Americium hydrolysis reactions

Bemkopf, M.F., and J.I. Kim, 1984, Hydrolysis Reactions and Carbonate Complexation of Trivalent Americium in Natural Aquatic Systems, Inst, for Radiochem., Report No. RCM-02884 (Technical University of Munich, Munich). In German. 

The trivalent lanthanide metals include lanthanum(III) through lutetium(III). The lanthanide oxides have a variety of uses from semiconductors, glasses, solid-state lasers and catalysts. There is quite a difference in the extent to which the hydrolysis reactions of the lanthanide metals have been studied. Typically, the light lanthanides have been studied to a greater extent than the heavier lanthanides. Neodymium(III) has received the most attention due to the perception that it can be used as an analogue for the trivalent actinide metals, in particular, americium (III). All of the isotopes of promethium are radioactive. 

Desire, Hussonnois and GuUlaumont (1969) determined stability constants for the species AnOH + for the actinides, plutonium(III), americium(III), curium (III), berkelium(III) and californium (III) using a solvent extraction technique. The stability constants obtained for americium(III) and curium(III) are two orders of magnitude larger than other similar data available in the literature. The stability constants of the lanthanide(III) and actinide(III) ions are very difficult to obtain using solvent extraction due to problems associated with attainment of maximum extraction into the solvent phase before the narrow band of pH between the onset of hydrolysis reactions and the precipitation of solid hydroxide phases. Consequently, the data of Desire, Hussonnois and GuUlaumont (1969) are not retained in this review. 

As trivalent americium has a smaller ionic potential than the ions of plutonium it hydrolyses to a much lesser extent than the various plutonium ions. However, like Pu3+, hydrolytic reactions and complex formation are still an important feature of the aqueous chemistry of Am3+. Starik and Ginzberg (25) have shown that Am(III) exists in its ionic form from pH 1.0 to pH 4.5 but above pH 4.5 hydrolysis commences and at pH 7.0 colloidal species are formed. The hydrolytic behaviour of Cm(III) resembles that of Am(III). 

The same three studies also gave stability constants for americium(V) hydrolysis species, and again, only the data of Runde and Kim (1994) can be accepted. These authors reported stability constants of log Pi = 3.62 0.27 and log P2 = 5.89 0.22 for Am020H(aq) and Am02(0H)2, respectively, at 25 "C and in 5 mol 1 NaCl. Again, the first hydrolysis constant can be expressed in terms of reaction (2.5) (M = Am02, p=l,q=l) with the reaction also being isoelectronic. The stability constant derived is... 

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