Electrochemical and Spectroscopic Studies of Some Less Stable Oxidation States of Selected Lanthanide and Actinide Elements

The technique of simultaneous observation of electrochemical and spectroscopic properties (spectroelectrochemistry) at optically transparent electrodes (OTE's) has been applied to the generation and characterization of some less stable oxidation states of selected lanthanide and actinide elements (Ce, Pr, Sm, Eu, Tb, Yb, U, Np, Am, and Cm) in complexing and noncomplexing aqueous solutions. Optically transparent electrodes used in this study included reticulated vitreous carbon (RVC) and metal screen OTE's and a new OTE made from porous metal foam (PMF).

Cyclic voltammetry at microelectrodes was used in conjunction with spectroelectrochemistry for the study of oxidation-reduction (redox) couples. In some cases additional analytical techniques were applied for identification of electrochemically generated oxidation state species; these included solution absorption, solid-state reflectance, and laser Raman spectroscopies, X-ray powder diffraction and thermogravimetric-mass spectral analyses, radiochemical measurements, and spectrophotometric and potentiometric redox titrimetry.

The formal reduction potential (E°') values of the M(III)/M(II) redox couples in 1 M KCl at pH 6 were found by voltammetry to be -0.34 ± 0.01 V for Eu, -1.18 ± 0.01 V for Yb, and -1.50 ± 0.01 V for Sm. Spectropotentiostatic determination of E°' for the Eu(III)/Eu(II) redox couple yielded a value of -0.391 ± 0.005 V. Spectropotentiostatic measurement of the Ce(IV)/Ce(III) redox couple in concentrated carbonate solution gave E°'equal to 0.051 ± 0.005 V, which is about 1.7 V less positive than the E°' value in noncomplexing solution. This same difference in potential was observed for the E°' values of the Pr(IV)/Pr(III) and Tb(IV)/Tb(III) redox couples in carbonate solution, and thus Pr(IV) and Tb(IV) were stabilized in this medium. The solution absorption spectra and redox properties of these aqueous species are reported. A solid Tb(IV)-containing compound was also prepared at an RVC electrode in carbonate solution.

The U(VI)/U(V)/U(IV) and U(IV)/U(III) redox couples were studied in 1 M KCl at OTE's. ²³⁷Np, ²⁴³Am, and ²⁴⁸Cm were studied in containment gloved box facilities. Spectropotentiostatic measurement of the Np(VI)/Np(V) redox couple in 1 M HCl0₄ gave an E°' value of 1.140 ± 0.005 V. Np(VII) was generated by electrolysis of Np(VI) in 2 M Na₂C0₃ at pH 13, and the solution absorption and laser Raman spectra were recorded, and an E°' value of 0.46 ± 0.01 V for the Np(VII)/Np(VI) couple was found by voltammetry.

Oxidation of Am(III) was studied in concentrated carbonate solution, and a reversible cyclic voltammogram for the Am(IV)/Am(III) couple yielded E°' = 0.92 ± 0.01 V in this medium; this value was used to estimate the standard reduction potential (E°) of the couple as 2.62 ± 0.01 V. The solution absorption spectrum and redox behavior of Am(IV) were compared to those of Am(V) and Am(VI) in this same medium.

Attempts to oxidize Cm(III) in concentrated carbonate solution were not successful which suggests that the predicted E° value for the Cm(IV)/Cm(III) redox couple may be in error.