Anal. Chem. 1982, 5 4 , 1367-1371
LITEXATURE CITED (1) Delahay, P. "Advances in Electrochemlstry and Electrochemical Englneering"; Interscionce: New York, 1961; Vol. I , pp 254. (2) Feng, Q. S.;Liu, G. L. MuaXueXueSao 1965, 3 1 , 291. Feng, Q. S. HuaXue Xue6ao 1966, 32, 7. (3) Feng, Q. S.; Lln. S. R. Anal. Chem. 1961, 5 3 , 1006. (4) O'Dea, J. J.; Osteryoung, J.; Osteryoung, R. A. Anal. Chem. 1961, 53, 695. (5) Koutecky, J. Collect. Czech. Chem. Commun. 1958, 18, 597. (6) Galus, 2. "Fundamentals of Electrochemlcal Analysls"; Ellis Horwood: 1976. (7) Koutecky, J. Collect. Czech. Chem. Commun. 1955, 2 0 , 116. (8) Bhat, T. R.; Iyer, R. K. Z . Anorg. Chem. 1965, 335, 331.
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(9) Wang, E. K.; Chang, R. 6 . HuaXue XueSao 1965, 3 1 , 18. (10) Moussa, A. A.; Abou-Romla, M. M.; Ghaiy, H. A. Nectrochim. Acta 1974, 19, 957. (11) Gottschalk, G. Z . Anal. Chem. 1956, 759, 257. (12) Hanus, V. Chem. Zvesti 1954, 8 , 702. (13) Delahay, P.; Oka, S.J . Am. Chem. Soc. 1960, 82, 329. (14) Schwarz, W. M.; Shain, I. J . Phys. Chem. 1965, 69, 30. (15) Kern, D. M. H. J . Am. Chem. Soc. 1954, 76, 1011. (16) Marcoux, L.; O'Brien, T. J. P. J . Phys. Chem. 1972, 7 6 , 1666. (17) Cheng, H. Y.; MrCreery, R. L. J . Electroanal. Chem. 1977, 85, 361.
RECEIVED for review September ,14, 1981. Accepted March 9, 1982.
Anodic Stripping Voltammetry for Evaluation of Organic-Metal Interactions; in Seawater Stephen R. Piotrowicx," M. Springer-Young, Jorge A. Pulg, and Mary Jo Spencer' National Oceanic and A tniospheric Administration, Atlantic Oceanographic and Meteoroiogical Laboratories, Ocean Chemistry and Biology Laboratory, 430 1 Rickentlacker Causeway, Miami, Florida 33 149
Dlfferentlal pulse anodlc strlpplng voltammetry (DPASV) is used to study aspects of the speciation of Cd, Cu, and Zn In seawater at natural levels of these metals. The speclatlon at natural pH of these metals appears to be a dynamic process changing wlth tlme constants on the order of hours to days. The technlque Is used to study the interactlon of these metals with marlne fulvlc acids. Results suggest that marlne fulvlc acids Interact wlth these metals to varying degrees and in a manner similar to that observed In natural samples. There Is llttle or no lnteractlon of Cd wlth the marlne fulvlc acids tested, strong lnteractllon wlth Zn, and varying degrees of lnteractlons with Cu. Ttie extent of these lnteractlons wlth Cu appears to be related to dlfferlng structural features of the fulvlc acids.
Developing an understanding of the biogeochemical cycles of trace metals in the sea has been difficult because seawater is not a simple electrolyte and the concentrations of most metals are in the nanomolar and below range. Inorganic speciation has been ashiessed with equilibrium models using known stability constartis and making assumptions regarding activity coefficients at extreme dilutions (1-5). The presence of relatively high concentrations of organic matter in seawater, however, considerably complicates any analysis of speciation. Organic matter in seawater has been found to complex trace metals, especially copper (6-8). Equilibrium models including organic ligands in fresh and salt water generally employ known complexing ligands (9),make assumptions on the range that the metal-organic ligand stability constant might be, or infer what the stability constant would have to be in order for it to successfully compete with inorganic ligands in a speciation model (10). Determinations of stability constants in fresh and coastal waters have generally concluded that metal-organic ligand stability constants are in the range of lo6and 1O1O (11) and, therefore, that built organic matter is not important in the speciation of trace metals in these waters. Present address: Department of E a r t h Sciences, University N e w Hampshire, Durham, NH 03824.
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Several studies, however, have found that a large portion of the Cu and Zn present in seawater appears to be organically complexed (12,13). Recent evidence suggests that most of the copper present in seawater is complexed or otherwise coordinated with organic ligands with apparent stability constants greater than 1O1O (14). Microbiological techniques have indicated that Cu-organic ligand stability constants in seawater may be in the range of 10l2(15). Stability constants of this magnitude are sufficient such that thermodynamic considerations alone would predict that some metals in seawater should be present as an organic complex. Determining which metals are bound, to what extent they are bound, and the kinetic stability of the complexes are some of the questions that must be investigated prior to developing a complete understanding of the chemistry and cycling of these metals in seawater. A variety of methods have been employed to assess complexation in seawater, with electrochemical methods among the most popular. The main advantages in using electrochemical methods are that the sensitivities are such that minimal sample handling of the sample is necessary, thus, minimum potential for alteration of the sample. Anodic stripping voltammetry (ASV) is the most widely used of these techniques (e.g., ref 16-20). ASV labile metal determinations at natural pH measure that metal present as free ion, simple inorganic complexes, and possibly weak organic complexes (21, 22). Subsequent acidification should release those metals tied up in stronger organic complexes and organic and inorganic colloids and that portion absorbed on particulate matter. Even with the best equipment available the technique had been limited to use where concentrations were in the range of 50 nM and greater for Cu and several nanomolar for Cd and Zn. This has been caused as much by contamination problems as by technology. The development and refinement of the thin Hg-film/glassy carbon electrode (23,24)provide a technique with sufficient sensitivity to be used for direct determinations on surface, open-ocean seawaters (25). The concentrations of Cu and Zn in these waters are tenths of nanomoles per liter and hundreths nanomolar for Cd. A combination of electrode technology and ultraclean handling techniques has provided us with a system capable of evaluating
Thls article not subJectto U S . Copyright. Published 1982 by the Amerlcan Chemlcal Society
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complexation of trace metals a t open-ocean levels. EXPERIMENTAL SECTION Equipment. The polarograph we employ is a PAR Model 384 (EG&G Princeton Applied Research, Princeton, NJ) which has been interfaced with a Tacussel ED1 rotating electrode (Astra Scientific International, Santa Clara, CA). The electronic interface was accomplished by duplicating the necessary control circuitry of the Model 301 static drop electrode that the PAR 384 was originally designed for use with. This circuitry was then miniaturized and mounted on the back of the electrode stand we constructed. The electrode is held in a L-shaped 2.5 cm thick Plexiglass stand that can be easily be secured on board ship. The rotating electrode is supported at the bottom by a 3 cm thick, 7.5 cm X 12 cm Teflon platform which is attached at right angles to the vertical portion of the stand and at the top by a 2.5 cm thick, 11 cm X 7.6 cm Plexiglass arm which is tightened using nylon screws. The gas control solonoid valve is also mounted on the stand. Besides the central well for the rotating electrode, small holes are placed in the Teflon support platform for the reference and working electrodes and gas inlet. A small hole is drilled in the front of the platform to allow additions to the sample using micropipettors. The reference electrode is of the Ag/AgCl type and is placed in a 4 mm, seawater filled, Teflon bridge tube equipped with an acid cleaned Vycor tip. The use of heat shrinkable Teflon tubing in the system permits the construction of clean and watertight seals. The counting electrode is a Pt wire. The purge gas is ultrahigh purity Ar (Linde) passed through a high-temperature catalytic scrubber (General Electric GO-GETTER, Altech Associates,Arlington Heights, IL) to remove oxygen and is then rehumidified by passing through an in-line gas washing tube containing seawater prior to entering the sample cell. The sample cells are Teflon polarography cells (Savillex Corp., Minnetonka, MN) which are held in place against the Teflon electrode support using a stainless steel laboratory jack. The Teflon block is recessed at the base to accept this cell and center it under the electrodes. Once in place, the cell does not need to be removed except to change samples. The working electrode consists of a 6 mm diameter glassy carbon disk (Tokai Mgf. Corp., c/o IMC Corp., NY). The electrical contact is made using a silver epoxy paste (ECCOBOND solder, Emmerson and Cuming, Inc., Canton, MA). The disk is mounted in a stainless steel tip and the entire assembly made watertight using heat shrinkable Teflon tubing. The assembly screws onto the rotating electrode outside the Teflon block. The working surface of the glassy carbon is rough polished with wet and dry sandpaper and then is taken to a high gloss with diamond polishing compounds (Metadi, Buehler Ltd., Evanston, IL) of increasing fineness finishing with y-alumina. The entire ASV system, and especially the electrode, is mounted and operated in a Class 100 clean bench inside a Class lo00 clean room on board ship. This clean room is half of a portable laboratory designed for trace metal research. The remaining half of the laboratory is a changing room for clean room clothing an contains a floor-to-ceiling vertical laminar flow clean bench for sample transfers. Water sampling is conducted by using clean techniques (13). Samples are transferred from the water sampling bottles to Teflon storage bottles using clean techniques inside the anteroom clean bench. Once samples are transferred from the anteroom to the clean room, all sample handling and analysis procedures are conducted in the Class 100 clean bench. Cleaning Procedures and Reagents. Teflon and plasticware (except Plexiglass) were cleaned by using the procedure of Settle and Patterson (26) which consists of concentrated HN03 soaking for a week at 70 "C, followed by ultrapure 0.05% HN03 soaking at 70 "C for a week, and storing with 0.5% ultrapure HN03. The Teflon block and Vycor electrode tip were soaked in concentrated, redistilled HN03 for 1 month. Standards were prepared from commercial atomic absorption standards (Alfa) by diluting with reverse osmosis/double deionized pretreated water and acidified to pH
