Redox Chemistry of Third Phases Formed in the Cerium/Nitric Acid/Malonamide‐n‐Dodecane Solvent Extraction System

Abstract

AbstractUpon solvent extraction of the colorless solution 0.1 M Ce(NO3)3⋅6H2O in 3 M HNO3 with DMDBTDMA (N,N′‐dimethyl‐N,N′‐dibutyltetradecyl malonamide) at a concentration of 0.5 M in n‐dodecane (also a colorless solution), a yellow third phase is formed. In keeping with previous observations reported in the historical literature of solvent extraction, the yellow color was initially interpreted as an indication of the presence of tetravalent cerium, which is, coincidentally, yellow in aqueous HNO3. The valence quandary arising from the visual assessment led us to extract a freshly prepared solution of 0.1 M cerium(IV) nitrate, which was obtained by exhaustive bulk electrolysis of 0.1 M Ce(NO3)3⋅6H2O in 3 M HNO3. A red third phase was obtained. Under ambient light, the red third phase reverts to the yellow color of the third phase formed upon solvent extraction of CeIII. These observations, in combination with results obtained by UV/Visible and X‐ray absorption spectroscopies as well as electrochemical studies of the yellow and red third phases, demonstrate that the colors are directly correlated with changes in Ce valence, where CeIII obtains in the yellow third phase and CeIV in the red one, and the formation of coordination complexes with DMDBTDMA. By use of three‐phase electrode voltammetry and controlled potential electrolysis techniques, the one‐electron CeIII/CeIV redox chemistry in the third phases was examined in a manner not realized beforehand. These results, demonstrating reversible electrochromism, stand at the intersection of the fields of electroanalytical chemistry, on the one hand, and separations science, on the other, thus providing original insights into third phase phenomena and ion transfer across the aqueous–organic liquid interface in the extraction system.