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Anal. Chem. 1982, 5 4 , 998-1000
(3) Barker, 0.C. “Progress in Polarography”; Zuman, P., Koltoff, I. M., Eds.; Interscience: New York, 1962; Vol. 2, Chapter 19. 1 Schmldt, H.; von Stackelberg, M. “Modern Polarographic Methods“; Academlc Press: New York. 1963: Chanter 1. Rlfkin, S. C.; Evans, D. H. Anal. &em. 7976, 4 8 , 2174-2179. Dlllard, J. W.; O’Dea, J. J.; Osteryoung, R. A. Anal. Chem. 1979, 51, 115-119. Leon, L. E.; Sawyer, D. T. Anal. Chem. 1981, 53, 706-709. Keiler, H. E.; Osteryoung, R. A. Anal. Chem. 1971, 4 3 , 342-348. Dlllard, J. W.; Hanck, K. W. Anal. Chem. 1976, 4 6 , 218-222. Blrke, R. L. Anal. Chem. 1976, 50, 1489-1496. Parry, E. P.; Osteryoung, R. A. Anal. Chern. 1965, 3 7 , 1634-1637. MacGiliavry, D.; Rideal, E. K. R e d . Trav. Chem. Pays-Bas WS7, 56, 1013-1 02 1, Biondl. C.; Bellugi, L. J . Elecfroanal. Chem. 1970, 24, 263-270. Salto, Y. Rev. Polarogr. 1968, 15, 178-186. Aoki, K.; Osteryoung, J. J . Electroanal. Chem. 1961, 122, 19-35. Myers, D. J.; Osteryoung, R. A. Anal. Chem. 1974, 46, 2089-2092. Shain, I.; Martin, K. J.; Ross, J. W. J . Phys. Chem. 1961, 65, 259-26 1. Oldham, K. 6.; Parry, E. P. Anal. Chem. 1986, 38, 867-872. Anderson, J. E.; Bond, A. M. Anal. Chem. 1981, 53, 504-508. Christie, J. H.; Osteryoung, R. A. J . Nectroanal. Chem. 1974, 49, 301-311. Dayton, M. A.; Brown, J. C.; Stutts, K. J.; Wightman, R. M. Anal. Chem. 1980, 52, 946-950. Cummings, T. E.; Elvlng, P. J. Anal. Chem. 1976, 50, 480-488. Kuwana, T.; Bablttz, D. E.; Hoh, G. J . Am. Chem. SOC. 1960, 82, 5811-5817. Adams, R. N. “Electrochemistry at Solid Electrodes”; Marcel Dekker: New York, 1969; Chapter 5. Wightman, R. M.; Cockrell, J. R.; Murray, R. W.; Burnett, J. N.; Jones,
S . 6. J . Am. Chem. SOC. 1976, 98, 2562-2570. (26) Perone, S. P.; Kretlow, W. J. Anal. Chem. 1966, 38, 1760-1763. (27) Panzer, R. E.; Elving, P. J. J . Nectmchem. SOC. 1972, 179, 864-874. (28) Gonon, F. G.; Fombarlet, C. M.; Buda, M. J.; Pujol, J. F. Anal. Chem. 1981, 53, 1386-1389. (29) Snyder, L. R.; Kirkland, J. J. “Introduction to Modern Liquid Chromatography”, 2nd ed.; Wiley-Interscience: New York, 1979; Chapter 13. (30) Klplniak, W. J . Chromatogr. Scl. 1961, 19, 332-337.
Kenneth J. S t u t t s Mark A. Dayton R. Mark Wightman* Department of Chemistry Indiana University Bloomington, Indiana 47405 RECEIVED for review April 20, 1981. Resubmitted November 9, 1981. Accepted February 11, 1982. This research was supported by the National Science Foundation (Grant No. BNS 81-00044). M.A.D. is a combined Medical-Ph.D. candidate, Indiana University. R.M.W. is the recipient of a Research Career Development Award from the National Institutes of Health (Grant No. KO4 NS 00356) and an Alfred P. Sloan Fellow.
Exchange of Comments on Evaluation of the Copper Anodic Stripping Voltammetry Complexometric Titration for Complexing Capacities and Conditional Stability Constants Sir: Tuschall and Brezonik carried out an anodic stripping voltammetry (ASV) titrimetric procedure with Cu and Co as titrants and several organic ligands as analytes (1). Their stated objective was to evaluate its use for estimating complex conditional stability constants (2). Careful consideration of their research design, particularly their choice of model systems, raises questions about the validity of their conclusions. Estimation of conditional stability constants from the titration is based on the ability of ASV to distinguish between uncomplexed metal and metal bound to complexes which dissociate slowly. Two conditions or criteria must be fulfilled (1) the metal complex which is formed during titration must dissociate slowly and (2) the complex must be reduced at a potential sufficiently separated from the reduction of the noncomplexed metal (2). Separate detection of uncomplexed and complexed metal depends on selecting a preelectrolysis potential in a region where only uncomplexed metal is reduced. Selection must be made for the conditions of the titration, which means that both metal forms must be present. Tuschall and Brezonik selected their preelectrolysis potential on the basis of a separate solution of uncomplexed metal. This is clearly inadequate. When Tuschall and Brezonik (1) find that Cu complexes of histidine, p-hydroxycinnamic acid, gallic acid, pyrogallol, and desferal are reduced a t their arbitrarily selected preelectrolysis potential of -0.3 V vs. SCE, it simply means that these complexes are unsuitable for evaluation of the method. On the other hand, Tuschall and Brezonik find that Cu complexes of bovine serum albumin and Cu and Cd complexes of EDTA are not reduced significantly at the selected preelectrolysis potentials, although, this information alone is insufficient for deciding whether these complexes are suitable for evaluating the procedure. No evidence is given that uncomplexed metal is reduced separately from complex reduction
in the presence of excess ligand. The polarographic literature (3) shows that CdEDTA is suitable for the evaluation and the estimate of its conditional constant by Tuschall and Brezonik (1)was in excellent agreement with literature values, These authors (1) assume that CuEDTA should behave similarly although they give no evidence to support this view. For example, the CuEDTA pseudopolarograms they present for an equimolar mixture of Cu and EDTA show one reduction, that of the complex. These pseudopolarogramswould reveal two waves if the complex and uncomplexed Cu reduction could be resolved. When their estimate of the CuEDTA conditional c o n s h t is much lower than literature values, the authors fault the procedure although it is likely that CuEDTA is inappropriate for their purpose. The polarographic literature of CuEDTA reveals that, under some conditions, CuEDTA is reduced reversibly or quasi-reversibily ( 4 ) and that its reduction is sensitive to buffer capacity ( 5 ) and to the presence of alkaline earth ions (6). Pecsok ( 4 ) estimated the formation constant from Ellz data obtained below pH 6 and found agreement with other methods. This would imply a mobile equilibrium (rapid dissociation and association), at least at low pH values. On the other hand, Wheelwright et al. (7) estimated stability constants of rare earth-EDTA complexes polarographicallyby measuring the Cu displaced upon addition of rare earth to solutions equimolar in Cu and EDTA. Their values agreed with potentiometric estimates and indicated that uncomplexed Cu was accurately measured polarographically in the presence of CuEDTA complexes. This agreement implies that CuEDTA dissociates very slowly. The pseudopolarogramsin Tuschall and Brezonik’s Figure 1 if analyzed by plotting E vs. log ( i / ( i d - i)) would show that CuEDTA reduction is nonreversible (8). In addition, the plot of their stripping currents at -0.3 V vs. SCE against CM/(CL
0003-2700/82/0354-0998$01.25/00 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 6, MAY 1982
Cu ~m~enlroMr UM
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Cyclic voltammogram of
M Cu in
M EDTA,
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Figure 3. (A) Stripping current plotted as a function of copper concentration for preelectrolysis potentlals -0.250 and -0.275 V and 5 pM EDTA and Ca and 50 mM acetate, pH 7. Solid line freehand drawn through origin. Bars indicate variance. Preelectrolysis time 120 s. (B) Same data as (A) plotted vs. CM/(CL- CM). Lines freehand drawn through origin.
-0,45 -063
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Pseudopolarogram in solid circles and plot of log ( / / ( i m x E in open clrcles. Dashed llne is slope of -6.6 V-’. Solution conditions 50 p M Cu, EDTA, and Ca and 50 mM acetate, pH 7. Preelectrolysis time was 60 s. Figure 2.
1 ) ) vs.
- CM),though expected to follow a straight line, is curved downward. This curvature could result from a linear dependence of current on cMand not on CMf (CL - cM). The magnitude of these currents is likely the result of CuEDTA reduction at the foot of the CuEDTA pseudopolarogram and not the result of unconiplexed Cu reduction or kinetic dissociation. A series of experiments was performed to test this hypothesis and to establish whether or not CuEDTA as employed by Tuschall and Brezonik is suitable for evaluating the titrimetric procedure.
EXPERIMENTAL SECTION A Princeton Applied Research Model 174 polarographic analyzer and a Model 301 hanging mercury drop electrode assembly were used along with a Hewlett-Packard Model 7004X-Yrecorder. All chemicals were reagent, grade. Deaeration was carried out with seaford grade nitrogen. The scan rate for both cyclic voltammetry and ASV was 100 mV/s. Preelectrolysis times were 60 or 120 s followed by a 30 s quiescent period and the application of a linear scan. RESULTS AND DISCUSSION Figure 1, the cyclic voltammogram of a solution of M EDTA, M Cu, M Ca, and 1 M acetate and pH 7, demonstrates that the reduction of CuEDTA is nonreversible
and that no separate wave exists for the reduction of noncomplexed Cu. Figure 3, a pseudopolarogram for a solution 5 pM EDTA, 5 pM Cu, 5 pM Ca, and 50 mM acetate and pH 7, along with analysis of the data as E vs. log (i/(imm - i)), indicates substantial irreversibility. The half-wave potential for the pseudopolarogram is -0.42 V which is in excellent agreement with the polarographic E l l z at this pH (4). ASV was carried out with 0.5, 1.0, and 2.0 pM Cu concentrations in solutions with the same background constituents as used for the pseudopolagram. Freelectrolysis was -0.250 and -0.275 V vs. Ag/AgCl. Figure 3 shows the stripping currents plotted against c u concentration and against c M / ( c - CM). The most important feature is the dependence of currents on potential which indicates that CuEDTA rather than uncomplexed Cu is reduced. Stripping currents could be interpreted as linear either with total Cu concentration or with cM/(cL- cM).Estimates of 107.2at -0.250 V and 106.8 at -0.275 V were obtained for the conditional constant by assuming that the current is linear with c M / ( c L - CM). When ASV was carried out a t elevated concentrations (10,20, and 40 pM Cu in W4M EDTA, Ca, and 1M acetate at pH 7), so that more accurate measurements could be made, the currents were strictly linear with total copper concentration which again indicates that the complex is reduced. At least for the model conditions established by Tuschall and Brezonik, CuEDTA is unsuitable for evaluation of the titrimetric procedure. Although Tuschall and Brezonik were unable to demonstrate that any of their model systems except CdEDTA were suitable for evaluation of the procedure, they offer criticisms about its accuracy and its applicability to natural waters. They first point out that “inaccuracies” will arise if complexes dissociate rapidly or are reduced directly at the preelectrolysis potential. This is simply a restatement of the criteria set forth above. The level of criticism, which says that the basic principles and limitations must be recognized by the analyst, applies to any technique used today for estimating stability constants and challanges the knowledgeable analyst to work within them. For example, the complexation capacity can be determined even when “directly reducible” complexes are encountered. The technique requires only that the current sensitivity increase for additions after the end point. The current sensitivity will increase if uncomplexed metal in excess of ligand is reduced at a potential sufficiently separated from the reduction potential of the complex (9). The potential separation can be determined with a simple pseudopolarogrm or with cyclic voltammetry at elevated concentrations. Tuschall and Brezonik ( 1 ) speculate that inaccuracies will result from a “lack of plating potential for some environmentally significant complexes” on the basis of analogy with their model complexes. The question of whether free histidine, refering to their model amino acid, or any other simple amino acid is present in natural waters and involved in any envi-
~
1000
Anal. Chem. 1982, 5 4 , 1000-1001
ronmentally significant complexes is not addressed by their work. In summary, because of shortcomings in their experimental design, Tuschall and Brezonik (1) did not evaluate properly the ASV titrimetric procedure. Their observations can be explained by considering the principles of the procedure and the electrochemistry of metal complexes. Their conclusion that the procedure is not accurate when correctly applied cannot be accepted on the basis of their work. LITERATURE CITED (1) Tuschall, J. R.; Brezonlk, P. L. Anal. Chem. 1981, 53, 1986-1989. (2) Shuman, M. S.; Woodward, G. P., Jr. Anal. Chem. 1973, 45,
(4) Pecsok, P. L. Anal. Chem. 1953, 25, 561-564. (5) Bril, K.; Krumholz, P. J. Am. Chem. SOC. 1954, 58,339-344. (6) Rajput, A. R.; Ohzeki, K.; Kambara, T. Frezenius’ Z . Anal. Chem. 1977, 288, 41-42. (7) Wheelwright, E. J.; Spedding, F. H.; Schwarzenbach, G. J. Am. Chem. SOC. 1953, 75,4196-4201. (8) Shuman, M. S.; Cromer, J. L. Anal. Chem. 1979, 51, 1546-1550. (9) Hanck, K. W.; Dillard, J. W. Anal. Chim. Acta 1977, 89, 329-338.
Mark S. Shuman Department of Environmental Sciences & Engineering School of Public Health University of North Carolina at Chapel Hill Chapel Hill, North Carolina 27514
2032-2035. (3) Tanaka, N.; Tamamushi; Kodama, M. Z . Phys. Chem. (Wlesbaden) 1958, 14,141-155.
RECEIVED for review December 21, 1981. Accepted December 21, 1981.
Sir: The results of Shuman for CuEDTA are interesting and supplement our work by explaining why p’ cannot be determined accurately for this complex by the Cu ASV titration method. As suggested by the pseudopolarogram in our Figure 2, CuEDTA exhibits irreversible reduction kinetics at the mercury electrode, and a sufficient amount of complex is reduced at the plating potential for free Cu2+to cause a major error in P’. Explaining the results of CuEDTA nonetheless does not improve the reliability of the Cu ASV titration method, and the major conclusion of our paper remains intact. Additionally, Shuman’s point that the irreversible reduction of CuEDTA caused inaccurate results is inconsequential considering that naturally occurring organic ligands, such as polypeptides and humics, are expected to form complexes with copper that are irreversibly reduced also (1). Although we were unable to obtain correct results for both p’ and CL by this method using a variety of model organic ligands, we did not conclude that the method will never work. It is possible that the copper complexes for some organic ligands are sufficiently ASV nonlabile to yield correct values for P’ and CL, even though we did not find any such ligands. However, the number and diversity of compounds for which we demonstrated that the method does not work casts serious doubt on the accuracy and general reliability of the method, especially in applications involving unknown mixtures of organic ligands (e.g., natural waters). Our paper and the work of others indicate that free metal ions, inorganic complexes, and copper complexes with both small organic ligands (e.g., pyrogallol, amino acids) and relatively large organic compounds (e.g., desferal) are all ASV labile (hence not measured by the Cu ASV titration method). Nonetheless,workers who have applied the method to natural waters generally obtain results that can be interpreted as a typical titration curve for weak ligands, with an equivalence point that implies some component in the water forms ASV nonlabile complexes. In the recent literature it often is assumed that this component is the high-molecular-weight humic fraction and some have interpreted results (CL and P’) obtained by the method as a measure of the binding capacity and binding strength of the humic fraction. Recent studies cast doubt even on this interpretation. Using czclic voltammetric procedures, Buffle et al. (2) and Greter et al. (3) concluded that complexes of lead with humic material are ASV labile, but because the diffusion coefficient for Pb-humic complexes was found to be lower than that for free Pb2+,peak current (i,) was lower for a given concentration of complexed Pb than for the same concentration of free Pb2+. Recent experiments in our laboratory show this also to be the case for Cu and Cd complexes with the organic fraction from humic-colored surface waters in Florida. Cyclic voltammetry showed that Cu-humic and Cd-humic complexes were readily reducible at potentials 120 mV and 30 mV (respectively)below
the reduction potentials of the free metal ions. The requirement that a complex be ASV nonlabile thus does not hold for humic complexes either, and the curves observed in Cu ASV titrations of natural water ligands thus may be a product of transport phenomena (differences in diffusion rates for free ions and macromolecular complexes) rather than a measure of intrinsic differences in electrochemical reactivity between these species. Our selection of a plating potential for copper was not arbitrary, as Shuman implies. The value we used (-0.3 V vs. SCE) has been used commonly in ASV studies of copper complexation in natural waters, and, for our conditions of analysis, it is the highest potential at which reduction of free copper occurs a t a rate independent of applied potential. Shuman’s conclusion that CuEDTA is a poor model because no separate wave was observed for the reduction of noncomplexed copper is illogical. Both the pseudopolarogram and cyclic voltammogram that he refers to were performed by using equimolar levels of copper and EDTA, and, at pH 7, essentially all copper was present as CuEDTA. Hence, under the experimental conditions, no separate wave i s expected. Furthermore, there is no reason to believe that the reduction potential of uncomplexed copper would depend on the presence or absence of CuEDTA, as Shuman suggests. Originally, we used a solution devoid of EDTA to construct the pseudopolarogram for the reduction of uncomplexed copper and a separate solution to determine the reduction potential of CuEDTA ( 4 ) . The two waves were sufficiently separated to allow plating of Cu2+ without reducing CuEDTA substantia!!y ( 4 ) . Subsequently, we investigated the behavior of Cu2+in the presence of CuEDTA by constructing a single pseudopolarogram for a solution of EDTA and a 2-fold excess of copper. The resulting pseudopolarogram showed two distinctly separate waves-one for Cu2+ and one for CuEDTA-at potentials identical with those reported for separate solutions (4). Thus, contrary to Shuman’s speculation, pseudopolarograms suggest that CuEDTA is a suitable model for evaluating the ASV method. Finally, there seems to be a certain faulty logic to Shuman’s criticism of our work. In essence, he states that if the method gives incorrect results for a particular compound, it is the “fault” of the compound and not the method. By this definition, the method (or any method) then never can be shown to be inaccurate; it simply is “inapplicable”. We have shown that the Cu ASV titration method is not applicable to a wide variety of organic complexes chosen as models of the organic ligands in natural waters. By implication, the applicability of the method to studies on naturally occurring dissolved organic matter (DOM) must be called into question. Because of the heterogeneous and poorly defined nature of aquatic DOM, thermodynamic stability constants are not known for this-material in any absolute sense, making it difficult to
0003-2700/82/0354-1000$01.25/00 1982 American Chemical Society
