The Photolysis of Persulfate

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Abstract
The photochemical efficiency, φ, for the decomposition of persulfate by light of λ254 mμ is determined by comparison with φ for the uranyl oxalate actinometer. The persulfate content was followed by the method of Kurtenacker and Kubina and in representative cases the hydrogen ion produced also was determined. The equation S2O8=+H2O+hv = 2HSO4−+O2/2 accounts quantitatively for the data. Evidence is given to show that the main reaction is not the decomposition of SO4−. Six tenths of a mole of persulfate is decomposed per Einstein absorbed by persulfate in dilute neutral and alkaline solutions when all oxidizable material is absent. Acetic acid increases this yield to unity but non‐oxidizable ions have a depressing effect. Many of the observations of Morgan and Crist also are verified. At constant ionic strength, φ remains nearly unchanged in alkali, but drops abruptly to less than 0.01 when solutions are acidified suggesting the formation of a weak acid containing photochemically inert persulfate. Absorption spectra and conductivity measurements, however, give no indication that such a weak acid is formed. The explanation offered for these facts is that the hydrogen ion associates with the thereby stabilizes the persulfate ion after the latter absorbs a photon. The stabilizing effects of other ions, except perhaps bisulfate, is much less than hydrogen which is thought to be related, among other things, to their larger size. Hydrogen peroxide was not found in any of the solutions.