Anal. Chem. 1984, 56, 1127-1131 (4) Wangen, L. E.; Woodward, W. S.; Isenhour, T. L. Anal. Chem. 1971, 43, 1605-1614. ( 5 ) Naegell, P. R.; Clerc, J. T. Anal. Chem. 1974, 46, 739A-744A. (6) Gronneberg, T. 0.; Gray, N. A. E.; Eglinton, G. Anal. Chem. 1975, 4 7 , 415-41 8. (7) Pesyna, G. M.; Venkataraghavan, R.; Dayvinger, H. E.; McLafferty, F. W. Anal. Chem. 1976, 4 8 , 1362-1368. (8) Blaisdell, 8. E. Anal. Chem. 1977, 49, 180-186. (9) McLafferty, F. W. Anal. Chem. 1977, 4 9 , 1441-1443. (10) Van Marlen, G.; Van Den Hende, J. H. Anal. Chim. Acta 1979, 772, 143-1 50. (11) Blaisdell, E. E.; Gates, S. C.; Martin, F. E.; Sweeley, C. C. Anal. Chim. Acta 1980, 777, 35-43. (12) Raznikov, V. V.; Tal’roze, V. L. Dokl. Phys. Chem. (Engl. Trans/.) 1966, 770, 597-599. (13) Jurs, P. C.; Kowalski, B. R.; Isenhour, T. L. Anal. Chem. 1969, 4 7 , 21-27. (14) Jurs, P. C.; Kowalski, E. R.; Isenhour, T. L.; Rellley, C. N. Anal. Chem. 1969, 4 7 , 1949-1953. (15) Bender, C. F.; Shepherd, H. D.; Kowalskl, E. R. Anal. Chem. 1973, 45, 617-618. (16) Lam, T. F.; Wilkins, C. L.; Brunner, T. R.; Saltberg, L. J.; Kaberline, S. L. Anal. Chem. 1976, 4 8 , 1768-1774. (17) Gray, N. A. E. Anal. Chem. 1976, 48, 2265-2268. (18) Ritter, G. L.; Isenhour, T. L. Computers Chem. 1977, 1 , 243-250. (19) Lpwry, S. R.; Isenhour, T. L.; Justice, J. E.; McLafferty, F. W.; Dayringer, H. E.; Venkataraghavan, R. Anal. Chem. 1977, 49, 1720-1 722. (20) Bugard, D. R.; Perone, S. P.; Wiebers, J. L. Blochemistry 1977, 76, 1051-1057. (21) Buchs, A.; Duffield, A. M.; Schroll, G.; DJerassi,C.; Delfino, A. E.; Buchanan, E. G.; Sutherland, G. L.; Felgenbaum, E. A.; Lederberg, J. J . Am. Chem. SOC. 1970, 92, 6831-6838. (22) Delfino. A. B.; Buchs, A. B. Heiv. Chim. Acta 1972, 55, 2017-2029. (23) Buchanan, E. G.; Smith, D. H.;White, W. C.; Gritter, R. J.; Feigenbaum, E. A.; Lederberg, J.; DJerassi, C. J. Am. Chem. SOC. 1976, 98, 6168-6178. (24) Dayringer, H. E.; McLafferty, F. W. Org. Mass Spectrom. 1977, 72, 53-54.
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(25) Gribov, L. A.; Elyashberg, M. E.; Serov, V. V. Anal. Chlm. Acta 1977, 96, 75-96. (26) Date, C. J. “An Introduction to Database Systems”; Addison-Wesley: Reading MA, 1975. (27) Martin, D. “Database Design and Implementation”; Van NostrandRelnhold: New York, 1980. (28) Martin, J. “Computer Data-base Organization”; Prentice-Hall: Englewood Cliffs, NJ, 1977. (29) Ullman, J. D. “Principles of Database Systems”; Computer Science Press: MD, 1980. (30) Yost, R. A.; Enke, C. G. Anal. Chem. 1979, 5 7 , 1251A-1264A. (31) Wong, C. M.; Crawford, R. W.; Barton, V. C.; Brand, H.R.; Neufeld, K. W.; Bowman, J. E. Rev. Sci. Insfrum. 1983, 5 4 , 996-1004. (32) Brand, H.R.; Yamauchi, R. K. “PLTLIB Graphics Library for Tektronix 4025 and 4027 Terminals”; Lawrence Livermore National Laboratory: Livermore, CA, 1981; UCID-18981. (33) Atkinson, T. V. “MULPLT: A Multiple Data Set, Flle Based Data Plotting Program”; Department of Chemistry, Michigan State University: East Lanslng, MI, 1981.
RECEIVED for review March 25, 1983. Resubmitted October 31, 1983. Accepted January 30, 1984. Work performed at Lawrence Livermore National Laboratory was under the auspices of the U S . Department of Energy under Contract No. W-7405-ENG-48. Work performed at Michigan State University was partially supported by National Institutes of Health Grant GM28254. Reference herein to any specific commercial products by manufacturer does not imply its endorsement by the United States Government or the University of California. The opinions of authors expressed herein do not necessarily reflect those of the United States Government and shall not be used for advertising or product endorsement purposes.
Lead-Selective Neutral Carrier Based Liquid Membrane Electrode Ern6 Lindner, Kl6ra T6th, and Ern6 Pungor Institute for General and Analytical Chemistry, Technical University, H-1521 Budapest, Hungary Felix Behm, Peter Oggenfuss, Dieter H. Welti, Daniel Ammann, Werner E. Morf, Ern6 Pretsch, and Wilhelm Simon* Department of Organic Chemistry, Swiss Federal Institute of Technology, CH-8092 Zurich, Switzerland
Certain synthetlc, llpophllic oxa- and dloxadlcarboxyllc amides act as lead-selectlve neutral carrlers In liquid-membrane electrodes. Lead Is detected as monovalent permeating specks of the type PbX’ (X: OH-, CI-, NO,-, CH,COO-). Membranes based on N,N-dloctadecyl-N’,N’-dlpropyl-3,6dloxaoctanedlamlde reject alkali metal Ions by a factor of at least lo3 and alkaline-earth metal Ions by at least lo4.
Several electrically neutral crown ethers (1-4) and a large number of nonmacrocyclic ionophores based on dioxa diamides and related compounds have found diverse applications in ion-selective electrodes especially for group 1A and 2A cations (5). Analytically relevant selectivity for UO?+ has been found for membranes based on selected dioxa diamides (6, 7). As expected, a replacement of coordinating oxygen atoms by sulfur atoms leads to selectivity of certain ligands for transition or B metal ions such as Cd2+ (8). Although dicyclohexyl-18-crown-6 complexes Pb2+clearly stronger than group 1A and 2A cations (9),there is no report on an ana-
lytically relevant lead sensor based on the incorporation of this ligand into membranes. Here we describe a liquidmembrane electrode with relevant selectivity for lead which is based on a nonmacrocyclic ion carrier.
EXPERIMENTAL SECTION Reagents. Double quartz distilled water and chemicals of the highest purity available were used throughout. Synthesis. The structures of ligands 1 to 6 are given in Figure 1. The synthesis of ligands 2 (IO),3 ( I I ) , 4 (IO), and 5 (12) has been described earlier. Ligand 1 was prepared according to the procedure in ref 11. The ‘H NMR, I3C NMR, IR, and mass spectra of the product are in accordance with its structure. Anal. Calcd for CzzH4,Nz04 (400.60): C, 65.94; H, 11.07; N, 6.99. Found: C, 65.52; H, 11.04; N, 6.92. Ligand 6 was prepared according to the procedure in ref 13. The ‘H NMR, 13CNMR, IR, and mass spectra of the product are in accordance with its structure. Anal. Calcd for C51H102N204 (807.39): C, 75.87; H, 12.72; N, 3.47. Found: C, 75.62; H, 12.29; N, 3.33. Membranes. The matrix of the membranes was PVC, S 704, high molecular (Lonza AG, Basel, Switzerland), with bis(1-bu-
0003-2700/84/0356-1127$0 l .50/0 0 1984 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
3 -
u u Figure 1. I o n carriers discussed.
tylpentylladipate (BBPA) (purum, Fluka AG, Buchs, Switzerland) as plasticizer. Two different concentrations of ionophore have been used for the transport experiments (3 w t 9'0 ionophore, 64 wt % BBPA, 33 wt % PVC) and the potentiometric measurements (1wt % ionophore, 66 wt % BBPA, 33 wt % PVC). The membrane preparation has been described earlier ( 1 4 ) . Electrode Systems. Cells of the type Hg; Hg&, KC1 (satd)lbridge electrolytelsample solutionllmembranellO.1M MgC12, 0.001 M Mg(OAc), (PH 4.0), AgC1; Ag have been used. The external reference electrode was a double-junction calomel electrode, Philips R 11. The pH determinations were carried out with a Philips GA 110 glass electrode. The following bridge electrolytes were used: 3 M KC1 for the determination of selectivity factors and electrode functions in chloride solutions, and 1M KN03 for electrode response studies in chloride-free solutions. EMF Measurements. The EMF measurements were carried out at 20 2 "C using a 16-channel electrode monitor; each channel was equipped with a FET operational amplifier AD 515 KH (Analog Devices, Norwood, MA; input impedance, lo1' 0 / 2 pF; bias current, C150 fA; capacity neutralization). Data acquisition was performed with an Intel Data System (Intel Corp., Santa Clara, CA) in combination with a display terminal, ADDS Regent 20 (Applied Digital Data Systems Inc., Hauppauge, NY), a Wenger Print Swiss Matrix Printer (Wenger Datentechnik, Basel, Switzerland), and our own software. Activity Coefficients. The activity coefficients yi of the ions in aqueous solutions were calculated according to the DebyeHuckel approximation -0.51.~tI~/~ log yi = (1) 1 + 0.33aZ1/2 where I = 0.5xi z:ci is the ionic strength. The parameters, a,used are listed in Table I. Values for the complexes of lead have been estimated. Complexation Equilibria. The activities of lead ion and its associates in solution at equilibrium have been calculated for a given total lead concentration by using an iterative algorithm. The thermodynamic complex formation constants listed in Table
*
Table I. Values of the Parameters a, as Used in Equation 1
a
ion
a
OH-, Cl-, NO; CH,COO' Hg2+ PbZ+ PbCl+, PbNO,', PbOH+,Pb(CH,COO)+
3.0 5.0 8.0 4.0 4.0a
Estimated.
Table 11. Thermodynamic Formation Constants, K , for the 1:l Associationa of pb2+with Different Anions (15, 1 6 ) anion
log K
c1NO; CH,COO' OH'
1.10 1.10 2.05 7.51
a Higher associates were not taken into account since their influence is small under the experimental conditions used here.
I1 and the activity coefficients defined above were applied. Selectivity Factors, Electrode Response. The selectivity factors were determined by the separate solution method (SSM) using a 0.001 M Mg(OAc), buffer of pH 4.0 ( 1 7 ) . The concentration of the aqueous metal chloride solutions was 0.1 M, except for PbClz where it was 0.01 M. Experimental data were corrected for changes in the liquid-junction potential using the Henderson formalism (18). Transport Experiment. Cell Assemblies. In the electrodialysis experiments, the cell consisted of two electrolyte compartments (cylinders of 2.5 cm diameter and 5 cm length) separated by a cation-permselective membrane (disk of 0.5 cm diameter and 0.01 cm thickness). Both compartments were equipped with Ag, AgCl electrodes (disks of 1cm diameter) and provided with magnetic stirring.
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
NMR Measurements. 13Cand 'H NMR spectra were recorded at 50.323 MHz and 200.133 MHz, respectively, on a 4.70-T Bruker WP 200 SY Fourier transform spectrometer. A 10-mm NMR tube containing 97.4 mg (0.243 mmol) of 1 in 2.099 g (47.67 mmol) of deuterated acetonitrile and 2 drops of MelSi was pepared. After initial 13C NMR and lH NMR spectra were obtained, a small amount of lead nitrate was added. The sample was spun in order to accelerate dissolution for at least 6 h before the next set of spectra were taken. This process was repeated until no more salt could be dissolved. The corresponding final shifts are represented by broken lines since no exact final concentration can be given. 13C NMR Measurements. The following conditions applied: spectral width, 10.0 kHz; acquisition time, 1.6384 s; data table size, 32K 24-bit words; radio frequency pulse duration, 12 p s (ca. 40'). Broad band decoupling of approximately 2 W with 250 L/h ambient temperature air cooling was applied during the acquisition period. In order to reduce the heating of the sample by proton irradiation, the decoupler power was switched to the minimum 0.5 W required to maintain the NOE for 5.0 s after each acquisition period. Usually about 1500 transients were taken. MelSi was used as internal shift standard. 'H NMR Measurements. 'H NMR spectra were obtained using the decoupler coil of the 10-mm broad band probe head as transmitter/receiver coil, so that both nuclei could be measured on the same sample without need to exchange probe heads. The quality of the resulting lH spectra was sufficient 40 measure accurate shift differences: spectral width, 2.0 kHz; dcquisition time, 4.096 s; 16 K data table size. A relaxation delay of 5.0 s was used. A total of 128 transients resulting from 2.0 p s rf pulses (ca. 8.4') were accumulated at approximately 22 "C.
log [Pb2']k
-2
-4
-6
-0
- 10 0
1
2
3
4
5
1129
P
Figure 2. Ion activities, calculated by using the parameters given in Tables I and 11, as a function of pH for two total lead concentrations for the system PbCI,, HCI, M Mg(OAc),.
Procedure. For the determination of the cation transport numbers on the specified carrier membranes, the anode compartment of the electrodialysis cell was filled with a solution of M Pb(OAc), whereas a 10" M Mg(OA& solution was used in the cathode compartment. A constant voltage was applied over the membrane. The transfer of the PbZt-carrying species through the membrane led to an increase of the total lead concentration in the cathode compartment which was studied by flameless atomic absorption spectrometry. Three to six probes of 10 p L were collected every 15 to 70 min. Finally, the transference number of Pb2+was obtained as the ratio of the charge equivalent of transferred species and the current-time integral. Appuratus: power supply, batteries with regulator; digital voltmeter type 2433 Philips, Eindhoven, The Netherlands; amperemeter, type 150 B, Keithley Instruments, Cleveland, OH; recorder, type 1100 W+W electronic, Basel, Switzerland; atomic absorption spectrometer, type 300 (graphite cell type HGA 72), Perkin-Elmer, Ueberlingen, West Germany (GFR).
RESULTS AND DISCUSSION The hydrolysis of lead salts in aqueous solutions sets an upper limit to the pH range within which electrode measurements can be effected; the lower limit is given by hydrogen ion interference. In addition, Pb2+associates with anions Apresent in the sample solution (Figure 2). The selectivity factors, as given in Figure 3 for membranes based on ligands 1 to 6 (Figure l),have therefore been determined on 0.1 M solutions of the metal chlorides (except for PbC12where the
'Ot
log
pH 4.0 0,001M Mg(0AC)Z
'bAM
1
0
-PbA'
-PbA'
-PbA'
-F bA'
-PbA'
-PbA'
3 -
4 -
5 -
6 -
-PbA*
-1
-2
-3
-4
-4
-6
1 -
2 -
00PA
Figure 3. Selectlvity factors, log ftA,, for solvent polymeric membranes based on ligands 1 to 8 (column 7: ligand-free membrane). Equation 2 was used for the evaluation of the data. The 0.1 M solutions of the chlorides were buffered to pH 4 with M Mg(OAc),/HCI. For PbCi,, lo-* M solutions were applied and the EMF values were extrapolated to IO-' M assuming a theoretical slope of 59 mV.
ANALYTICAL CHEMISTRY, VOL. 56, NO. 7, JUNE 1984
1130
Table 111. Potentiometric Selectivity Factors,a , a Liquid-Membrane Electrode Based log K & & H . ~for on Ligand 2
---8
-6
-4
-12
-8
-10
a.
-6
log aPbOH
log aPbOH
b.
-8
-6
-4 log ‘PbOH
-1
-4.8 H’ -4.2 a Equations 3 and 4 were used for the curve-fitting procedure. The E , values obtained thereby are 450 mV and 350 mV, respectively.
C.
Figure 4. EMF response of a liquid membrane electrode cell based on ligand 2. PbOH’ acthritles were calculated by using the parameters given In Tables I and 11. The response curve was calculated with the $$H,J values In Table 111.
concentration was 0.01 M) using a 0.001 M magnesium acetate/hydrochloric acid buffer of pH 4.0. Under these conditions a slope of the electrode response to solutions of lead chloride of 57.6 f 3.5 mV (standard deviation; range of linear regression, 10-5-10-2 M PbCl,; Pb2+ activities calculated by using parameters in Table I and 11) was measured. This indicates that monovalent cations are the predominant permeating species. Evidently, carrier complexes with ion pairs of the type PbA+ are formed in the membrane. Since all anionic species present in the sample solution (Cl-, OH-, and CH,COO-) form relatively stable associates with Pb2+ (see Table 11), it is not easy to assess in detail the individual contributions to the electrode response. Therefore the selectivity factors given in Figure 3 were estimated by using a simplified equation
where the subscript PbA refers to the total contribution to the cell potential by all lead monocomplexes. Figure 3 clearly shows that carriers 1to 6 induce selectivity for PbA+ beyond that characteristic of a corresponding membrane without ionophores (column 7 in Figure 3 ) . Membranes based on ligand 2 are by far the best in terms of lead selectivity. This ligand was studied in greater detail. In order to evaluate the contributions by the different lead species, the electrode response has been described by the following extended equation:
K$gbH,PbOAcaPbOAc
log KPP6H.J -1.8 -1.8
J PbCl* PbNO: PbOAc’ Pb2+
Table IV. Transport Numbersa for PbZ+at Different Transmembrane Voltages as a Function of the pH ligandb
pH
1
3.6
1v
3v
7v
0.46 t 0.06
0.60 * 0.17 1.86 t 0.10
0.42 I0.03 1.17 t 0.12 0.85 2 0.05 1.65 t 0.11
5.4 3.6
2
5.4
1.22 t
0.12
a Mean of 5-7 values measured within a maximum of 5 h, 95% confidence limits. Membrane compositions: 3 wt % 1, 63.4 wt % BBPA, 33.6 wt % PVC; 2.9 wt % 2, 64.4 wt % BBPA, 32.7 wt % PVC.
EMF
EMF
--
-7
-5
-3
-1
-7
-5
-3
-1
‘‘9 aPbN03
b. Figure 5. EMF response of a liquid membrane electrode cell based on ligand 2. &NO3+ and Pb2+activities were calculated by using the parameters given in Tables I and 11. The same response is plotted as a function of PbOH’ in Figure 4b.
+ KfgbH,PbaPb1’2 + $K!gbH,flJ1’zJ) (3)
For the sake of a more precise estimation of the novel selectivity factors introduced here, further EMF measurements were carried out on Pb(N03)2solutions at different pH values (see Figure 4). The corresponding electrode responses were described by eq 4
The selectivity factors compiled in Table I11 were obtained by fitting the curves in Figure 4 on the basis of eq 3 and 4, respectively. Figure 4b indicates that a satisfactory fit is obtained even for sample solutions of pH 1.2. The electrode response curves in Figure 4 are given in terms of the calculated PbOH+ activities, since this is the ion present
in all three cases. This does not imply, however, that the electrode selectively measures PbOH+ under these conditions. In fact, the response data can also be plotted vs. the logarithm of the activities of PbN03+ and Pb2+ (Figure 5 ) . These response curves suggest that all species contribute to the EMF. Further evidence for the presence of monovalent permeating species was obtained from electrodialysis experiments (Table IV). The transport numbers obtained for Pb2+with a membrane based on ligand 2 approach a value of 2 at a relatively high pH value of the sample solution, corroborating the transference of monovalent permeating species of the type PbA+. Transport numbers below 1, as obtained at low pH values indicate that lead-free species partly contribute to the electric current across the membrane. Ligands of the type 2 indeed interact with lead. Because of the low solubility of 2 in CD3CN, an NMR study of this interaction was carried out on ligand 1. When increasing
1131
Anal. Chem. 1084,56,1131-1 135
LITERATURE CITED
A '13~
[ppml
20
10
0 0
02
04
06 MOLES Pb(N03)2 / MOLES LIGAND
Figure 6. Chemical shifts of the carbonyl carbon atoms and of the CH,CO protons of ligand 1 induced by the addition of Pb(NO& in CD,CN. The dashed lines represent values measured on a saturated solution.
amounts of Pb(N0J2 are added to a fixed amount of 1 in CD,CN, rather large 13Cand lH chemical shift changes of the ligand are induced up t o a molar ratio of near 1:l (Figure 6). The initial slopes of the curves in Figure 6 indicate, however, that the complexes formed are not exclusively a 1:1 stoichiometry. memA study has shown that brane electrodes of the type described here are attractive for the end-point detection in the titration of sulfate with aqueous solutions of lead ions. Registry No. 1,43133-06-8; 2,72469-41-1; 3,58726-79-4; 4, 72469-42-2; 5, 74267-27-9; 6, 89322-23-6; pbC1+, 19511-77-4; PbN03+, 12311-63-6; PbOH+, 12168-64-8; Pb(CHSCOO)+, 72954-17-7; Pb2+,14280-50-3; Pb, 7439-92-1.
(1) Petrgnek, J.; Ryba, 0. Anal. Chim. Acta 1074, 72, 375-380. (2) Masclni, M.;Pallozzi, F. Anal. Chim. Acta 1074, 73, 375-362. (3) Tamura, H.; Kimura, K.; Shono, T. J . Electroarlal. Chem. Interfacial Electrochem. 1080, 115, 115-121. (4) Fung, K. W.; Wong, K. H. J . Necfroanal. Chem. Interfaclal Electrochem . 1980, 11 1 , 359-368. (5) Ammann, D.; Morf, W. E.; Anker, P.; Meler, P. C.; Pretsch, E.: Simon, W. Ion-Sel. Electrode Rev. 1083, 5 , 3-92. (6) Senkyr, J.; Ammann, D.; Meier, P. C.; Morf, W. E.: Pretsch, E.: Simon, W. Anal. Chem. 1979, 51, 786-790. (7) Bertrand, P. A.; Choppin, G. R.; Rao, L. F.; Bunzii, J. G. Anal. Chem. 1083, 5 5 , 364-367. (8) Schneider, J. K.; Hofstetter, P.; Pretsch, E.; Ammann, D.; Simon, W. Helv. Chlm. Acta t080, 63, 217-224. (9) Christensen, J. J.; Hill, J. 0.; Izatt, R. M. Science 1071, 174, 459-467. (IO) Oesch, U.; Ammann. D.; Pretsch, E.; Simon, W. Helv. Chim. Acta 1070, 62, 2073-2078. (11) Ammann, D.; Bisslg, R.; Guggi, M.; Pretsch, E.; Simon, W.; Borowitz, 1. J.; Weiss, L. Helv. Chlm. Acta 1075, 58, 1535-1548. (12) Pretsch, E.; Ammann, D.; Osswald, H. F.; Guggi, M.; Simon, W. Helv. Chlm. Acta 1080, 63, 191-196. (13) Kirsch, N. N. L.; Funck, R. J. J.; Pretsch, E.; Simon, W. Helv. Chim. Acta 1077, 60, 2326-2333. (14) Erne, D.; Morf, W. E.; Arvanitls, S.; Clmerman, 2.; Ammann, D.; Simon, W. He&. Chlm. Acta 1070, 62, 994-1006. (15) Sili6n, L. Q.; Marteii, A. E. "Stability Constants of Metal-Ion Complexes"; The Chemical Society, Burlington House: London, 1964; Speclai Publication No. 17. (16) Siil6n, L. G.; Marteii, A. E. "Stablilty Constants of Metal-Ion Complexes Supplement No 1"; The Chemical Society, Burilngton House: London, 1971; Speck1 Publication No. 25. (17) Guiibauit, 0. G.; Durst, R. A.; Frant, M. S.; Freiser, H.; Hansen, E. H.; Light, T. S.; Pungor, E.; Rechnitz, G.; Rice, N. M.; Rohm, T. J.; Simon, W.; Thomas, J. D. R. Pure Appl. Chem. 1078. 46, 127. (18) Morf, W. E. "The Principles of Ion-Selective Electrodes and of Membrane Transport. Studies In Analytical Chemistry 2"; Akad6mlal KiadB: Budapest (Elsevier: Amsterdam, New York), 1981.
RECEIVED for review December 12,1983. Accepted February 1, 1984. This work was partly supported by the Schweizerischer Nationalfonds zur Forderung der wissenschaftlichen Forschung.
Differential Pulse Anodic Stripping Voltammetry of Cadmium(I I) with a Rotating Membrane-Covered Mercury Film Electrode Edward E. Stewart and Ronald B. Smart*
Department of Chemistry, West Virginia University, Morgantown, West Virginia 26506-6045
A rotatlng membrane-covered mercury fllm electrode (MCMFE) was constructed by placlng a dlalysls membrane over a glassy carbon rotatlng disk electrode and plating a thin mercury fllm onto the electrode surface through the membrane. Dlfferentlal pulse anodlc strlpplng voltammetry of cadmhm was used to evaluate the effects of pH, rotatlon rate, deposltlon time, and concentration on the MCMFE. The response was llnear from 4.0 X 10-o M Cd2+ to 1.07 X 10" M Cd2+ wlth a standard devlatlon of f9.80 X 10-lo M Cd2+for a 1.78 X 10" M Cd2+solutlon (RSD & l l . l % ) and a standard devlatlon of f6.44 X lo-' M Cd2+ for a 1.78 X lo-' M CdZ+ solution (RSD &3.64%). The llmlt of detection was estlmated to be 8.6 X 10-l' M. These results compared favorably to the bare mercury fllm electrode (MFE) for llnear scan anodlc stripping voltammetry.
It is now a well accepted fact that trace metals play a very important role in the ecology of aquatic organisms. Gaining an 0003-2700/84/0356-1131$01.50/0
adequate understanding of thisrole requires that the geochemical transport and biological interactions of trace metala be thoroughly investigated. However, in light of recent research it is apparent that a knowledge of total metal concentration is insufficient. Nielsen and Wium-Anderson ( I ) inferred that even though deep-sea water is rich in nutrients it is unsuitable for the growth of phytoplankton because it had a higher ratio of ionic to organically bound copper than surface waters. Black (2) also found that ionic copper was more toxic to aquatic organisms than organically bound copper. Furthermore, he concluded that the most stable copper complexes were the least toxic. Other authors (3-7) have also concluded that strongly bound metals are less toxic than uncomplexed metal. Electrochemicalmethods offer promise in solving the problems associated with chemical speciation. Differential pulse polarography (DPP), DC polarography,and ion-selective electrodes (ISE), while being quick, inexpensive, and nondestructive, seem to lack the required sensitivity. Because of its inherent sensitivity and nondestructive nature, anodic stripping voltammetry (ASV) appears to be the method of choice. One serious problem with voltammetry at mercury surfaces is the adsorption of humic substances and other naturally occurring 0 1984 American Chemlcal Society
