphur trioxide.