Chemistry of Cerium and Other Lanthanides

A predominant feature of the atomic structure of the lanthanide group is the sequential addition of 14 electrons to the 4f subshell (Table 1). The / electrons do not participate in bond formation and in ordinary aqueous solutions all of the lanthanides exhibit a principal (III) state. The common (III) state confers a similarity in chemical properties to all lanthanide elements. Some of the lanthanides can also exist in the (II) state (Nd, Sm, Eu, Tm, Yh) or in the (IV) state (Ce, Pr, Nd, Tb, Dy). Except for Ce(IV), Eu(II), and Yb(II), these unusual lanthanide oxidation states can only be prepared under drastic redox pressure and temperature conditions, and they are not stable in aqueous solutions. Cerium (IV) is a strong oxidizing agent 

Atomic number Element Radius of (III) ion (A0) Electronic configuration1 Oxidation states  

Most lanthanide compounds are sparingly soluble. Among those that are analytically important are the hydroxides, oxides, fluorides, oxalates, phosphates, complex cyanides, 8-hydroxyquinolates, and cup-ferrates. The solubility of the lanthanide hydroxides, their solubility products, and the pH at which they precipitate, are given in Table 2. As the atomic number increases (and ionic radius decreases), the lanthanide hydroxides become progressively less soluble and precipitate from more acidic solutions. The most common water-soluble salts are the lanthanide chlorides, nitrates, acetates, and sulfates. The solubilities of some of the chlorides and sulfates are also given in Table 2. 

Lanthanides form soluble complexes with many inorganic and organic substances however, the nature of the bonding in these complexes has not been completely determined. There is evidence for either ionic or covalent bond formation or a combination of both. Lanthanides are complexed by inorganic ions, but not as readily as are the transition elements. The inorganic complexes are not as important 

Citric acid and nitriloacetic acid (NTA) lanthanide complexes were used in the earliest ion exchange separations of lanthanides from fission product mixtures (Kf = 3.2 for Ce(H3 Cit.)3 and Kf = 10.8 for CeNTA2) (Sillen and Martell, 1964). More recently such polyamino-polycarboxylic acids as ethylenediaminetetraacetic acid (EDTA), 1,2-diaminocyclohexaneacetic acid (DCTA), and diethylenetriaminepentaacetic acid (DTPA) have been prepared. Their lanthanide complexes are very stable (Table 3) and have been widely used in analysis and separation of lanthanide mixtures. They have also been used experimentally to remove internally-deposited 144Ce and other radioactive lanthanide nuclides from animals and man (Foreman and Finnegan, 1957 Catsch, 1962 Balabukha et al., 1966 Palmer et al., 1968 among others). 

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