Curium complex oxides

Its stability then decreases progressively until we reach curium where aqueous solutions containing the tetra-positive state must be complexed by ligands such as fluoride or phosphotungstate. Even then, they oxidize water and revert to cur-ium(lll). The expected drop in I4 between curium and berkelium provides Bk" (aq) with a stability similar to that of Ce (aq), but the decrease in stability is then renewed, and beyond californium, the +4 oxidation state has not yet been prepared [2, 10, 15]. 

Especially interesting in a discussion of radionuclide speciation is the behaviour of the transuranium elements neptunium, plutonium, americium and curium. These form part of the actinide series of elements which resemble the lanthanides in that electrons are progressively added to the 5f instead of the 4f orbital electron shell. The effective shielding of these 5f electrons is less than for the 4f electrons of the lanthanides and the differences in energy between adjacent shells is also smaller, with the result that the actinide elements tend to display more complex chemical properties than the lanthanides, especially in relation to their oxidation-reduction behaviour (Bagnall, 1972). The effect is especially noticeable in the case of uranium, neptunium and plutonium, the last of which has the unique feature that four oxidation states Pum, Pu, Puv and Pu are... 

Since the uranyl ion is so obviously its own category, it is very interesting to compare with the analogous species formed by transuranium elements. M = Np, Pu and Am form all three MO 72 and MO which are, by no means, the most stable oxidation states of their elements, and which tend toward reduction by the radiochemical products concomitant with the high specific radioactivity of the isotopes normally studied of plutonium and americium (whereas e.g. 244Pu with the half-life 82 million years would not present this problem). Contrary to some reports in literature, it does not seem that curium (and the subsequent elements) form such dioxo complexes. 

Plutonium Purification. The same purification approach is used for plutonium separated from sediments or seawater. In case reduction may have occurred, the plutonium is oxidized to the quadrivalent state with either hydrogen peroxide or sodium nitrite and adsorbed on an anion exchange resin from 8M nitric acid as the nitrate complex. Americium, curium, transcurium elements, and lanthanides pass through this column unadsorbed and are collected for subsequent radiochemical purification. Thorium is also adsorbed on this column and is eluted with 12M hydrochloric acid. Plutonium is then eluted from the column with 12M hydrochloric acid containing ammonium iodide to reduce plutonium to the non-adsorbed tervalent state. For seawater samples, adequate cleanup from natural-series isotopes is obtained with this single column step so the plutonium fraction is electroplated on a stainless steel plate and stored for a-spectrometry measurement. Further purification, especially from thorium, is usually needed for sediment samples. Two additional column cycles of this type using fresh resin are usually required to reduce the thorium content of the separated plutonium fraction to insignificant levels. 

The only complexes known (Table 90) are those of -ketoenolates with (n-C8Hi7)3PO and (Bu"0)3PO, prepared by solvent extraction from aqueous solutions of the actinides(III) with a mixture of the /S-ketoenol and the P-oxide. There is also solution chemistry evidence for the formation " of Am[CF3COCHCO(2-C4H3S)]3-2(Bu"0)3PO. Am[CF3COCHCOCF3)3-2(Bu"0)3P0 is volatile at 175 and a gas chromatography study of this complex and the curium analogue has been reported. ... 

M (IV) The most stable oxidation state of thorium is -i- 4. Pa, and Np are stable but are oxidized by Oj. Pu + is stable in acid solutions with low plutonium concentration. Americium, curium and californium can be oxidized to the -H 4 state with strong oxidizing agents like persulfate, pulse radiolysis or other strong oxidation and complexation techniques. Bk is relatively stable as a result of the half-filled shell, 5f. ... 

There is no evidence for an oxidation state in solution less than ii. A fluoride complex of tetravalent curium was obtained in aqueous solution when separately prepared CmF4 was dissolved in concentrated (15 m) MF solution (M = alkali-metal ion) [21,159]. Even under these conditions, and using the self-... 

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