Hydrogen Selenide, H₂Se

Hydrogen Selenide, H₂Se was discovered in 1817 by J. J. Berzelius as the gaseous product of the interaction of hydrochloric acid and iron selenide or potassium selenide.

1. Hydrogen and selenium combine on heating:
   
   H₂ + Se ⇔ H₂Se.
   
   The action begins appreciably near 250° C., and with rise of temperature there is an increasing percentage of hydrogen selenide in the equilibrium mixture until a maximum (exceeding 50 per cent.) is reached near 570° C., above which temperature the percentage decreases. On account of this behaviour, selenium heated in one part of a tube containing hydrogen gives a crystalline deposit in the cooler parts of the tube, the apparent sublimation being due to the formation of hydrogen selenide at the higher temperature and its partial decomposition by cooling away from the heated space. If the cooling is effected rapidly it is possible to check the readjustment of the equilibrium. The gas can be purified by cooling in liquid air, when the free hydrogen remains uncondensed.
2. Aqueous solutions of selenious acid yield small quantities of hydrogen selenide when submitted to the action of nascent hydrogen.
3. Phosphorus pentaselenide, obtained by heating together an intimate mixture of selenium and red phosphorus, when warmed with water gives a steady stream of hydrogen selenide.
4. From metallic selenides. - Aluminium selenide is easily decomposed, even by water, with formation of hydrogen selenide:
   
   Al₂Se₃ + 6H₂O = 2Al(OH)₃ + 3H₂Se.
   
   Many metallic selenides yield the gas when treated with dilute mineral acids. For example, when aluminium or magnesium selenide is dropped slowly into the acid a continuous stream of hydrogen selenide is generated, which can be dried by means of phosphorus pentoxide. If required in a high degree of purity, the dried gas can be liquefied, e.g. by means of a mixture of ether and solid carbon dioxide, and obtained pure by re-vaporisation.
5. Fairly pure hydrogen selenide can be obtained by heating selenium with several times its weight of resin or paraffin wax, the temperature in the latter case being 335° to 340° C. The organic impurities are easily removed, being almost entirely eliminated at 60° C. A good yield of the gas is obtained.

Hydrogen selenide is a colourless gas the odour of which at first suggests hydrogen sulphide, but afterwards is unpleasantly pungent, causing headache and affecting the mucous membrane of the nose in such a way as to induce a form of catarrh.

The liquefied substance solidifies at -64° C. and boils at -42° C. under 760 mm. pressure, the critical temperature being +138° C. One litre of the gas at N.T.P. weighs 3.6715 grams. The solubility in water at 4° C. is 3.77 volumes per unit volume of solvent, whilst at 22.5° C. water dissolves 2.70 times its own volume of the gas. The gas is also soluble in molten selenium, being freed on solidification. The combination of hydrogen gas and amorphous selenium is accompanied by an absorption of 16.0 Cals. per gram-molecule of hydrogen selenide produced; with the use of the more stable monoclinic and metallic forms, the values are 17.0 and 17.4 Cals. respectively.

Hydrogen selenide is not decomposed by daylight but it is sensitive to ultra-violet light. In the presence of moisture, oxygen rapidly decomposes the gas, but if the oxygen and the gas are both dry there is no action. It burns with a blue flame, producing water and selenium or selenium dioxide, according to the supply of oxygen.

When passed over heated iron boride, the gas interacts, forming iron selenide and selenium boride. With water no stable compound is produced, although the existence of an unstable crystalline hydrate has been observed. Sulphur dioxide and selenium dioxide oxidise a solution of hydrogen selenide, the product in the latter case being red selenium:

SeO₂ + H₂Se = 3Se + 2H₂O.

With ammonia, hydrogen selenide reacts at the ordinary temperature with the formation of a white crystalline mass of ammonium selenide, (NH₄)₂Se; the same compound is produced by passing hydrogen selenide into a concentrated aqueous solution of ammonia at 0° C., the air in the apparatus having been displaced by nitrogen.

Hydrogen selenide exhibits stronger acidic properties than hydrogen sulphide, and many of the heavy metals may be precipitated as selenides from aqueous solutions of their salts under conditions generally analogous to those required for the precipitation of sulphides; manganous selenide may be precipitated from slightly acid solution. The selenides so obtained, however, are usually impure, owing to deposition of the metal or of selenium or the formation of complexes and double salts, and the pure selenides are best prepared by the methods outlined below.

In the wet way selenides are best produced by allowing the solution of the metallic salt to drop slowly into a saturated aqueous solution of hydrogen selenide which is mechanically stirred. In this way the selenide is formed in the presence of excess of hydrogen selenide, no excess of metal ions being at anytime present in the solution. The alkali selenides may be obtained by the action of hydrogen selenide on the corresponding carbonates in aqueous solution in an atmosphere of nitrogen, followed by crystallisation from solution.

The selenides may also be obtained by direct combination of the elements, either by passing selenium vapour over the heated metal in a vacuum or by heating the metal with selenium in a crucible, the reaction being started by means of a magnesium fuse. In the case of potassium the reaction is explosive. The compounds may also be obtained by the action of hydrogen selenide on the heated metal, on the heated metallic chloride or its vapour, preferably in the presence of nitrogen, or by reduction with hydrogen or carbon of an oxysalt such as a selenite. Selenides have also been prepared by electrolytic methods.

The selenides of the alkali metals are white when pure, but generally appear pink owing to the presence of free selenium or of polyselenides, as also do the alkali hydroselenides; moreover they readily oxidise in air, with separation of selenium. The selenides of the alkaline earth metals are described as pink, but it is questionable whether they have yet been obtained in a pure condition.

Aluminium and magnesium selenides are very similar light brown powders, unstable in air. Zinc and iron (ferrous) selenides are more stable in air, the zinc compound being citron-yellow and the iron compound black and metallic in appearance. The latter becomes brown in air owing to oxidation. Ferric selenide is difficult to obtain pure. Cadmium selenide, which is dark brown, is very stable in colour and is used as a pigment. With thallium, selenium is said to form three distinct compounds, but analyses of these compounds have led to discordant results. The selenides of aluminium, chromium and uranium cannot be prepared in the wet way. Nickel selenide, unlike the sulphide, shows no tendency to form a colloidal solution.

As regards solubility in water, the general behaviour of the selenides is analogous to that of the sulphides. Aqueous solutions of the alkali and alkaline earth selenides gradually undergo oxidation by atmospheric oxygen and selenium separates, generally as the amorphous red variety, but sometimes as the crystalline modification.

In general, the metal selenides correspond in composition with the sulphides, are much less stable, oxidise quickly in air, and when prepared in the wet way cannot be dried without some decomposition. They are generally deeper in colour than the corresponding sulphides, a fact which is probably associated with the higher atomic weight of selenium.

Organic derivatives of hydrogen selenide of the types R.SeH and R₂Se are known. These are generally prepared by distilling an alkali hydrogen selenide or alkali selenide with the corresponding alkylsul- phate; thus for ethyl selenomercaptan:

KHSe + C₂H₅KSO₄ = C₂H₅SeH + K₂SO₄;

and for dimethyl selenide:

K₂Se + 2CH₃KSO₄ = (CH₃)₂Se + 2K₂SO₄.

With ketones in the presence of concentrated hydrochloric acid, hydrogen selenide yields heavy red oils, which decompose on distillation.

Poly selenides

Poly selenides of the alkali metals, of the type M₂Se_x, where x may be as many as 5, are known, but although organic derivatives such as benzyl diselenide (C₇H₇)₂Se₂ are stable, no hydrogen diselenide analogous to the sulphur and oxygen compounds has been obtained.

A thioselenide of composition Na₂SeS₂.5H₂O has been prepared.