The photosensitised decomposition of nitrogen trichloride.—Part I

Abstract

The present work is introductory to a detailed study of the nature of the induction period of the photochemical reaction between hydrogen and chlorine. Whatever the mechanism finally adopted for this process, it is essential that it should admit of a clear interpretation of the remarkable inhibition exerted by small traces of ammonia or nitrogen trichloride; conversely, if the full details of this inhibition be thoroughly understood there seems a possibility that they will yield valuable evidence as to the nature of the photochemical union of hydrogen and chlorine. As is well known Burgess and Chapman traced the cause of the induction period to the existence of ammoniacal impurities in the water of the actinometer, and by special purification they were able to eliminate the inhibition almost completely; further, artificial induction periods were produced by the addition of small quantities of ammonia solution to the antinometer. The intense nature of the inhibition is shown by an interesting calculation of Chapman, who shows that one part of nitrogen trichloride in 10 6 parts of hydrogen and chlorine cuts down the rate of formation of hydrogen chloride by 100 times. By a different technique, in which water was eliminated form the reaction system. Norrish discovered that the length of the induction period is directly proportional to the weight of ammonia added to the reaction mixture of hydrogen and chlorine, and the sharpness of the onset of the photochemical reaction at the end of the period of induction is well characterised by the Draper effect, as shown in one of his curves which is reproduced in fig. 1.