1882
Anal. Chem. 1980, 52, 1882-1885
Determination of Extraction Constants for Lead(II), Zinc(II), Thallium(I), and Manganese(I1) Dithiocarbamates by a Two-step Extraction Method L. H. Shen,’ S. J. Yeh,’ and J. M. Lo Institute of Nuclear Science, National Tsing Hua University, Hsinchu, Taiwan 300, Republic of China
A two-step extraction method with radiometry was developed to determine the relatively low equilibrium constants for the extraction reaction of some metal dithiocarbamates, M(DTC), Mm+ 4- mHDTC(,, + M(DTC),(,g) mH+. The distribution ratio of the metal in two phases was determined by its radiotracer activlty. The dithiocarbamk acid in the organic phase was separated and shaken with a suprastoichiometric amount of Cu2+ labeled with 64Cu. The material balance for the two-step extraction method was derived. The extraction constants for diethyldithiocarbamates and pyrrolidinecarbodithioates of Pb(II), Zn( 11), TI( I), and Mn( 11) were determined by this method.
+
Dithiocarbamate anion (DTC-) is one of the most important complexing agents for heavy metals in trace analysis. Diethyldithiocarbamate (DDC) and pyrrolidinecarbodithioate (PDC) are two of its derivatives commonly used in analysis. Extraction constants for metal dithiocarbamates, M(DTC),, can be used to predict the optimum conditions for separation of a metal or a group of metals. So far information on the extraction constants is still scanty (1-6), although many authors have qualitatively investigated the extraction of metal dithiocarbamates (Le., extraction order) (7-11). Recently we have developed two methods for determining extraction constants of M(DTC),: one is specific for those of high value (12) and the other is specific for those of low value. Our methods seem to be more accurate and convenient than others published in literature. In this article, the determination of low extraction constants for some metals will be delineated.
THEORY All metals involved in the Theory are marked with appropriate radioisotopes, and equal volumes of organic and aqueous phases are employed. When a metal ion, M“+, is extracted with a dithiocarbamate ligand, the following two equations describe the equilibrium reached: Mn+ mHDTC(,,,) M(DTC),(,,,) + m H + (1)
+
M“+
+ mDTC-
M(DTC),(,,,)
(2)
where the subscript “org” represents organic phase and no subscript represents aqueous phase. The extraction constant can be written as shown in eq 3 or 4 where K ~ ~ ~ and T o , [M(DTC)rnI(orgj[H+lrn = [Mrn+][HDTC],,,,,rn
Ka‘TC)m
- [M(DTC),I K‘(DTC)m
(3)
(or,)
- [Mrn][DTC-]m
(4)
constant”, respectively (6, 1 3 ) . The acidity a t which the HDTC is equally distributed between the aqueous and organic phases (i.e., [DTC-] = [HDTC],,,) is defined by the pH1/2 value according to eq 5 where Km is the formation constant
LIlog m
-
log
~ g i h T c ) , ~(5)
and Pmc is the partition constant (13). In the present work, K@‘td,Tc),is determined by the procedure described below, whereas K M W ) ,is calculated from the experimental K ~ $ T c ) , value by use of eq 5. Either Kg’;d,Tc)m or KM(mC),can be used as the equilibrium constant for the extraction of metals ( 1 4 , 12, 13). T o find KgltTc), in eq 3, one can measure [H+] with an accurate pH meter and [M(DTC),](,,)/[M”+] from the distribution ratio of the metal in the two phases, i.e. T’M
CM(org)
[M(DTC)inI(org)
CM(aq)
[ Mn+]O(~m+
= --
(6)
where cM(org) and CM(aqj are total concentrations of the metal, M, whatever its chemical form, in the organic phase and the aqueous phase, respectively, and C Y M ~=+ C M ( ~ ?[M”’] )/ (13). The asterisk implies that the distribution ratio is measured by radioactivity in the study. The value of [HDTC](,,,j in eq 3 is usually not easy to estimate accurately. This difficulty can be solved by a two-step extraction method with radiometry as described below. The optimum condition for the determination of K ~ $ T c ) , with eq 3 is that the pH of the solution should be much less than pHII2, because only under this condition would [HDTC],,,,) be significant. If pH > P H ~ , practically ~, all HDTC,,,,) will be converted to DTC- in the aqueous phase ( 1 4 , 1 5 ) . The [HDTC],,,, term in eq 3 can be determined by shaking an appropriate amount of organic phase with an excess of Cu2+ solution immediately after the organic phase was separated by centrifugation. By this second step of the extraction procedure, dithiocarbamic acid in the organic phase is completely converted to copper dithiocarbamate, because the latter possesses a very high extraction constant (2-4,12). In addition, copper will replace M in M(DTC), readily and equilibrium is rapidly attained. This can be represented as
( m + 1)Cu2++ 2HDTC(,,,) + SM(DTC),(,,,) + ( m + l)Cu(DTC),(,,,) + 2Mm++ 2H+ ( 7 ) By material balance [HDTCl(,rgj = 2[Cu(DTC),I,,,gj’ - m[M(DTC)mI(org) + m[M(DTC)rnI(org)’ (8)
KMcm), are called “acid extraction constant” and “extraction Present address: Institute of Nuclear Energy Research, Lungtan,
Taiwan 325, Republic of China.
0003-2700/80/0352-1882$01 .OO/O
KM(DTC),
1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER 1980
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 12, OCTOBER 1980
Table 111. Extraction Constants of Various Dithiocarbamates in the Chloroform/Water System a t 25 ( l i m ) log K@&,,,
metal ion
metal complex
this work
lit.
i
0.1 C
( l i m )log KM(DTC), this worku lit.
Pb( DDC), 2.77 3.88,bsC2.87,b8C3.4,d 2 . 5 , e 3.97s 9.49 9.6-10.8," 1 0 . 0 8 b ~ C Pb( PDC), 2.00 8.13 Zn(I1) Zn( DDC), 1.26 1.48,b,C1.27,b3C1.3,d 1.2,e 1.20f 7.98 8.1-8.3,a 7.96's' Zn( PDC), 0.83 0.2h 6.96 6.3h TW) TlDDC 0.54 -0.53,b,C O.ObsC 7.26 5.6gbrc TlPDC 0.33 6.46 Mn(I1) Mn( DDC), 0.27 - 2.2 l b , C 6.99 4.00b2C Mn( PDC), - 2.50 3.63 All the ( l / r n ) log K M ( D T C ) values , are calculated from the experimental (lim)log K$,C'P,,c,m values by eq 5, where for HDDC is 6.72 (see ref 1)and for HPDC is 6.13 (see ref 6). CCl, was used as the organic solvent. Reference 2. Reference 3. e Reference 4. f Reference 5. Reference 1. Reference 6. Pb(I1)
Here all primes designate the concentrations after extraction by the addition of Cu2+ solution, Le., the equilibrium concentrations of eq 7, and no prime represents the concentrations which only involve eq 1. [Cu2+]initia1 and [Mm+]initial are the initial concentrations of Cu2+ and Mm+solutions. By substituting eq 6 in eq 9 and 10, one can obtain
Equations 11-13 are transferred into eq 8, so that
+
4 + )
m r *1 ~ [ Mr~t M + I i n i t(1 ial - 1
r*M
(14)
From eq 14, [HDTC](,,,, can be conveniently obtained from the experimental results-the distribution ratios of copper and the metal, M.
EXPERIMENTAL SECTION Reagents. Diethyldithiocarbamic acid (HDDC) and pyrrolidinecarbodithoic acid (HPDC) were prepared fresh before each set of experiments as follows: Aqueous solutions of sodium diethyldithiocarbamate (NaDDC) and ammonium pyrrolidinecarbodithioate (APDC) were shaken with chloroform by the stepwise addition of perchloric acid until pH 1-2 was reached. The radiotracers 64Cu,65Zn,'04Tl, and 56Mn,used in the experiment, were produced by irradiating Cu metal, Zn metal, T1N03, and MnS04.2H20,respectively, in the Tsing Hua open pool reactor at the university (neutron flux N 2 X 10" neutrons cm-* 5-l). The nuclide *lOPbwas purchased in its nitrate form from Radiochemical Centre Ltd., Amersham, England. It was radiochemically purified before use by removing its daughter nuclides according to van Erkelens' procedure (16). All the chemicals used were the products of E. Merck Co., West Germany. Subboiling redistilled water was used throughout (17). All the containers were washed by detergent, immersed in 8 N HNOBovernight, and then rinsed with the subboiling redistilled water several times. Procedure for t h e Determination of K$&,-),. All the metals involved in the procedures were marked with appropriate radioisotopes. A specific metal ion solution was prepared, whose pH value was adjusted to the desired value and ionic strength was set at 0.1 with KC1 or KN03 (see Tables I and 11). Ten milliliters of the metal ion solution was transferred into a 25-mL groundstoppered quartz tube (i.d. 3 cm). Then 10 mL of HDDC or
-
HPDC chloroform solution of an appropriate concentration was added. The mixture was shaken for 5 min to reach equilibrium (2). After centrifugation of the mixture for 1 min, 5 mL of the organic phase was immediately separated into another groundstoppered quartz tube. Then 5 mL of a suprastoichiometric amount of CuZf solution (pH -2, in 0.1 N KCl or 0.1 N KNOJ was added to chelate with HDTC(,,,, and extracted into organic solution (see Tables I and 11). Three milliliters each of both phases in the first and second steps of the extraction were drawn off for activity measurements. The distribution ratios of the metal and copper in two phases at the two steps of the extraction could thus be determined. The aqueous phase from the first step extraction was used for pH measurement. Then [HDTC],,,,, can be calculated with eq 14. Finally, the K&&), value was evaluated from eq 3. Activity Measurements. A well-type NaI(T1) crystal connected to a single channel analyzer was used to measure the counting samples with a single nuclide. For those counting samples with two radionuclides (Le., the metal investigated and copper), a Ge(Li) detector connected t o a 4096-channel pulseheight analyzer was used. The activity of *lOPbwas corrected for different absorptions by aqueous solution and by organic solution due to its low energy (47 keV).
RESULTS AND DISCUSSION
Extraction constants of various metal dithiocarbamates differ over several decade orders of magnitude (2-4,l.Z). The aim of the present work was t o develope a convenient and accurate method for determination of relatively low extraction constants for some metal dithiocarbamates. As described previously, the two-step extraction method with radiometry was found t o be rather promising for this purpose. With this method only the distribution ratios of the metals (Le., the metal under investigation and copper) in two phases were required. Radiometry seems to be the best available way to simultaneously determine these distribution ratios. This technique is analogous to our previous work on the determination of relatively high extraction constants of metal dithiocarbamates (12),where the only experimental data which had t o be measured were also the distribution ratios of the metals involved. T h e data and values for the determination of extraction constants for various metal dithiocarbamates in the chloroform/water system are shown in Tables I and 11. Usually the distribution ratio of the metal, r*M,in the first step of the extraction was measured by a NaI(T1) scintillation counter. However, the distribution ratios of the metal and copper, r.M' and rsCu',in the second step of the extraction were simultaneously measured by a Ge(Li) detector connected t o a 4096channel pulse-height analyzer. Due to the low stability of dithiocarbamic acid ( 5 , 18-20), it was necessary to spearate the organic phase in the first step of the extraction as soon as possible after centrifugation for 1 min, and then CU*+was added for the second step of the extraction. The reason for using cupric ion in the second step
Anal. Chem. 1980, 52, 1885-1889
is t h a t extraction constants of Cu(DTC)* are so high that practically all HDTC,,,, will be converted to Cu(DTC)2 and extracted easily and quantitatively into the organic layer. Thus the two-step extraction method described here provides a good way to determine the free-ligand concentration in the first step of the extraction. Radiometry incorporated into this technique makes the two-step extraction method more valuable. In principle, determination of extraction constants by the present method can be extended to other metal dithiocarbamates as long as their extraction constants are lower than t h a t of copper dithiocarbamate (7-12). Table I11 summarizes the K$iTC), and K M ( D T C ) , values obtained in the present study. The former was experimentally observed, whereas the latter was calculated by eq 5. It is found t h a t the extraction constants of the metal diethyldithiocarbamates are generally higher than those of the metal pyrrolidinecarbodithioates. All of the literature values are also listed in Table I11 for comparison.
LITERATURE CITED (1) Still, E. Suom. Kemistlseuran Tied. 1964, 73, 90-106. (2) Stary, J.: Kratzer, K. Anal. Chim. Acta 1968, 4 0 , 93-100
1885
(3) Ooms, P. C. A.; Brinkman, U. A. Th.; Daa, H. A. ECN[Rep.]1976, No. 77-018. (4) Ooms, P. C. A.; Brinkman, U. A. Th.; Das, H. A. Radiochem. Radioanal. Len. 1977, 31, 317-322. (5) Bajo, S . ; Wyttenbach, A. Anal. Chem. 1979, 57, 376-378. (6) Likussar, W.; Boitz, D. F. Anal. Chem. 1971, 4 3 , 1273-1277. (7) Wyttenbach, A.; Bajo, S. Anal. Chem. 1975, 4 7 , 1813-1817. (8) Wyttenbach, A.; Bajo, S. Anal. Chem. 1975, 47, 2-7. (9) Bode, H.; Tusche, K. J. Fresenius I?. Anal. Chem. 1957, 157, 41 4-427. (10) Wickbold, R. Fresenius' Z . Anal. Chern. 1956, 752,259-262. (11) Eckert, G. Fresenius' 2. Anal. Chem. 1957, 155, 23-25.. (12) Yeh. S. J.; Lo, J. M.; Shen, L. H. Ana/. Chem. 1980, 5 2 , 528-531. (13) Ringbom, A. "Complexation in Analytical Chemistry": Interscience: New York/London, 1963. (14) Bode, H. Fresenius' 2 . Anal. Chem. 1955, 144, 165-186. (15) Stary, J. "The Solvent Extraction of Metal Chelates"; Permagon Press: Oxford, 1964. (16) van Erkelens, P. C. Anal. Chim. Acta 1962, 26, 32-45. (17) Kuehner, E. C.; Alvarez, R.; Paulsen, P. J.; Murphy T. J. Anal. Chem. 1972, 4 4 , 2050-2056. (18) Aspiia, K. I.; Sastrl, V. S.; Chakrabarti, C. L. Talanta 1969, 16. 1099-1102. (19) Joris, S. J.; Aspiia, K. I.; Chakrabarti, C. L. Anal. Chem. 1969, 4 7 , 1441-1445. (20) Scharfe, R. R.; Sastri, V . S.; Chakrabarti, C. L. Anal. Chem. 1973, 45, 413-41 5.
RECEIVED for review June 26,1979. Accepted June 23,1980.
Glancing Incidence External Reflection Spectroelectrochemistry with a Continuum Source Joan P. Skully and Richard L. McCreery' Department of Chemistry, The Ohio State University, Columbus, Ohio 43210
A previous report demonstrated that light reflected from an electrode at small angles could be used to make optical absorption measurements on electrogenerated chromophores. A large sensitivity enhancement over previous methods was realized due to the relatively long effective path length. The present report discusses the extension of the technique to carbon and platinum electrodes and the use of a continuum rather than laser light source. For all combinations of source and electrode, the Sensitivity of the method is about 100 times that of a comparable experiment using an optlcally transparent electrode, and the magnitude of the enhancement can be calculated from geometric considerations. I n addition, the effect of the beam entering the side of the diffusion layer was considered, and it was found that this effect can be neglected if the experiment is designed properly.
T h e external reflection geometry has been used for spectroscopic monitoring of electrochemical events both a t the electrode surface and in the nearby solution (1-4). Conventional, single-pass reflection geometries have inherently low sensitivity due to short effective optical pathlength, but recent reports describe the use of multiple reflections ( 5 ) or glancing incidence angle (6, 7) t o improve the effective path length. When the light beam approaches the electrode at a small angle relative t o the surface, the light traverses a path in the diffusion layer which is long compared to the path for an optically transparent electrode (OTE) operated in the usual normal incident configuration. An initial report on this approach 0003-2700/80/0352-1885$01 .OO/O
demonstrated sensitivity enhancements of factors of 100-200 over an OTE experiment under similar conditions (6). The importance of this enhancement lies in the fact that absorbances for OTE spectroelectrochemical experiments are very small due to the short effective pathlength and become even smaller as the time of the experiment decreases (8). While OTE approaches have been very valuable for a variety of applications, their use for short-lived electrogenerated chromophores has been limited to special cases where strong absorbers are involved or where extensive time averaging is permitted (9). When weak chromophores or irreversible chemical reactions are involved, the OTE approach may not yield results with sufficient signal t o noise ratio. Since the absorbance for an electrogenerated chromophore is time dependent, it is convenient to compare sensitivities of spectroelectrochemical techniques to that for a normal incident beam passing through an CITE. For this geometry, the absorbance for a reduced species generated from a nonabsorbing oxidized species is given hy eq 1 ( I , 8). As shown
previously (6),the absorbance measured with a reflected beam is higher by a factor of 2/(sin a ) , where a is the angle of the beam relative to the electrode. This factor will be referred to as the enhancement of absorbance relative to an OTE, and the complete equation for the reflection approach is given by eq 2. The initial work using the glancing incidence geometry
C 1980 American Chemical Society
