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(3) McBrlde, P. T.; Janata, J.; Comte, P. A,; Moss, S.D.; Johnson, C. C. Anal. Chim. Acta I 9 7 9 107, 239-245. (4) Shiramlzu, B. S.; Janata, J.; Moss, S.D. Anal. Chim. Acta 1979, 108, 161-167. (5) Ammann, D.; Jenny, H.-B.; Meier, P. C.; Simon, W. “Electroanalysis in Hygiene, Environmental, Cllnlcal and Pharmaceutical Chemistry”; Smlth, W. F., Ed.; Elsevler: Amsterdam, 1980; pp 3-10. (6) Meier, P. C.; Ammann, D.; Morf, W. E.; Simon, W. “Medical and Blological Applications of Electrochemlcal Devices”; Koryta, J., Ed.; Wiley: New York, 1980; pp 13-91. (7) Steiner, R. A.; Oehme, M.; Ammann, D.; Simon, W. Anal. Chem. 1979, 51, 351-353. (8) O’Doherty, J.; Garcla-Diaz, J. F.; Arrnstrong, W. McD. Science 1979, 203, 1349-1351. (9) Guggl, M.; Oehme, M.; Pretsch, E.; Slmon, W. Helv. Chim. Acta 1876, 59, 2417-2420. (IO) Moss, S. D.; Johnson, C. C.; Janata, J. I€€€ Trans. Biomed. Eng. 1978, BME-25, 49-54.
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(11) Gullbauit, G. G.; Durst, R. A.; Frant, M. S.;Freiser, H.; Hansen, E. H; Light, T. S.; Pungor, E.; Rechnitz, G.; Rice, N. M.; et at. Pure Appl. Chem. 1878. 48. 127-132. (12) Osswald, H. F. Ph.D. Dissertation, ETH 6480, Swiss Federal Institute of Technology, Zurich, 1979. (13) Durst, R. A. NBSSpec. fubl. (U.S.) 1989, No. 374. (14) Oesch, U.; Slmon, W. Anal. Chem. 1980, 52. 692-700. (15) Lindner, E.; Toth, K.; Pungor, E. Anal. Chem. 1978, 48, 1071-1078. (16) Morf, W. E.; Lindner, E.; Simon, W. Anal. Chem. 1975, 47, 1596- 160 1. (17) Morf, W. E.; Simon, W. “Ion-Seiectlve Electrodes in Analytlcal Chemistry”; Frelser, H., Ed.; Plenum Press: New York, 1978; Vol. 1, pp 211-286. (18) Fjeldy, T. A.; Nagy, K. J . Electrochem. SOC.1980, 727, 1299-1303.
RECEIVED for review May 1, 1981. Accepted July 20, 1981.
Evaluation of the Copper Anodic Stripping Voltammetry Complexometric Titration for Complexing Capacities and Conditional Stability Constants John R. Tuschall, Jr.,* and Patrlck L. Brezonlk‘ Depariment of Environmental Engineering Sciences, University of Florida, Gainesville, Florida 326 I I
Several model organic compounds were titrated with copper or cadmium and analyzed by anodic stripping voltammetry. The condltlonal stability constant of CU-EDTA was found to be lo’.’ by the tltrimetrlc ASV procedure, whlch Is far below lllerature values (lo’*.*). Accurate condltlonal stability constants for other model organic compounds with copper could not be determlned by thls method due to the reduclble nature of the metal-organic complex. Implications of these results relative to the use of this procedure for natural water studles are discussed.
Interest in the nature of metal-organic complexes in natural waters has prompted the use of numerous methods to specigte heavy metals and to estimate the quantitative stability of metal-organic complexes. One commonly used method involves titrating water samples that contain excess ligand with a heavy metal, and analyzing the “uncomplexed” metal by anodic stripping voltammetry (ASV) (1-4). The end point of such a titration is interpreted as a measure of the “complexing capacity” of a water sample. This procedure was adapted by Shuman and Woodward (2,3)to include the direct determination of a conditional stability constant (p’) for metal-ligand complexes that are not reduced under the conditions of metal ion analysis. The complexometric titration procedure is a simple technique that can be used at metal, ligand, and pH levels found in natural waters. On the other hand, problems can arise from dissociation of kinetically labile complexes at the mercury electrode and from sorption of organic compounds onto the electrode during ASV titrations (2,4-6). For example, Hanck and Dillard (5) reported up to 34% relative error in determining the complexation capacities of ethylenedinitrilotetraacetic acid (EDTA) solutions using indium as the titrant, Present address: Department of Civil and Mineral Engineering, University of Minnesota, Minneapolis, MN 55455.
and they assumed that complex dissociation caused the low results. Shuman and Woodward (2) investigated the accuracy of their procedure by titrating EDTA with cadmium and concluded that it gave accurate results. However, copper is commonly used to titrate natural water samples (2, 4, 7-9) because it forms more stable complexes with Tost organic ligands than does cadmium (10). Although many measurements of complexing capacities and apparent stability constants have been reported for copper with natural (mixed) organics using this method (8,9,11-13), a systepatic evaluation of the copper ASV titration using organic compounds with known copper-stability constants has not been reportad. This paper describes the effect of complex dissociation on the accuracy of conditional stability constants that were determined with a titrimetric ASV procedure (2) using model organic compounds.
EXPERIMENTAL SECTION Titrations were performed according to the procedures described by Shuman and Woodward (2,3),except that the stripping step of ASV was performed in the differential pulse (DP) mode rather than the linear (DC) mode. Instrumentation consisted of a PAR Model 174 polarographic analyzer using a Kemula-type hanging mercury drop electrode (HMDE). A saturated calomel reference electrode (SCE) was connected to the solution by a 0.01 M KN03 bridge. Instrument settings were 50 mV modulation amplitude and 1 s-l pulse rate. Pseudopolarograms were constructed by the method of Figura and McDuffie (14) to determine appropriate plating potentials. Cadmium was plated at -0.8 V vs. SCE and scanned at 5 m V d . Copper was plated at -0.3 V vs. SCE and scanned at 2 m V d . The initial potential was applied for 60-300 s with stirring, followed by a 15-s quiescent period. Analyses were performed with 50-mL samples containing 30-50 mM sodium acetate buffer, except for Desferal, .which contained 1 mM KN03 as the supporting electrolyte. All electrolytes were purified by electrolysis. Nitrogen gas used to deoxygenate and stir each test solution was regulated by an in-line ball flow meter. The pH was adjusted with NaOH after deoxygenation and solution pH was checked at the end of each titration t o ensure constant pH. A gel-filled
0003-2700/81/0353-1986$01.25/00 1981 American Chemical Society
PrNALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981
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pH electrode was used so that chloride contamination of samples was avoided. Solutions were prepared from distilled and deionized (MilUi-Q) water, and all chemicals were reagent grade except Desferal (Ciba-Geigy). Metal solutions were added in microliter quantities to avoid dilution, and prior to analysis, sampleii were equilibrated 15 min after each addition of metal. Rapid equilibration of each ligand with metal was verified by analyzing solutions that, had equilibrated in quartz test tubes up to 3 h. In all cases, equilibration was achieved within 15 min. Analyses were duplicated after each metal addition, and each organic compound was titrated at least twice. Linear portions of titration curves were modeled by least-squares linear regression. With solutions of E:DTA, a broad peak was observed by w and others (5) at 0.0 V, which obscured the initial copper peaks. This peak, which decreased in size as the titration with copper progressed, was possibly due to the cathodic shift in the oxidation of mercury by uncomploxed EDTA. Therefore, 5 pM Ca was added to suppress this peak during the titration of ElDTA with copper. The thermodynamic stability constants (8) from Sillen and Martell (15) and Ringbom (16) were converted to conditional stability constants (pl) by the method of Ringbom (16) so that a direct comparison could be made between the literature values and those determined here. The conditional values were obtained by using the relatioinship (16) @'M[An
=
[MLI -PML. [M'I---aMmaLn
(1)
where M' is the metal not complexed by L, Idfis the liganid not complexed by M, CYM = [M'l/[M]fr,, and a~ = [L']/[L]f,,. For instance, in the case of our titration of EDTA with copper, with hydrogen and calcium as competing cations ~ E D T A=
[Y'I [Ylfree
After substituting thie appropriate constants (pK4 = 10.34; pK3 = 6.24; pK2 = 2.75; pK1 = 2.07; and pPcay = 10.7) (1611 and concentrationsfor our analysis (pH 7.0; pCa2+5 5.3), the a~value was calculated to be 105.4. Similarly, the ac,, was calculated to be loo" for pOH = 7.0 and acetate concentration of 0.03 M. Therefore, the thermodynamic stability constant for Cu-EIDTA @ = 1Ola6) can be converted to the conditionalvalue @') as follows:
Figure 1. DPASV titration curve of (A) 5 ~ L EDTA M with copper: (B) lod M EDTA with cadmium; and (C) plots of metal (C,) and ligand (C,)concentrations for lnitlal additions of copper and cadmium. 2X
A CU-NTA 0 Cu-EDTA+Ca 0
It should be noted that as Cu2+replaced the calcium complexed by EDTA during the titration, the free calcium concentration, M to 10-5.3M. Thus, the con[Ca2+],increased from ditional stability constant (@'= 1012.8)represents a lower limit based on the extreme case (i.e., [Ca2+l= [ C a l ~= 10-5.3M).
-
-
RESULTS AND DISCUSSION One of the requisite criteria for performing the titrimetric ASV procedure is that the metal-organic complex be nonreducible a t the mercury electrode at a potential significantly separated from the reduction of ionic metal (2). Therefore, we selected EDTA, a ligand that forms a strong complex with copper (0 = 101s.s)(15),to examine the accuracy of a conditional stability constant (p') that was determined by using the titrimetric ASV procedure. Although the titration curve of 5 p M EDTA (Figure l ) , and the pseudopolarogram (Figure 2), indicate that th1e Cu-.EDTA complex is predominantly nonreducible at a plating potential of -0.3 V, @' for Cu-EDTA wm found to be lo'.?, which1 is over 5 orders of magnitude lower than other published values (p' = 1012.6)(15,16) for similar solution conditions. This discrepancy was apparently caused by dissociation of the complex throughout the initial stages of the titration ([Cu]
