Samarium, Europium, and Ytterbium Electrode Potentials in LiCl-KCl Eutectic Melt

Samarium, europium, and ytterbium displace the alkali metals from their molten chlorides, forming the divalent species Sm (II), Eu (II), and Yb (II) in solution. Progressive coulometric reduction of solutions of SmCl 3 , EuCl 3 and YbCl 3 in LiCl ‐ KCl at 450 ° C , enabled the redox potentials to be measured vs . 1 m Pt ( II ) ‐ Pt ( O ) : Sm ( III ) − Sm ( II ) − 1.729 ± 0.006 V ; Eu ( III ) − Eu ( II ) − 0.554 ± 0.007 V ; Yb ( III ) − Yb ( II ) − 1.375 ± 0.003 V . Potentials of ytterbium systems starting from both YbCl 3 and YbCl 2 were followed over a ∼200°C interval and from ∂ E ° ' / ∂ T ( Yb ) − ∂ E deg ; ' / ∂ T ( Ag ) = 0.66 × 10 − 3 V deg − 1 , Δ S° and △H° for the reaction YbCl 3 → YbCl 2 + 1 / 2 Cl 2 in LiCl ‐ KCl at 450 ° C were found to be 22 cal/deg −1 mole −1 and 54.5 kcal/mole −1 , respectively, close to the values for pure YbCl 3 decomposition. No evidence of disproportionation of Sm(II), Eu(II), and Yb(II) was found but scrupulous precautions were needed to keep the solvent free from impurities which readily react with the highly reducing ions, especially Sm(II). Considerable experimental details are included.