Synergistic effects in ternary substrate1 + substrate2 + extractant systems are illustrated by the extration of uranyl ion from mixed acetate and chloride solns. According to the cone packing model, ligand packing around the uranyl equatorial plane should be neither overcrowded nor undercrowded. The sum of the ligand fan angle (FA) around the uranyl equatorial plane provides a useful criterion for estimating steric crowding. While FA of UO2(OAc)2(TBP)2, UO2(OAc)2TBP, UO2Cl2(TBP)2 and UO2Cl2(TBP)3 are 204, 164, 164, and 203? resp., none favors formation of stable extracted complexes. The FA of UO2(OAc)Cl(TBP)2 is 184?indicating the most favorable packing. The synergistic effect based on the formation of the extracted complex UO2(OAc)Cl(TBP)2 in mixed acetate and chloride solution was predicted. The prediction was confirmed by extration of uranyl ion from solns. containing NaOAc-HOAc and NH4Cl. At constant total concentration of solution, synergistic extration occurs at 1:3 acetate to chloride ratio which shifts a little on changing the concentration of the uranyl ion from 3.97 ?10-4 to 2.06 ?10-2 M. The extracted complex is mainly UO2(OAc)Cl(TBP)2 according to the slope method and an anal. of the uranyl and chloride concentration in the organic phase. Variation of the synergistic effect with pH was studied. A similar effect was found for other extractants and diluents.

Key words: CONCENTRATION, URANYL COMPOUNDS, PHOSPHORIC ACID TRIBUTYL ESTER, EXTRACTION, CHLORINE ION, SYNERGISTIC EFFECT, EQUILIBRIUM CONSTANT, STERIC EFFECT, ACETIC ACID P, SYNERGIC SOLVENT EXTRACTION