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Anal. Chem. 1982, 54, 2116-2117
Exchange of Comments on Evaluation of the Copper Anodic Stripping Voltammetry Complexometric Titration for Complexing Capacities and Conditional Stability Constants Sir: Tuschall and Brezonik (I) in a recent paper criticize the titrametric anodic stripping voltammetry (ASV) method ( 2 , 3 )for determining total ligand concentration (CL) values and conditional stability constants (@’) for copper(I1) complexes. The authors conclude that the method is unsuitable for titration of natural water samples or natural organic matter. In contrast, our recent paper ( 4 ) concludes that the ASV method is suitable for titration of soil fulvic acid with cu2+. We contend that Tuschall and Brezonik (1)did not properly test the approach because of a poor choice of ligands and wrong application of theory (5). The measured stripping current (is) has one or more of the following components depending on the behavior of the system The current id is due to diffusion, ik is due to dissociation of the complex, and iredis due to reduction of the complex if the reduction potential of the complex is not well separated from that of metal ion present in the complex. y is the proportionality constant between plating and stripping currents. We will now examine how much each of the current components contributes to is for the titration of EDTA by Cu2+. The general reaction of interest for 1:l complexation is
where the primes designate all metal ion and ligand species not in ML. By rearrangement
pM’ = log p’
[L’I + log [MLI
A t pH 7 log p’ is approximately 13 for CU(EDTA)~-.The pM’ values during the titration are as follows: 50% titration, pM’ = 13; 90% titration, pM’ = 12; 99% titration, pM’ = 11; and 99.999% titration, pM’ = 8. Since the detection limit for ASV (5) is about lo3 M (pM’ = 9), is has virtually no id component during the titration. Therefore, is is comprised of ik and/or ired and does not correspond to free [Cu2+]. Since Shuman’s equations are derived for diffusion-controlled processes ( i d is the main component) with some ik contribution, they cannot be applied to the Cu2+-EDTA titration at pH 7 . It is unsurprising that the “measured @’ value” is far below the correct value. Previously Chau et al. (6) demonstrated the difficulty of obtaining p’ for CU(EDTA)~+ by ASV, and Eisenreich et al. (7) determined an erroneously
low value. (The latter group calculated p’ incorrectly and believed they obtained the correct value.) Consider the other results in the paper of Tuschall and Brezonik ( I ) . Experiments failed with Cu2+and a variety of ligands (Table I, ref l),because the ligands are all at least partially reducible. In contrast, they obtained the correct p’ value for Cd(EDTA)2+because it is not reduced, and the lower log p’ value of 8.5 allows measurement of id due to sufficient free Cd2+before the titration end point. Only in the Cd2+EDTA experiment is the measured is predominantly due to id.
In our Cu2+-SFA paper ( 4 ) the correct conditions for Shuman’s equations (2, 3) also exist. The Cu2+-SFA complexes are not reduced (ired = O), and p’ is about lo5 to lo6 assuring a mainly diffusion-controlledprocess. The resulting CL and p’ values are similar to those we obtained by voltammetric methods (8). For example, at pH 6, p’ is 3 X lo5 by ASV (4) and 1 X lo5 by spectrofluorometry. We conclude that Tuschall and Brezonik (1) obtained incorrect p’ values for Cu2+because they chose inappropriate ligands to test Shuman’s model (2, 3 ) . Although ASV can measure nanomolar metal ion concentrations, researchers must use it with care. Implicit and explicit assumptions are involved in determining the free metal ion concentrations from the measured is and the subsequent calculation of p’. LITERATURE CITED Tuschall, J. R., Jr.; Brezonik, P. L. Anal. Chem. 1981, 5 3 , 1986- 1989. Shuman, M. S . ; Woodward, G. P., Jr. Anal. Chem. 1973, 4 5 , 2032-2035. Shuman, M. S.; Cromer, J. L. Environ. Sci. Techno/. 1979, 13, 543-545. Bhat, G. A.; Saar, R. A.; Smart, R. B.; Weber, J. H. Anal. Chem. 1981. 53.2275-2280. Bond; A. M. “Modern Polarographic Methods in Analytical Chemistry”; Marcel Dekker: New York, 1980; Chapter 3, p 438. Chau, Y. K.; Giichter, R.; Lum-Shue-Chan, K. J. Fish. Res. Board Can. 1974, 31, 1515-1519. Eisenrelch, S.J.; Hoffmann, M. R.; Rastetter, D.; Yost, E.; Maier, W. J. Adv. Chem. Ser. 1980, No. 189, 135-176. Ryan, D. K.; Weber, J. H. Anal. Chem. 1982, 5 4 , 986-990.
Gajanan A. Bhat James H. Weber* Department of Chemistry Parsons Hall University of New Hampshire Durham, New Hampshire 03824
RECEIVED for review January
11, 1982. Accepted June 28, 1982. National Science Foundation Grant No. OCE 79-10571 provided partial funding for this research.
Bhat and Weber (1)use the same faulty logic expressed by Sir: The correspondence of Bhat and Weber (1)is mainly Shuman ( 3 )in stating that because Cu-EDTA produced era reiteration of discussion presented both in our original paper roneous results, the organic ligand is suspect, but not the ASV (2) and in a recent exchange of comments ( 3 , 4 ) ;Le., for copper method. Our contention is that the method has not been ASV titrations of several pure organic compounds we were validated for copper with any ligand that has a known p’ value, unable to obtain correct p’ values because of kinetic dissoand therefore the method should not be used for natural-water ciation or direct reduction of complexed copper. We are well organics until it is demonstrated that the method produces aware that if neither phenomenon were occurring, undeaccurate results. tectable to barely detectable currents would have been observed in the case of Cu-EDTA. Instead, we obtained current Contrary to Bhat and Weber’s assertion ( I ) that we are values much higher than expected, thus producing a FCuEDT*condemning Shuman and Woodward’s calculation model (5, that was lo5 lower than the predicted value. 6) to estimate @’, we instead are primarily questioning the 0003-2700/82/0354-2116$01.25/00 1982 American Chemical Soclety
