Lanthanide ions redox potentials

The local structure around the lanthanide ions with differing redox potentials Eu(III)/(II) (-0.35 V vs. NHE), Yb(III)/(II) (-1.05 V), and Sm(III)/(II) (-1.55 V) in Ti02 particles was investigated by EXAFS. The photocatalytic reaction and EXAFS studies were also carried out for a calcined Yb(III) ion adsorbed-Ti02 catalyst [157], The photocatalytic activity of these lanthanides toward methyl blue photodecomposition was very similar, suggesting that adsorbed lanthanide ions on TiOz particles scarcely assist the high-photocatalytic activity of TiOz catalyst. However, the way in which the catalysts were activated was of high importance. The photocatalytic activity of the calcined Yb/TiOz... 

Table 2 Redox potentials of lanthanide ion couples M3 + /M2+ in aqueous solution...

The standard electrode potentials for all the rare earths have similar values and are comparable with the redox potentials of alkaline earth metals [144], Thus the lanthanides are strong reducing agents, and form trivalent ions easily. Both europium and samarium can exist in both trivalent and divalent states and the divalent states are not stable in aqueous solutions. Cerium can exist in both tetravalent and trivalent states in solution but Ce(III) is the most stable. 

The known oxidation states of the actinides are given in Table 20-3. With the exception of Th and Pa, the common oxidation state, and for trans-americium elements the dominant oxidation state, is +3, and the behavior is similar to the +3 lanthanides. Thorium and the other elements in the +4 state show resemblances to HfIv or Ceiv, whereas Pa and the elements in the +5 state show some resemblances to Tav. Exceptions to the latter statement are the dioxo ions MOJ for U, Np, Pu, and Am that are related to the M02+ ions in the +6 state. Redox potentials are given in Table 20-4. 

In the various solvent-extraction circuits employed in this process, use is made of a solution of D2EHPA in kerosene as the extractant. The selective recovery of the various metals is achieved by careful control of the equiUbrium pH value of the aqueous phases in the multistage extraction and stripping operations. After the leach liquor has passed through two separate circuits, each of which comprises five stages of extraction and four of stripping, the europium product is obtained initially as a solution of europium(III) chloride. Further purification of the product is accomplished by reduction with amalgamated zinc to Eu +, which is by far the most stable of the divalent lanthanide ions with respect to the reduction of water cf. the redox potentials of the Eu /Eu and Sm /Sm + couples, which are —0.43 and —1.55 V respectively ). Sulfuric acid is added to the... 

The multiplicity of oxidation states of the light actinides can be utilized to accomplish very efficient separation of these elements from the lanthanides. Except for actinium (only trivalent), the actinide ions to plutonium either exist predominantly in higher oxidation states [Th(IV), Pa(IV, V)] or can be interconverted with relative ease among any of four oxidation states (III, IV, V, VI). The upper two oxidation states exist in aqueous solutions as the dioxocations AnOj or AnO - The relative strength of complexes formed by the actinide cations in these oxidation states is An(IV) > An(VI) > An(III) > An(V), which order also applies to the separation reactions involving these cations. The dominant oxidation states for the light actinides are Ac(III), Th(IV),Pa(IV or V),U(IV or VI), Np(IV or V). For plutonium, the redox potentials indicate nearly equal stability for all four oxidation states in acidic solutions. The tri-, tetra-, and hexavalent oxidation states are most important in separations. 

The light actinide elements differ from their 4f analogues, the lanthanides, in their ability to exist in common solutions in oxidation states III through VI. The potential appearance of multiple oxidation states is of particular importance for the transport characteristics of redox-sensitive actinides, such as Np and Pu. Figure 2-2 shows the reported redox potentials for U, Np, Pu, and Am ions at acidic, neutral, and basic pH. For U, Np, and Am the redox potentials between the oxidation states differ sufficiently enough so that one or two states are usually favored over all others. The redox potentials of Pu in the oxidation states III, FV, V, and VI are all remarkably similar around approximately 1.0 V. Therefore, under certain conditions Pu can coexist in up to four oxidation states in the same solution. Table 2-1 summarizes the oxidation states of the actinides and highlights the cnvironmentully most relevant ones. Some of the oxidation states listed,. such as Pat III) or Put VII), can be synthesized only under extreme conditions, far from those round in natural. systems. In the III and IV oxidation. states, the actinides loini hydiated An and An ... 

Cheng and co-workers recently reported Fc-cyclopeptides 12-15 (Scheme 5.4) and showed that they acted as redox-switchable cation receptors [26]. These Fc-cyclopeptides exhibited strong anodic shifts of their electrode potentials in the presence of alkaline earth metals and lanthanides. The extent of the anodic shift can be correlated with the charge density of the metal ion with a bias toward binding of lanthanides, alkaline earth metals, and the least sensitivity to alkaline metals [26]. Chowdhury et al, reported the syntheis and interaction of cychc Fc-Histidine conjagates 16 with metal ions (Scheme 5.5). Electrochemical measurements showed that the compound exhibited cathodic shifts in the order Na Li K+>Cs which in the order of their ionic sizes and suggest that the observed shift relates to the cavity of the compound [27]. 

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