Stability of europium(II) in aqueous nitrate media

In the lanthanide series, Eu(III) is most easily reduced to its divalent state. Reduction of Eu(III) has been studied extensively in aqueous media that are insensitive to reducing conditions. Recently, it has been reported that reduction of Eu(III) is also feasible in aqueous nitrate solutions and that Eu(II) remained sufficiently stable in these media to conduct separation experiments. However, additional fundamental research on the reduction efficiency of Eu(III) and stability of Eu(II) in these media has not been reported yet. In this paper, cyclic voltammetry, magnetic susceptibility measurements, UV-vis absorption spectroscopy and X-ray absorption near edge structure (XANES) spectroscopy were used to gain more insights into the reduction of Eu(III) in aqueous nitrate media. Within the parameters used in this work, near-quantitative reduction of Eu(III) could be achieved within 120 min in highly concentrated nitrate salt solutions, using both chemical and electrochemical reduction techniques. Moreover, Eu(II) was remarkably stable in these solutions, showing just a small percentage of back-oxidation after 5 h in a sealed measurement cell.