The oxidation of sulphur at low pressures

Abstract

The reaction between sulphur vapour and oxygen has been examined by Norrish and Rideal* for pressures of oxygen from 0·1 to 1 atmosphere at temperatures from 200° to 400° C. Between the temperatures of 235° and 305° C., they found that the reaction was confined to the sulphur surface and the walls of the vessel, while below 200° C. there was no appreciable reaction. Several investigators')* have found that the oxidation of sulphur is accompanied by phosphorescence. Emeleus studied the phosphorescent combustion in order to examine the reaction products, which he found consisted of sulphur dioxide and a small amount of the trioxide. During the course of the investi­gation he found that sulphur dioxide and certain organic vapours inhibit the glow, and he concluded that the oxidation is a chain reaction similar to the phosphorescent oxidation of phosphorus. Semenoff and Rjabinin§ have investigated the reaction between sulphur vapour and oxygen at fairly low pressures of oxygen and have obtained very interesting results. Their reaction vessel, which contained the solid sulphur, was maintained at a temperature between 80° and 120° C. and oxygen was introduced to a pressure of 20 mm. of mercury, or less. Any SO 2 or SO 3 formed was immediately condensed in a side tube which dipped into liquid air. Under these conditions they observed in general no reaction between sulphur vapour and oxygen. If, however, an electric discharge were passed through the vessel a reaction started which continued after the removal of the discharge. This reaction was accompanied by a luminescence, and its rate was practically independent of the pressure of oxygen when this was below 1 mm., until an oxygen pressure of about 0·2 mm. was reached, when the reaction ceased. It was found that the residual oxygen pressure varied with the temperature of the reaction vessel, but was not changed by addition of inert gas or by changing the diameter of the vessel as is the case with the residual oxygen pressure in the oxidation of phosphorus As the investigation* of the oxidation of phosphorus below the lower critical limit had proved interesting, it was decided to carry out a further examination of the sulphur oxidation. Experimental. To find if it were possible to obtain a stable chain oxidation of the sulphur vapour, the apparatus shown in fig. 1 was used. R is the reaction vessel, which is divided by the ground joint J, so that various filaments can be easily placed in the vessel. Borated copper wire passes through a pinch seal in the upper part of joint J, and is soldered to degassed nickel wire, which serves as a support for the various filaments. The vessel is enclosed in an electric oven H 1 , so that the temperature can be maintained at about 120° C. The small tube S contains solid sulphur, and is enclosed in a small electric oven H 3 . M is a sulphuric acid manometer which is read by means of a microscope with an eyepiece scale (1 division of the eyepiece scale corresponds to 0·024 mm. Hg). T 2 is a small “tap-pipette” to allow small quantities of oxygen to be transferred from the oxygen reservoir O into the reaction vessel. A is a reservoir for gases other than oxygen. C is a side tube which is placed in liquid air, and thus serves as a condenser for any SO 2 and SO 3 formed.