in soluble or adsorbed Cr were noted between station classes. O n t h e other hand, a significantly larger percent of Co was transported with organics at polluted stations, with soluble and adsorbed forms accounting for about 40%. These results agree with a model for Cr and Co in sewage (21) which suggests that Cr exists as the solid Cr (111)hydroxide and that Coexists principally in soluble a n d adsorbed forms. In general, Co, which had no known waste input, showed a high degree of phase stability between station classes, whereas Cr and Sb, elements which had large concentration increases due to waste inputs, showed large differences among modes of transport between station classes.
Literature Cited (1) Wilson, A. L., “Concentrations of Trace Metals in River Waters: A Review”, WRC Tech. Rep. T R 16, 1976. ( 2 ) Krauskopf, K. B., Geochim. Cosmochim. Acta, 9,1-32 (1956). (3) Jenne, E. A., in “Trace Inorganics in Water”, ACS Publ. #73, pp 337-87,1968. (4) Turekian, K. K., Geochim. Cosmochim. Acta, 41, 1139-44 (1977). (5) Gibbs, R. J., Science, 180,71-3 (1973). (6) Gupta, S. K., Chen, K. Y., Enuiron. Lett., 10, 129-58 (1975). (7) Luoma, S. N., Jenne, E. A,, in “Trace Substances in Environmental Health”, X. pp 343-51, 1976. (8) Helz, G. R., Huggett, R. J., Hill, J. M., Water Res. ( G B ) ,9,631-6 (1975). (9) Staffers, P., Summerhayes, C., Forstner, V., Patchineelam, S. R., Enuiron. Sei. Technol., 11,819-21 (1977). (10) Holmes, C. W., Slade, E. A., McLerran, C. J., ibid., 8, 255-9 (1974).
(11) Burrows, K. C., Hulbert, M. H., in “Marine Chemistry in the
Coastal Environment”, ACS Symp. Ser. 18, pp 382-93 (1975).
(12) Haynie, C. L., MS thesis, Universityof North Carolina, Chapel
Hill, N.C., 1974. (13) Jackson, M. S. “Soil Chemical Analysis”, Prentice-Hall, Englewood Cliffs, N.J., 1964. (14) Sokal. R. R.. Rohlf., F. J.., “Biometrv”. Freeman. San Francisco. Calif., 1969. (15) Bruland, K. W., Bertine, K., Koide, M., Goldberg,E. D., Enuiron. Sei. Technol., 8,425-32 (1974). (16) Andren, A. W., Lindberg, S.E., Bate, L. C., “Atmospheric Input and Geochemical Cycling of Selected Trace Elements in Walker Branch Watershed”, ORNL-NSF-EATC-13, Environmental Sci. Div. Publ. No. 728,68 pp, Oak Ridge National Lab, Tenn. (17) Turekian, K. K., Scott, M. R., Enuiron. Sci. Technol.. 1,940-4 (1967). (18) Angino, E. E., Schneider, H., “Trace Element Mineralogy and Size Distribution of Suspended Material Samples from Selected Rivers in Eastern Kansas”, Kansas Water Resources Research Inst. Rep. No. 169, 1975. (19) Shuman, M. S., Smock, L. A., Haynie, C. L., “Metals in the Water, Sediments and Biota of the Haw and New Hope Rivers, North Carolina”, Water Resources Res. Inst., University of North Carolina, Rep. No. 124, 1977. (20) Crecelius,-E. A , , Bothner, M. H., Carpenter, R., Enuiron. Sci. Tpchnol., 9,325-33 (1975). (21) Morel, F.M.M., Westall, J. C., O’Melia,C. R., Morgan, J. J., ibid., pp 756-61. I
,
Receiued f o r reuieu; Aprii 16, 1976. Resubmitted January 23, 1978. Accepted Maji,5, 1978. Funding for this project receiued from the Water Resources Research Institute of the Unicersity of North Carolina, Project No. A-07O-NC.
Application of the Rotated Disk Electrode to Measurement of Copper Complex Dissociation Rate Constants in Marine Coastal Samples Mark S. Shuman* and Larry C. Michael Department of Environmental Sciences and Engineering. School of Public Health, University of North Carolina, Chapel Hill, N.C. 27514
A rotating disk electrode technique was used t o estimate dissociation rate constants of copper chelates formed in marine coastal samples, to measure the extent of Cu chelation in these samples, and to establish a n operational definition for labile and nonlabile metal complexes based on a kinetic criterion. Samples collected off the mid-Atlantic coast showed various degrees of chelation toward copper. A first order dissociation rate constant for copper chelates was estimated t o be of t h e order of 2 s-1. Anodic stripping voltammetry (ASV) procedures are used t o estimate t h e degree of metal-organic complexation in marine and fresh waters and the concentration of organic ligand sites in solution available for binding with added metal (called t h e water’s “complexation capacity”). Complexed metal is estimated from the ratio of the ASV current a t natural p H values to t h e current obtained after sample oxidation or a t some low p H value where the metal is assumed uncomplexed (1-4). Complexation capacity is determined from t h e endpoint of a titration of the organic chelates with metal ( 4 , 5 ) . Shuman and Woodward (6, 7) demonstrated t h a t this titration could be used for estimating conditional formation constants. All these procedures rely on the assumption that ASV can distinguish between “labile” and “nonlabile” complexes. T h e use of the terms labile and nonlabile is unfortunate because lability refers to the ability of a complex in solution to make and break bonds rapidly and t o rapidly exchange a ligand within the metal coordination sphere for one outside this 0013-936X/78/0912-1069$01.00/0
sphere. Lability as a term applied to the ASV experiment has an operational definition related t o the observed stripping current. Reduction of the metal ion during ASV pre-electrolysis at the mercury electrode lowers its concentration near the electrode and disturbs t h e equilibrium between t h e complex and its components. A complex t h a t dissociates rapidly enough t o maintain equilibrium is called a labile complex, and requires a rate of dissociation at least as rapid as the rate of metal mass transfer to the electrode surface. ASV currents result from the reduction of metal ions supplied by dissociation of labile complexes. A nonlabile complex in this terminology is one t h a t dissociates very slowly and does not produce a current. Complexes with dissociation rates t h a t fall between t h e operational definition of labile and nonlabile complexes contribute to ASV currents to an extent t h a t depends upon their dissociation rate and their Concentration. Shuman and Woodward (6) corrected for these contributions by varying ligand concentration, but the procedure is not recommended for natural water samples because it necessitates dilution or concentration of t h e sample. Shuman and Michael (8) introduced a technique t h a t has sufficient sensitivity for kinetic measurement a t very dilute solutions. I t combines ASV with the rotating disk electrode (RDE) and provides a method for measuring kinetic dissociation rates in situ and a method for distinguishing labile and nonlabile complexes kinetically, consistent with the way they are defined. T h e RDE is essentially a flow system with solution transported across t h e face of the disk electrode in lam-
@ 1978 American Chemical Society
Volume 12, Number 9, September 1978
1069
inar flow at a rate dependent on the rate a t which the electrode is rotated. T h e time available for a complex t o dissociate as it moves across the disk can be manipulated by varying t h e rotation rate. The dichotomy between “labile” and “nonlabile” complexes is operationally and unambiguously defined, and information about metal speciation is obtained from one series of experiments without resort t o unrelated procedures such as photo-oxidation or drastic reduction of solution pH. In addition, rate constants for kinetic dissociation of complexes are obtained. Copper toxicity to fresh water and marine phytoplankton in metal buffer solutions (TRIS or EDTA) has been studied as a function of Cu activity measured with an ion selective electrode (9, 10). In this media noncomplexed Cu as opposed to total Cu appears responsible for inhibiting growth. Related evidence indicates humic materials in natural waters suppress metal toxicity by complexation (11,12).Duursma and Morgan ( 1 3 )have suggested that metal availability to organisms may not be influenced simply by metal complex stability but may be related to the kinetic dissociation rates of complexes as well. Testing this hypothesis has been hampered by the lack of techniques capable of measuring kinetics a t concentrations as low as those found in natural waters. No in situ measurements of complex dissociation rates in natural waters have been reported to date although the dissociation rates of iron-humate complexes in synthetic solutions of high concentration have been estimated by a stopped flow method (14). T h e objectives of the work reported here were to demonstrate the utility of the R D E technique for estimating dissociation rate constants of copper chelates in natural waters, for measuring the extent of Cu chelation in these samples, and for establishing a n operational definition for labile and nonlabile complexes based on kinetic measurements.
Procedures T h e utility of a mercury-plated rotated disk electrode for measuring dissociation kinetics of metal complexes in natural waters was suggested by earlier work with model systems of Cd-NTA and Cd-EDTA solutions (8).A detailed description of the technique, its mathematical development, the equipment, and the general procedures used for each experiment is available in that reference. Briefly, anodic stripping voltammetry is combined with the R D E to allow kinetic measurements a t a very low metal concentration. Electrolysis of the uncomplexed metal (the complex itself is not reduced a t t h e applied potential) is described by the overall reaction:
ML
& M(I1) + L kb
M(I1)
+ 2e-
-
(1)
M(o)
and is carried out a t constant potential for a period of 5-10 min for a series of electrode rotation rates, w, in the range of about 50-3000 rpm. T h e amount of electricity passed during electrolysis, Q k , is measured by integrating the oxidation (stripping) current and is a function of the kinetic dissociation constant of the complex, hi, the ratio of uncomplexed to complexed metal, K , and the rotation rate. In general, Qk increases as hi and K increase (increased availability of M ) and decreases as the experimental parameter, w,is increased (less time for dissociation). The data are plotted as the ratio Q k / Q O , o r its inverse, as a function of rotation rate where Q,, is the amount of electricity that would be passed during electrolysis i f all the metal present were reducible. When Qk/Q
