On the absorption spectrum of sulphur trioxide and the heat of dissociation of oxygen

Abstract

Much work has been done by different workers on the heat of dissociation of oxygen and in the present work I have tried to determine this value from the continuous absorption spectrum of SO 3 -vapour. Experiment .-The absorption spectrum of the sulphur trioxide vapour was obtained with a hydrogen discharge tube as the source of continuous light. The photograph was taken on a Leiss quartz spectrograph. The SO 3 -vapour was prepared by distilling pure fuming sulphuric acid, the gas thus obtained being collected in a glass absorption tube, fitted with quartz ends. The ab­sorption was found to be continuous, beginning from the long wave-length ca . λ 3300, with no trace of bands which could be assigned to SO 3 , just as in the case of saturated halides. Different lengths of the tube as well as different pressures were tried with no appreciable change in the position of the long wave-length limit. As the SO 3 -vapour is normally partly dissociated into SO 2 and O 2 , generally some bands of the SO 3 gas appear. These can be easily eliminated by comparison with the absorption spectrum of SO 2 . But it is possible to eliminate the SO 2 bands from the plate by putting an excess of oxygen in the absorption chamber, and then filling it up with SO 3 -vapour. According to the law of mass action the partial pressure of SO 2 is very con­siderably reduced by the addition of O 2 , hence the bands due to SO 2 are expected to become weakened: this was found to be the case.