Acetylacetone as Analytical Extraction Reagent I ncrease in Selectivity with (E thyle nedinitrilo)tetra acetic Acid and Analytical Separation of Uranium from Bismuth ANOOP KRISHEN and HENRY FREISER University of Pittsburgh, Pittsburgh 7 3, Pa. b The ability of acetylacetone to act both as chelating agent and solvent has established it as an analytical extraction agent. In order to improve the utility and versatility of this reagent, introduction of competing complexing agents into the system was found to be of great advantage. Extraction behavior of uranyl, copper(II), lead(ll), and bismuth(ll1) acetylacetonates was studied. The selectivity of separations was enhanced when (ethylenedinitri1o)tetraaceticacid was used as the competing complexing agent. Extraction with acetylacetone in the presence of EDTA, followed by polarographic estimation, was successfully utilized to develop a method for separation of uranium in the presence of as much as a 5000-fold excess of bismuth.
S
STUDIES on extraction behavior of metal acetylacetonates conducted by Steinbach and Freiser (10, 11) and Rydberg (6, 7’) have established the position of this reagent in the dual role of a solvent and a chelating agent. At the same time, the development of suitable theoretical equations by Kolthoff and Sandell (3) and Irving and Williams ( I ) , as well as determination of formation and distribution constants, have provided a better understanding of the processes involved. The possibilities of analytical separations by the use of acetylacetone can be enhanced if another complexing agent is introduced into the system. By acting as a masking or sequestering agent, the second reagent lowers the effective concentration of certain metal ions, while others may not be affected to the same extent. The decreased concentration of one of the metal ions can result in separation of the others by extraction. The choice of (ethylenedinitril0)tetraacetic acid (ethylenediaminetetraacetic acid) as the competing complexing agent was dictated by its great versatility in forming negatively charged chelates with a large number of metal ions and its effectiveness in acting as a masking agent in various systems (6, 8, YSTEYATIC
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12). The addition of EDTA causes a
marked increase in the selectivity of extraction, which results in the possibility of a larger number of separations than would be possible without the masking agent. This paper considers the extraction behavior of uranyl, lead(II), copper(II), and bismuth(II1) acetylacetonates, both with and without the presence of EDTA, as a function of pH. REAGENTS
Acetylacetone. Commercial acetylacetone was purified by the method described by Steinbach and Freiser (11).
Standard Metal Ion Solutions. URANIUM.A weighed quantity of uranium oxide, U,08, was dissolved in nitric acid, fumed with sulfuric acid, and made up to volume to give a stock solution containing 5 mg. of uranium per ml. COPPER. The stock solution of copper sulfate was standardized by electrodeposition and diluted for use. BISMUTH. A weighed amount of bismuth trichloride (Baker Analyzed) was dissolved in dilute hydrochloric acid to give a stock solution containing 25 mg. of bismuth per ml. Preparation of Acetylacetonates. LEAD. An aqueous solution of lead nitrate and acetylacetone was made ammoniacal; the resulting nThite precipitate of lead acetylacetonate was air-dried and recrystallized from hot benzene. Analysis showed 51.20% lead; theoretical value was 51.11%. URANYL.The yellow precipitate formed when an aqueous solution of uranyl nitrate and acetylacetone was made ammoniacal was recrystallized from hot benzene to give uranyl acetylacetonate. The precipitate contained 50.83% uranium: theoretical value was 50.84%. COPPER. Technical copper acetylacetonate was recrystallized from 95% alcohol. The copper found was24.25%; theoretical value was 24.29%. PROCEDURE
Extraction. In equilibrium studies, the organic phase was prepared by dissolving weighed amounts of re-
crystallized uranyl, lead(II), and copper(I1) aoetylacetonates in watersaturated acetylacetone to give about 10-3M solutions in each case. The aqueous phase, both with or without ethylene(dinitri1o)tetraacetic acid, was prepared by adjusting the pH with sulfuric acid and sodium hydroxide and saturating with acetylacetone. I n the case of bismuth, the metal ion was originally in the aqueous phase and the volume \vas made up to 25 ml. finally. Ten milliliters of each of the tJvo phases was pipetted into a dry 50ml. volumetric flask and shaken for 2 hours on a mechanical shaker in a water bath thermostated a t 25’ C. When extraction was complete, the pH of the aqueous phase was measured with a Beckman Model G pH meter. A suitable aliquot of either of the pha,qes was pipetted out for determination of metal concentration. Analysis. URANIUM,LEAD, AND COPPERI N ACETYLACETOXE PHASE. The organic phase was filtered through paper, made up to a suitable volume, and the absorbance was read against water-saturated acetylacetone in 1em. silica cells, using a Beckman Model D U spectrophotometer fitted with a photomultipler unit. The concentration was read from standard curves prepared by measuring the absorbance of similar solutions containing weighed amounts of recrystallized acetylacetonates. The absorption peaks of uranium, lead, and copper a t 365, 347, and 345 mp, respectively, were found to be suitable for analytical purposes. BISMUTH( 4 ) . One milliliter of the acetylacetone phase was taken up in 5 ml. of 2hTnitric acid, made up to 10 ml., then deaerated for 5 minutes with nitrogen. Bismuth was determined with a Leeds &- Northrup Electrochemograph, Type E. The wave was found a t a half-wave potential of -0.11 volt with respect to the saturated calomel electrode. URAKIUJIIK AQUEOUS PHASE.The method of Souchay and Faucherre (9), using 0.111.1 EDTA and 2M sodium acetate as a supporting electrolyte, was found t o be suitable in the presence of dissolved acetylacetone. The half-wave potential was shifted to -0.47 volt but the wave height was not affected. COPPER, Kolthoff and Lingane’s procedure ( 2 ) was modified by replacing
gelatin with a 0.002% solution of Triton X-100 (Rohm & Haas Co.) as the maximum suppressor, which was found to be satisfactory. The presence of dissolved acetylacetone shifted the halfwave potential to -0.10 volt. DISCUSSION
OF RESULTS
The extraction curves without ethylene(dinitri1o)tetraacetic acid, as obtained from the experimental data, are shown in Figure 1. The pHlit values for copper(II), uranyl, and lead(I1) are 1.12, 1.65, and 5.64, respectively. Within experimental error, the points for the extraction of copper from the acetylacetone to the aqueous phase fall on the same curve as those for the extraction from water to acetylacetone. This shows that equilibrium was attained in this case. In the case of bismuth(II1) it was not possible to study the extraction beyond pH 1 as the oxychloride precipitates out a t higher pH values. The slopes of the curves for uranyl, copper(II), and lead(I1) are essentially the same and in agreement with the slopes for extraction curves for divalent metal species calculated by Irving and Williams (1). The wide separation of pHliz values for copper(I1)-lead(I1) and uranyllead(I1) acetylacetonates in a wateracetylacetone system shows that these metals can be separated from each other by an extraction process similar to the one for copper(I1) and zinc(I1) (11). Separation of uranyl and copper(II), however, is not possible and presence of bismuth would be troublesome in all cases due to precipitation. Utilizing the pH1Iz and distribution coefficient values obtained from Figure 1, it is possible to calculate the formation constants (12) for uranyl acetylacetonate. The logarithm of the formation constant is found to be 13.89. S o value for the formation constant of uranyl acetylacetonate in aqueous solution exists in literature. The extraction behavior of metal acetylacetonates in %e presence of a tenfold excess of EDTA is shown in Figure 2. The plots of percentage extracted into the organic layer us. p H shorn that the extraction of bismuth and copper(I1) acetylacetonates has been greatly lowered, while in the case of uranyl, there has only been a shift in the position of the curve to give a pH1 value of 2.26. The precipitation of bismuth(111),even a t pH values as high as 8, has been completely eliminated and separation of uranium can be effected nithout the precipitation of bismuth(II1). The over-all effect is the increase in specificity of extraction with acetylacetone. This is a result of the differences in the stabilities of the acetylacetonates and the corresponding EDTA complexes. Wherever the EDTA com-
d e x is stabler. it decreases the formation bf the acetylacetonate and the extractability by acetylacetone is lowered. Separation of Uranium from Bismuth. Solutions of uranyl sulfate containing 0.1 and 1.0 mg. of uranium were mixed with different amounts of
bismuth trichloride solution. A solution of the disodium salt of E D T A was added t o give a bismuth t o E D T A ratio of 1 to 30. The DH of the mixture was raised to 7.5-by careful addition of IN sodium hydroxide. Then approximately 10 ml. of acetylacetone was added and the mixture
PH
Figure 1. Extraction of acetylacetonates by acetylacetone
%E 50
1 - - - - 2
3
4
5
BismuthC g
6
7
8
OH
Figure 2. Extraction of acetylacetonates by acetylacetone in presence of (ethylenedinitri1o)tetraacetic acid VOL. 29, NO. 2, FEBRUARY 1957
289
ACKNOWLEDGMENT
Table I.
25 125
500 500 500 500
Separation of Uranium from Bismuth
0.100 0.100 0.100 1.000
1.000 1.000
0,099 0.097 0.097 0.980 0.976 0.987
shaken for 10 minutes. The acetylacetone phase was separated, filtered, and made up to a volume of 10 ml., of which 2 ml. was withdrawn by a pipet into a 10-ml. borosilicate glass volumetric flask. The flask was very gently warmed until the liquid was reduced in volume to about 0.5 ml. Then the supporting electrolyte was added and the resulting solution deaerated for 5 minutes in a 10-ml. Lingane-Laitinen H-type polarographic electrolysis cell. The polarogram was then recorded and the concentration of uranium found from suitable calibration curves. The results are shown in Table I. The separation of uranium from a 5000-fold excess of bismuth did not present much difficulty. The amount of bismuth was limited only by the low
0.101
0.099 0,099 0.995 0.991 1.002
The authors gratefully acknowledge the financial assistance of the U. S. Atomic Energy Commission in this work.
+o. 001 -0.001
-0.001
-0.005 -0,009 $0,002
solubility of disodium ethylene(dinitrilo)tetraacetate, which must be present in a thirtyfold excess. The amount of uranium found is consistently low, as might be expected from the extraction curves (Figure 2 ) . Complete extraction is not reached in the case of uranium, but it is a simple matter to allow a correction factor, as the percentage extraction for uranium acetylacetonate is 98.48% a t pH values higher than 6.5. When this correction is applied (Table I), the results fall within the experimental error of =t1%. Experimental data not presented here indicate that this method for separation of uranium would be equally applicable in the presence of copper, zinc, and lead.
LITERATURE CITED
Irving, H., Williams, J. P., J. Chem, SOC.1949, 1841. Kolthoff, I. M.;, Lingane, J. J., “Polarography, p. 494, Interscience, New York, 1952. Kolthoff, I. hl., Sandell, E. B., J.A m . Chem. SOC.63, 1906 (1941). Lingane, J. J., IND.ENG.CHEnr., ANAL. ED. 15,583 (1943). Pribil, R., Jelinek, M., Chem. Listy 47, 1326 (1953). Rydberg, J., Arkiv Kemi 8, 115 (1955). Ibid.,9, 95 (1955). Sharma, R. N. S., hfalik, A . K., 2. anal. Chem. 148, 179 (1958). Souchay, P., Faucherre, J., -4naL Chim. Acta 3, 252 (1949). Steinbach, J. F., Freiser, H., ASAL, CHEY.25, 881 (1953). Ibid., 26,375 (1954). Taylor, R. P., Ph.D. thesis, Princeton University, 1954. RECEIVEDfor review June 25, 1956. Accepted October 31, 1956. Presented in part, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1956. Contribution from Chemistry Department, University of Pittsburgh, Pittsburgh 13, Pa.
Analytical Solvent Extraction of Molybdenum Using Ace tylace t one JAMES P. McKAVENEY I and HENRY FREISER University of Pittsburgh, Piftsburgh 73, Pa.
FAcetylacetone has been found to be a selective extraction reagent for molybdenum in ferrous materials. Molybdenum(V1) is extracted from a 6N sulfuric acid solution while copper, tungsten, and chromium, which normally interfere in the colorimetric procedure for molybdenum, are not extracted at this acidity. The molybdenum is determined as the thiocyanate after extraction.
A
IS UNIQUE in the field of solvent extraction because it can be used as both a solvent and as a reagent. It functions as a reagent by first forming a metal chelate with the metal ion. The neutral metal CETYL-4CETOXE
1 Present address, Crucible Steel Co. of America, Research Laboratory, 234 Btwood St., Pittsburgh 13, Pa.
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ANALYTICAL CHEMISTRY
chelate which is thus formed, being more of an organic nature than inorganic, is then extracted by the excess acetylacetone in the organic layer. The percentage of metal ion extracted into the organic layer is principally a function of the pH of the aqueous layer and the oxidation state of the metal ion. As early as 1900, Urbain (8) discovered that the acetylacetonate of thorium could be extracted by chloroform from an aqueous solution acidified with sulfuric acid. In 1949 Abrahamczik (1) used a 1 to 4 mixture of acetylacetone and carbon tetrachloride to separate small amounts of iron, aluminum, and manganese from a weakly ammoniacal solution prior to the determination of magnesium with Titan yellow. Steinbach and Freiser (5-7), using acetylacetone as both solvent and reagent,
have made a detailed study on the extraction charatteristics of aluminum(II1) , beryllium(II), copper(II), cobalt(III), chromium(III), iron(III), nickel(II), vanadium(IV), zinc (11), and zirconium(1V). I n the present work the study has been extended to include iron(II), molybdenum(V-I), manganese(I1) , titaniumIIV), and tungsten (VI). In particular, this work deals with the extraction of molybdenuni from ferrous materials. REAGENTS AND APPARATUS
Chloroform, reagent grade. Acetylacetone (2,4 - pentanedione) , Carbide and Carbon Co. reagent is ordinarily of sufficient purity as received. If any appreciable discoloration is present, the solvent is distilled.
