Americium aqueous oxidation-reduction

Newton, T. W and F. B. Baker Aqueous Oxidation-Reduction Reactions of Uranium, Neptunium, Plutonium, and Americium. In if. F. Gould (Ed.), Lanthanide/Actinide Chemistry, Advances in Chemistry Series, Vol. 71, p. 268. Washington American Chemical Society 1967. 

Aqueous Oxidation-Reduction Reactions of Uranium, Neptunium, Plutonium, and Americium... 

In contrast to the lanthanide 4f transition series, for which the normal oxidation state is +3 in aqueous solution and in solid compounds, the actinide elements up to, and including, americium exhibit oxidation states from +3 to +7 (Table 1), although the common oxidation state of americium and the following elements is +3, as in the lanthanides, apart from nobelium (Z = 102), for which the +2 state appears to be very stable with respect to oxidation in aqueous solution, presumably because of a high ionization potential for the 5/14 No2+ ion. Discussions of the thermodynamic factors responsible for the stability of the tripositive actinide ions with respect to oxidation or reduction are available.1,2... 

T. W. Newton, The Kinetics of the Oxidation Reduction Reactions of Cranium, Neptunium, Plutonium, and Americium in Aqueous Solution, TID-26506, U.S. Energy, Research, and Development Administration (ERDA) Technical Information Center, Washington, D.C., 1975. 

In contrast to americium, the oxidation of Cm(iii) to Cm(iv) is achieved only with the strongest oxidizing agents and only one report [48] claims evidence for an oxidation state greater than iv. Transient divalent and tetravalent states have been observed in aqueous perchlorate media using pulse radiolysis techniques [49], Attempts have been made to induce Cm(iii)-Cm(iv) oxidation chemically (using ozone [50] and perxenate [51]) or electrochemically [52], These early attempts have failed, an effect clearly not attributable solely to radiolytic reduction. 

Extensive treatments of oxidation-reduction kinetics of the aqueous ions of uranium through americium are found in Chapters 5-8, in the critical monograph of Newton [264], and in a recent review [273]. 

Americium and californium have been prepared by the reduction, using noble metals and hydrogen at temperatures greater than 1110 °C, of the oxides MOj 5. A new determination has been made of the heat of dissolution of Am in aqueous HCl, and the standard enthalpies of a series of Am compounds and ions have been reported (Table 1). The standard electrode potential of the Am -Am" couple was +2.06 + 0.01 V, making the metal only slightly more electropositive than Pu. 

Other. The pyridine- and A-oxopyridine-2-carboxylates, Am02L2, have been obtained from aqueous solutions of americium(VI) and the acid HL the A-oxide 2-carboxylate is formed as the dihydrate.di-(2-ethylhexylphosphoric acid (HDEHP) solutions have been used to selectively extract Am from Cm in such systems rapid reduction of Am to lower oxidation states is a problem. 

In addition to the aqueous raffinates from the solvent extraction cycles of the Purex process, an actinide bearing waste stream will arise from the washing of the TBP/OK solvent prior to its recycle to the first cycle. These wastes will typically contain actinides in a mixed NajCOs/NaNOs solution which also contains HjMBP and HDBP. The uranium present will form soluble U complexes with carbonate, as discussed in Section 65.2.2.l(i). Carbonate complexation of Pu also leads to solubility in alkaline solutions and in Na2C03 media precipitation did not occur below pH 11.4, although precipitates did form on reduction to Pu One Pu" species precipitated from carbonate media has been identified as Pu(0H)3-Pu2(C03)3 H20. In 2M Na2C03 media, Np is oxidized by air to Np above pH 11.7 while Np either precipitates or is reduced above pH 13. The potential of the Am /Am " couple, in common with those of other actinides, becomes more cathodic with increasing carbonate concentration. In the total bicarbonate plus carbonate concentration range 1.2-2.3 M all the americium oxidation states from (III) to (VI)... 

Americium and curium can be obtained from the aqueous waste of the Purex process. This americium is a mixture of Am and " Am. Isotopically pure Am, the decay product of " Pu, can be obtained from aged plutonium. Solvent extraction and ion-exchange procedures are used to recover americium from waste streams. Americium metal is produced by lanthanum reduction of the oxide, followed by vacuum distillation of the americium at 1400°C. 

Americium, unlike europium, does not have a divalent state in aqueous solution this was long greeted with some surprise. However, Am(n) has been prepared in the solid compounds AmQj, AmBr2, and Aml2 and demonstrated as a dilute solution in Cap2 [99-102,105]. Reduction conditions used to prepare Eu and Sm " do not reduce Am (aq) to Am (aq) [229]. Myasoedov et al. claim electrochemical evidence for unstable Am(ii) in acetonitrile, instantly oxidized by water in the solvent [230]. Sullivan et al. [356] formed transient Am(ii) by pulse radiolysis, with an absorption maximum at 313 nm, and tj/2 5 x 10 s for disappearance. 

Other than as a transient aqueous species and species co-precipitated from melts, and possibly in CmO, Cm(ii) is unknown. Pulse radiolysis, producing OH radicals as oxidant and the aquo electron as reductant, produced changes in aqueous americium and curium perchlorate solutions. The new absorbances were attributed to transient formation of Cm(n), 240 nm, and Cm(iv), 260 nm [49]. A recent study has shown that the two polarogiaphic waves earlier found for acetonitrile solutions of hydrated Am " and Cm " perchlorates are not, as claimed, evidence for Am(ii) and Cm(n), but in fact correspond to reduction of hydration water and to the An(in) An(0) reduction [117]. 

All actinides from thorium to californium form tetravalent oxidation states. For the three elements of highest atomic number, however, viz. americium, curium, and berkelium, the hydrated ions are too strongly oxidizing to be stable in aqueous solution [7,10]. Their rates of reduction nevertheless vary widely, in the order Bk + < Am < Cm + < Cf, with Bk" being by far the most resistant species. This is also the order of thermodynamic stability, as indicated by the oxidation potentials of the couples [11]. 

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