Compounds of Sulphur

Although sulphur in the compact form appears to be a comparatively inactive element, yet when in a fine state of division it reacts with many metals, and at elevated temperatures it combines directly with nearly all other elements (exceptions being nitrogen, iodine, beryllium, gold, platinum and iridium) to form stable binary compounds, the reactions often occurring with great vigour.

In its most stable compounds with hydrogen and the metals, sulphur in Compounds of Sulphur is usually bivalent, but it possesses the property of combining with these elements in proportions exceeding the valency requirements, forming series of polysulphides of the type R₂S_x, where x may be as high as 6. The polysulphides of hydrogen are unstable oily liquids of uncertain freezing-points, and on heating undergo decomposition.

Towards the halogens, sulphur is electropositive. Combination with fluorine and chlorine takes place at ordinary temperatures, the product in the former case being the hexafluoride, SF₆, whilst with chlorine, sulphur monochloride, S₂Cl₂, is first formed, then the dichloride, S₂Cl₂, and finally the tetrachloride, SCl₄. A monofluoride and a mono- bromide are also known, but no compound of sulphur and iodine has been isolated.

Sulphur is also electropositive towards oxygen, and in the oxides and their derivatives the element usually exhibits a valency of 4 or 6. The two stable oxides, SO₂ and SO₃, are strongly acidic, giving rise to sulphurous and sulphuric acids, respectively, from which numerous other oxyacids are derived. An intermediate unstable sesquioxide, S₂O₃, is known, but it does not appear to be the anhydride of hydro-sulphurous acid, H₂S₂O₄, as its formula would suggest, since this acid cannot be obtained from it. Products have also been described purporting to contain a higher oxide variously formulated as S₂O₇, SO₄ and S₃O₁₁, from which the peracids are derived, but the identity of such an oxide has not yet been satisfactorily established.

The following is a list of the oxyacids of sulphur; those in brackets are only known in the form of their derivatives, whilst those marked

with an asterisk are known in solution, but have not been isolated:

(Sulphoxylic acid, H₂SO₂).
*Hydrosulphurous acid, H₂S₂O₄.
*Sulphurous acid, H₂SO₃.
Sulphuric acid, H₂SO₄.
Pyrosulphuric acid, H₂S₂O₇.
*Thiosulphuric acid, H₂S₂O₃.
Permonosulphuric acid, H₂SO₅.
Perdisulphuric acid, H₂S₂O₈.
*Dithionic acid, H₂S₂O₆.
*Trithionic acid, H₂S₃O₆.
*Tetrathionic acid, H₂S₄O₆.
*Pentathionic acid, H₂S₅O₆.
(Hexathionic acid, H₂S₆O₆).

Arranged in order of increasing strength, the sulphur acids are as follows:

H₂S, H₂SO₃, H₂S₂O₄, H₂S₂O₃, H₂SO₄, H₂S₂O₆, H₂S₃O₆.

Hydrogen sulphide is the weakest acid, di- and tri-thionic acids are approximately of equal strength, whilst sulphurous acid is the weakest oxyacid in the list.

On replacement of one hydroxyl group in sulphuric acid, SO₂(OH)₂, by such univalent radicals as -NH₂, -NO₂, -F or -Cl, a series of sulphonic acids may be obtained, from which a large number of derivatives, both inorganic and organic, have been prepared.

The most important and the most stable compound of sulphur and hydrogen is the well-known hydrogen sulphide, but, in addition, at least three well-defined compounds, H₂S₂, H₂S₃ and H₂S₅, are known, and there is evidence that a hexasulphide, H₂S₆, also exists.