Anal. Chem. 1989, 6 1 , 1621-1623
arm, carrying an etheral functionality, would be best for the lithium fifth coordination role.
ACKNOWLEDGMENT Preparation of the solvent-polymeric membranes and the emf measurements were conducted in the laboratories of Professor Dr. W. Simon at ETH-Zentrum in Zurich, Switzerland.
LITERATURE CITED Rechnitz, G. A,; Eyal, E. Anal. Chem. 1972, 4 4 , 370. Petranek, J.; Ryba, 0. Anal. Chim. Acta 1974, 72, 375. Ammann, D.;Pretsch, E.; Simon, W. Anal. Lett. 1972, 5, 843. Erne, D.; Morf, W. E.; ArvanRis, S.; Clmerman, 2.; Ammann, D.; Simon, W. Helv. Chim. Acta 1979. 62, 994. Morf, W. E.; Ammann, D.; Bissig, R.; Pretsch, E.; Simon, W. In Progress in khrocyclic Chemishy; Izatt, R. M., Christensen, J. J., Eds.; Wiley: New York, 1979; pp 1-61. Oisher, U. J . Am. Chem. SOC. 1982, 704, 4006. Amann, D.;Morf, W. E.; Anker, P.; Meier, P. C.; Pretsch, E.; Simon, W. Ion-Sel. Elecfrode Rev. 1983, 5 , 3.
1821
(8) Simon, W.;Morf, W.E.; Meier, P. C. Struct. Bond. 1973, 76, 113. (9) Kimura, K.; Yano, H.; Kitazawa, S.; Shono, T. J . Chem. SOC., Perkin Trans. 2 1988, 1945. (10) Kimura, K.; Oishi, H.; Miura, T.; Shono, T. Anal. Chem. 1987, 59, 2331. (11) Shoham, G.; Christianson, D. W.; Bartsch, R. A,; Heo, G. S.; Oisher, U.; Lipscomb, W. N. J . Am. Chem. SOC. 1984, 106, 1280. (12) Shoham, G.; Lipscomb, W. N.; Oisher. U. J . Chem. SOC.. Chem. Commun. 1983, 208. (13) Heo, G.S.;Bartsch, R. A.; Schiobohm, L. L.; Lee, J. G. J . Org. Chem. 1981, 4 6 , 3574. Bartsch, R. A.; Heo, G. S.; Kang, S. I.; Liu, Y.; Strzeibicki, J. J . Org. Chem. 1982, 4 7 , 457. Oisher, U.; Frolow, F.; Bartsch, R. A.; Pugia, M. J.; Shoham, G. J . Am. Chem. SOC.,in press.
RECEIVED for review December 23, 1988. Accepted May 1, 1989. G.S. thanks the Bat-Sheva Foundation of Israel for financial support. Research conducted a t Texas Tech University was by a grant from the Robert A* Foundation.
Electrochemical Study of the Mechanism of Cadmium Extraction with Dithizone Wei-hua Yu and H. Freiser* Strategic Metals Recovery Research Facility, Department of Chemistry, University of Arizona, Tucson, Arizona 85721
A current-scanning polarographic study of extraction processes involving dlthlzone and its metal chelates using the ascending water electrode (AWE) has been conducted. Of the metals examined, Cd( I I ) gives a wave that arises from the transfer of an unusual, charged, mixed ligand complex species, Cd(OAc)Dz,-. Other metal ions that form extractable dlthlzonates could be Indirectly determined by their effect on the dlthironate wave.
of the double layer, inasmuch as there is not a common ion transferring across the aqueousforganic interface. As no charge flows through the electrode,i.e., no charging or discharging occurs, the potential is reproducible. The net effect is a shift in the +$E of 106 f 7 mV in the positive direction, compared to the electrode containing TMA'Cl. The electrolyte used in the 1,2-dichloroethane (DCE) phase was 0.01 M THA+,TPB-,prepared by mixing THA+,Br-dissolved in DCE and aqueous Na+,TPB- in stoichiometric proportions. Dithizone (Eastman Kodak Co.) was purified by recrystallization. 1,2-Dichloroethane (DCE) (Aldrich Chemical Co.) was used as received. All other reagents were analytical reagent grade.
INTRODUCTION This represents a continuation of our electrochemical studies of the transfer processes associated with metal ion extractions utilizing the ascending water electrode (AWE) (1-3,which has proven to be a useful approach to the elucidation of the details of the chemistry of solvent extraction. In this report, the behavior of diphenylthiocarbazone (dithizone), a weak monobasic acid with an aqueous pK, of 4.7 (8), a widely used extractant whose sulfur atom bonding site results in a fair degree of selectivity, is examined, and its mechanism of extraction of cadmium is elucidated in this paper.
RESULTS AND DISCUSSION When current scan polarography is carried out on a HDzDCE solution containing 0.01 M THA+,TPB- as supporting electrolyte in contact with 0.2 M sodium acetate aqueous solution, a well-defined cathodic wave is obtained (Figure 1). The features of this wave are as follows: 1. The limiting current of the cathodic wave is proportional to the initial concentration of HDz in DCE. 2. The limiting current is proportional to the square root of the height of the head of the aqueous reservoir. 3. The half wave potential shifts 49.5 mV i= 2.0 more positive per unit increase in pH in the range 6.4 to 10.7. 4. The logarithmic analysis shows the slope of 59.3 mV f 3.0. These characteristics suggest that the diffusion-controlled cathodic wave represents the transfer of the dithizonate anion from aqueous to organic phase. This is quite analogous to the behavior of other acidic extractants (1-4). When Cd ion is present in the aqueous solution, another cathodic wave having a more positive half-wave potential, which is completely distinct from the deprotonated wave, appears (Figure 2). The minimum pH of the appearance of the new wave is 5.4, which is lower than that for the depro-
EXPERIMENTAL SECTION The apparatus and procedures for current scan polarography at the AWE have been described earlier (2-8). The electrolytic cell used earlier was slightly modified in order to improve the stability of the organic reference electrode. The elimination of tetramethylammonium chloride, TMA+,Cl-, from both aqueous and organic reference solutions (which contain 1M LiCl and 0.01 M tetraheptylammonium tetraphenylborate, THA+,TPB-, respectively) of the electrode resulted in stable readings for at least a month, whereas the earlier organic reference had to be replaced at least weekly. With the new arrangement, the potential of the organic reference electrode is largely determined by the charge
0003-2700/89/0361-1621$01.50/0Q 1989 American Chemical Society
1622
ANALYTICAL CHEMISTRY, VOL. 61, NO. 15, AUGUST 1, 1989 41'
I-
16.0
12.0
- I 200mV
Figure 1. Polarography of dithizone: aqueous phase, 0.2 M sodium acetate of pH 8.0 f 0.1: DCE solution, 0.01 M THA-TPB, HDz (mM) (1) 0 and (2) 0.4. .70
dl
'1-
2.30
1.50 pOAc
Figure 4. Effect of acetate on the limiting current of the complex wave: aqueous phase, 0.5 M magnesium sulfate, 0.2 mM cadmium sulfate: DCE solution: 0.01 M THA-TPB, 1.5 mM HDz.
Figure 2. Polarography of dithizone in the presence of cadmium: aqueous phase, 0.2 M sodium acetate, cadmium sulfate (mM) (1) 0 and ( 2 ) 0.8; DCE solution, 0.01 M THA-TPB, 0.4 mM HDz. i
125
t
\i .70
1.90 pOAc
Flgure 5. Effect of acetate on the half-wave potential of the complex wave: aqueous phase, 0.5 M magnesium sulfate, 0.2 mM cadmium sulfate; DCE solution, 0.01 M THA-TPB, 1.5 mM HDz. Figure 3. Effect of cadmium concentration: aqueous phase, 0.2 M sodium acetate of pH 7.5 f 0.1, cadmium sulfate (mM) (1) 0.08, ( 2 ) 0.1, (3) 0.2, (4) 0.4, and (5) 1: DCE solution, 0.01 M THAaTPB, 0.8 mM HDz.
tonated wave. No wave is observed in the absence of acetate in aqueous phase. The new wave must result from the electrochemical transfer of a hitherto unobserved chelate anion of Cd, in which both dithizone and acetate must be involved. Effect of HDz. The two separate cathodic waves can be observed when HDz is in excess (Figure 3). At constant [Cd2+],increasing [HDz], has no effect on the chelate anion wave, but the dithizonate wave grows, since the former depends on Cd and the latter is related to the fre HDz. The relation between the limiting current of the chelating wave and the height of the aqueous reservoir demonstrates that this wave also is diffusion-controlledwhen HDz is in large excess. Effect of Cd. When Cd is in mild excess ([HDz], = 0.6 mM, [Cd] > 0.4 mM), the half-wave potential of the chelating wave becomes more negative while the limiting current remains constant, as might be expected by its control by the concentration of HDz. The limiting current is independent of the column height of the aqueous reservoir, characteristic of a kinetically controlled wave. Unless both HDz and acetate are in excess, the kinetically controlled wave will be observed.
Effect of Acetate. When [Cd2+]is 0.2 mM and [HDz] is 1.5 mM, no chelate anion wave appears until the [OAc-] is at least 2 mM. The limiting current of the wave rises with increasing concentration of acetate, reaching a plateau at [OAc-] = 0.2 M (Figure 4). The half-wave potential shifts 50 mV more positive per decade increase of [OAc-] (Figure 5), indicating that one acetate is present in the transferring anion. Logarithmic analysis of the diffusion-controlled chelate wave (when both [HDz], and [OAc-] in aqueous solution are in large excess over [Cdz+])gives a slope of 61 mV f 5.7, signifying that the transferring species causing the chelate anion wave is monovalent. Taken together with the evidence that one OAc- is present, as deduced above, Cd(Dz),OAc- must be the transferring anionic species. The limiting current reaches half the plateau value, a point at which half the cadmium is in the anionic acetate complex from, at pOAc = 1.9, which suggests a stepwise formation constant of this species from the neutral CdDz2 of The result of the following experiment supports the assumption of a chelate anion having a 1:2 CdDz ratio: A series of polarograms is run,increasing the concentrationof cadmium from 0.02 to 0.2 mM, while keeping the amount of HDz constant and in excess, e.g., 0.8 mM. The limiting current
ANALYTICAL CHEMISTRY, VOL. 61, NO. 15, AUGUST 1, 1989
1623
Table I"
[Cdl, m M
i, PA
0.04 0.08 0.1 0.2
64.6 57.12 54.4 38.08
io, PA (4)
5.44 6.8
12.24
"i, and i, stand for the currents of the deprotonated wave and chelate wave, respectively. Scheme I (0)
HDz
(a)
HDz
Cd( Dz)20A6
1 I .lr H+
+ Dz-
2Dz-
+ Cd2+
-
Cd(Dz)2
+ OAc-
-
Cd(Dz)20A6
of the dithizonate wave decreases, and the chelate anion wave appears and increases with increasing [Cd] (Table I). The effect of the cadmium on the size of the dithizonate wave can be derived with the help of the following equations: il
= K(C - nM2)
(1)
i2
= K(C - nM2)
(2)
where C, Mi, and ii are the initial concentration of HDz, the concentration of Cd, and the limiting current of the dithizonate wave, respectively, and n is the number of dithizonates in the chelate. Taking the ratio il/i2, the value of n can be readily obtained il
-=i2
c-nM, C-nMz
(3)
From the data in Table I, n is found to be 2,proving that the transferring complex ion is Cd(Dz),OAc-, a five-coordinated cadmium complex (9). The process can be depicted as shown in Scheme I. The half-wave potential for the diffusion controlled wave can be calculated from the following equation:
When formate is substituted for acetate, it behaves similarly, but because it is a weaker ligand than acetate, the cathodic wave begins to appear at higher formate concentrations and higher pH, with a more negative half-wave potential than when acetate is present. Although no other metal ion that forms extractable chelates with dithizone gives rise to a characteristic wave, either anodic or cathodic, the presence of the metal ion in the aqueous phase can be detected by its effect on reducing the limiting current of the dithizonate wave. For example, when lead(I1) is present instead of Cd(II), no additional wave appears, but the size of the dithizonate wave decreases. This signifies that only Cd forms the unusual anionic mixed ligand under our experimental conditions. Instead, the simple neutral extractable complex is formed, consuming dithizone and transferring without electrochemical trace into the DCE phase. Further, the effect on the dithizonate wave could form the basis for an indirect determination of those metals that form dithizonate complexes.
ACKNOWLEDGMENT The authors gratefully acknowledge the helpful discussions with S. Muralidharan. Registry No. Cd, 7440-43-9; dithizone, 60-10-6; acetic acid, 64-19-7. LITERATURE CITED Yoshida, 2.; Freiser, H. Elechoanal. Chem. 1984, 762, 307. Yoshida. Z.; Freiser, H. Inorg. Chem. 1984, 23, 3931. Yoshida, Z.;Freiser, H. J . Electroanal. Chem. 1984, 779, 31. Lin, S.; Freiser, H. J . Elechoanal. Chem. 1985, 797, 437. Lin, S.; Zhao. 2.; Freiser, H. J . Elechoanal. Chem. 1988, 210, 137. Lin. S.; Freiser, H. Anal. Chem. 1987. 59, 2834. Yu, W.; Freiser, H. Anal. Scl. 1987, 3 , 401. Morrison, G. H.; Freiser, H. Solvent Extract/on in Analyticel Chemistry; John Why and Sons: New York. 1957. (9) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry; 3rd ed.; pp. 515 (1972).
(1) (2) (3) (4) (5) (6) (7) (8)
RECENEDfor review January 3,1989. Accepted April 21,1989. The research was supported by a grant from the National Science Foundation.
