NOTES
Dee., 1958
1601
The author wishes to acknowledge discussions with Prof. P. D. Bartlett.
greater absorption of energy, Ar attains but a low maximum yield before its trend is reversed by its impurities. Inhibition by NZl4if not due to initial impurities, may be explained by the formation NOTE ON RADIATION-INDUCED of NH,6 which has i.p. of only 10.23 e.v. and could EXCHANGE O F HYDROGEN readily inhibit by charge transfer. Presumably, that small portion of the reaction BY S. C. LIND chain remaining when inhibition is complete (Fig. Oak Ridge National Laboratory, 1 Oak Ridge, Tennrssee 4)2 is the part propagated by free atoms? ApReceived July 81, 1968 parently the very long chain reaction between H2 I n their recent paper on the influence of ions of and Dz escaped earlier detection on account of the inert gases on the radiation-induced reaction presence of Hg vapor which, like Xe, inhibits the H2 D2 = 2HD, Thompson and Schaeffer2 ion propagation. Almost unattainable purity is dealt with a reactant having ionization potential necessary to avoid some suppression of sensitive (i.p.) midway among the five noble gases. Xenon long-chain reactions. Particularly substancea with and krypton, with i.p. below that of Hz (or Dz), lower ionization potential are to be avoided. suppress by charge exchange a hitherto unknown (4) The authors also have informed me that nitrogen depressed long-chain reaction between H2 and Dz, while He, instead of promoting as its higher i.p. would predict. Ne and Ar, having higher i.p. than Hz, do not do (5) J. C. Jungera, Bull. sac. chim. Belg., 41,393 (1932). so. This the authors interpret, correctly I believe, as indicating that the long-chain reaction is propagated by ions rather than by free atoms or SPECIFIC EFFECTS OF CATIONS ON radical^.^ Without entering here into the question RHODAMINE B EQUILIBRIA1 of which hydrogen ion is the propagator, I wish BY R. W. RAMETTE AND T. R. BLACKBURN to call attention to further conclusions t o be drawn from their results. Contribution from Leighton Hall of Chemistry, Carleton College, Northfield, Minnesota Comparison of their Fig. 22 for the influence of Received July fi6, 1068 additions of He, Ne or Ar with Figs. 3 and 4, which show inhibition by Xe and Kr, seems convincing I n both kinetics and equilibrium research in evidence that Ne, Ar and He, having i.p. above aqueous solutions there is frequent need for the that of H2, make small contributions in the op- variation of hydrogen ion concentration in the posite (positive) direction, also by charge transfer. range from 0.1 to 1.0 M . T o minimize the simulThe authors do not stress this, apparently because taneous variation of activity coefficients it has been the accelerating effect is almost negligible in com- commoii practice to add an inert electrolyte which parison with inhibition. This is so because, in- maintains constant ionic strength throughout a stead of breaking short a very long chain as Xe series of solutions. Although the ionic strength and K r do, only a little ionization is being added continues to be a useful generalization a t low values, by exchange with He+, Ne+ or Ar+, t o the direct there is ample evidence2that inert electrolytes show ionization of H2 by a-irradiation. Nevertheless, increasingly individualistic effects as the ionic the complete series presents an outstanding example strength becomes greater than about 0.05. Thereof ion exchange of H2+ in either direction ac- fore, when studies are made in a series of solutions cording to the relative i.p.'s of varying pH a t constant ionic strength the observed effects are not entirely due to changing He (24.58); Ne (21.58); Ar (15.77); HP(15.43); Kr (14.0); Xe (12.13). acidity. formation ---t + inhibition A common way of varying acidity a t constant Explanation of the subsequent drop following the ionic strength is to use mixtures of strong acid, rise with Ne, Ar and He (Fig. 2)2 requires some usually hydI ochloric or perchloric, with the corconsideration. (The authors have kindly in- responding alkali metal salt. The present work formed me that He also begins a sharp and con- was undertaken to compare the effects upon a tinuing drop at higher concentrations (20-38%) not chosen acid-base equilibrium of various 1:1 inert shown in their Fig. 2.) Impurities from some chlorides a t an ionic strength of unity. The source apparently inhibit the reaction and over- choice of rhodamine B for the equilibrium system come the initial rise. If the impurity were present was based upon several considerations. Rhodain the initial gases, He would appear purest, Ne mine B equilibria have been well characterizeda next and Ar least. The contribution of He is so that unsuspected behavior is minimized, The small due to its low stopping power. That of Ne orange-violet step in the equilibrium system can be reaches a maximum a t 3% Ne, which accounts studied nearly independently of other equilibria almost fully for its observed contribution of lo%, and the acid dissociation constant is about 0.2, if one takes into account that its specific ionization which makes it suitable for study in the acidity is more than threefold that of Hz. Despite its range mentioned above. The spectral characteristics of the dye are such that the equilibrium
+
-
(1) Operated for the United States Atomic Energy Commission by the Union Carbide Corporation. (2) 8. 0.Thompson and 0. A. Schaeffer, J . A m . Chem. Soc., 80.553 (1958). (3) H.Eyring, J. 0. Hirschfelder and H. 8. Taylor, J . Cham. Phys.. 4,479 (1936); P. C. Capron, Bull. 8 0 ~cha'm. . Belo., I S , 222 (1935); W. Mund and M. Van Meersche. ibid.. 67, 88 (1948).
(1) Based upon a thesis submitted by T. R. B. for the degree of B.A. with honors, 1958. (2) See, for example, A. A. Noyes, J . A m . Chem. SOC.,46, I098 (1924); F. A. Long and W. F. McDevit, Chem. Revs.,51, 119 (1952). (3) R. W. Ramette and E. B. Sandell, J . A m . Chem. SOC..1 8 , 4872 (1956).
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