Photochemical after-effect in the bromine-oxalate reaction

Abstract

A marked photochemical after-effect has been observed in the oxalic acid-bromine reaction in the presence of excess of potassium bromide. The after-effect persists for several hours after cutting off the illumination, and increases with increasing periods of pre-illumination up to a certain stage. If the illumination is continued for longer periods the after-effect shows a diminution. The study of the retarding action of hydrobromic acid which is a product of the reaction may explain this diminution. When the bromine that has been photochemically removed in the light reaction is restored in the end solution in the dark immediately after the completion of the photo-reaction, no 'secondary after-effect' is observed. Under suitable, conditions the photochemical reaction between potassium oxalate and bromine is followed by a marked photochemical after-effect which persists for several hours. No 'secondary after-effect' could be detected in this reaction. The 'decay' of the photochemical after-effect is very rapid just after the illumination is withdrawn and is relatively slower afterwards. It appears that the active intermediate product formed in the light reaction reacts with bromine giving the photochemical after-effect and is at the same time destroyed in other ways. A tentative mechanism of these after-effects has been suggested involving the formation of the univalent oxalato ion C₂O₄'.