A NEW VALENCE STATE OF AMERICIUM, AM(VI)¹

Abstract

The foll&ng evidence has been obtained for t h e existence of americium in the formal valence of plus sixr (1) ammonium peroxydisulfate in 0e2M n i t r i c or perchloric acids t o a no$luoride-precipitable e t a t e e ( 2 g f l l .fp a the characteristic pi& of Am(II1) changes t o a s t r o n g yellaw color. ( 2 ) "he absorption spectrum of suoh an oxidized solution was measured w i t h a Beckilram model DU speotrophotomter using q u a r t z micro c e l l s and Om1 -s e n s i t i v i t y scale. attended by a growth of h ( V ) absorption b u t no increase in the Am(II1) absorpticm. Ameriefum(I1I) ie completely oxidized byThe decrease w i t h time of these new absorption peaks isThe rate of decrease of the 991 peak is meh greater i n the absence of s u l f a t e , and the u l t r a B i o l e t absorption i s altered.& .the addition of sodium acetate, a yellow compound of t e t r a h e d r a l s t r u c t u r e was obtsiaed which was isomorphous w i t h Pthe analogous NaM *Acornpounds (Ba i s U or Pu). The r e f r a c t i n index of sodium americyl a c e t a t e has the expected value of 10528 (compamd with 1.501 and 1.518 fer the uranium and plutonium coqounds, respectively). There were no b ( I I 1 ) o r Am(B) absorption l i n e s in t h i s ompound{ The compound WBS dried w i t h acetom, t r e a t e d w i t h excess ferrous amonium s u l f a t e and back-titrated with c e r i c sul@$ate.f The amount of americum was determined by radio-assay. A value of 2.9 was obtained f o r the number of equivalents on reducing Am(@ t o Am(II1).

Recent advances in f-element separations based on a new method for the production of pentavalent americium in acidic solution

“…Oxidation to Am VI continued aer cooling for another 5-10 min, following which reduction to Am V commenced, with Am V continuing to grow in for the duration of the experiment. As expected, [5][6][7]18,21 the Am V yield rapidly decreased with increasing acidity, consistent with reduction by produced H 2 Finally, Fig. 2.…”

“…The rst reported preparation of Am VI was by Asprey et al, in 1951. [1][2][3] They used an excess of solid ammonium peroxydisulfate, (NH 4 ) 2 S 2 O 8 , added to 0.2 M nitric, perchloric or sulfuric acid solutions containing 0.002-0.035 M Am III . At 85 C, the pink Am III color was replaced by a straw-yellow color, and a sharp UV/Vis absorbance peak was reported at 992 nm, with another minor peak at 666 nm.…”

Exploring Soft Donor Character of the N-2-Pyrazinylmethyl Group by Coordinating Trivalent Actinides and Lanthanides Using Aminopolycarboxylates

“…The electrode potentials of americium redox couples, as reported by Schulz and later by Silva et al, show that the electrochemistry of this actinide is very diverse in aqueous environments. , The Latimer diagram shows the stable trivalent cation may be oxidized to Am 4+ (−2.62 V vs SCE) to [Am V O 2 ] + (−1.727 V), and to [Am VI O 2 ] 2+ (−1.683 V) in 1.0 M HClO 4 ; however, the large reduction potentials demonstrate that access to the higher valence states requires powerful oxidizing agents. , Asprey et al used ammonium peroxydisulfate in HNO 3 and HClO 4 and confirmed the presence of hexavalent americium using spectroscopy. , Numerous studies followed, showing hexavalent accessibility with ozone (Ag + -catalyzed), sodium bismuthate, − and copper periodate − oxidants. However, the main limitations of the redox approach are the thermodynamic instability of AmO 2 2+ (because it undergoes autoreduction immediately upon formation), the low charge density ( z 2 / r ) of AmO 2 + , and the immediate reduction of Am 4+ back to Am 3+ via water oxidation. − …”

Americium

“…Early exploratory work by Asprey et al (1950), the discoverers of the reaction between S 2 O 2À 8 and Am(III) that produces Am(VI), established that the reaction proceeded in the concentration range from 10 À8 to 10 À1 M Am(III), implying a low-order dependence of the rate on Am(III) concentration. They further found that acidities greater than a few tenths molar were deleterious, presumably due to the acid-catalyzed decomposition path of S 2 O 2À 8 .…”