Studies in the chemical behaviour of sulphur compounds (disulphur monoxide)

Abstract

Earlier observations made by several workers in the field indicated that a small amount of lower oxide of sulphur could be prepared when sulphur was slowly oxidised by air at normal pressure. The yield of the lower oxide increased when the oxidation was carried out by oxygen at very low pressures. A supply of oxygen at a pressure of 1.5–0.5 cm was found to give nearly forty per cent of lower oxide. An extension of the above analogy stimulated the use of many other oxygen substances such as metallic oxides, nitrates, nitrites, chlorites, bromates, iodates, permanganates and dichromates etc., in the oxidation of sulphur. In addition to this it was also found interesting to study the low?pressure oxidation of many metallic sulphides, instead of sulphur itself. In the study of the oxidation of sulphur by metallic oxides, the following oxides were used: oxides of copper, silver, nickel, cobalt, iron, chromium, arsenic, antimony, lead and manganese. About two grams of A.R. cupric oxide was heated with five times its weight of elemental sulphur in vacuum up to 400°C. The gaseous product evolved in the reaction was condensed in a trap cooled by liquid oxygen. The reaction product condensed in the trap was cherry?red in colour along with a whitish deposit. The cherry?red product condensed in the liquid?oxygen trap could not be distilled without decomposition into sulphur and sulphur dioxide, thereby suggesting it should be a polymerised product. By the analysis of the elemental sulphur left in the flask and also the sulphur dioxide formed primarily and from the decomposition of the polymerised product, it was found possible to calculate the amount of sulphur and oxygen that had taken part in the reaction. By such calculations, the ratio of total sulphur to total oxygen that has taken part in the reaction was found to vary from 1.4–1.6, indicating the presence of a sulphur?rich oxide in the reaction products. However, it was also observed that the reaction product should be a mixture of sulphur dioxide with any one of the lower oxides as earlier claimed to exist or with all of them together, i.e., sulphur dioxide should have mixed with either sulphur monoxide, or with disulphur dioxide or with disulphur monoxide or with all the three lower oxides together. To ascertain the nature of the cherry?red product formed in the above reaction, it was found necessary to separate the cherry?red product from the sulphur dioxide. To achieve this, the gaseous products of the reaction between cupric oxide and elemental sulphur in vacuum were made to pass through traps cooled to different temperatures before the entire product was condensed in the trap cooled by liquid oxygen. Such a fractionation study indicated that it was possible to separate a small amount of the cherry?red product from the mixture when a trap cooled to –80°C was placed before the trap cooled by liquid oxygen. The analysis of the small amount of the cherry?red product condensed at –80°C indicated that the lower oxide could be disulphur monoxide. The cherry?red product condensed at this temperature was found to be practically free from any free sulphur dioxide. However, since the amount condensed at –80°C was very small, further confirmative evidence was needed. Assuming that the cherry?red lower oxide to be disulphur monoxide and that it undergoes decomposition to sulphur and sulphur dioxide according to the equation, it is possible to calculate the percentage of S?O formed in the reaction. The yield varied from 25–36%. It was also noticed that at 300–320°C the yield was maximum. Large excess of sulphur was essential to produce a good yield of the lower oxide. Vacuum was a crucial factor; at normal pressure only sulphur dioxide was formed. The nature of the metallic oxide also influenced formation—only oxides of copper, nickel, cobalt and chromium gave the lower oxide, whereas manganese, lead, silver, arsenic and antimony gave only sulphur dioxide. Metallic nitrates such as those of copper, nickel, cobalt, lead, bismuth, silver, thorium and uranium behaved similarly to oxides, giving good yield of disulphur monoxide along with nitrogen oxides. The reaction proceeded in two stages:

Decomposition of nitrate to oxide + oxygen + nitrogen oxides. Reaction of freshly formed oxide with sulphur.

Freshly formed oxides were much more effective oxygen sources than pre?formed oxides. Oxidation of metallic sulphides (Cu, Ni, Co, Ag, Pb, Mn, Cr, Mo) at 0.5–1.5 cm oxygen pressure produced cherry?red lower oxide only in cases of copper, nickel and cobalt sulphides. The reaction involved decomposition of sulphide ? metal + sulphur, followed by oxidation of sulphur. Yield was small. Further confirmation was obtained by preparing a solution of the oxide in carbon tetrachloride (yellow solution) and studying its reactions: (i) Decomposition by mercury: Gave SO? + elemental sulphur (3 atoms of S per SO? ? S?O). (ii) Reaction with anhydrous HI: All sulphur reduced to H?S and oxygen to water, giving iodine. Ratio I:S = 3 ? confirms S?O. (iii) Alkaline hydrolysis: Gave only sulphide, sulphite and thiosulphate. (iv) Acid hydrolysis: Gave sulphur, sulphite and thiosulphate via formation of thiosulphurous acid. (v) Homogeneous hydrolysis: Enhanced thiosulphate formation. (vi) Reaction with cyanide: Formed thiocyanate + cyanate, but incomplete due to decomposition of S?O. Attempts to form S?O from polythionates failed; only SO? + S were obtained. Small amount of S?O was formed when SOCl? vapours were passed over heated silver sulphide at low pressure, but product was contaminated by thiopyl chloride.