Thermodynamic Study on the Synergistic Extraction of Cobalt(II) with Acetylacetone and Trioctylphosphine Oxide

Abstract

Thermodynamic quantities (enthalpy and entropy changes) are very important for explaining the extraction behavior of the metal ion. However, information concerning thermodynamic quantities for chelate extraction is not sufficient to discuss the extraction behavior. Hellwege and Schweitzer examined the temperature effect on the extraction of cadmium(II) with 8-quinolinol, and reported that the enthalpy and entropy changes for all elementary steps in the extraction system.1 By using thermodynamic quantities, Kandil et al. indicated that the synergistic effect of tributylphosphate (TBP) on the extraction of lanthanides with 2-thenoyltrifluoroacetone (TTA) is caused by an expansion of the coordination number of the lanthanides ion.2 In previous work, the authors studied the synergistic extraction of cobalt(II) with TTA and trioctylphosphine oxide (TOPO) thermodynamically, and the mechanism for the adduct-formation was clarified to proceed according to the expansion of the coordination number of cobalt(II). 3In the present work, to obtain thermodynamic quantities for synergistic extraction system, the extraction of cobalt(II)-acetylacetone(HR)-TOPO was investigated thermodynamically, and the mechanism of synergistic extraction was discussed qualitatively. Experimental ReagentsAcetylacetone and cobalt(II) nitrate hexahydrate were obtained from Wako Pure Chemicals, and TOPO was obtained from Dojindo Ltd. The above reagents were used without further purification. Benzene and all other chemicals used were of guaranteed grade. The water used to prepare the reagent was obtained from a Milli-Q water purification system (Millipore Ltd.). ApparatusA Hitachi (Model Z-6100) polarized Zeeman atomicabsorption spectrophotometer was used for the determination of cobalt(II). A Hitachi (Model U-2000A) spectrophotometer was used for measuring the absorbance. A Fisher Scientific (Model AC-15) pH analyzer and an Iwaki (Model SHK-101B) shaker equipped with an Iwaki (Model CTR-330) thermoregulator were used for pH measurements and shaking the glass vessels, respectively. Procedure Synergistic extraction of cobalt(II)Seven cubic centimeters of an aqueous solution containing 2.0×10 -5 mol dm -3 cobalt(II) and 0.1 mol dm -3 sodium perchlorate were taken in a glass vessel. An equal volume of a benzene solution containing 0.1 mol dm -3 acetylacetone and 2.0×10 -5 -0.1 mol dm -3 TOPO was added, and the mixture was shaken for 3 h at various temperatures. After the phases were allowed to separate, the concentration of cobalt(II) remaining in the aqueous phase was determined with an atomicabsorption spectrophotometer, and the pH was also measured. Distribution and dissociation of acetylacetoneSeven cubic centimeters of an aqueous solution containing 0.1 mol dm -3 sodium perchlorate was taken in a glass vessel. An equal volume of a benzene solution containing 8.0×10 -5 mol dm -3 acetylacetone was added, and the mixture was shaken for 1 h at various temperatures. After the phases were allowed to separate, the pH in the aqueous phase was measured. Th...

Extraction Equilibrium of Zinc(II) and Cadmium(II) by Mixtures of Primary Amine N1923 and 2-Ethylhexyl Phosphonic Acid Di-2-Ethylhexyl Ester

“…A variety of reagents have been used as extractants in this field, among which organophosphorus reagents have attracted much attention. Many authors have studied the extraction and separation of divalent metal ions by mixtures of neutral organophosphorus reagents and other extractants such as chelating reagents, carboxylic acids, etc. − For instance, Tayeh et al investigated the extraction of zinc by mixtures of tri- n -octylphosphine oxide and chelating extractant and conducted a spectroscopic study. Preston 4 studied the selective solvent extraction of cadmium by mixtures of carboxylic acids and trialkylphosphine sulfides and determined the synergistic effects in detail.…”