A DIRECT N-15 NMR-STUDY OF CERIUM(III)-NITRATE COMPLEXES IN AQUEOUS SOLVENT MIXTURES

A study of the complex formation which occurs between cerium(III) and nitrate ions in aqueous solvent mixtures has been carried out by a direct, low-temperature, nitrogen-15 (N-15) NMR technique. At temperatures in the range of -95 to -110-degrees-C, ligand exchange is slow enough to permit the observation of separate N-15 NMR signals for bulk nitrate, and this anion in the cerium(III) principal coordination shell. In water-acetone-Freon-12 mixtures, the spectra reveal the nitrato complexes do not form consecutively. Rather, signals are observed for Ce(NO3)2+, Ce(NO3)21+, and only two other higher order complexes, even at very high NO3- to Ce(III) mole ratios. Signal area evaluations were used to identify the possible higher order complexes. At comparable salt concentrations in aqueous-methanol mixtures, only Ce(NO3)2+ and Ce(NO3)21+ are formed, reflecting a decreased tendency for complexation in media of higher dielectric constant.