I am realistic enough to recognize that it is essentially impossible to discourage research oriented toward the use of chronopotentiometry for the study of adsorption. I suggest, therefore, that this research could most profitably be directed towards discovering an accurate method for the determination of chronopotentiometric transition times. If a consensus can be reached on this point, and if it proves possible to automate this method, then chronopotentiometry might gain moderate stature as a tool for the study of adsorption.
ACKNOWLEDGMENT The author is grateful for the frequent and cogent criticisms of Fred C. Anson, Joseph H. Christie, and William P. Schaefer. RECEIVED for review August 4, 1966. Accepted February 9, 1967. This work was supported in part by a predoctoral fellowship from the U. S. Public Health Service, Division of General Medical Sciences. Contribution No. 3399 from the Gates and Crellin Laboratories of Chemistry.
Use of Organic Additives to Induce Selective Liquid-Liquid Extraction of Niobium with Thenoyltrifluoroacetone Aart Jurriaansel and Fletcher L. Moore Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn.
A new, highly selective method for the liquid-liquid extraction of niobium(V) is described. The method is based on the ability of n-butanol in the aqueous phase to enhance the formation of an extractable niobium chelate with thenoyltrifluoroacetone. Niobium can be recovered quantitatively from aqueous solutions of concentrated hydrochloric acid or hydrochloric acidsulfuric acid mixtures. Excellent separation of niobium from most metal ions is effected with 0.5M thenoyltrifluoroacetone-xylene solution. FORSEVERAL YEARS chemists have been puzzled by the inextractability of niobium from aqueous acidic solutions with 2-thenoyltrifluoroacetone (TTA). The highly selective extraction of zirconium with TTA (1, 2) from acidic solutions is extensively used for analytical and preparative purposes. Niobium is essentially inextractable under the conditions employed for zirconium. Ordinarily, one would expect the more highly-charged niobium ion with its smaller ionic radius to exhibit even higher extractability than zirconium with TTA. The negligible chelation and extraction of niobium from aqueous solution were postulated to be a reflection of its pronounced hydration characteristics. About 1954 it was observed (3) that 95Nbtracer formed by the decay of 95Zrtracer in 0.5M TTA-xylene solution was extremely difficult to strip from the organic phase with various strong acids. Such behavior suggested that niobium does form a highly stable chelate complex with TTA in the absence of water, Further, several exploratory experiments at that time indicated that the extraction of g5Nbtracer with 0.5M TTA-xylene increased from 3 x in 3.6M hydrochloric acid to 45 %in 3.6M hydrochloric acid-37 (vjv) acetone mixtures. Recently we have studied the extraction behavior of niobium with TTA from mixed aqueous-organic solutions. These studies have led to a new, highly selective liquid-liquid extraction method for niobium. 1 Present address, South African Atomic Energy Board, Private Bag 256, Pretoria, Republic of South Africa.
(1) F. L. Moore, ANAL.CHEM.,28,997 (1956). (2) F. L. Moore, “Metals Analysis with TTA,” Symposium on Solvent Extraction in the Analysis of Metals, ASTM Spec. Publ. No. 238 (1958). (3) F. L. Moore, unpublished data, Oak Ridge National Labora-
tory, Oak Ridge, Tenn, 1954.
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EXPERIMENTAL Apparatus. An internal sample methane proportional counter was used for alpha counting. A NaI(T1) well-type gamma scintillation counter, x2 inches, was used for gamma counting. Reagents. 2-Thenoyltrifluoroacetone (TTA, M.W. = 222) is available from Columbia Organic Chemicals Co., Columbia, S. C. Other chemicals used were analytical reagent grade. Procedure. Pipet 1 ml of g5Nbtracer (1.5 X IO6 gamma c.p.m.) into a 50-ml glass centrifuge tube. Add appropriate amounts of distilled water, hydrochloric acid, and butanol (or other organic solvent under study) to obtain 5 ml of a solution which is 7N hydrochloric acid. Extract the aqueous phase for 5 minutes with 5 ml of 0.5M TTA-xylene solution, using high speed motor stirrers equipped with glass paddles. After the extraction, centrifuge the tubes in a clinical centrifuge for 2 minutes. Count 1-mI aliquots of each phase for S5Nbradioactivity in a well-type gamma scintillation counter.
RESULTS AND DISCUSSION Preliminary experiments verified that the presence of acetone in aqueous hydrochloric acid solutions enhanced the extraction of 95Nb tracer with 0.5M TTA-xylene solutions. However, the coextraction of acetone produced large volume changes, rendering further studies difficult. Use of 0.5M TTA dissolved in the relatively aqueous insoluble ketone, methyl isobutyl ketone, resulted in about 80 extraction of 95Nb from 7N hydrochloric acid solution. Further experiments with methyl isobutyl ketone containing no TTA gave similar recoveries, suggesting that the niobium extracted as an ion association chloro complex and not as a chelate. At this time we observed that various alcohols were considerably more effective than ketones for enhancing the chelation and extraction of niobium with TTA. Moreover, volume changes of the phases were negligible. The extraction of 96Nbtracer with 0.5M TTA-xylene from aqueous solutions of 7N hydrochloric acid containing varying amounts of methanol, ethanol, propanol, and butanol is shown in Figure 1. The extractability of 95Nb tracer increases with increasing concentration of alcohol in the aqueous phase. With methanol or ethanol the extraction behavior was somewhat erratic; however, with propanol or butanol results were quite reproducible above 10 volume %.
x
Table I. Extraction of V6Nb Tracer with 0.5M TTA-Xylene from Hydrochloric Acid and Hydrochloric Acid-Butanol Mixtures as a Function of Hydrochloric Acid Concentration Aqueous phase HCl, N Butanol, %(v/v) 1.1 1.1 2.3 2.3 4.7 4.7 7.0 7.0 9.4 9.4
96Nbtracer extracted,
z
13.7 52.6 11.1 74.9 14.1 98.0 12.5 99.7 2.1 99.6
10
10 10 10 10
Table 11. Extraction of Niobium Carrier from 7N Hydrochloric Acid-10 % (v/v) Butanol with 0.5M TTA-Xylene Solution
10
20 30 ALCOHOL CONCENTRATION, volume 'lo
0
Figure 1. Extraction of 95Nb tracer with 0.5M TTA-xylene from 7N hydrochloric acid solution as a function of alcohol concentration
Equilibrium is attained rapidly in these systems. Thus, from aqueous solutions of 7N hydrochloric acid-10z (v/v) butanol, equal volume portions of 0.5M TTA-xylene extracted 99.5,99.5,and 92.7% g5Nbtracer iny0.5, 2, and 5 minutes, respectively. Ususlly chelation and extraction of metal ions with TTA from such acidic solutions in the absence of organic additives is considerably slower (1, 2). The extraction of g5Nbtracer with 0.5MTTA-xylene from hydrochloric acid and hydrochloric acid-10 (v/v) butanol mixtures as a function of hydrochloric acid concentration is shown in Table I. Butanol markedly enhances the extraction of niobium tracer with TTA-xylene solutions from all acidities tested. For further evaluation an aqueous phase of 7N hydrochloric acid-10 (vjv) butanol was selected. The extraction of milcro amounts of niobium from 7N hydrochloric acid-1097, (v/v> butanol with 0.5MTTA-xylene is shown in Table 11. A (decreasein extraction is observed with increasing concentration of niobium. This effect is marked at niobium concentrations greater than 0.2 mg per ml.; it probably results from the increased tendency of niobium to form inextractable hydrolytic and/or polymeric species at the higher concentrai5ons. Similar effects have been reported for zirconium (1, 4 ) and tin (5) in TTA systems. The extraction of 95N13tracer with 0.5MTTA-xylene from aqueous solutions containing 1097, (v/v) butanol as a function of sulfuric and hydrochloric acid concentrations is shown in Table 111. The striking absence of any inhibitory effect by sulfuric acid on the extraction of niobium is of considerable practical importance.
(4) R. E. Connick and W. H. Reas, J. Am. Chem. SOC.,13, 1171 (1951). (5) J. R. Stokely and F. L. Moore, ANAL.CHEM., 36, 1203 (1964).
Niobium concn., m/ml isobutanol > propanol > ethanol > methanol; this is also the order of increasing dielectric constant. The lower extractability with tertiary butanol is presumably due to steric effects. In the absence of any organic additive, the extraction of the 96Nbtracer was about 10%. The marked enhancement by butanol on the extraction of the niobium-TTA chelate may possibly be explained as follows: The niobium ion is very probably oxygenated and highly hydrated. Butanol decreases the dielectric constant of the aqueous medium, thereby promoting the formation of a TTA chelate. Moreover, butanol displaces any residual coordinated water molecules from the chelate. The resulting VOL. 39,
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Table IV. Effect of Various Organic Additives on the Extraction of 95Nb Tracer from 7N Hydrochloric Acid Solution with 0.5M TTA-Xylene Volume in O6Nbtracer Organic additive aqueous phase extracted, % Isobutanol 2 87.5 Tertiary butanol Ethylene glycol
5 10 10 10 20 30
Propylene glycol
40 10
Glycerol Dimethylformamide Dimethylsulfoxide
99.4 99.8 41.1 4.4 1.9 0.9 0.6 3.8 2.4 2.2 4.7
20 30 5 10 15 20 20
2.5 1.7 9.8 2.0
Table V. Extraction of Selected Nuclides from 7 N Hydrochloric Acid-10 (v/v) Butanol with 0.5M TTA-Xylene Solution Nuclide Tracer extracted, % 184Cs
86Sr 1osRu
162-4Eu 288U
24lAm
6gFe 96Zr-Nb 18Ta
