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Preparation end Properties of Europium
filled (4/14) structures are reached as soon as possible as in the case of Eu (4/7 6s2) and Yb (4/14 6s2), they are also maintained as long as possible
i. e.f Gd (4/7 Ed1 6s2) and Lu (4/14 bd1 6s2). It is, however, of less chemical importance whether the ground state configurations for the rare earths is 4/n 5d1 6s2 or 4fn +1 6s2 because either choice leads to the same configuration i.e., 4/n for the tripositive state of the rare earths. Moreover, since the energy difference between these two configurations is so small, the chemical propert es are rarely affected.
We have just seen that in the case of the rare earths the previously unoccupied, well-shielded 4/ shell is filled with increasing atomic number. The belated filling of the 4/ shell gives rise to the well know lanthanide contraction. The effect of the lanthanide contraction is seen even in Tc and Re. ThiF contraction ip characteristic of both the metals and the cations in different oxidat’ >n states. Similar curves to Fig. 7 with discontinuities at Eu and Yh arc« obtained by plotting atomic radii and ionic radii against atomic number of the rare earths. The ionic radii of the rare earths  in various oxidation states are given in Table 7. The ionic radii of the divalent ions of Sm and Eu compare closely with the Ca2+ and Sr2+.
Table 7. Ionic radii of rare earths
Element M2+ M*+ M4+
Oxidat m States
The normally stable oxidation state of the rare earths is the trioositive one with the 4fn ground state configuration. The divalent ions of all rare earths have been prepared by rfidu* Ing the trivalont ions with y-rays in CaF2 matrix The tetrapositive state of Ge and the higher oxidation states of Pr in PreOn and of Tb in ThiO? were recognized even by the
early workers. Asprey  has presented evidence for Nd4+ and Dy4+ in double fluorides with Cs and Rb. Direct evidence for their presence has been obtained from the absorption spectra. The compounds NaPrFs, Na2PrF« K^PrF«, R^PrF« and Cs2PrFe have been prepared , and the tetravalency of the praseodymium has been confirmed by analysis [183, 184], absorption spectra , magnetic measurements  and by X-ray powder diagram [183,184]. The reader will find the review by Asprey and Cunningham  on the unusual oxidation states of the lanthanides and actinides very valuable.
Urbain and Bourion  first prepared the divalent europium salt EuCl2. In weakly acidic aqueous solution Eu2+ is quite stable in the absence of oxidizing agents. The divalent ions of Sm, Eu and Yb are strong reducing agents and follow the order Sm2+ > Yb2+ > Eu2+. The oxidation potentials (E°) for the reaction
M2+ -► M3+ + © (10)
of Sm2+, Yb2+ and Eu2+ are ~0.9, 0.578 and 0.43 V respectively [187—190]. The standard oxidation potentials (estimated values) for the M° M3+ couple are found to decrease with increasing atomic number along the rare earth series (Fig. 8).
Fig. 8. A plot of estimated oxidation potentials of the rare earths against
Preparation and Properties of Europium
The oxidation of Eu2+ at various HC1 concentrations and at different pressures of hydrogen, oxygen and air has been studied \191]. The rate of oxidation is extremely sensitive to the concentration of oxygen. In the absence of oxygen the oxidation process can be represented as
M2+ + H+ - M3+ + 1/2 H2 (11)
and in acidic (HC1) solution EuCk takes many hours to oxidize, whereas SmCl2 and YbCh react completely in 10 mins. The photochemical oxidation of Eu2+ in aqueous solution has been studied by Douglas and Yost \192].
The kinetics of the reduction of Eu3+ by Yb2+ and oxidation of Eu2+ by V3+, Cr3+, Ti3+ and Ce4+ have been studied by King [.193]. Meier and Garner  have shown the exchange reaction Eu2+—Eu8+ to be rapid and first order with respect to both Eu2+ and Eu3+ as well as first order in chloride ion concentration. The rate is, however, independent of hydrogen ion concentration.
A well defined cathodic wave at —0.9 V and an anodic wave at
— 0.3 V were observed by the oscillographic polarographic technique  for Eu3+ in a nitrate medium. No interference was encountered in the presence of La, Pr, Nd, Gd, Tb and Tm. The normal redox potential for the Eu3+—Eu2+ system at pH 1 was measured  as — 0.428 V agreeing with other data —0.43 V  and —0.429 V . Variation of pH (0—6) and introduction of other rare earth impurities (~ 20%) have no substantial effect on the redox potential of the system. The equilibrium constant of Eu3+—Eu2+ system was found  to be 1.78 X 107 at 25° C.
Eckardt and Holleck  reported a reversible reduction of Eu2+-(EDTA) in the presence of excess of the ligand which had a half wave potential (E1/2) of —1.1 V. The rate of calcium exchange between Ca2+ and Eu2+-(EDTA) ions has been investigated by a polarographic method . A rate constant of 1580 ± 180 1/mole/sec at pH 9.3 was observed . The Ei^’s for Sm, Eu and Y in acetone containing N(C2H5)4C104 have been measured  with the following results.