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in the form of the complex, Cu^O^". A mechanism which leads to the rate law of equations ( 1) and ( 2) can be obtained by postulating the oxidation of copper in this complex to the tripositive state. Culx(C204)r + &08-->.CuXXI(C204)2~+ so4 -+ S04-( 10)Cu"i(C204)r ->• CuI(C204)" + 2C02 ( 12)It is postulated that the rate-determining step is reaction (12), and therefore the rate is first-order with respect to the copper catalyst and zero-order with respect to oxalate and peroxydisulfate. This accounts for the second term on the right-hand side of equation ( 2), but does not account for the first term. Although it would be convenient to ascribe the first term to the residual copper impurity of the solutions, the spectrographic analyses showed that the impurity was too small for this by a factor of five.One striking effect on the copper catalysis in this region of concentration is inhibition of the uncatalyzed reaction. The curves of Fig. 3 are very straight and show none of the autocatalytic character of the curves of Fig. 2-this despite the fact that the time for 50% reaction is about the same for the uncatalyzed reaction as for the experiment with 1 X 10-7 M CuS04. Apparently the S04-radicals are removed by reactions ( 11) and ( 15) before they can initiate the chain-reaction with reaction (5).The increase in rate obtained with a large increase in peroxydisulfate concentration can be explained as due to the simultaneous uncatalyzed reaction. This seems especially likely as the curves resemble those for the uncatalyzed reaction.The slight decrease in rate observed with an increase in oxalate concentration may be due to a competition between the reactions of the catalyzed and uncatalyzed mechanisms. A more exact explanation of this phenomenon is not readily apparent.Acknowledgments.-I am deeply indebted to Professor Don M. Yost for his aid, encouragement, and for many helpful conversations. I also wish to express my thanks to Professor William H. Cone and Professor Herbert A. Young for their interest and suggestions, and to Mr. John Voth for the spectrographic analyses.Davis, Calif.

the two methods, while in poor agreement, are of the same order of magnitude. The value of Kv> calculated from pH titration may be in error because of formation of colloidal ferric hydroxide, since at points slightly beyond those used for calculation it was apparent that precipitation was taking place.The polarographic results depend upon the value of Kb, which may be in error.Bobtelsky and Jordan2 titrated an iron(III) solution with sodium citrate and found an increase in conductivity as the citrate was first added. In this titration the reactions represented by equations 8 and 9 would be expected to occur.The hydrogen ion produced by the second of these reactions would cause the pH to decrease. A check at the beginning of this titration showed that the pH did decrease, thus causing a conductivity increase as reported by Bobtelsky and Jordan.As the pH decreases one would expect relatively more of the material reacting to follow equation 8 and also the competitive reaction Cit-+ H+-> HCit-(15)The hydrogen ion concentration would then decrease, resulting in a conductivity decrease, which would continue even after all of the iron was complexed because of 15. Finally the conductivity would start to increase because of the addition of sodium citrate. The breaks in the conductivity curve would, according to this explanation, bear no direct relationship to the complex ion composition.Definite points of difference exist with Lingane5 and Meites6 with regard to the polarographic measurements. Lingane has claimed that the half-wave potential may be represented by an equation linear in pH from 4 to 12. The slope given by this equation is intermediate between the slopes of the two portions of our plot, and may have resulted from Lingane's larger but less detailed range of study. The differences with Meites arc harder to explain since we have found only a single wave at all pH values. Meites suggested that his differences from Lingane might be the result of the fact that Lingane had used 0.005% gelatin. We did not use gelatin, but have results corresponding more closely with those of Lingane. Acknowledgment.--We would like to express our thanks to Dr. S. R. Dickman who originally suggested the study of the citrate complexes of iron(II) and assisted by giving summer support to one of us (C.M.S.) from U.S. Public Health funds under his direction, as the investigation first started.

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