An Electrochemical and Spectroelectrochemical Study of Ln(II) (Ln = Sm, Eu, Yb) Species in NaCl-2CsCl Melt
Analysis of cyclic voltammograms showed that Ln(III)/Ln(II) reduction/oxidation is an one-electron diffusion controlled reversible process. Diffusion coefficients of LnCl63-species (Ln = Sm, Eu, Yb) were determined in the studied temperature range and are described by the following equations:
lgD = -3.42 – 1417/T (±0.02) for Sm,
lgD = -2.68 – 2057/T (±0.01) for Eu,
lgD = -1.95 – 2397/T (±0.02) for Yb.
Formal standard red-ox potentials of Ln(III)/Ln(II) couples in NaCl-2CsCl based melts were determined in the studied temperature range using cyclic voltammetry:
E*Sm(III)/Sm(II) = -2.899 + 8.01·10-4 T (±0.04) V,
E*Eu(III)/Eu(II) = -1.191 + 3.5·10-5 T (±0.03) V,
E*Yb(III)/Yb(II) = -2.592 + 8.48·10-4 T (±0.02) V.
Gibbs free energy change of the formation of LnCl3 from LnCl2 in NaCl-2CsCl based melts is described by the following equations:
ΔG* = -278.17 – 0.0759 T, kJ/mol for Sm,
ΔG* = -126.83 – 0.0318/T, kJ/mol for Eu,
ΔG* = -250.07 – 0.0818 T, kJ/mol for Yb.
For the reaction
LnCl2 (melt) + ½ Cl2 (gas) → LnCl3 (melt)
the equilibrium constants can be expressed by the following equations:
lgK* = -4.057 + 14616/T (for Sm),
lgK* = -1.663 + 6625/T (for Eu),
lgK* = -4.297 + 13084/T (for Yb).
A series of spectroelectrochemical experiments was performed to study potentiostatic reduction of Ln(III) ions. The concentration changes of LnCl63-species in the melt were followed by recording the absorption spectra. For all three elements studied the absorption related to Ln(III) ions decreased as the electrolysis progressed and the example of the spectra recorded in ytterbium containing melts is presented in Fig. 1. No new obvious peaks that could be attributed to Yb(II) species were formed but the absorption below 600 nm increased. Changes of the absorption of Yb(III) ions showed essentially linear dependence on the amount of electricity passed during the electrolysis, Fig. 2. Analysis of the melt after the electrolysis showed that Yb concentration in the melt remained essentially unchanged and the mean oxidation state of ytterbium decreased to 2.83. Diffuse reflectance spectrum of the quenched melt contained additional peaks between 350-450 nm, Fig. 3.
Additional spectroscopy experiments were performed on reduction of Sm(III), Eu(III) and Yb(III) species in NaCl-2CsCl melts using metallic zirconium. In this case again the concentration of Ln(III) species in the melt and the mean oxidation state of Ln ions decreased with time.