FT-IR study of third phase formation in the U(VI) or Th(IV)/HNO3, TBP/alkane systems

The infrared reflectance spectra of the third phases formed in the Systems UO2(NO3)(2)/HNO3/20%TBP in n-dodecane and Th(NO3)(4)/HNO3/20%TBP in n-octane gave evidence for the presence in solution of a significant amount of weakly bonded molecular nitric acid. From the correlation between the ratio of the areas of the bands at 1672 cm(-1) and 1648 cm(-1), characteristic of weakly intermolecularly hydrogen-bonded nitric acid and nitric acid strongly bonded to TBP, respectively, the molecular HNO3 concentration was determined. The presence of these two bands in the spectra of the third phase samples provides evidence that only part of the HNO3 is directly and strongly bound to the TBP phosphoryl group. The ratio of the weakly intermolecularly hydrogen-bonded HNO3 to that bound directly to P=O group of TBP was much higher for the uranium than for the thorium third phases formed under comparable conditions. The estimated amounts of the weakly intermolecularly hydrogen-bonded HNO3 were about 47% and 30% of the total HNO3 present in the uranium and thorium systems, respectively. In the uranium third phase, the TBP hemisolvate of HNO3 (TBP . 2HNO(3)) was recognized as the predominant species with accompanying very small amount of monosolvate (TBP . HNO3). In the thorium system the hemisolvate of HNO3 was also present, but the monosolvate was found to be the major species. When the thorium concentration in the third phase was increased, a conversion of monosolvate into hemisolvate was observed. Analysis of the infrared spectra for both systems indicated that the nitrate anions form bidentate chelates with the studied metals.