Arsenic and Sulphur

The occurrence in Nature and the historical importance of the two sulphides, realgar, As₂S₂, and orpiment, As₂S₃, have already been discussed. The existence of a third sulphide, arsenic pentasulphide, As₂S₅, is also well-established, and two others of composition As₃S and As₄S₃ probably exist as definite compounds. Berzelius reported several other products which he assumed to be sulphides. Three of these approximated in composition to the formulae As₁₂S, As₂S₁₀ and As₂S₁₈, but they were undoubtedly mixtures of arsenic sulphides with arsenic or sulphur. Investigations of the system As-S have so far established only the existence of the di- and tri-sulphides. A study of the photochemical reaction between yellow arsenic and sulphur dissolved in carbon disulphide, which results in the formation of insoluble products, suggests the possibility of the existence of other sulphides, probably similar to those of phosphorus. The properties of the precipitates are neither those of known sulphides of arsenic nor of mixtures of the two elements. The composition of the products varies with the proportions of arsenic and sulphur dissolved.

Freezing Point Curves of Binary System As-S

The system As-S has been studied by Borodowski and Jonker. The freezing point curve (fig.) determined by the former is not continuous, because mixtures containing 20 to 60 per cent, of arsenic were so viscid that definite solidifying points could not be obtained. The form of the curve shows that both orpiment and realgar exist in two forms. The yellow or α-form of orpiment is stable at low temperatures and is converted at 170° C. into the red β-form, which melts at 300° C. The stable a-form of realgar is red and changes at 267° C. into the black β-form, which melts at 307° C. and boils at 565° C. (at 760 mm.). β-realgar forms a discontinuous series of mixed crystals with arsenic, and apparently orpiment and sulphur form mixed crystals. A break in the curve at 301° C. may be due to transformation of α- to β-arsenic, or to the formation of the unstable subsulphide As₃S. The existence of tetrarsenic trisulphide, As₄S₃, is not indicated.

Boiling Point Curves of Binary System As-S

The sublimation curve of the system As-S (fig) was obtained from analyses of the vapour in contact with the boiling liquid. A saturated solution of arsenic in realgar boils at 534° C. and a liquid of composition As₂S₃ resembles natural orpiment in boiling at 707° C. The form of the sublimation curve shows that the vapour of the disulphide, As₂S₂, is largely dissociated, whilst the trisulphide, As₂S₃, distils unchanged. This is not in accordance with the work of Szarvasy and Messinger who, from vapour density determinations, concluded that the vapour of the disulphide was considerably associated. The sublimation curves do not show any indication of the formation of a pentasulphide.