Synergistic extraction of uranyl ion with acylpyrazolones and dicyclohexano-18-crown-6

Abstract

Synergistic extraction of uranyl ion with acylpyrazolones such as 1-phenyl-3-methyl-4-trifluoroacetylpyrazolone-5 (HPMTFP, pKa=2.7), 1-phenyl-3-methyl-4-acetylpyrazolone (HPMAP, pKa=3.8) or 1-phenyl-3-methyl-4-benzoylpyrazolone-5 (HPMBP, pKa=4.2) in combination with dicyclohexano-18-crown-6 (DC-18-C6) has been studied at various fixed temperatures. The results indicate that the equilibrium constants of the organic phase addition reaction, log Ks, at 30°C are almost constant, viz., 2.72, 2.69 and 2.84, respectively, for the above three systems. The similarity and low log Ks values with DC-18-C6 as compared with TBP systems with these pyrazolones appears to arise due to the limitation to the approach of the large crown ether molecule in bonding with the uranyl chelate. This is in contrast to the fact that the relative basicities of the two donors (equilibrium constant for nitric acid uptake) are comparable. Thermodynamic data for chelate extraction with HPMTFP evaluated by the temperature coefficient method indicates that a hydrated chelate is extracted into the organic phase. Also, the organic phase addition reaction with DC-18-C6 is stabilized by exothermic enthalpy change, the entropy change counteracting in all the three cases.