Graduate Thesis Or Dissertation
 

The study of fluo-tantalum species by solvent extraction

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  • The extraction of tantalum fluoride complexes by large cationic extractants (tetraphenylarsonium ion, TPA, and crystal violet) and by methyl isobutyl ketone (MIBK) has been investigated as a function of fluoride concentration using ¹⁸² Ta as tracer. Extractions by the cationic extractants into chloroform were carried out in the absence of a constant ionic medium because of the high extractability of perchlorate; aqueous phases contained only HF. Dependence of the distribution ratio Φ of Ta upon extractant concentration is approximately linear; at HF concentrations above 10⁻3 M the distribution is independent of metal concentration (10⁻⁷ to 10⁻⁶ M) indicating absence of polynuclear Ta species in this range. Both TPA and crystal violet exhibit a distribution maximum at 0.4 M HF (approximately 1 x 10⁻² M [F⁻]) which is probably due to the extraction of TaF₆⁻ through ion pair formation; the extraction drops sharply at higher fluoride concentrations. At HF concentrations below 10⁻³ M evidence was found for hydrolytic polymerization with one of the products extracting at about 10⁻⁴ M HF. Distribution of ¹⁸²Ta between pre-equilibrated MIBK and MIBK-saturated aqueous 1.0 M perchlorate (constant [H⁺] = 0.40, 0.70 and 1.0 M) and 2.0 M perchlorate (constant [H⁺] = 2.0 M) media maintained by HC1O₄ and NaC1O₄ was studied as a function of free fluoride concentration calculated from known HF and HC1O₄ concentrations. Increasing distribution ratios with increasing (H⁺) within the 1.0 M perchlorate system, as well as comparison of the function Φ [(F⁻)] with that expected from published tantalum fluoride formation constants, indicate the primary extracting species to be TaF₆⁻ paired with the hydrated and solvated hydronium ion. A shift in the distribution maximum to lower fluoride concentrations with increasing (H+ ) is probably due to changes in activity coefficients. This shift continues, accompanied by a change in slope of the distribution curve and a decrease in the maximum extraction, as the background electrolyte concentration is increased from 1.0 M to 2.0 M HC1O₄; the reasons for this behavior are not clear but may involve changes in hydration and solvation numbers of the extracting species as well as activity effects. Mutual solubilities of MIBK and the various aqueous media have been investigated. MIBK solubility in perchlorate solutions increases with HC1O₄ concentration. Extraction of NaC1O₄ into MIBK is practically negligible but the distribution ratio of HC1O₄ is significant, increases with acid concentration and is accompanied by the extraction of water. Evidence was found for the dissociation of HC1O₄ in MIBK. Hydrofluoric acid extracts as molecular HF and appears to be monomeric at concentrations up to about 1 M in the MIBK phase. Significant and changing aqueous phase MIBK solubilities and MIBK phase HC1O₄ and HF concentrations make it questionable whether reliable tantalum fluoride formation constants can be obtained from distribution data in this system.
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