586
Anal. Chern. 1987, 5 9 , 586-592
(9) Jackson, R. E.; Patterson, R. J. Water Resour. Res. 1982, 18, 4, 1255-1268. (IO) Lindberg, R. D.: Runnels, Donald D. Science (Washington, D.C.) 1984, 225,925-927. (11) Barcelona, M. J.; Garske, E. E. Anal. Chem. 1983, 55,965-967. (12) Schwarzenbach, R. P.; Glger, W.; Schaffner. C.; Wanner, 0. Environ. Sci. Technol. 1985, 19,322-327. (13) Sato, M. €con. e o / . 1960, 55,928. (14) Andreae, W. A. Nature (London) 1955, 175,859-860. (15) Cooper, W.; Zika. R. Science (Washington. D.C.)1983, 221, 71 1-712. (16) Zika, R. E.; Saltzman. E.; Chameides, W. L.: Davis, 0. D. J. Geophys. Res., B 1982, 87,5015-5017. (17) Walker, W. H.; Bergstrom, R. E.; Walton, W. C. Preliminary Report on the Ground- Water Resources of the Havana Region in West-Central Illinois ; Illinois State Water Survey/Illinois State Geological Survey Coop. Report 3, 1985. (18) Naymlk, T. G.; Sievers, M. E. Ground Water Tracer Experiment (I)at Sand Ridge State Forest, I L ; State Water Survey Contract Report No. 317. - .. , 1983 - -- . (19) Naymik, T. G.;Sievers, M. E. Ground Water 1985, 23, 746-752. (20) Barcelona. M. J.: Glbb, J. P.; Helfrich, J. A,; Garske, E. E A Practical Guide for Ground Water Sampling; prepared for USEPA, RSKERL, Ada. OK. and EMSL. Las Vegas, NV, State Water Survey Contract Report No. 374, Nov. 1985. (21) Schock, M. R.; Garske, E. E. Ground Water Monit. Rev. 1986, 6, 79-84. (22) Kok, G. L Atmos. Environ. 1980, 14, 653-656. (23) Skoog, D. A.; West, D. M. I n Fundamentals of Analflical Chemistry, 4th ed.: Saunders College Publishing: New York, NY, 1982; pp 755-761. (24) Van Baalen, C.; Marler, J. E. Nature (London) 1988, 951. (25) Zika, R. G.;Saltzman, E. S. Geophys. Res. Left. 1982, 9. 231-234. (26) Larsen, I.L.; Hartmann, N. A. Wagner, J. J. Anal. Chem. 1973, 45, 1511-1513. (27) Perschke, H.; Broda, E. Nature (London) 1961, 190,257-258. (28) Hem, J. D. U.S. Geological Survey Water Supply Paper No. 1473, 2nd ed.. 1970
(29) Schumb, N. C. ACS Moncgr. Ser. 1955, No. 128, 421. (30) Maloney, S.M.: Manem, J.; Mallevialle, J.; Frlssinger, F. Environ. Sci. Technol. 1986, 20, 249-253. (31) Perdue, E. M. ACS Symp. Ser. 1979, No. 93,99-114. (32) ACS Committee on Environmental Improvement. Anal. Chem. 1980, 52, 2242. (33) Lazrus, A. L.; Kok, G. L.; Gilin, S.N.; Lind, J. A,: McLaren, S. E. Anal. Chem. 1985, 57, 917-922. (34) Zika, R. G. Short-Lived Oxidants in Natural Waters, Extended Abstract 52:Conference on Gas-Liquid Chemlstry in Natural Waters, Brookhaven National Laboratory, April 1984. (35) Kok, G. L., Thompson, K., Lazrus, A. L.; McLaren, S. E. Anal. Chem. 1986, 58, 1192-1 194. (36) Hwang. H.; Dasgupta, P. K. Anal. Chem. 1988, 58, 1521-1524. (37) Winograd, 1. J.; Robertson. F. N. Science (Washington, D . C . ) 1982, 216, 1227-1230. (38) Dean, R. B.; Dixon, W. J. Anal. Chem. 1951, 23, 636-638.
RECEIVED for review June 16,1986. Resubmitted September 18, 1986. Accepted October 15, 1986. The support of the USEPA-R. S. Kerr Environmental Research Laboratory and the Campus Research Board of the University of Illinois, Urbana-Champaign, is gratefully acknowledged. The work was supported in part by donations of material and equipment by Q.E.D., Inc., Ann Arbor, MI; Fluorocarbon, Anaheim, CA, and Du Pont, Wilmington, DE. Although the research described in this article was funded wholly or in part by the United States Environmental Protection Agency, through its cooperative agreement program, it has not been subjected to the Agency’s peer and policy review and, therefore, does not necessarily reflect the views of the Agency, and no official endorsement should be inferred.
Specificity of the Ion Exchange/Atomic Absorption Method for Free Copper(I I) Species Determination in Natural Waters Jamal A. Sweileh, Dale Lucyk, Byron Kratochvil,* and Frederick F. Cantwell* Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
Concentratlons of the free copper( 11) species (Cu2+) measured by the Ion exchange/atomlc absorptlon (IEX) method In the presence of various concentratlons of the ligands citrate, giyclnate, phthalate, sallcylate, chloride, and fulvate are compared to concentratlons measured wtth a cupric Ion selective electrode (ISE) and/or to concentrations calculated from known metal-llgand formation constants. The IEX method is conslderably more sensltive for Cu2+ than the ISE method but is subJed to Interference from cationic and neutral copper complexes as well as from fllterable collold copperhydroxo specles at higher pH values. Accurate values of Cu2+ concentration are obtained by both methods In the presence of anknlc copper-ilgand complexes. Slnce fulvate, which Is the principal ligand present in natural freshwaters, forms anionic copper complexes, the IEX method possesses adequate selectMly for measuring Cu2+ at trace levels In such waters. The complextng capacity of an actdk lake water wlth a very low dissolved organic carbon content was measured as 3.0 X lo-’ M by monkorlng Cu2+concentratlon by the IEX method during titration with copper nitrate.
The determination of concentrations of individual metalcontaining species as opposed to determining the total con-
centration of a metal is important, especially in the environmental, toxicological, and clinical fields. In previous studies it has been shown that a column-equilibration technique, employing a strong-acid-type cation exchange resin, can be coupled to an atomic absorption spectrophotometer to provide an “ion selective probe” for measuring the concentration of free (hydrated) metal ion species in the presence of kinetically labile complexes of the metal ion (1-3). In this technique the sample solution is passed through a small cation exchange resin column until complete breakthrough of the free metal ion has occurred and equilibrium has been achieved between the resin and sample solution. After a water wash, the sorbed metal ion is eluted from the resin and measured by atomic absorption spectroscopy. As is true for any species-selective method, including ion selective electrode (ISE) potentiometry, the term “selective” is relative, for there are always some conditions under which a method experiences interference. This study investigates interferences in the ion exchange (IEX) technique, which involves ion exchange column equilibration followed by atomic absorption, to determine free, hydrated Cu(I1) (hereinafter referred to as Cu2+)in natural waters. In the study, Cu2+ concentrations are measured by the technique in solutions containing known concentrations of ligands that are commonly found in natural waters or that contain functional groups similar to those present in ligands found in natural waters.
0003-2700/87/0359-0586$01.50/0 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 4, FEBRUARY 15, 1987
The measured Cu2+concentrations are compared to concentrations obtained by ISE potentiometry and to concentrations calculated from known metal-ligand complex formation constants. Cu2+is important in natural waters because it is the most toxic species of dissolved copper to fish, plants, and other aquatic organisms (4-8) and because copper sulfate is widely used for the control of algal growth in freshwater lakes and reservoirs (9, 10).
THEORY This subject is treated in greater detail in ref 11. When a ligand which is present in the test solution but not in the standard solution forms a copper complex that sorbs on the resin, then the IEX method yields an erroneously high value for Cu2+. Consider a ligand L- that forms a complex CuL+ cu2+ + L- s CuL+ (1) If CuL+ is sorbed along with Cu2+,then the sorption equilibria are
Cu2+ + 2RNa s R2Cu + 2Na+
(2)
and
CuL+ + RNa s RCuL
+ Na+
(3) The distribution coefficients, A, for the species Cu2+and CuL+ are defined as
XcU2+= [ R ~ C U/][Cu2+]
(4)
= [RCUL]/[CuL+]
(5)
and The distribution ratio for copper in the standard solution is given by
DCuJ=0= Xcu2+(Ycu2+J=o
(6)
where D c ~ is~the = ratio ~ of the total copper concentration in the resin to the total copper concentration in the solution and is measured experimentally by running a standard solution (containing no L-) through the IEX procedure. The quantity acuz+JI0is the fraction of copper in the standard solution that is present as the Cu2+species (12) and has a value less than 1if species such as CuOH+ or CuN03+are present. In the test solution which contains L- the distribution ratio for copper is given by
D c u ~ z o= Xcu2+acu2+~zo + ~ C ~ L + W ~ L + L # O (7) where acUz+~+o and a ~ ~are + the fractions ~ + ~of copper in the test solution present as the Cu2+ and CuL+ species. Dividing eq 7 by eq 6 gives
+ ~CUL+~CuL+L#O (8) Xcu2+acu2+~,o In the IEX method the peak area, PA (absorbance seconds), is measured for the eluted copper peak. PA is proportional to the concentration of copper in the resin,,C (moles/gram); DCUJZO -Dcuc=o
~CU~+~CU2+L#O
according to
P A = (GS/F)Cc,,
(9)
where the proportionality constant includes the sensitivity of the atomic absorption spectrophotometer, S ((absorbance/mol)/mL), the weight of resin in the column, G (grams), and the flow rate of eluate into the nebulizer, F (mL/s). Since peak area is directly proportional to the distribution ratio of copper, it has been shown (11)that
PAL#o/PAL=o= D c u ~ z o / D c u ~ = o
(10)
If the only copper-containing species sorbed by the resin is Cu2+(and perhaps CuOH+ as discussed below), then XcuL+ =
587
0 and the resin is selective (specific) for Cu2+. Combining eq 8 and 10 gives
PALz o / P A ~ = o= acUz+czo/ ~ c ~ z + , L = o
(11)
In this equation the ratio a C U 2 + ~ Z O / ~ C U 2 + C =is0 identical with the relative fraction of Cu2+in the test solution compared to the fraction (taken as 1) of Cu2+ in the standard solution. When this ratio of peak areas for test solution and standard solution, PAL,o/PAL=o, is multiplied by the Cu2+concentration of the standard solution, the Cu2+concentration of the test solution is obtained. If, on the other hand, CuL+ is sorbed, the resin is not selective for Cu2+. From eq 8 and 10
and multiplying PAL+o/PAL=o by the Cu2+concentration of the standard solution will yield an erroneously high concentration of Cu2+for the test solution. The above concepts can be generalized for the simultaneous sorption of other Cu-L complexes and of copper complexes with other ligands. In the experiments discussed below the ligands L- present czO ~ ~ c ~ in the test solution are known so that a ~ u ~ +and can be predicted for any ligand concentration and pH from available equilibrium constants. The experimentally measured value of PAL+o/PAL=o can then be compared with the value predicted from eq 11to determine the specificity of the method for Cu2+.
EXPERIMENTAL SECTION Apparatus and Glassware. The instrument used for the ion exchange column equilibration experiments was similar to the one described previously ( I ) , with a few modifications. A third pump tube and an additional Cheminert four-way valve were used with the peristaltic pump to maintain a flow of water to the nebulizer of the atomic absorption spectrometer during the column loading step. An additional sample injection valve (Model 202-00, Altex Inc., Berkeley, CA) with a 135-pLinjection loop was placed between valve V3 and the resin column. This valve permitted injection of a standard copper solution to check for flow rate changes or instrument drift. The sample solution reservoir and the small ion exchange column (containing either 2 or 10 mg of Dowex 50W-X8) were thermostated at 25 f 0.5 "C with a circulating water bath. The copper in the column eluate was monitored with a Model 4000 atomic absorption spectrometer (Perkin-Elmer). Measurement of solution pH was made with an Accumet 320 meter (Fisher) using a 13-639-90combination electrode (Fisher). Potentiometric measurement of Cu2+concentration was made at 25.0 f 0.1 "C with an Orion 94-29 cupric selective electrode, an Orion 90-02 double junction Ag/AgCl reference electrode with 0.1 M NaN03 as the salt bridge, and an 825 MP Accumet meter (Fisher). Ultraviolet-visible absorption spectra of molecular species in solution were made with an HP 8451A spectrophotometer (Hewlett-Packard), and flameless atomic absorption measurements were made with a PE 5000 atomic absorption spectrometer, P E HGA 2200 graphite furnace, and PE AS-1 autosampler (Perkin-Elmer). All labware used for handling solutions was cleaned with detergent solution, rinsed with tap water, soaked overnight in either 30% (v/v) nitric acid or in 3:l (v/v) H2S04/HN03,and rinsed with distilled deionized water. Reagents and Chemicals. A stock copper(I1) solution was prepared by dissolved ACS reagent grade copper foil (Matheson, Coleman and Bell) in 1% (v/v) nitric acid and diluting with water. The buffer component 2,6-dimethylpyridine (2,6-lutidine, Terochem, Edmonton, AB) was distilled at atmospheric pressure and the middle fraction (132-136 "C) was collected. A sample of fulvic acid which had been extracted from the Bh horizon of a podzol from Armadale, Prince Edward Island, Canada, was supplied by
2
+
~
=
~
588
ANALYTICAL CHEMISTRY, VOL. 59, NO. 4, FEBRUARY 15, 1987
C. H. Langford, Concordia University, Montreal, and was used without further treatment. Analytical grade 100-200mesh Dowex 50W-X8strong-acid-type cation exchange resin (J. T. Baker) was treated as described in the literature (13). Its ion exchange capacity was 5.1 mequiv/g on a dry basis. Water was distilled and passed through a mixed-bed ion exchange resin column (Amberlite MB-1, Rohm and Haas). All other chemicals were analytical or reagent grade and were used as received. Swamping electrolyte was prepared from a stock solution of 0.500 M sodium nitrate that had been passed through a bed of Chelex 100 resin (analyticalgrade, 160-200 mesh, sodium form) (Bio-Rad) to remove trace copper. Test Solutions and Blanks. A series of test solutions, each 1.00 x W7M in total dissolved copper, was prepared to contain variable ligand concentrations but constant total sodium concentration (0.100 M) and constant lutidine buffer concentration (loT3M) as follows: A volume of 0.500 M NaN03 solution that would yield a final total Na+ concentration, including the Na+ from the ligand solution,of 0.100 M was transferred into a 400-mL beaker. The calculated volume of ligand solution was added, followed by 5.00 mL of 0.05 M lutidine and enough water to adjust the volume to about 200 mL. The pH was adjusted to 6.0 with dilute HN03. This was followed by dropwise addition to the stirred solution of 10.00mL of daily prepared 2.50 X lo4 M copper standard solution. In some cases the pH had to be readjusted to 6.0 after adding the copper solution. The contents of the beaker were quantitatively transferred to a 250-mL volumetric flask and diluted to volume. These test solutions were used for the Cu2+ determination by the IEX method. For each test solution a corresponding blank solution was prepared in an identical manner, with the copper omitted. For many of the ligand systems the Cu2+concentration was also determined by ISE potentiometry. For this purpose five test solutions were prepared, each containing 0.100 M NaNO,, M lutidine buffer, and 1.00 X 10+ M totaldissolved copper. Four of these solutions contained varying concentrations of the ligand. The fifth, containing no added ligand, was used as a standard to calibrate the electrode-meter system. These solutions were prepared in a manner similar to that described above. The total dissolved copper concentration in these solutions was 10 times that used in the IEX measurements. Lake Water Sample. The Bonneville Lake water sample was supplied by M. Papineau of Environment Canada, Inland Waters Directorate, Quebec Region. It was an integrated sample (0-4 m depth) collected by helicopter on October 3,1985, divided into four 1-L subsamples, and shipped in ice. In our laboratory the subsamples were allowed to stand at rcmm temperature for several hours before the pH was measured (pH 5.2). The subsamples were then filtered (0.45 pm) through acid-washed 47-mm-diameter Nylon 66 filters (Rainin Instruments Co., Woburn, MA). For each 1-L subsample a new filter was used. The first 200 mL of filtrate was used to rinse the receiving flask and discarded. The lake water sample solutions were prepared for free copper determination as follows: To 20 mL of 0.500 M NaN03 solution was added 2.00 mL of lutidine buffer (0.05 M); the pH was adjusted to 5.2 by dropwise addition of dilute nitric acid; into this solution was delivered 78.00 mL of the filtered lake water sample. The final pH was 5.25 f 0.05. A blank test solution was prepared by the same procedure, except that the lake water sample was replaced with an equivalent volume of filtered distilled water. A standard copper solution (1.00 x M) was also prepared for calibration as described above except for pH adjustment to 5.2. In another experiment the complexing capacity of the lake water was measured. Lake water solutions were prepared for this experiment as follows: Eighty milliliters of the filtered lake water was dispensed into each of eight 100-mLpolyethylene bottles and spiked with 0, 10, 25,50,100,200,300, and 400 pL of a standard copper solution (5 x 10" M). The final pH did not drop by more than 0.06 unit from the initial value of 5.2. The next day 20.00 mL of a solution composed of 0.500 M NaN03 and 0.005 M lutidine adjusted to pH 5.2 was added to each bottle. After the sample was shaken, the Cu2+concentration was measured in these solutions by the IEX method. Cu2+by IEX. Flow rates were 5 to 6 mL/min for all solutions. Test or blank solutions were pumped through a 10-mg ion ex-
change resin column for 15 min, a period of time found sufficient to achieve equilibrium at pH 6. During this resin loading step the column effluent was directed to waste to prevent plugging of the atomic absorption (AA) nebulizer by the high salt concentration. The resin was next backwashed for 4 min to remove interstitial solution, during which time the column effluent was directed to the nebulizer. Then eluent (2 M "OB) was pumped through the resin column. A 50-s integration of the eluted copper peak area was triggered 15 s after switching from water to eluent flow. Before the next test or blank solution was pumped, the resin column was washed briefly with water. Cu2+in a test sample is calculated from the ratio of peak areas of the test solution to the standard solution, each corrected for the appropriate blank area (eq 11). Cu2+by ISE. Measurement and electrode cleaning procedures were similar to those of Blaedel and Dinwiddie (14). At the start of each series of measurements the ISE was polished and then placed in a stirred solution containing 10 gg/mL Cu(NO&, 0.100 M NaN03, and lo-, M lutidine (pH 6) for several hours. Before each measurement in a series, the electrode was placed in the above solution until it reached a stable potential (14),rinsed with water, placed in 0.025 M H2S04until it reached a constant potential, rinsed with water, and blotted dry. The electrode was calibrated by placing it in a blank solution and adding successive small increments of copper nitrate solution. A constant potential was reached 2-7 min after each addition. Five calibration points were used between [Cu2+]values of 1 X lo6 and 1 X M. Electrode responses were nonlinear, and precision was unsatisM. factory, at Cu2+concentrations below 1 x Measurements of Cu2+in ligand-copper solutions were performed in duplicate with frequent electrode recalibration. For each solution the ratio of the fraction of Cu2+in test solution ( L # 0) to that in standard solution ( L = 0) was calculated as CfCU*+,L#O -- ~O(EL=~-EL+O)/S
acu2+,L=o
(13)
where E is the measured potential and S is the slope of the best-fit line to the pooled values of the two calibration points bracketing the point in question. RESULTS AND DISCUSSION Copper(I1)-Hydroxo and -Nitrato Species. A condition of the IEX method is that all solutions are buffered a t the same pH and that their electrolyte composition is swamped by the addition of the same large excess of an inert electrolyte such as NaN03. All solutions (test, standard, and blank) will therefore necessarily contain the same concentrations of OHand NO3-. In the IEX method the area of the eluted copper peak is proportional to the moles of copper sorbed on the resin. A fundamental assumption of the method is that the moles of sorbed copper are proportional to the concentration of Cu2+ in the solution with which the resin was equilibrated. In solutions buffered at pH
