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ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
conditioned with water or a solution rather more basic than solvent B is eluted isocratically with solvent B. If one of the sample peaks is found a t this position, interference will, of course, occur. Corrosion of the stainless steel appears t o be a limiting factor in trace metal analysis by HPLC, given the equipment available a t this time. What is needed besides very pure reagents and solutions is an all-Teflon system or a t least one where the amount of stainless steel is severely limited (8). T h e detection system described here is ideally suited for use in conjunction with an HPLC investigation involving a wide variety of polyvalent metal ions. We have found it more than adequate for use in applying chemically bonded chelating agents as immobile phases ( I ) , but it should be readily adaptable for ion exchange and liquid-liquid reverse phase partition separations as well.
LITERATURE CITED (1) J. R. Jezorek and Henry Freiser, Anal. Chem., preceding paper in this issue. (2) K. Kawazu and J. S. Fritz, J . Chromatogr., 77, 397 (1973). (3) J. S. Fritz and J. N. Stary, Anal. Chem., 46, 825 (1974). (4) S. Shibata, in "Chelates in Analytical Chemistry", H. Flaschka and A. J. Barnard, Ed., Vol. 4, Marcel Dekker, New York, 1972, pp 116-165. (5) K. N. Munshi and A. K . De, Anal. Ctem., 36, 2003 (1964). (6) L. G. Sillen and A. E. Martell, Ed., Stability Constants of Metal-Ion Complexes", Suppl. No. 1 , Special Pub. No. 25, The Chemical Society, London, 1971. (7) M. D. Arguello and J. S. Fritz, Anal. Chem., 49, 1595 (1977). (8) W. A. MacCrehan, R. A. Durst, and J. M. Beliama, Anal. Left., 10, 1175 (1977).
RECEIVED for review August 24, 1978. Accepted November 9, 1978. This work was conducted with the financial assistance of the U.S. Department of Energy.
Lead, Cadmium, and Zinc Bis(diethy1dithiocarbamate) and Diethyldithiocarbamic Acid as Reagents for Liquid-Liquid Extraction Sixto Bajo" and Armin Wyttenbach Swiss Federal Institute for Reactor Research, 5303 Wurenlingen, Switzerland
The extraction constants of Pb(DDC),, Cd(DDC),, and Zn(DDC),-where DDC is used to denote the diethyldithiocarbamate anion-were determined for the system H,O/CHCI,; log K,, was found to be 7.94 f 0.09, 5.77 f 0.05, and 2.39 f 0.02. Solutions of Cd(DDC)2 and Zn(DDC), In CHCI, were shown to be stable for at least 40 days. When solutions of Cd(DDC),, Zn(DDC),, or HDDC as reagents in CHCI, were used to extract acid aqueous solutions, their titer deteriorated as a function of pH and time; however, this deterioration is much less rapid than the destruction of a solution of NaDDC. The title compounds therefore can be considered suitable reagents for extractions from acid aqueous solutions.
Extraction of metals as their diethyldithiocarbamates (the anion (C2H5)2NCS2- will in the following be denoted as DDC) is becoming more and more popular. T h e reagent is traditionally introduced into the aqueous phase as NaDDC or into the organic phase as NH2(C2H5)DDC.Substitution of these reagents by metal diethyldithiocarbamates (dissolved in an organic diluent) results in an improved stability in contact with acid aqueous solutions and in an increased specificity of the extraction. Examples for the application of metal dithiocarbamates as reagents can be found in many publications (1-11). Selectivity of these extractions is governed by the extraction constants of the reagents (11,which unfortunately are not well known for the practically important system CHC13/H20. Furthermore, safe application of these reagents requires some knowledge about the stability of their titer. T h e present work was undertaken in order to contribute to the knowledge of metal dithiocarbamates as reagents by determining the extraction constants of Pb(DDC)2,Cd(DDQ2, and Zn(DDC)2and by measuring the stability of their solutions (as well as of HDDC) in CHC13 alone and in contact with 0003-2700/79/0351-0376$01 0010
various acids. Both points are prerequisites to the appropriate use of these reagents.
EXPERIMENTAL Extractions. Extractions were done in 250-mL separatory funnels at room temperature (21 "C) on a shaking machine with a capacity of 6 cm and a frequency of 6.6 s-l. The phases were then separated by decantation. Reagents. The solid products Pb(DDC)*, Cd(DDCI2, and Zn(DDC& were prepared as described before ( I ) . They were weighed to prepare solutions in CHC1, of known content, which were stored in dark bottles. Solutions of HDDC in CHC1, were prepared by shaking equal volumes of Zn(DDC),/CHCl, solutions and 2 M HCl for 30 s. Determination of the Extraction Constants. Solutions of the different metal diethyldithiocarbamates in CHC1, (30 mL) were shaken with aqueous solutions of HC104 (100 mL). The concentration of the M(DDQ2 as well as the acidity was varied within the limits given in Table I, whereas the concentration of C 1 0 in ~ the aqueous phase was kept constant at 1 M with NaC104. The concentration of the metals in the organic phase both before and after the experiment was determined by evaporating and wet ashing aliquots which were then analyzed by complexometry (Zn, Cd) or by atomic absorption (Pb). Control of the Titer of Solutions of Cd(DDC),, Zn(DDC)*, and HDDC in CHC1,. The variation of the titer with time was checked by periodically extracting a radioactive aqueous metal solution (100 mL) with aliquots of the organic solutions (30 mL) under conditions that were substoichiometric with respect to the reagent. The activity extracted was measured and taken to be proportional to the titer. The following systems were used: (a) for Cd(DDCI2and Zn(DDC)*in CHCl,: Hg2+('03Hg) in 1 M HCl, extraction time was 2 min; (b) for HDDC in CHCl,: Cu2+(62Cu) in 0.1 M HC104, and Zn2+ (65Zn)in a citrate buffer of pH 5, extraction time was 4 min. Control of the Titer of Organic Solutions in Contact with Acid Aqueous Solutions. Aliquots of the organic solutions (30 mL) were shaken for various times with 100 mL " 0 3 , HC104, HCl, or H2S04of different concentrations. Thereafter an excess of CuL+(64Cu)was added t o the aqueous phase and shaking was C 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 3, MARCH 1979
377
Table I. Extraction Constants for Zn(DDC),, Cd(DDC),, and Pb(DDC), for the System H,O/CHCl, no. of log K,, range of range of initial range of determimetal [H'] used concentration C used F found nations this work literature values Zn2+
0.01-0.06
Cd2+ Pb"
0.1-1.0 0.1-1.0
(1.7-5.2) X (1.1-1.8) x (0.2-1.6) x
lo-,
0.24-0.60 0.42-0.85 0.51-0.96
continued for another 4 min. The activity extracted was taken to be proportional to the titer. Counting was done with a well-type NaI(T1) crystal, taking care of counter dead-time, background, and radioactive decay as usual.
RESULTS AND DISCUSSION Extraction Constants. T h e extraction of a metal from the aqueous phase into an organic phase (subscript org) as well as the reverse process can be described by
Mn+ + HDDC,,,
* M(DDC),,,,, + nH+
(1)
T h e equilibrium constant for this process is called the extraction constant K,, (for the given organic phase) and can be written as
[M(DDC)nIorg[H+I" Kex =
[Mn+l [HDDCInorg
(2)
The determination of the extraction constant is usually done under conditions of a large excess of extractant (12). Our approach is different in that the total concentration of extractant is exactly stoichiometric to the total concentration of the metal. With the assumptions (which are fulfilled in our case) that M(DDC), is the only extracted complex, that Mn+ exists in the aqueous phase only as aquo ion and that (HDDC),,, >> (HDDC),, + (DDC),,, the following relations hold a t equilibrium:
[M(DDC)nIorg= F
X
C
(3)
where C is the original concentration of M(DDC), in the organic phase and F the fraction of the metal staying in the organic phase a t equilibrium. For stoichiometric reasons i t follows t h a t
[HDDClorg = n ( C - [M(DDCln)orgl= nC(1 - F) (4) Vorg F) 7(5)
[Mn+]= { C -
Substituting Equations 3, 4, and 5 into Equation 2 gives: P
T i
where V and Vorgare the volumes of the aqueous and of the organic phase. I t can be seen from Equation 6 that under the conditions of stoichiometry as used here, K,, can be calculated from a determination of F and [H+] alone. There is no need to introduce other constants (ase.g. the acid dissociation constant or the distribution constant of the reagent) or to add auxiliary complexants into the aqueous phase. Although this procedure is limited to a certain range of K,,, we think that apart from the cases studied here it could also be applied to In(DDC), and TI(DDC) or to other types of chelates. Extraction times used in these experiments were 0.5 min for Cd and P b , and 4 min for Zn. I t was assured that these times were long enough to reach equilibrium and not long enough t o allow for a noticeable destruction of the reagent (see later). T h e results are given in Table I. T h e individual determinations of log K,, for every element show a relative standard
7 11 10
2.39 i 0.02 5.77 ? 0.05 7.94 t 0.09
2.2 ( 1 3 ) ,2.3 z 0.2 ( 1 4 ) 5.6 ( 8 ) , 5 . 4 ( 1 3 ) ,5.50 0.05 ( 1 4 ) 7.0 ( 1 3 ) , 5.0 t 0.1( 1 4 ) +_
deviation of approximately 1%; this can be taken to indicate the internal consistency of the results. Agreement with literature values is fair for Zn and Cd, but only moderate for Pb. For this latter element, the same authors (13,14) have published two very different values. Stability of Solutions of Zn(DDC), and Cd(DDC)2in CHCl,. Both reagents were studied a t concentrations of 5 x M, and 5 x lo-' M for a period of 40 days. M, 5 x No deterioration of their titer exceeding the experimental accuracy of 2% was found. These reagents therefore seem to be perfectly stable up to 40 days. These findings are a t variance with the statement t h a t Zn(DDC), M in CHC1,) looses 10% of its titer within 4 days, and that more dilute solutions are even less stable (15); it is suspected that in these cases the apparent instability is due to the quality of NaDDC or to the method used for the preparation of the Zn(DDC)2. Stability of Solutions of HDDC in CHCI3 The solutions M and showed a decrease in their titer studied were 3 X which followed a complicated pattern. They were stable for 6 to 7 min, then deteriorated with an apparent half-life of 7 min to a value that was approximately 12% of the original value; this residual titer then stayed constant from 60 to 220 min. Experiments in which the titer was determined by extraction of Cu2+ or of Zn2+ showed identical results. In separate experiments the variation of the absorbance of the organic phase with time was measured a t 350 nm, the wavelength given by Bode (16) for HDDC; this variation paralleled the change in the titer. It seems thus reasonable to attribute the residual titer to HDDC and not to some stable decomposition product. In order to verify if the observed decomposition might be due to traces of HC1 in the organic phase originating from the preparation of the HDDC, solutions of HDDC in CHC1, were washed with a buffer of p H 4 immediately after their preparation. However their behavior was identical to the unwashed solutions. Stability of Solutions of HDDC, Zn(DDC),, and Cd-
(DDC)2 in CHCl, That Are Being Shaken with Acid Aqueous Solutions. All solutions showed a decrease of their titer which followed first-order kinetics: the apparent half-lives are given in Table 11. Joris et al. (17,181 showed that aqueous solutions of HDDC decompose according to
h[HDDC] ( 7 ) where h (the true rate constant) is 0.1 S S ~ . The apparent rate constant h' is in general pH-dependent, but in the region used in our work ( p H 0-3) can be considered constant and equal to k . Equation 7 can also be used to describe the deterioration of the titer of a two-phase system if it is assumed that (i) the decomposition of HDDC in the aqueous phase is the only reaction leading to a deterioration of the titer; (ii) the establishment of the equilibrium between the two phases given by Equation 2 (which involves transportation of Me(DDC), and HDDC across the phase boundary as well as dissociation and formation of Me(DDC), and of HDDC) is very fast with
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ANALYTICAL CHEMISTRY, VOL. 51,
NO.3,
MARCH 1979
Table 11. Decomposition of Solutions of HDDC (3.4 X l O - ' M ) , Zn(DDC), (1.7x M ) , a n d Cd(DDC), (1.7x M in CHCl, While They Are Shaken with Acid Aqueous Solutionsa mocompound
HDDC
larity of acid HNO,
HCIO, 52 (81)
1.0
71 70 68 57
0.10 0.010 0.0010
Zn(DDC), Cd(DDC),
1.0 0.10 0.01 1.0 0.10 0.010
41 88
229
HC1 H,SO,
(81)
(81)
(81) (81) 82 (87) 245 (186) 8 3 (139) 479 (500) (2200)
74
99 23 7
90 99 240
78 34 6
1525
Values given are half-lifes in minutes for the rate of destruction of the reagent. Values in parentheses were calculated according to Equation 8. a
respect t o the destruction of HDDC. Assumption (i) is sustained by t h e observed stability of Zn(DDQ2 and Cd(DDQ2 in the organic phase alone. If we denote with Q the sum of all forms of DDC in the two-phase system, it follows t h a t
where k' is t h e apparent rate constant as used in Equation 7 , and cy is the ratio of the concentrations of all forms of DDC in the aqueous phase to all forms of DDC in the organic phase: [HDDC] ff=
[HDDCI.,,
+ [DDC-I
+ n[M(DDC),I,,,
(9)
can be calculated with the aid of K,,, the partition coefficient (2360 for CHC1,/H20 (19))and the acid dissociation (20)). Its value depends on the constant K H D D C (4.47 X p H of t h e aqueous solution. T h e half-lives as calculated from Equation 8 are given in Table 11. T h e agreement with the experimental data is fair, thus lending support t o the assumptions made in deriving Equation 8. I t can be seen that, under otherwise identical conditions, the stability of a given M(DDC), against acid cy
PHDDC
decomposition increases with Kex;it is therefore to be expected t h a t solutions of Pb(DDC)2-which have not been tested in this work-are even more stable than those of Cd(DDC)2. The observed stability of solutions of HDDC in CHC13 that are shaken with aqueous acid solutions is hard to understand. Although the observed half-life is consistent with t h e mechanism used above for M(DDC),, we have found much shorter half-lives for organic solutions that are not shaken with aqueous solutions. For the time being we cannot advance any explanation for this apparent contradiction. T h e experimental results given in Table I1 show t h a t the half-life for the decomposition of organic solutions of metal diethyldithiocarbamates in contact with acids is of t h e order of hours. This contrasts favorably with the half-life of about 10 s for NaDDC dissolved in acid aqueous solutions (17, 18) and avoids many difficulties arising from the fast decomposition of NaDDC.
LITERATURE CITED A. Wyttenbach and S. Bajo, Anal. Chem., 47, 1813 (1975). S. Bajo and A. Wyttenbach, Anal. Cbern., 48, 902 (1976). J. KuEera, Radiochem. Radioanal. Leb., 24, 215 (1976). P. A. Schubiger and 0. Muller, Radiochem. Radioanal. Lett., 24, 353 (1976). P. A. Schubiger. 0. Muller, and W. Gentner, J . Radioanal. Chem., 39, 99 (1977). J. M. Lo, J. C. Wei, and S. J. Yeh, Anal. Chem., 49, 1146 (1977). J. M. Lo, J. C. Wei, and S. J. Yeh, Anal. Chim. Acta, 93, 301 (1977). S. Bajo and A. Wyttenbach, Anal. Chem., 49, 158 (1977). S. Bajo and A. Wyttenbach, Anal. Chem., 49, 1771 (1977). S. Bajo and A. Wflenbach, "Extraction liquide-liquide de I'arsenic, de I'antimoine, du s616nium et du tellure par le di6thyldithiocarbamate de zinc", EIR-Report No. 336, Swiss Federal Institute for Reactor Research, Wurenlingen, March 1978. E. Pernicka, P. A. Schubiger, and 0. Muller, Ana/ysf(London), 103, 475 (1978). J. Stary and K. Kratzer, Anal. Cbim. Acta, 40, 93 (1968). P. C. A. Ooms, U. A. Th. Brinkman, and H. A. Das, "Separation of metal ions from an aqueous solution by displacement reactions in liquid-liquid extraction systems", ECN-Report No. 77-018. Stichting Energionderzoek Centrum Nederland, Petten, December 1976. P. C. A. Ooms, U. A. Th. Brinkman, and H. A. Das, Radiochem. Radioanal. Lett., 31, 317 (1977). D. A. Beardsley, G. 8. Briscoe, J. Ruzicka, and M. Williams, Talanfa. 14, 879 (1967). H. Bode, Fresenius Z . Anal. Chem., 142, 414 (1954). K. I. Aspila, V. S.Sastri, and C. L. Chakrabarti, Talanta, 16, 1099 (1969). S. J. Joris, K. I.Aspila, and C. L. Chakrabarti, Anal. Chem., 41, 1441 (1969). H. Bode and F. Neurnann, Fresenius Z . Anal. Chern.. 169,410 (1959). H. Bode and K. J. Tusche, Fresenius Z . Anal. Chem., 157, 414 (1957).
RECEIVED for review May 8, 1978. Accepted November 10, 1978.
