The kinetics of the decomposition of potassium ferrocyanide in ultra-violet light and the kinetics of the reverse (thermal) reaction have been investigated. The study was made in presence of air, in nitrogen and in oxygen using a glass electrode for measuring the changes of pH. Oxygen has no influence on the velocity of the process. From the reaction schemes suggested in the literature, only that involving the formation of Fe(CN)S. H203-is in accord with the experiments. A mechanism of photoreaction and reverse reaction is given. The addition of potassium ferricyanide inhibits the reaction as the ferricyanide ion acts as an internal filter. In the light the process has a negative temperature coefficient ((210 = 0.7). The energy of activation of the reverse process, which represents a reaction between Fe(CN)5. H203-and the cyanide ion, is about 24 kcal/mole. Fe(CN)S. H203is decomposed by prolonged exposure to light, but this process is very slow so that the velocity with which the equilibrium is reached is easily measured.
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