Separation of Zinc(ll), Nickel(ll), and Cobalt(l1) with Dithizone Based on Differences in Rates of Extraction B. E. McClellan’ and P a t s y Sabe12 Department of Chemistry, Murray State Unicersity, Murray, K y . 42071
The extraction constants ( K * ) for zinc(ll), cobalt(ll), and nickel(l1) were determined for the extraction of the metal ion from an aqueous phase into a chloroform solution of dithizone. The extraction constant for nickel (0.60) was significantly different from that reported by Koroleff (1.2 x 10-3). This difference is attributable to the slow formation of the nickel dithizonate. As Zn(ll), Co(ll), and Ni(ll) cannot be separated by extraction with dithizone without the use of masking agents, the optimum conditions for the separation of the ion pairs Ni-Zn, Zn-Co, and Ni-Co, based on the differences in the rates of formation of the metal dithizonates, were determined. More than 99% of Zn could be extracted while only 2.5% of Ni extracted. The optimum conditions determined for the separation of Zn from Co resulted in extraction of 96% Zn with only 8.6% Co extracting. The separation of Co from Ni was less effective, because of the similarity in extraction rates and extraction curves. The extraction rates decrease in the order Zn, Co, Ni. Analysis of synthetic samples of the pairs of ions using the separation method gave acceptable results except in the case of Co-Ni.
SEVERAL I N ~ E S T I G A T O R S(1-8) have studied the rate of extraction for a number of metal ions from an aqueous phase into an organic phase containing a chelating agent. Bolomey and Wish (9) used the marked difference in the rates of extraction to separate Fe(II1) from Be(I1) with very dilute solutions of thenoyltrifluoroacetone (TTA) in benzene. McKavney and Freiser found Cr(II1) to extract extremely slowly with acetylacetone except at relatively high pH values (IO). Numerous metals, including Fe(II1) and AI(III), form stable chelates much more rapidly at lower pH values and can be separated from Cr(II1). Irving, Andrews, and Risdon (11) have separated Hg(I1) from Cu(1I) with dithizone based on the more rapid extraction of Hg(I1). Only a very limited number of separations of metal ions by solvent extraction based on differences in extraction rates have been reported, and these have been done with metal ions of rather widely differing extraction rates. Therefore, the authors consider a general study of the separation of metal ions by solvent extraction based on differential kinetics merits attention. Metal ‘To whom inquiries should be addressed. *Present address, Department of Chemistry, Memphis State University, Memphis, Tenn. 38111
(1) H. Irving and R. J. P. Williams, J . Chem. SOC.,Part IZZ, 1949, 1841. (2) H. Barnes, Analyst, 72, 469 (1947). (3) A. Walkley, Proc. Australian Chem. Inst., 9,29 (1942). (4) J. Bjerrum and K. G. Poulsen, Nature 169. 463 (1952). (5) R. W. Geiger and E. B. Sandell, Anal. Cltim. Acta, 8, 197(1953). (6) C. B. Honaker and H. Freiser, J . Phys. Chem., 66,127(1962). 39, 295 (1967). (7) J. S . Oh and H. Freiser, ANAL.CHEM., (8) Zbid.,p 1671. (9) R. A. Bolomey and L. Wish, J . Amer. Chem. SOC.,72, 4483 ( 1950). (10) J. P. McKavney and H. Freiser, ANAL.CHEM., 29, 290 (1957). (11) H. Irving, G. Andrews, and E. J. Risdon, J . Chem. SOC.,Part I, 1949,541.
ions which under conditions at which quantitative extraction normally occurs may extract very rapidly were considered in this work. Koroleff (12) has reported the values for the extraction constants ( K * ) for the extraction of a number of metal ions from aqueous solution into solutions of dithizone dissolved in CC14 and CHC13. These values were suspected of being erroneous for metal ions, such as Ni(II), which extract relatively slowly with dithizone. In this work, shaking periods of up to 10 days were used in order to ensure that equilibrium had been attained. A significantly higher value of K* for Ni(I1) was obtained. McClellan and Freiser (13) showed that the rates of formations of the metal dithizonates ( i e . , the rates of extraction) decrease rapidly in the series Cd(II), Zn(II), Co(II), and Ni(I1). The rate constants, which vary from about 109M-’ min-l for Cd(I1) to 8.0 X lO4M-’ rnin-’ for Ni(II), should be sufficiently different to allow the separation of the ions by extraction based solely on the differences in the rates of extraction. This would be of interest and practical value as the separation of Zn(II), Co(II), and Ni(I1) by extraction with dithizone is not possible without the use of masking agents, since the extraction curves (pH cs. E ) lie very close together (14). The determination of the optimum conditions of pH, dithizone concentration, and shaking time for the maximum separation of the ion pairs Zn-Ni, Zn-Co, and Co-Ni was the primary purpose of the present work. EXPERIMENTAL Apparatus. Tracer counting of gamma emission was done with the Nuclear-Chicago Scaler, Model 8775, equipped with a scintillation detector. The detection of beta emission (63Ni) was done with a Baird-Atomic Scaler, Model 135, equipped with a Geiger-Muller gas-flow detector. Absorption measurements were done with a Beckman D U spectrophotometer or with a Beckman DB-G spectrophotometer connected to a Sargent Linear-Log Recorder, Model SRL. The metal ion concentrations of the nonradioactive samples were determined with the Beckman Atomic Absorption Accessory for the D U equipped with the appropriate hollowcathode discharge tube. An Eberbach water-bath mechanical shaker was used for agitation for all the extractions. Distilled water was deionized with an Illco-Way deionizer (Illinois Water Treatment Co., Rockford, Ill.). A Beckman Zeromatic I1 and a Beckman Model 72 were used for pH measurements. Reagents. Dithizone, supplied by the J. T. Baker Chemical Co., was purified and standardized by the method of Landry and Redondo (25). Enough 60Co, ‘j5Zn,or 63Ni, obtained from International Chemical and Nuclear Corp., Pittsburgh, Pa., was added to
(12) F. Koroleff, Thesis, Univ. of Helsingfors, Finland, 1950. 36,2262 (1964). (13) B. E. McClellan and H. Freiser, ANAL.CHEM., (14) Jiri Stary, “The Solvent Extraction of Metal Chelates,” Macrnillan, New York, 1964, p 142. (15) A. S . Landry and S . F. Redondo, ANAL.CHEM., 26,732 (1954). VOL. 41, NO. 8, JULY 1969
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ANALYTICAL CHEMISTRY
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Figure 2. Per cent extraction of Co(I1) 6s. pH
-15 minutes . 10 hours 34 hours [HDZ] 2.4 X 10-'M
aqueous solutions of the metal perchlorates to give a count rate of at least 10,000 cpm/3 ml. All other chemicals were of Reagent Grade quality, and deionized distilled water was used in all cases. Procedure. A 10-ml aliquot of the dithizone dissolved in chloroform was pipetted into a 125-ml separatory funnel or a 50-ml glass bottle equipped with a polyethylene liner and plastic cap. The diihizone concentration varied from 2.4 X 10-5M to 1 X 10-3M. A 5-ml aliquot of the appropriate buffer was pipetted into the funnel or bottle along with a 5-ml aliquot of the appropriate metal ion solution. The buffer concentration was kept constant at /I = 0.025 and the metal ion concentration was 1 X 10-jM. All systems contained equal volumes of organic and aqueous phases. The funnel or bottle was stoppered and mechanically shaken for the desired time period. The funnel or bottle was removed from the shaker and allowed to stand for several minutes to ensure complete separation of the phases. Three-milliliter aliquots containing radioactive 65Znor 6OCo were counted for gamma activity to determine the per cent extraction. Samples containing G3Ni were analyzed by determining the counting rate of the very low energy beta emissions in the organic phase. Two milliliters of the organic phase was pipetted into a planchet and the solvent evaporated under a heat lamp. The counting rate of the radioactive nickel in both phases was determined with a Geiger-Muller detector system. The analyses of the solutions containing no radioactive materials were done by atomic absorption using the appropriate hollow cathode lamp. The instrument was zeroed with an aqueous solution of the same concentration of buffer solution as the samples. The instrumental parameters of lamp current, wavelength, and burner height were optimized for each metal determined. The flame gases, air and acetylene, were at pressures of 20 psig and 4 psig, respectively. The large bore capillary and the heating mode were used in all measurements. The absorbances of the metal ions were determined in the aqueous phase and the concentrations read from previously prepared calibration curves.
6
PH
Figure 1. Per cent extraction of Zn(I1) cs. pH I
5
__ 20 hours [I-IDZ] = 1.95 X 10T4M
RESULTS AND DISCUSSION
Figure 1 shows the per cent extraction of zinc us. pH at various shaking times. These data show that equilibrium is attained after 10 hours. Actually, a 1-hour shaking time was found adequate for equilibration for all but the most acidic samples. From this equilbrium extraction curve, the value for K*, the extraction constant, was calculated. The distribution ratio (D)for a chelate extraction system is:
where
K,
=
formation constant of the metal chelate
KDx
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distribution ratio of the metal chelate
K,
=
acid dissociation constant of the reagent [H+I [R-I ___ [HRI distribution ratio of the reagent
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=
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fully formed metal chelate reagent
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Table I. Extraction Constants, K* Metal ion K* determined& K* from literatureb Zinc 8.7 4.4 Nickel 0.60 1 . 2 x 10-3 Cobalt 0.08 0.032 a Ionic strength = 0.025. * Ionic strength = 0.1.
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Figure 3. Per cent extraction of Ni(1I) us. pH 6-6 15 minutes A--A 1 hour 0-0
10 days, isotope (63Ni)analysis
tracer, 63Ni. The atomic absorption determination is complicated by the presence of large amounts of sodium ion added as buffering compounds. It was necessary to prepare all samples and standards at a constant ionic strength (0.025) by addition of NaC104. The counting of 63Niis likewise difficult because of the extremely low energy beta (0.067 MeV) emitted. Table I lists the K* values determined and those determined by Koroleff (12). The K* value determined by Koroleff for nickel is significantly lower than the value obtained after extraction for a 10-day period. This would be expected, since Koroleff used a shaking time of only 1.5 hours, a period not sufficient to reach equilibrium. At 5Ox extraction, using 2.42 X 10-4M dithizone, the theoretical value calculated for nickel from the previously reported value of K* for the pHliz is 4.31. The observed pHllS at 2.42 X dithizone after a 10-day shaking period is 3.72. Since
0-0 10 days, atomic absorption analysis . , . Av of two 10-day extractions [I-IDZ] = 2.4 X 10-4M
Cobalt and nickel extract slower than zinc, and longer shaking periods were necessary to attain equilibrium, especially at low pH values. Figure 2 shows the extraction curves for cobalt at various shaking times. Figure 3 shows a plot of the data obtained for the extraction of nickel with dithizone. Nickel was found to extract very slowly at low pH values. The spectra of the organic extracts from a nickel solution of pH 6 after a 5-minute shaking time, and after shaking overnight (12 hours) were obtained. The spectrum of the 5-minute extract showed no evidence of the characteristic nickel dithizonate, and was due only to uncomplexed dithizone. The spectrum of the extract shaken overnight, although still showing some evidence of uncomplexed dithizone by a shoulder at about 600 mp, had the characteristic nickel dithizonate spectrum as determined by Math, Fernando, and Freiser (16). To ensure that equilibrium was attained in order to get a valid value for K * , two sets of samples were shaken for a period of 10 days. The per cent extractions were determined on one set by atomic absorption analysis, and on the other by use of an isotopic (16) K. S. Math, Q. Fernando, and H. Freiser, ANAL.CHEM.,36, 1762 (1964).
a lower pHijz will result in the higher value for K* observed in this study. To determine the magnitude of the effect of different ionic strengths on the extraction constants, a series of extractions of cobalt were done at ionic strengths of 0.025, 0.05, and 0.10 using 8.55 X lO+M dithizone and an extraction time of 19.5 hours. The results of this study showed that the ionic strength had no appreciable effect on the per cent extraction. Tablc I1 shows the optimum conditions of pH, dithizone concentration, and extraction time determined for the maximum separation of the ion pairs Ni-Zn, Zn-Co, and Ni-Co. For the separation of Ni and Zn, acetate buffer giving a pH of 5.5 gave the best separation. The separation of Ni and Zn was the most efficient of the three ion pairs separated because of the larger difference in rates of extraction (13). The second order rate constant for the formation of the zinc dithizonate is higher by a factor of 4.62 X 103 than that for nickel dithizonate, while the factor for zinc/cobalt is 7.12 X 10' and for cobalt/nickel is 6.50 X 101. Also, the extraction curves shown in Figure 4 indicate that the curves for cobalt and nickel lie much closer together than for zinc and cobalt. This, of course, adds to the difficulty in separation of cobalt and nickel. The curves shown in Figure 4 are calculated curves plotted from the determined values of K* and assuming
Table 11. Optimum Conditions for Separation of the Ion Pairs Ni-Zn, Zn-Co, and Ni-Co
Ion pairs Ni, Zn Zn, Co Ni, Co
PH 5.5 5 .O
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1.00
x x
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Ext. time
Nickel
One-half hour One-half hour One-half hour
2.5
...
32.5
ZE
Zinc 99.0 95.9
...
Cobalt ... 8.6 95.8
VOL. 41,NO. 8,J U L Y 1969
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Table 111. Separation of Samples of Nickel from Zinc Micrograms Micrograms Corrected microtaken found grams found Ni Zn Ni Zn Ni Zn 14.68 16.35 14.4 15.7 14.7 15.8 15.8 14.4 15.7 14.7 14.4 15.7 14.7 15.8 14.3 15.4 14.7 15.6 Av 14.7 15.7 Av %error 0.1% 4.0%
Table IV. Separation of Samples of Cobalt from Zinc Micrograms Micrograms Corrected microfound grams found taken co Zn co Zn co Zn 16.35 16.35 14.5 15.8 15.9 16.5 15.9 14.5 15.9 16.5 15.7 15.6 16.4 14.3 14.5 15.8 15.9 16.5 Av 15.8 16.5 Av %error 3.4% 0.9%
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Table V. Separation of Samples of Nickel from Cobalt Micrograms Micrograms Corrected micrograms found taken found Ni co Ni co Ni co 14.68 16.35 11.8 14.0 17.5 14.6 12.0 14.0 17.8 14.6 12.0 14.0 17.8 14.6 Av 17.7 14.6 20.4% 11.0% Av %error
a reagent concentration of 1 X 10-4M. To the knowledge of the author, this is the first time that the extraction curve for nickel with dithizone has been reported to lie on the acid side of cobalt. This is a reflection of the slowness with which nickel extracts in the acid region. As shown above, several days are required to attain equilibrium. The effect of metal ion concentration on the separation efficiency was studied. As expected, no metal ion dependence was observed. Simulated unknowns containing the ion pairs Ni-Zn, Co-Zn, and Ni-Co were each submitted to separation and analysis using the method. The metal ion concentration, prepared from the pure metals, in each case was 2.5 X lO-5M. All determinations were done by atomic absorption spectrometry using calibration curves previously prepared by using standard solutions of the single metal ions. The separation of nickel from zinc was done with 3.12 X 10-4M dithizone at a p H of 5.5 and an extraction period of one-half hour. The concentration of the nickel was determined in the aqueous phase. The zinc was back-extracted with 10 ml of O.1NHCl for 10 minutes. The results are shown in Table 111. The amount found in each determination was corrected for losses due to incomplete extraction of the ion or for the co-extraction of the other ion contained in the mixture. For example, in the separation of Ni(I1) and Zn(II), 15.7 pg of Zn(1I) and 14.4 pg of Ni were recovered. This cor-
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Figure 4. Calculated extraction curves for Zn(II), Co(II), and Ni(I1)
- - - Zn(I1)
__ Ni(I1)
. . . Co(I1)
responded, from previous data, to 99 % extraction of Zn(I1) and the co-extraction of 2.5% Ni(I1). The Zn(I1) was corrected by 15.7/0.99 = 15.8 pg and the loss of Ni(I1) was corrected by 14.4/0.975 = 14.7 pg. The separation of cobalt from zinc was done at p H 5.00 by extracting the solution containing both metal ions for onehalf hour with 1 X lO-4M dithizone. The unextracted cobalt was determined in the aqueous phase. The zinc was backextracted from the organic phase with 0.1N HCI by shaking for 10 minutes. The results are shown in Table IV. The micrograms found were corrected and the percentage error was calculated as was done in the separation of nickel and zinc. Nickel was extracted from cobalt with 1 X 10-3M dithizone for one-half hour at a pH of 6.1. The nickel and cobalt were determined in the aqueous phase, Cobalt could not be quantitatively back-extracted. The results of this separation are shown in Table V. The micrograms found were corrected and the percentage error was calculated as was done in the separation of nickel and zinc. This determination was not accurate because of the small difference in extraction rates and closeness of pH curves, and because of difficulties in back extraction. Therefore, the separation of cobalt and nickel based on differential extraction rates is not recommended. RECEIVED for review January 21, 1969. Accepted April 30, 1969. Presented before the Southeastern Regional Meeting, American Chemical Society, Tallahassee, Fla., December 1968. Financial assistance from the Murray State Committee on Institutional Studies and Research is gratefully acknowledged.
