Have you any evidence for or against this temporary release of a carboxyl group to give a tetradentate EDTA intermediate? D. H. Busch.-Your suggestion is quite good and is of a nature which had not occurred to us. I do not believe that we have any strong evidence for or against this intermediate; however, the data in basic media will test your mechanism very nicely. A single factor troubles me. I fail to see why the dissociation of a carboxyl group would be rapid, and therefore not rate determining, while the dissociation of a bromide ion is slow and rate determining (data in acid). In fact, I would expect acid acceleration rather than base acceleration if the replacement of a carboxyl group by a water molecule was the first step in the process. I suspect that protonation of the coordinated carboxyl groups might labilize them. As you recall, we did observe that a slight acid acceleration occurs in the systems studied at pH ~0; however, the reacting species was also shown to be altered. It might be [Co(H2Y)Br],THE AQUEOUS CHEMISTRY OF INORGANIC FREE RADICALS.1 I.

A kinetic study has been made of the reaction of peroxydisulphate ion and hydrogen peroxide in aqueous solution at 30°C. In three different ranges of concentration of the reactants the rate law approaches three different limiting forms : (i) first order in peroxydisulphate and half order in hydrogen peroxide, (ii) first order in peroxydisulphate and (iii) half order in peroxydisulphate. The proposed mechanism involves a chain reaction, with the SO,-, OH, and H02 radicals carrying and terminating the reaction chain.The present paper is concerned with the kinetics of the reaction of persulphate ion and hydrogen peroxide, with the results yielding detailed information about the reaction mechanism and about various aspects of the chemistry of the H02, SO4 and OH radicals.When hydrogen peroxide, hereafter referred to as peroxide, reacts with persulphate ion under the conditions outlined below, the stoichiometry of the reaction does not deviate appreciably from that given by eqn. (I) : S 2 0 g -+ H202+ O2 +2HSOq.

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Oxidation of -Octyl Mercaptan by Ferricyanide 1233 low values of s found, it would still be very desirable to perform a similar test with a solute of larger molecular weight, so that the full potential accuracy of modern ultracentrifuges could be realized.Two points which may confer advantage on the Rayleigh method of determining s by the Gutfreund-Ogston procedure must be mentioned in addition to those discussed earlier. These are: (i) the cylindrical lens magnification factor is not directly required, and errors in its equivalent, the fringe separation, are relatively unimportant; and (ii) as measurements are based on determining the positions of intensity minima, the results are independent of exposure time.Acknowledgments.-The author is greatly indebted to Professor J. W. Williams for his interest in, and critical comments on, this work, and to Professor L. J. Gosting for many discussions. The work was supported by a grant from the National Institutes of Health, No. A3030 (C6).

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