Anal. Chem. 1994,66, 1242-1248
Dithiocarbamate Extraction and Au( I I I ) Back Extraction for Determination of Mercury in Water and Biological Samples by Anodic Stripping Voltammetry Jem-Mau Lo' and Jun-Der Lee Institute of Nuclear Science, National Tsing Hua University, Hsinchu 30043, Taiwan, Republic of China
A novel method employed for determining trace mercury in water and biological samples was developed in the study. The mercury was preconcentratedfrom the water samples and the digested biological samples through means of diethyldithiocarbamate extractionin conjunction with A 3 +back extraction. The preconcentrated solution, which simply contained the enriched mercury ion with the possible transition-metal ions and nonreacted Ad+, was next transferred to an electrochemical cell for detecting mercury via differentialpulse anodic stripping voltammetry (DPASV) through the usage of a gold film carbon electrode. A d + was selected as a back-extracting agent in light of its standing as highest extraction constant with diethyldithiocarbamate, being greater than that of Hg2+. Besides, the excess nonreacted A 3 + in the preconcentrated solution could be used in situ as the source of generating the gold film of the working electrode in the DPASV. Sufficient evidence of high feasibility in the determinationof mercury for different water samples was provided in this present work. Reliability and accuracy of this novel method were confirmed from the NBS biological standard reference materials analysis. Mercury detection has received extensive attention during the past 3 decades from environmental and biological related studies, partially owing to the menace associated with its high toxicity to human beings. Among the most sensitive analytical methods for mercury are cold vapor atomic absorption spectr~metry,I-~ cold vapor atomic fluorescence ~pectrometry,~ neutron activation a n a l y ~ i s ,etc. ~ - ~ Electrochemical analytical technique may essentially become yet another one of the conventional methods employed for detecting mercury in the category associated with high sensitivity. However, detection of mercury by electrochemical analysis has found scarce application and is rarely found in the routine analysis of either environmental or biological samples. Availability of a suitable working electrode for electrochemical analysis specific for mercury is apparently technically infeasible, as indicated from several previous studies.1°-21 These studies concluded that (1) Hatch, W. R.; Ott, W. L. Anal. Chem. 1968, 40, 2085-2087. (2) Topping, G.; Pirie, J. M. Anal. Chim. Acta 1972, 62, 2012-203. (3) Welz, B.; Melcher, M.; Sinemus, H. W.; Maier, D. At. Spectrosc. 1984, 5, 37-42. (4) Morita, H.; Sugimoto, M.; Shimomura, S.Anal. Sci. 1990, 6, 91-95. (5) Lo, J. M.; Wei, J. C.; Yeh, S.J. Anal. Chem. 1977, 49, 1146-1147. (6) Lo, J. M.; Wei, J. C.; Yeh, S.J. Anal. Chim. Acta 1978, 97, 311-316. (7) Yu, J. C.; Lo, J. M.; Wai, C. M. Anal. Chim. Acta 1983, 154, 307-312. (8) Lo, J. M.; Wei, J. C.; Yang, M. H.; Yeh, S. J. J . Radioanal. Chem. 1982, 72,
571-585.
(9) Rook, H. L.; Gills, T. E.; LaFleur, P. D. Anal. Chem. 1972,44, 11 14-1 117. (10) Vydra, F.; Stulikova, M.; Petak, P. J . Elecrroannl. Chem. 1972, 40, 99-106. (11) Luong, L.; Vydra, F. J . Electroanal. Chem. 1974, 50, 379-386. (12) Ulrich, L.; Ruegsegger, P. Z . Anal. Chem. 1975, 277, 349-353.
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gold is the preferable electrode material, with attempts made at fabricating it through several designs into a suitable working electrode for detecting mercury by anodic stripping voltammetry (ASV). For example, gold disk or rod electrodes,'@-l6 gold fiber e l e ~ t r o d e s , gold ~ ~ J ~twin disk electrode^,^^^^^ gold film carbon electrodes,21etc. have already been developed for this purpose. A preseparation or preconcentration procedure is generally considered as essential to couple with instrumental analysis in the determination of a trace amount of mercury in various samples, except for neutron activation analysis which can directly or nondestructively detect mercury in some of the matrices. The preconcentration procedure serves the dual purposes of removing the major matrix material as well as increasing the concentration of mercury for the subsequent instrumental analysis. A preconcentration procedure employed via a two-step extraction combining dithiocarbamate extraction and Hg(I1) back extraction was previously established and successfully applied in the analysis of environmental and biological samples for trace metal^.^^-^^ However, mercury is excluded from the scope of the detectable analytes via the procedure mentioned above in light of the fact that the metal itself functions as the back-extracting agent. In the present work, Au(II1) is used, in lieu of Hg(II), as the backextraction agent in a similar two-step extraction procedure. Subsequently, mercury can be enriched by the preconcentration procedure from the various samples. Furthermore, the preconcentration procedure employed through means of Au(II1) as back-extracting agent is found to be appropriately connected to the subsequent instrumental analysis by electrochemical analytical technique. Differential pulse anodic stripping voltammetry (DPASV), which employs a gold film carbon electrode as a working electrode, is applied toward detecting mercury in the preconcentrated solution. The proposed preconcentration technique notably possesses a unique feature that the Au(III), which has functioned in the (13) Fukai, R.; Herynh-Ngoc, L. Anal. Chim. Acta 1976, 83, 375-379. (14) Taddia, M. Microchem. J. 1978, 23, 64-70. (15) Golimowski. J.; Gustavsson, I. Z.Anal. Chem. 1984, 317, 481. (16) Jangner, D.; Josefsson, M.; Aren, K. Anal. Chim. Acta 1982, 141, 147-156. (17) Huliang, H.; Jagner, D.; Renman, L. Anal. Chim. Acta 1987, 201, 1-9. (18) Huliang, H.; Jagner, D.; Renman, L. Anal. Chim. Acta 1987,201,269-273. (19) Sipos, L.; Nurnberg, H. W.; Valenta, P.; Branica, M. Anal. Chim. Acta 1980, 115, 25-42. (20) Sipos, L.; Valenta, P.; Nurnberg, H. W.; Branica, M. J. Electroanal. Chem. 1977, 77, 263-266. (21) Allen, R. T.; Johnson, D. C. Talanra 1973, 20, 799-809. (22) Lo, J. M.; Yu, J . C.; Hutchison, F. I.; Wai, C. M. Anal. Chem. 1982, 54, 253&2539. (23) Labuda, J.; Vanickova, M. Anal. Chim. Acta 1988, 208, 219-230. (24) Lo, J. M.; Sun, H . J.; Lin, Y. P.: Wu, K. L. Anal. Chim. Acta 1989, 224, 133-139.
0003-2700/94/0366-1242$04.50/0
0 1994 American Chemical Society
role as the back-extracting agent, can also be in situ used as the source of gold film coated on a glassy carbon electrode for subsequent DPASV.
EXPER IMENTAL SECTION Reagents. The chemical compounds, HgO, ZnCl2, CdClTH20, Pb(N03)2, and CuC1~2H20 of analytical reagent grade were purchased from E. Merck Co., Germany. The stock solutions (ca. 100 mg/L) of the metal ions were prepared by dissolving appropriate amounts of the compounds in 0.10 M HNOs and diluting to 250 mL with pure water. This solution was then adjusted to pH 1.6 and stored in polyethylene bottles. Standard solutions of metals were prepared from the stock solution by appropriate dilution with pure water. Radioactive Hg2+ solution was obtained from the neutronirradiated HgO in the THOR reactor of the university and prepared in the same procedure as nonradioactive Hg2+ solution as mentioned above. Pure water was obtained from a two-stage distillation by a quartz apparatus of the deionized water which was produced via an ion exchange column and reverse osmosis equipment (Micropore, Inc., Hsinchu, Taiwan). All Pyrex glassware and polyethylene bottles were washed by a detergent solution, then immersed in 10% nitric acid for at least 24 h, and finally washed 3 times with pure water. Extracting Agent. Two derivatives of dithiocarbamate (DTC), sodium diethyldithiocarbamate (NaDDC) and ammonium tetramethylenedithiocarbamate (APDC) used were of analytical reagent grade (E. Merck Co.). One gram of NaDDC or APDC or both was dissolved in 100 mL of pure water, stirred well, and then shaken with 2 folds of 10 mL of chloroform. The organic phase was discarded, removing any possible impurity of mercury and any other metals, e.g. Zn, Cd, Pb, Cu, etc., already present in the reagents. The aqueous phase is referred to as the purified extracting agent used in the following. Back-Extracting Agent. Gold metal (99.99%) was purchased from Koch-Light Co., Colnbrook, England. An appropriate amount of the gold metal was weighted and dissolved in aqua regia. The solution was repeatedly evaporated on a water bath to near dryness so as to remove hydrochloric acid (adding water to avoid drying out). This solution was finally diluted with pure water and adjusted to pH 0.63 with 65% H N 0 3 . The Au(II1) solution was prepared to be 300 mg/L as a back-extracting agent. Its pH value was adjusted to 3 before use. Water Samples. Tap water was collected from the laboratory. Lake water came from Cheng Kung Lake on the campus, with seawater coming from the coast of the Taiwan Strait near Hsinchu, Taiwan. The water samples were taken and separately filled in a 20-L PE bottle. Subsequently, the water samples were filtered through a 0.45-pm Millipore membrane and stored after acidification to pH 1.5 with 1.0 M nitric acid. Biological Samples. NBS standard reference materials Citrus Leaves (SRM 1572), Tomato Leaves (SRM 1573), Pine Needles (SRM 1575), and Bovine Liver (SRM 1577a) were used. The biological samples were weighed, 0.5 g, and digested via a microwave machine (MDS-IlD, CEM Co.). The samples were placed into 120-mL Teflon PFA vessels into which 10 mL of 65% H N 0 3 and 5 mL of 37% HCl were
added. Immersion of the sample in the strong acids was maintained for at least 1 h. Thereafter the microwave digestion was conducted by adjusting the power to 50%and maintaining for 8 min, which was followed by 100% power for 4 min. The digested solution was then cooled down to room temperature. Finally, the solution was filtered by filter paper and adjusted to a pH value of 4.0. Apparatus. Voltammetric measurement was taken in an electrochemical cell, Le., a 20-mL covered beaker with the three electrodes (i.e., working, reference, and auxiliary) being immersed in the electrolyte solution. A glassy carbon electrode was employed as the working electrode, a silver/silver chloride electrode as the reference electrode, and an inert conductive material Pt wire as the auxiliary electrode. The three electrodes, as well as the tube used for N2 purging, were inserted through the four holes via an O-ring on the cell cover. The multipurpose microprocessor-based voltammetric analyzer used was purchased from Bioanalytical Systems, Inc. (Model BAS- 1OOB). Two-stepPreconcentration Procedure. A 200-mL portion of the water samples or the digested biological samples (both unspikedor spiked with a certain amount of Hg2+) was placed into 250-mL Pyrex separator funnels. The solutions were adjusted to an appropriate pH value by 0.0 1 M HNO3 and/or 0.01 M NaOH. A 4-mL portion of the purified extracting agent was then added. Precisely 20 mL of chloroform was added into the funnels and the mixture was shaken vigorously by an up-and-down shaking machine for 20 min. After a 5 min wait to allow the phases to separate, the organic phase was transferred into flask and 5 mL of a 300 mg/L Au3+ solution was then ad ed for back extraction. The solution mixture was shaken for 20 min which was sufficient enough for completing the back extraction. The separated aqueous phase was adjusted to a pH value of 1.O with 5.0 M HNO3, with 0.5 mL of 0.5 M KCl being added. This finally accumulated to 10 mL with pure water. The solution was immediately transferred to the electrochemical cell for DPASV. Differential pulse Anodic Stripping Voltammetry (DPASV). The solution mentioned above was purged with N2 and stirred for at least 5 min prior to analysis by DPASV. The glassy carbon electrode was polished with a very fine paper (wet and dry), alumina, and finally diamond paste before use. Analysis of Hg2+ in the preconcentrated solution was started by electrochemical formation of a gold film on the glassy carbon electrode by applying a deposition potential at -1000 mV for 150 s. Simultaneously, the Hg2+ ion was reduced to form HgO or else became amalgamated in the gold film during the deposition step. After deposition was terminated, the solution was purged with N2 to drive off any bubbles adhered on the gold film, and the mercury was then stripped from the gold film back into the solution. Differential pulse anodic stripping voltammetry was employed for the measurement of the mercury under the following conditions: stripping potential from -1000 to 700 mV, scan rate 20 mV/s, pulse amplitude 50 mV, sample width 17 ms, pulse width 50 mV, pulse period 200 ms, and stirring rate 20 rpm. The mercury was identified by the stripping potential at 450-500 mV and quantified by the corresponding oxidation peak current. Besides the analysis for Hg2+ in the preconcentrated solutions, the blanks including 0.025 M KCl and nonreacted Au3+ were also situated in the
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AnaEytical Chemistty, Vol. 66, No. 8, April 15, 1994
1243
Table 1. Efflclencles of Extractlon by DTC and Consecutlve Back Extractlons by Au3+ for Mercury In Varying pH Medla pH 1.0 pH 2.0 pH 3.0 pH4.0 pH 5.0 pH6.0 pH 7.0 1.extraction by NaDDC + APDC ( % ) back extraction (5%) 2. extraction by NaDDC (% ) back extraction ( % ) 3. extraction by APDC (7%) back extraction (%)
99.7 1.1
81.0 98.0 99.9 0.2
99.6 1.6 99.8 99.0 99.9 0.2
medium of 0.025 M KCl and 90.00 mg/L Au3+with the pH value being adjusted to 1.0 and run through the DPASV procedure.
RESULTS AND DISCUSSION The two-step extraction procedure (Le., extraction by DTC and back extraction by metal ion) has been previously verified as being an efficacious preconcentration technique employed for enriching trace metals in environmental or biological samples with simultaneous elimination of substantial amounts of matrix components. It is noted that the metal ion employed as the back-extracting agent is selected on the basis of the rationale that the stability constant or extraction constant of the DTC complex of the metal is high and greater than those of the metals of interest to be analyzed. For example, Hg2+ and Pd2+ have been used in previous works as the backextracting agent simply as a result of their standing with highstability constants with dithiocarbamate.22-26 Determining the degree or order of extraction constants of the DTC complexes of various metals was the focus of several previous research effort^.^^-^^ The practical analytical works involving separation based on the magnitude of extraction constant of metal dithiocarbamates have also been given a great deal of attention, Le., both the usages of the metal ion as a backextracting agent as previously mentioned22-26and of metal dithiocarbamate as an agent for selectively extracting metal ion of i r ~ t e r e s t . ~However, - ~ ~ ~ ~ surprisingly ~~~ gold is never used as the agent in any of the analytical works despite the fact that the extraction constant of gold dithiocarbamate is recognized as the highest among all of the metal dithiocarbamates investigate^.^^-^^ All of the metals which can form a complex with dithiocarbamate should be referred to as the detectable analytes when Au3+ is used as the back-extracting agent; in addition, a certain instrument is employed for the subsequent detection. This speculation has become realized in one of our recent works determining trace metals in seawater by inductively coupled plasma atomic emission spectrometry in conjunction with a preconcentration procedure by DTC extraction associated with Au3+ back e ~ t r a c t i o n .Besides, ~~ the Au3+ back extraction is found to be particularly useful in the analysis for ( 2 5 ) Lo, J. M.; Lin, Y. P.; Lin, K. S.Anal. Sci. 1991, 7, 455458. (26) Sachsenberg, S.;Klenke, Th.; Krumbein, W. E.; Zeeck, E. Fresenius’ J. Anal. Chem. 1992, 342, 163-166. (27) Shen, L. H.; Yeh S. J.; Lo, J. M. Anal. Chem. 1980, 52, 1882-1885. (28) Yeh, S. J.; Lo, J. M.; Shen, L. H. Anal. Chem. 1980, 52, 528-531. (29) Lo, J. M.; Shen, L. H.; Yeh, S. J. J . Radioanal. Nucl. Chem. 1985,88,313-
323. (30) Lo, J. M.; Wai, C. M. Anal. Chim. Acta 1983, 148, 327-330. (31) Briscoe, G. E.; Humphries, S . Talanro 1969, 16, 1403-1419. (32) Lo, J . M.; Tseng, C. L.; Yeh, S. J. Anal. Chim. Acta 1981, 126, 191-197. (33) Chermette, H.; Colonat, J.; Tousset, J . Anal. Chim. Acta 1977.88, 331-338. (34) Lo. J. M.; Lin, C. C.; Yeh, S. J. Anal. Chim. Acta 1993, 272, 169-178. (35) Wyttenbach, A.; Bajo, S.Anal. Chem. 1975, 47, 2-7. (36) Wyttenbach, A.; Bajo, S.Anal. Chem. 1975, 47, 1813-1817. (37) Lo, J. M.; Lin, K. S.; Wei, W. C. Unpublished work.
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Analytical Chemistry, Vol. 66, No. 8, April 15, 1994
99.8 98.5 99.8 99.2 99.8 97.8
99.8 98.1 99.7 99.6 99.9 98.2
99.9 97.9 99.9 99.3 99.9 97.8
99.9 97.8 99.9 99.7 99.8 97.9
99.6 98.4 99.8 99.3 99.9 98.4
pH 8.0 99.9 97.8 99.8 99.2 99.9 97.8
those of metals with a high extraction constant with DTC, e.g. Hg, Pd, ..., etc. As demonstrated in this present work, mercury in its form of DTC complex in chloroform becomes back extractable by Au3+, probably the sole metal ion being employable as the back-extracting agent. The feasibility of determining trace mercury in water and biological samples by a preconcentration procedure with DTC extraction and Au3+ back extraction coupled with anodic stripping voltammetry is demonstrated in the following. The efficiency of extracting mercury either in pure water or in seawater by dithiocarbamate-chloroform solution and consecutive back extracting by Au3+ was investigated by a radiotracer technique by means of radioactive 1989203Hg. These results are listed in Table 1. The extraction efficiency of Hg2+ by a mixture of DTC derivatives, NaDDC and APDC, could reach as high as 199% in the pH value range from 1 to 8. However, the back-extraction efficiency was found to be low at a pH value below 2 although higher than 99% at a pH value higher than 3. A close observation revealed that the Hg2+ could be extracted by NaDDC alone and back extracted by Au3+ both at high efficiency 199% from a pH value of 8 to even at pH down to 2. The Hg2+could also be extracted by APDC alone with 199% high efficiency through theentire pH value range 1-8. However, thesubsequent back extraction by Au3+ accompanying the extraction at pH 1-2 was found to be quite inefficient with efficiency as low as 510%. It is noticed here that NaDDC alone is a better choice as the extracting agent in the present work for determining mercury than that of using a mixture of NaDDC and APDC. This is despite the fact that the latter is generally accepted as the extraction agent for transition-metal ions.22,24-25 The result mentioned is equally applicable to both the cases of analyzing pure water and seawater. The separation of mercury by the two-step extraction is noted not to be affected by chloride ion, although both Au(II1) and Hg(I1) form strong chloride complexes in the water phase. This phenomenon is rationalized by acknowledgment of the exceptionally high stability constants being associated with Au(II1) and Hg(I1) dithioc a r b a m a t e ~ ~ which ~ , ~ ~is, even ~ ~ -over ~ ~ the sufficiently large stability constants of the chloride complexes of both metal^.^^,^^ The conditions suitable for the DPASV following the preconcentration procedure were systematically investigated in this work. Practically, the concentration of Au3+ should be controlled at a magnitude for the formation of an integral gold film as an efficient working electrode. The relationship between the stripping peak currents of mercury and the concentration of Au3+ in the electrolyte solution of 0.025 M KC1 at pH 1.O with deposition of 150 s is provided in Figure 1. The peakcurrent of Hg could reach maximum and maintain (38) Skibsted, L. H.; Bjerrum, J. J . Indian Chem. SOC.1977, 54, 102-108. (39) Smith, R. M.; Martell, A. E. Critical Stability Constants; Plenum: New York, 1976; Vol. 4.
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itself constant when the concentration of Au3+ rose to greater than 60.00 mg/L up to at least 150.00 mg/L. The Au3+ at even a higher concentration is predicted to not affect the peak current of Hg since the stripping potential of Au appears at 876 mV, which is higher than that of Hg at 450-500 mV (Figure 2). Theconcentration of Au3+as the back-extracting agent was chosen at 150.00mg/L in the practical analytical works. That is, a small portion of the gold would be consumed in the preconcentration procedure; however, a large portion (190 mg/L) would be remaining and subject to transfer to the electrolyte solution. Besides, the Au3+ could be used in situ as the source of gold film coated on the glassy carbon electrode, rendering it as an efficacious working electrode for DPASV. The effect of pH on the stripping peak current of mercury in the electrolyte solution of 0.025 M KCl containing 90.00 mg/L Au3+ is demonstrated in Figure 3. The peak current maintained itself at a constant value in acidic solution with a pH value
