April 5 , 1057
RADIATION CHEMISTRY WITH CYCLOTRON BEAMSOF VARIABLEENERGY
1565
that the contraction in ionic radius with increasing atomic number will cause the entropies to become more negative along the series. At present, however, it is not possible to evaluate these effects adeAmO+z(aq) H+(aq) = AmOl+(aq) 1/2Hz(g) (14) in combination with the value of - 1.60 v. reported quately. It does seem most probable, however, by Penneman and Asprey13 for this couple, to cal- that the entropy values assigned by Latimer to the culate SAmo,++(aq) - S A m O , + ( a q ) . Straight for- MOz+ type of actinide ions are too positive by some ward manipulation of the data yields - 16 & 4 e. u. 15 to 20 entropy units. The linear structure and for this difference. We then adopt, for the en- high formal charge on the MOz+ type of ion should tropy of AmOz++(aq), -18 e.u., the value given by permit close approach and marked ordering of water of hydration, as compared with the large Connick and McVeyl@for PuOz++, to obtain -2 for the entropy of Am02+(aq). We also adopt for alkali cations, t o which Latimer's entropy values Am+4(aq)the entropy of -77 e.u., given by Con- correspond. Use of the longer-lived isotope Am243in studies nick and McVey for P ~ + ~ ( a and q ) take the differon the disproportionation of AmOz+in 1 M H + may ence in entropy S~m+'(aq)- S ~ m + ~ ( ato q )be -47 e.u., the same value as for S ~ ~ + ~ (a qS )~ , + ~ ( a q )yield equilibrium values that will define the ratio found by Cohen and Hindman.l2 Finally the en- of the potentials of the 3-5 and 3-6 couples more tropy of Am@is assumed t o be 12 e.u., the same as precisely than is possible from the present data. In solutions of Am241of conveniently realizable that of UO. These entropy values in combination with the concentrations the disproportionation in 1 AI acid heat data of Table IV yield the following self- is completely masked by autoreduction. consistent potential scheme : Acknowledgments.-We wish to express our The more reliable values -1.83 v. are piven to the nearest r 0.01"~. and the less ret-2.38 v. I -24v -12v -1.60 V. q) Am+'(aq) -ArdOz+(aq) Am02 + +(aq) liable to 0.1 v. At best, Amo ___ A ~ n + ~ ( a 1.75 V. however, the potentials are uncertain by 0.05 v. and those involving Am+4(aq)by 0.2 v. appreciation to Mr. Herman Robinson for assistIt is recognized that differences in ground-state ance in the design and maintenance of the apparatus multiplicities of analogous actinide ions will lead and to Mrs. Winifred Heppler and Miss Lily Goda to variations in entropy for technical assistance in some parts of the work. .. of 1 t o 2 entropy .. units, and Spectrographic analyses were performed by Mr. (12) D. Cohen and J . C. Hindman, THIS JOURNAL, 7 4 , 4682 (1952). JohnG*Conway and Mr* 'CV. lZfcLaughlin* (13) . . R . A. Penneman and L. B. Asprey. American Chemical So-
HindmanI2 for the corresponding neptunium ion, we shall use our value of AH = 36.9 f 1.0 kcal. for the reaction
+
+
+
~
LIVERMORE, CALIFORNIA
ciety, Chicago, Illinois, 1950.
[CONTRIBUTION FROM THE
DEPARTMENT OF
CHEMISTRY,
BROOKIIAVEN NATIONAL
LABORATORY]
Radiation Chemistry Studies with Cyclotron Beams of Variable Energy : Yields in Aerated Ferrous Sulfate Solution1 BY ROBERTH . SCHULER~ AND AUGUSTINE 0. ALLEN RECEIVED OCTOBER19, 1956 Yields of ferrous ion oxidation and of hydrogen gas evolution in air- or oxygen-saturated solutions of ferrous sulfate in 0.8 N sulfuric acid have been determined for beams of helium ions and deuterons of various energies. From the results, the yields of total net water decomposed and of hydrogen atoms produced are found. The results are compared with those reported in the literature for various types of radiation, and are discussed in terms of the free radical model of water radiolysis,
A key phenomenon in the radiolysis of water and aqueous solutions is the variation of yields with the type of radiation. Ferrous sulfate solution is advantageously used for such studies, since the reaction mechanism is believed to be well under~ t o o d ~and - ~precise measurements of the yield are made with relative ease. A considerable number (1) Research carried out under the auspices of the IJ. S. Atomic Energy Commission. (2) Department of Radiation Research, Mellon Institute, Pittsburgh 13, Pa. (3) T.Rigg, G. Stein and J. Weiss, Proc. Roy. SOC.( L o n d o n ) . A a l l , 375 (1952). (4) F. S. Dainton and H. C. Sutton, T r a n s . Faraday Soc., 49, 1011 (1953). ( 5 ) A 0 Allen, Puoc. Inlcrn. Cow' Pcnrcfirl U s r s Alomir Energy (L'nifed S a l i o n s , .\-. Y . ) , 7 , 513 (195G).
of reports have appeared on yields found on irradiation of ferrous sulfate solutions with a wide variety of types of radiation. The cyclotron is one of the most useful radiation sources for studies of this kind, in that i t provides beams of charged particles of precisely known initial speed which can be varied a t will over a considerable range. Preliminary data on the effect of deuteron and helium ion beams on ferrous sulfate solutions have been published by the present authorse and further work has been performed with proton and deuteron beams by Hart, Ramler and Rocklin.' This paper (6) R. H. Schuler and A. 0. Arlen, THIS JOURNAL, 77, 507 (1955). (7) E. J. Hart, W J Ramler and S . R . Rocklin, Radinlion R f s e a r r h , 4 , 378 (1956).
1566
ROBERT
H. SCHULER
AND
presents more extensive and inore accurate data than our previous publication. Experimental Materials and Analytical Methods.-Chemicals were C.P. grade. Solutions were 1 or 10 m M in ferrous ammonium sulfate and were always 1 m M in NaCl and 0.4 M in HzSOl. Ferric iron was determined directly by measurement of the optical absorption at 305 mp in a Beckman DU spectrophotometer thermostated at 23.7'; the extinction coefficient was taken as 2174. The hydrogen evolved in the air-saturated solutions was determined by a method suggested by Ghormley and Hochanadel.8 The solution was sealed in a completely filled stirred cell, and after irradiation connected t o a vacuum line through a break-seal. T h e sample was degassed at Dry Ice temperature and the gas collected on activated charcoal at liquid nitrogen temperature. The hydrogen was then separated quantitatively from the charcoal along with small amounts of nitrogen and oxygen by raising the trap t o -130' (ethyl bromide-butyl bromide mush temperature). The resulting mixture was analyzed by combination of the oxygen present with the hydrogen on a hot platinum filament followed by combustion of the remaining hydrogen on hot copper oxide. Cyclotron Irradiation.-Most of the present work was done with deuterium or helium ion beams. The cyclotron also accelerates the hydrogen molecule ion; b u t the resulting protons are easily distinguished from deuterons as they have only half the range. Precautions were taken to ensure t h a t the deuteron beams used here were free of proton contamination. Helium ions, which have a slightly different ratio of charge to mass than deuterons, are accelerated under somewhat diff erent operating conditions, so t h a t the helium ion beams are always clean. Exposures were made at t h e end of a n evacuated pipe 10 meters long through which the deflected beam from the cyclotron passed. Only about 0.1% of the total deflected beam of 10 Ma. passed through a 1/4-inch diameter hole and was received by the irradiation cells. Since the beam is spread out by the fringing magnetic field of the cyclotron according t o the energy of the particles, the small fraction of the beam reaching our cell was extremely homogeneous in energy. Focusing magnets on the tube are available to increase the fraction of the beam being received by the cell, but ordinarily there was little occasion to use much focusing because low currents were desired. Solutions were contained in magnetically stirred cells of the Saldick type.9 Both mica and glass windows were used for entrance of the beam into the solution. Solutions could be run either sealed or left open t o the air. I n some runs the solution was saturated with pure oxygen by bubbling while stirring; the cell was then stoppered during irradiation. Mica window thicknesses (in mg./cm.2) were determined from the weight of known areas. The thicknesses of the glass windows, which were blown to the cell, were determined by focusing a microscope successively on the two surfaces; the thickness was then given by the length of travel of the microscope multiplied by the refractive index of the glass. Determination of Current .-The beam current absorbed in the solution was read from a wire sealed into the solution and connected to ground through a Higinbotham-Rankowitz integrator.'O The currents, as read, were corrected for the electric charge displaced from the insulating window into the solution, as discussed in detail elsewhere." The present current integrator gives results reproducible t o better than 0.17'. T h e instrument was recalibrated every day on which runs mere made. An earlier model of the current integrator gave poor results a t lower currents due to zero drift, and some of the low current determinations presented in Fig. 1 were monitored by activation and counting of appropriate weighed metal foils; copper was used for helium ion bombardments and titanium for deuteron bombardments. Determination of Beam Energies .-The energy was determined from the mean range of particles in aluminum ____-(8) J . A . Ghurrnley a n d C . J . Hochanadel, TEXIS J O E R N A L , 7 6 , 3351 ( I Y S l ) , and personal communications. (9) J . Saldick a n d A . 0 . Allen, J. Ciienz. P h y s . , 22, 438 (1954). (10) W. Higinbotham and S. Rankowitz, Rev. Sci. I n s t ? . , 22, 688 (19
