An Interpretation of the Photochemical Behavior of a Dye-Reducing Agent-Oxygen System on the Basis of a Switch-over of the Primary Processes

An example is presented in which the complicated photochemical behavior of a system involving dye, a reducing agent, and oxygen is interpreted on the basis of the rate constants of the elementary reactions, which predict the change in mechanism from a D–D (or D–O) to a D–R mechanism. The dye chosen is eosine, and the reducing agent is EDTA. Investigations have been made of the quantity of oxygen consumed by the aerated aqueous dye solution, of the rate of oxidative photobleaching, and of the photochemical yield of hydrogen peroxide; the influence of the EDTA concentration on these quantities has been studied in particular. This influence was especially remarkable on the third phenomenon; the yield of hydrogen peroxide shows a sharp maximum at a certain EDTA concentration, ∼1×10−2 m for 1.0×10−3 m and 1.0×10−4 m of dye, and ∼6×10−2 m for 1.0×10−5 m of dye. To give a quantitative account of the above phenomena, the rate constants for the following elementary reactions have been determined tentatively by the flash technique: (Remark: Graphics omitted.) Although further evidence for the existence of D+···O2− and D+···D− would be desirable, the above data as well as those already reported by Lindqvist and by our group, are well correlated with the present results.