Selenium Dioxide, SeO₂

Selenium Dioxide, SeO₂, may be prepared by the oxidation of selenium, either by burning in a stream of air or oxygen, or better by the use of a stream of nitrogen dioxide and oxygen, the latter process being capable of yielding a very pure product. If oxygen is passed in a rapid stream through fuming nitric acid and then over selenium heated to its melting-point, the selenium burns with a brilliant blue flame and a sublimate of dioxide is obtained. The more rapid the supply of oxygen the purer is the product. The dioxide thus obtained may be purified by sublimation in a current of pure dry oxygen. The oxidation appears to be greatly affected by catalytic influence, acidic substances acting as positive catalysts, while alkaline materials, including glass, act as negative catalysts. The dioxide is also precipitated when ozone is passed through a solution of selenium in selenium oxychloride.

The oxidation can also be effected in the wet way by the use of nitric acid. The crude dioxide prepared in this way can be purified by sublimation.

Sulphuryl fluoride in the absence of air reacts with selenium vapour in a glass vessel according to the equation:

2SO₂F₂ + SiO₂ + Se = 2SO₂ + SiF₄ + SeO₂.

Selenium dioxide forms lustrous, colourless, needle-shaped crystals, which readily sublime on warming. It has a density of 3.954 at 15° C. It can be fused by heating in a sealed tube to about 340° C., the liquid being orange-yellow in colour, whilst the vapour is yellowish-green; the change is reversible and the colour is not attributable to dissociation into selenium and oxygen. The crystals are hygroscopic and are soluble in water, alcohol, and sulphuric, selenic and acetic acids; one part of selenium dioxide dissolves in 2.67, 2.6 and 2.54 parts of water at 11.3°, 14° and 15.6° C., respectively. The solutions of the oxide in warm concentrated sulphuric and selenic acids are distinctly yellow, but become lighter in colour when cooled.

Selenium dioxide is an exothermic compound, the heat of formation being 57.2 Calories per gram-molecule, referred to vitreous selenium, whilst the value is somewhat less for the monoclinic element and less still for "metallic" selenium.

The absorption spectrum of selenium dioxide vapour contains groups of lines, particularly in the blue and violet.

The dioxide is easily reducible to the element; when aqueous solutions are exposed to light, reddish-brown amorphous selenium slowly separates. The freezing-point, solubility and vapour pressure curves for the aqueous solutions indicate the presence of one hydrate only, namely the monohydrate, SeO₂.H₂O.

Heated in a current of hydrogen or in a sealed tube with sulphur (in the absence of air), the element is liberated, sulphur dioxide also being produced in the latter case. In the absence of moisture, sulphur dioxide does not reduce selenium dioxide, but hydroxylamine, hydrazine and phenylhydrazine reduce the solution in absolute alcohol.

When heated with sodium chloride the dioxide gives a distillate of oxychloride:

2SeO₂ + 2NaCl = Na₂SeO₃ + SeOCl₂.

Hydrogen chloride is absorbed with formation of unstable additive compounds, SeO₂.2HCl and SeO₂.4HCl, both of which dissociate readily. Additive compounds are also formed with hydrofluoric, hydrobromic and hydrocyanic acids.

Sulphuryl chloride is without action on the dioxide, but thionyl chloride attacks it on warming, according to the equation:

2SOCl₂ + SeO₂ = SeCl₄ + 2SO₂.

Combination with sulphur trioxide readily occurs on warming, giving an easily fusible crystalline compound, SeO₂.SO₃, which does not fume in air; the same product is obtainable by dissolving the dioxide in fuming sulphuric acid. With ordinary hot sulphuric acid selenium is converted into the sulphoxide, SeSO₃, hydrogen selenide and some amorphous selenium.

Phosphorus pentachloride acts on selenium dioxide with the formation of the tetrachloride, whilst phosphorus trichloride yields brown amorphous selenium; in these reactions phosphorus oxychloride is also formed.

Carefully dried ammonia reacts with the dioxide in solution in absolute alcohol, the product having been variously described as ammonium selenosamate, NH₂.SeO₂.NH₄, and as an ammonium ethyl-selenite, C₂H₅.O.SeO₂.NH₄. Possibly the nature of the product is dependent on the exact conditions prevailing during the reaction. Phosphine reacts in a different manner, the main products being free selenium and diethyl selenide.

With a solution of selenium dioxide in isoamyl alcohol, ammonia throws down a white compound, which has been described as ammonium amylselenite, NH₄.C₅H₁₁.SeO₃. The reaction may be represented thus:

C₅H₁₁OH + SeO₂ = ,
= O + NH₃ =

This ammonium amylselenite decomposes slowly on standing in air, liberating red selenium. The action of light hastens the rate of the decomposition.3 Hydrogen sulphide produces a dark orange-coloured precipitate from the solution in isoamyl alcohol.

The oxidation potential of the system SeO₂-Se has been investigated.