May, 1954
403
ALKALICARBONATES OF NP(V), Pu(V) AND h ~ ( \ r )
ALKALI CARBONATES OF Np(V), Pu(V) AND Am(V)l BY J. P. NIGON,R. A. PENNEMAN, E. STARITZKY, T. K. KEENANAND L. B. ASPREY Los Alamos Scientific Laboratory, University of California, Los Alamos, New Mexico Received Decembcr 14, 1065
The existence of the XOz+ion in crystalline, double alkali carbonates containing Np(V), Pu(V) and Am(V) is established. X-Ray work by F. H. Ellinger and W. H. Zachariasen (reported in detail in the following paper) shows that the RbAni(V), NH4Am(V), NHaPu(V) and KPu(V)(low pH) compounds were hexagonal and had the typical composition: XAmOd203. A phase study of the three-component system Rb2C0s-H20-( AmOz)&ORconfirmed the existence of the compound RbAm02C03. Carbonate compounds containing Np( V), Pu( V) and Am(V) were precipitated from aqueous solutions of sodium, potassium, rubidium or ammonium carbonates. Three crystal phases resulted-hexagonal, orthorhombic and monoclinic-depending upon the radius of the alkali cation and the pH at which the compound was precipitated. These compounds are stable only in contact with carbonate solutions, and undergo alteration when washed with water.
The existence of the pentavalent state has been established for uranium, neptunium, plutonium and americium. The formula of the aqueous (V) ion has been the subject of considerable discussion but has not been completely characterized. The aqueous U(V) state is the least stable; however, Kraus and co-workers2s3 measured its hydrolysis and made predictions concerning its pH stability range, The aqueous pentavalent states of the series are unstable with respect to disproportionation into the (VI) oxidation state and a lower valence state, either (IV) or (III).2-6 The (VI) states of the series are reduced reversibly to the corresponding (V) state a t an electrode.2-9 Both Np(V) and Am(V) in aqueous perchloric acid have similar infrared absorption peaks of roughly similar shape but are broader and appear at lower energies than the NpOz++and AmOz++absorption peaks.1° Contrary to the behavior of U(V) and Pu(V), both Np(V) and Am(V) are stable over considerable p H ranges.ll For example, Pu(V) disproportionates in both low or high [H+].12 However, there have been very few solid compounds prepared containing these states. The principal ones of nTp(V) are the hydroxide and the 0xa1ate.l~ Pentavalent americium was discovered by Werner and Perlman, who oxidized Am(II1) in potassium (1) Presented in part by J. P . Nigon at the March, 1953, meeting of the American Chemical Society. This work was sponsored b y the AEC. (2) K . A. Kraus and F. Nelson, J . A m . Chem. SOC.,71, 2517 (1949). (3) K. A. Kraus, F. Nelson and G. L. Johnson, ibid., 71, 2510 (1949). (4) L. H. Gevantman and K. A. Kraus, “The C h e m k t w o f Plutonium (V). Stability and Spectrophotometry,” NNES, I V , 14B, Transuranium Elements, P a r t I, p. 500, McGraw-Hill Book Co., Inc., N e w York, N. Y . ,1949. ( 5 ) L. E. Asprey, S. E . Stephanou and R. A. Penneman, J . A m . Chem. Soc., 73, 5715 (1951). (6) R. Sjoblom and J. C. Hindman, ibid., 73, 1744 (1851). (7) L. E. Magnusson, J. C. Hindman and T. J. LaChapelle, ”Clremistry of Neptunium(V). Formal 0xidat.ion Potentials of Neytuninm Couples,” NNES, IV, 14B, Transuranium Elements, 1059 (Part 11). (8) W. E . Harris and I. R4. Kolthoff, J . A m . Chem. SOC.,67, 1484 (1945).
(9) R. A. Pennenian and L. B. AsJIrey, “The Formal Potential of the Am(V)-Am(V1) Cougle,” AECU-936. (10) L. H. Jones and R. A. Penneman, J . Chem. Phys., 21, 542 (1953). (11) J. C. Hindman, L. l3. Magnusson and T. J. LaChopelle, J . A m . Chem. Soc., 71, 687 (1949). (12) IC. A . Kraus, “Oaidation-Reduction Potentials of Plutonium Couples as a Function of pH.” “Transuranium Elements,” Part I , N N E S I V , 11B, p. 241, IllcGmw-Hill Book Co., New York, N. Y., 1949. (13) G . Gibson, D. M. Gruen and J. J . Katz, J. A m . Chenk. Soc., 74, 2103 (1952).
carbonate solution with sodium hypochlorite to an insoluble compound of unknown composition.14 Since this Am(V) compound was apparently crystalline, it was decided to determine its composition, ie., whether it was a salt of the hydroxide such as KOAm02, or double carbonate, etc. Furthermore, it was of interest to see whether analogous compounds containing Np(V) and Pu(V) could be prepared. Of additional importance would be to show the existence of the ion XOz+ in the solid, for determination of its dimensions and linearity by X-ray investigation.
Experimental Americium(V) Compounds .-The method of Werner and Perlmanl‘ using potassium carbonate as the solvent for Am(OH)3 and sodium hypochlorite as the oxidant was modified so that there was only a single alkali cation present. Thus, a solution of Am(1II) in otassium carbonate was warmed to -80” with potassium Rypochlorite, and a solution of Am(II1) in sodium carbonate was treated similarly with sodium hypochlorite. The potassium salt containing Am( V) crystallizes as orthorhombic bisphenoids which are optically biaxial negative with a small optic angle and have refractive indices CY = 1.593 f 0,002, @ and y = 1.596 =k 0.002. Washing the product with water resulted in the formation of isotroplc alteration rims which had a much higher index of refraction. Eventually whole crystals were altered to this isotropic substance and usually disintegrated. Crystals which were left for some time on the slide in air-dry condition and in ordinary atmosphere gradually altered to a compound which gave the absorption spectrum of trivalent americium and had an index of refraction lower than t,hose of the original crystals. I n this case, the external shape of the crystals was well preserved, the alteration being apparent, however, from the irregular, mottled extinction in polarized light. The sodium salt containing Am(V) crystallizes as sixsided plates which are optically negative with optic angle sensibly zero (later X-ray work indicates the structure is probably monoclinic). The .efractive indices are between 1.58 and 1.60. Closer determination of indices was not feasible because of the fine size and unstable nature of the crystals. As in the case of the potassium salt, washing with water resulted in a rise of the apparent index of refraction. Since the potassium and sodium compounds are crystallographically distinct, well defined species, it was considered highly probable that the Am( V) compounds contain alkali metal ions as an essential constituent; this work alone, however, did not answer the question concerning the carbonate content. I t was later found that the compounds could be washed without alteration using 0.1 ill carbonate. Subsequently, additional americium( V) compounds were made from ammonium carbonate and rubidium carbonate using either peroaydisulfate or ozone15 as the oxidant, resulting in predominantly a hexagonal phase. Since the Am(V) rubidium carbonate compound cannot
(1951).
(14) L. B. Werner and I. Perlinan, ibid.,73, 495 (18) T. K. Keenan and S. E. Stephanou, unpublished work, referred
t o in reference 10.
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J. P. NIGON, R. A. PENNEMAN, E. STARITZKY, T. I
