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Effects of competitive binding on the amperometric determination of copper with electrodes modified with chromotrope 2B
Talmra, Vol.38,No. 1, pp. 89-93, 1991 Printed in Great Britain. All rights mcrved
0039-9140/91 53.00 + 0.00 Copyright 0 1991 Pergmon Press plc
EFFECTS OF COMPETITIVE BINDING ON THE AMPEROMETRIC DETERMINATION OF COPPER WITH ELECTRODES MODIFIED WITH CHROMOTROPE 2B SONO IL CHA, KMEM K. KASEM and HECTOR D. AJSRIJGA* Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, NY 14853-1301, U.S.A. (Received 17 July 1989. Accepted 6 November 1989) Summary-Electrodes modified with Chromotrope 2B incorporated by ion-exchange into a polycationic fdm of electropolymerlxed [Ru(v-bpy)J2+ (v-bpy = 4-vinyl-4’-methyl-2,2’-bipyridyl) have been employed in the amperometric determination of copper in solution and exhibit very high sensitivity as well as linear calibration curves in the concentration range 7 x lO-8-l x IO-‘M. The effects of competing ligands, including chloride, bromide, oxalate, ammonia, acetate, citrate, borate, hmnic and fulvic acids, or the presence of competing metal ions such as cobalt or nickel on the uptake of copper by the modified electrodes have also been studied. The presence of competing ligands or metal ions decreases the analytical signal due to copper incorporation. The magnitude of this effect is dependent on the relative strength of coordination of the competing ligands for copper ions or of Chromotrope 2B for the competing metals, and also on the concentration of the interferents. The relevance of this work to speciation studies is discussed.
Chemically modified electrode8’4 are of great use in the development of analytical strategies and sensors, owing to the high specificity that can be achieved by appropriate choice of modifier, and the high sensitivity that can be achieved by preconcentration of the analyte at the surface-modified electrode. In addition, the methodologies and instrumentation involved are relatively simple. A large number of analytical applications of chemically modified electrodes have been reported.9-26 In our work, we have sought to exploit the advantages of polymer-modified electrodes for the determination of transition metal ions*“3 and organic functional gro~ps.*~*~ The methods have been based on preconcentration of the analyte (metal ion or organic species) at an electrode surface modified with polymers carrying reagents for the selective and sensitive determination of the species of interest. For the determination of transition metal ions we employ bifunctional or multifunctional polymer films containing electroactive centers and coordinating groups. The internal redox center is used to induce precipitation of the polymer on the electrode surface and thus allows precise control of the coverage and also serves in the determination of the number of immobilized
ligand sites, which is important as it allows a priori determination of the saturation response. A coordinating group is chosen that will bind strongly and selectively to the metal ion of interest. In addition, we have also employed carbon-paste electrodes, in which the polymer containing the ligand is mixed with the pasting material. This approach allows rapid renewal of the electrode surface. The analysis is based on the electrochemical determination of the amount of immobilized metal/ligand complex and can employ either a metal or ligand redox process to provide the analytical signal which is related to the concentration of the analyte in solution. We have demonstrated the applicability of this approach to the determination of iron, copper, cobalt, nickel and calcium. 18-23 We are also interested in assessing the utility of chemically modified electrodes in speciation studies, which are of great importance in analysis of environmental samples since the toxicity of metal ions is often strongly dependent on the form in which they are present. Speciation studies are difficult because the concentration levels are low, the given ion may be present in numerous forms, and the method of analysis must not only be sufficiently sensitive, but also not perturb the species distribution. Fundamentally, speciation involves analysis of the competitive equilibria between the metal
*Author for correspondence. TAL
M/I43
89
SF.ONG K. CIU et
90
ion of interest and the ligands present in solution. We have therefore initially investigated the response of electrodes modified with Chromotrope 2B (for structure see Fig. 1) for the determination of copper ions in solution in the presence of competing ligands or metal ions. EXPERIMENTAL
Reagents Chromotrope 2B (C2B) (Aldrich) was purified by three recrystallizations from an ethanol/ water mixture. The water was purified by a Hydro water purification train or a Millipore Milli-Q system. Buffer solutions and standard copper copper solutions were prepared from reagent grade materials. Acetonitrile (Burdick and Jackson distilled in glass) and dried over molecular sieves (4 A). Tetra-n-butylammonium perchlorate (TBAP) (G. F. Smith) was recrystallized three times from ethyl acetate and dried under vacuum at 75” for 72 hr. [Ru(vbpy)s]2+ (PF&* was prepared as previously described.27 All other reagents were of at least reagent grade quality and were used without further purification. Instrumentation Prior to use, platinum or glassy-carbon disk electrodes sealed in glass or teflon, respectively, were polished with 1 pm diamond paste (Buehler), rinsed with water, placed in an ultrasonic bath (in water) for 1 min and rinsed with acetone. Electrochemical cells of conventional design with three compartments separated by medium porosity sintered-glass disks were used throughout. Electrochemical experiments were performed with either an IBM Instruments EC 225 Voltammetric Analyzer or a BAS 100 Electrochemical Analyzer. Data were recorded on a Soltec X-Y recorder. Differential pulse voltammetric experiments were done with a 50 mV pulse amplitude and a sweep rate of 10 mV/sec. All potentials are referred to the sodium chloride saturated calomel electrode (SSCE) without regard to the liquid-junction potential. Procedures Electrodes were modified with a polymeric film of [Ru(v-bpy),12+ by reductively initiated *v-bpy = 4-vinyl-4’-methyl-2,2’-bipyridyl.
al.
polymerization from acetonitrile/O. lit4 TBAP solution as previously described.” The exact coverage was determined from the charge under the voltammetric wave for the Ru(III/II) process at about + 120 V. Typical coverages were 2-3 equivalent monolayers. The modified electrodes were immersed in a stirred 1OmM aqueous solution of Chromotrope 2B for 15 min. After rinsing with water, the electrodes were placed in contact (for 15 min, with stirring) with aqueous solutions of Cu(1) [obtained by the addition of a fivefold excess of hydroxylamine hydrochloride to Cu(I1) chloride solutions] at various concentrations and in the presence (or absence) of other metal cations or ligands, also at various concentrations. Afterwards, the electrodes were rinsed with water. Determinations (by differential pulse voltammetry) were performed in either aqueous trifluoroacetate buffer (pH 5.25) or in acetonitrile/O.lM TBAP. The currents were normalized with respect to the surface coverage, which was determined as described above. At least five replicate determinations were performed.
RESULTS AND DISCUSSION
Preliminary voltammetric characterization In the absence of copper, C2B did not exhibit any redox response in trifluoroacetate buffer at a glassy carbon electrode in the potential range between -0.5 and + 1.0 V. Upon addition of copper ions, a well defined wave was observed, which had a conditional potential of -0.05 V and that we ascribe to formation of a stable C2B/Cu complex. The electrochemical behavior of electrodes modified with poly-[Ru(v-bpy)$+ and loaded with C2B was also investigated before and after its exposure to an aqueous copper solution. Prior to exposure (Fig. l,A), the only electrochemical response observed between - 0.50 and + 1.40 V was that of the poly-[Ru(v-bpy),12+ at about + 1.20 V. After exposure, however, an additional redox response with a formal potential of - 0.05 V was observed (Fig. 1,B); consistent with the results in solution. When the electrochemical response of modified electrodes (again prior to and after exposure to copper-containing solution) was measured in acetonitrile medium, analogous results were obtained except that the wave for the C2B/Cu complex was at a formal potential of +0.45 v.
Amperometric determination
of copper
91
I +1.6
Fig. 1. Differential pulse voltamperograms for a glassy-carbon electrode modified with poly[Ru(v-bpy)p]2+ loaded with C2B prior to (A) and after (B) exposure to a SOpM solution of copper. Inset: structure of C2B.
Copper determination studies Electrodes modified with C2B were exposed to aqueous copper solutions (pH 5.25) at various concentrations, as described in the experimental section. Subsequently, the electrochemical response of the surface-immobilized C2B/Cu complex in either aqueous pH 5.25 buffer or acetonitrile/O. IM TBAP was obtained, with the waves at formal potentials of -0.05 and +0.45 V respectively, as the analytical signal. Figure 2 shows a plot of the logarithm of the normalized current (for the determination in acetonitrile medium) vs. the logarithm of the copper concentration in solution. A fairly good correlation (r = 0.98) was obtained over the concentration range 7.2 x lo-‘-1.1 x 1O-4M. This illustrates the sensitivity of the method and its wide dynamic range. A saturation response is obtained as shown by the levelling of the curve at the higher concentrations,
-5.0
-7.0
-6.0
-5.0 log
-4.0
-3.0
-2.0
PJI
Fig. 2. Calibration curve for the determination of copper with electrodes modified with poly-[Ru(v-bpy)$+ loaded with C2B.
because of coordination of all the available surface sites. This was corroborated by the fact that the current for the immobilized C2B/Cu complex did not increase with further increase in the solution concentration of copper. In addition, the observed currents correlated very well with our estimates for a completely metalated film, calculated from the experimentally determined surface coverage of the polymer on the electrode surface an suming complete neutralization of the charge due to the pendant [Ru(v-bpy),]‘+ groups, by the sulphonate side-chains in C2B. This is based on the fact that we could precisely determine the amount of surface-immobilized [Ru(v-bpy)J*+ (from its voltammetric response) and from this, determine the maximum amount of C2B that could be incorporated by ion-exchange. Since the Cu/C2B stoichiometry is known, the measurement of the redox response due to the immobilized metal/ligand complex for an ostensibly metal-saturated film yields the amount of C2B incorporated. The experimental and calculated values were always in good agreement. We have also previously showni that for similar ligands there is virtually no loss of ligand or complex from the electrode surface upon potential cycling or in the presence of high (1 .OM) electrolyte concentrations for extended time periods. At the low-concentration end, the response appears to level off at a copper concentration of about 5.4 x 10m8M. This, however, appears to be due not to the limit of detection of the technique, but to background levels of copper in the reagents employed.
SEGNG K. ctu et al.
92
Competitive binding studies
We have studied the effects of competitive binding of copper ions by other ligands or of other metals by the surface-immobilized ligand (C2B). These studies will be helpful in trying to apply the approaches described here to speciation studies since speciation fundamentally involves competitive equilibria. Competitive binding of copper by other ligands.
In the investigation of the effects of competing ligands on the determination of copper, the modified electrode was exposed to solutions of copper at a fixed concentration of 1 x lo-‘M, which also contained a competing ligand at various concentrations. In this manner, we studied the effects of chloride, bromide, oxalate, ammonia, acetate, citrate, borate, humic and fulvic acids as competing ligands. Figure 3 shows representative results obtained when the competing ligands were chloride, bromide, oxalate and hurnic acid. In all cases there was a diminution in the response for the surfaceimmobilized C2B/Cu complex and the magnitude of this effect was proportional to the solution concentration of the competing ligand. The linearity of the log(i/r) vs. log[competitive ligand] plots strongly suggests that indeed the observed effects are due to competitive binding between the surface-immobilized C2B and the
ligand in solution. In addition, for the cases where there are reliable values for the conditional stability constants (b’) of the copper complexes with the competitive ligands, we find that a plot of log(i/r) vs. log /?’ is also linear. In other words, the response depends on the conditional formation constants for the copper complexes under the specific conditions used.28 This supports our assertion that competitive binding effects are responsible for the observed diminution in the analytical signal, and more establishes that the relative importantly, strengths of coordination of the various ligands are maintained under the experimental conditions employed. This implies that the coordination properties of an interface (a modified electrode in this case) can be systematically and deliberately controlled by choice of the immobilized ligand as well as by the presence of other competitive ligands in solution. This should have significant implications for speciation studies, which we are currently exploring. Competitive binding of other metals by surface-immobilized Chromotrope 2B. We have also
performed effects of nation of C2B. The containing
1.10 F”“‘“““‘“-l
1.00
h
some preliminary studies on the other metal cations on the determicopper by electrodes modified with electrodes were exposed to solutions copper at a concentration of 5OpM
0.90
.-
$ 0.80 r - 0.0950
0.70
t
I.
*
1.0
*
I
I
I.
2.0 -log [CT]
I
-
r - 0.9762
I
O.Q50.01.0
3.0
3.0
-log
[Bi]
1.oo 0.90 4
0.30
4.0
-log
[Oxalate’4
-h
4.2
4.4 -log
4.6
4.8
[Humic Acid]
Fig. 3. Effects of competing ligands at various concentrations on the determination of copper at a solution concentration of 1 x 10-5M at electrodes modified with poly-(Ru(v-bpy),]*+ loaded with C2B.
5.0
Amperometric determination Table 1. Effects of competing metal ions on the determination of copper* at electrodes modified with C2B
M’ = cobalt M’ = nickel
[M’l/]Cul*
ir, arbitrary units
0.0 1.0 5.0 0.0 0.2 1.0
5.55 4.08 3.90 5.55 2.73 1.05
*[Cu] held constant at 50@4.
as well as either Co(I1) or Ni(I1) at various concentrations. The signal due to the immobilized C2B/Cu complex was then obtained. The results (Table 1) show that Ni(I1) had a more dramatic effect on the signal than Co(I1) did, which is consistent with the higher formation constant for the nickel complex. (The formation constants obtained in the literature were for Chromotrope 2Rz9 and we have assumed that the trends in coordination strength are the same for the closely related C2B.) The lack of additional data on formation constants precludes any further analysis, but the trends are consistent with differences in the strength of coordination. The presence of nickel gave rise to an additional redox response at -0.29 V. Thus, electrodes modified with C2B could be employed in the determination of nickel in solution.
CONCLUSIONS
We have demonstrated that electrodes modified with C2B can be employed in the determination of copper in solution at concentrations down to 7 x lo-*M, with a wide dynamic range. The presence of competing ligands or other metal ions affects the analytical signal in proportion to the strength of coordination of the other ligands with copper ions or of other ions with the surface-immobilized C2B. The presence of competitive equilibria suggests that this analytical approach might be applicable to speciation studies, and this is currently under investigation. Acknowledgements-This work was supported in part by the National Science Foundation. HDA is a recipient of a Presidential Young Investigator Award (1984-1989) and an A. P. Sloan Fellowship (1987-1991). SKC acknowledges support by the Korean Ministry of Education.
of copper
93 REFERENCES
1. R. W. Murray, in EIectroanalytical Chemistry, A. J. Bard (ed.), Vol. 13, p. 191. Dekker, New York, 1984. 2. I&m, Ann. Rev. Mater. Sci., 1984, 14, 145. 3. I&m, Act. Gem. Res., 1980, 13, 135. 4. L. R. Faulkner, Chem. Eng. News, 27 February 1982, 28. 5. H. D. Abru& Coord. Chem. Rev., 1988, 86, 135. 6. Idem, in Electroresponsive Molecular and Polymeric Systems, T. A. Skotheim (ed.), p. 92. Dekker, New York, 1988. 7. M. Fujihira, in Topics in Organic Chemistry, A. J. Fry and W. R. Britton (eds.) p. 255. Plenum Press, New York, 1986. 8. R. W. Murray, A. G. Ewing and R. A. Durst, Anal. Gem., 1987, 59, 379A. 9. R. P. Baldwin, J.-K. Christensen and L. Kryger, ibid., 1986, 58, 1790. 10. S. V. Prabhu, R. P. Baldwin and L. Kryger, ibid., 1987, 59, 1074. 11. J. A. Cox and P. J. Kulesza, J. Electroanal. Chem., 1983, 159, 337. 12. J. Gardea-Torresdey, D. Damall and J. Wang, ibid., 1988, 252, 197. 13. Y. Ikariyama and W. R. Heineman, Anal. Chem., 1986, 58, 1803. 14. J. A. Cox and B. K. Das, ibid., 1985, 57, 2739. 15. N. Oyama and F. C. Anson, J. Electrochem. SOL, 1980, 127, 247. 16. M. W. Espenscheid, A. R. Ghatak-Roy, R. B. Moore III, R. M. Penner, M. N. Szentirmay and C. R. Martin, J. Chem. Sot. Faraday Trans. I, 1986,82, 1051. 17. G. Nagy, G. A. Gerhardt, A. F. Oke, M. E. Rice, R. N. Adams, R. B. Moore III, M. N. Szentirmay and C. R. Martin, J. Eiectroanal. Chem., 1985, 188, 85. 18. A. R. Guadalupe and H. D. Abrmia, Anal. Chem., 1985, 57, 142. 19. L. M. Wier, A. R. Guadalupe and H. D. Abruiia, ibid., 1985, 57, 2009. 20. A. R. Guadalupe, L. M. Wier and H. D. Abrufia, Am. Lab., 1986, 18, No. 8, 102. 21. L. L. McCracken, L. M. Wier and H. D. Abrufia, Anal. Mt., 1987, 20, 1521. 22. H. C. Hurrell and H. D. Abruria, Anal. Chem., 1988,60, 254. 23. K. K. Kasem and H. D. Abrufia, J. Electroanal. Chem., 1988, 242, 87. 24. A. R. Guadalupe and H. D. Abrufia, Anal. Lett., 1986, 19, 1613. 25. A. R. Guadalupe, S. S. Jhaveri, K. E. Lui and H. D. Abruiia, Anal. Chem., 1987, 59, 2436. 26, S. K. Cha and H. D. Abruiia, ibid., 1990, 62, 274. 27. H. D. Abruiia, P. Denisevich, M. Umaiia, T. J. Meyer and R. W. Murray, J. Am. Chem. Sot., 1981, 103, 1. 28. H. A. Laitinen and W. E. Harris, Chemical Analysis, 2nd Ed., McGraw-Hill, New York, 1975. 29. 0. Prakash and S. P. Mushran, Chim. Anal. Paris, 1967, 49, 473.
0039-9140/91 53.00 + 0.00 Copyright 0 1991 Pergmon Press plc
EFFECTS OF COMPETITIVE BINDING ON THE AMPEROMETRIC DETERMINATION OF COPPER WITH ELECTRODES MODIFIED WITH CHROMOTROPE 2B SONO IL CHA, KMEM K. KASEM and HECTOR D. AJSRIJGA* Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, NY 14853-1301, U.S.A. (Received 17 July 1989. Accepted 6 November 1989) Summary-Electrodes modified with Chromotrope 2B incorporated by ion-exchange into a polycationic fdm of electropolymerlxed [Ru(v-bpy)J2+ (v-bpy = 4-vinyl-4’-methyl-2,2’-bipyridyl) have been employed in the amperometric determination of copper in solution and exhibit very high sensitivity as well as linear calibration curves in the concentration range 7 x lO-8-l x IO-‘M. The effects of competing ligands, including chloride, bromide, oxalate, ammonia, acetate, citrate, borate, hmnic and fulvic acids, or the presence of competing metal ions such as cobalt or nickel on the uptake of copper by the modified electrodes have also been studied. The presence of competing ligands or metal ions decreases the analytical signal due to copper incorporation. The magnitude of this effect is dependent on the relative strength of coordination of the competing ligands for copper ions or of Chromotrope 2B for the competing metals, and also on the concentration of the interferents. The relevance of this work to speciation studies is discussed.
Chemically modified electrode8’4 are of great use in the development of analytical strategies and sensors, owing to the high specificity that can be achieved by appropriate choice of modifier, and the high sensitivity that can be achieved by preconcentration of the analyte at the surface-modified electrode. In addition, the methodologies and instrumentation involved are relatively simple. A large number of analytical applications of chemically modified electrodes have been reported.9-26 In our work, we have sought to exploit the advantages of polymer-modified electrodes for the determination of transition metal ions*“3 and organic functional gro~ps.*~*~ The methods have been based on preconcentration of the analyte (metal ion or organic species) at an electrode surface modified with polymers carrying reagents for the selective and sensitive determination of the species of interest. For the determination of transition metal ions we employ bifunctional or multifunctional polymer films containing electroactive centers and coordinating groups. The internal redox center is used to induce precipitation of the polymer on the electrode surface and thus allows precise control of the coverage and also serves in the determination of the number of immobilized
ligand sites, which is important as it allows a priori determination of the saturation response. A coordinating group is chosen that will bind strongly and selectively to the metal ion of interest. In addition, we have also employed carbon-paste electrodes, in which the polymer containing the ligand is mixed with the pasting material. This approach allows rapid renewal of the electrode surface. The analysis is based on the electrochemical determination of the amount of immobilized metal/ligand complex and can employ either a metal or ligand redox process to provide the analytical signal which is related to the concentration of the analyte in solution. We have demonstrated the applicability of this approach to the determination of iron, copper, cobalt, nickel and calcium. 18-23 We are also interested in assessing the utility of chemically modified electrodes in speciation studies, which are of great importance in analysis of environmental samples since the toxicity of metal ions is often strongly dependent on the form in which they are present. Speciation studies are difficult because the concentration levels are low, the given ion may be present in numerous forms, and the method of analysis must not only be sufficiently sensitive, but also not perturb the species distribution. Fundamentally, speciation involves analysis of the competitive equilibria between the metal
*Author for correspondence. TAL
M/I43
89
SF.ONG K. CIU et
90
ion of interest and the ligands present in solution. We have therefore initially investigated the response of electrodes modified with Chromotrope 2B (for structure see Fig. 1) for the determination of copper ions in solution in the presence of competing ligands or metal ions. EXPERIMENTAL
Reagents Chromotrope 2B (C2B) (Aldrich) was purified by three recrystallizations from an ethanol/ water mixture. The water was purified by a Hydro water purification train or a Millipore Milli-Q system. Buffer solutions and standard copper copper solutions were prepared from reagent grade materials. Acetonitrile (Burdick and Jackson distilled in glass) and dried over molecular sieves (4 A). Tetra-n-butylammonium perchlorate (TBAP) (G. F. Smith) was recrystallized three times from ethyl acetate and dried under vacuum at 75” for 72 hr. [Ru(vbpy)s]2+ (PF&* was prepared as previously described.27 All other reagents were of at least reagent grade quality and were used without further purification. Instrumentation Prior to use, platinum or glassy-carbon disk electrodes sealed in glass or teflon, respectively, were polished with 1 pm diamond paste (Buehler), rinsed with water, placed in an ultrasonic bath (in water) for 1 min and rinsed with acetone. Electrochemical cells of conventional design with three compartments separated by medium porosity sintered-glass disks were used throughout. Electrochemical experiments were performed with either an IBM Instruments EC 225 Voltammetric Analyzer or a BAS 100 Electrochemical Analyzer. Data were recorded on a Soltec X-Y recorder. Differential pulse voltammetric experiments were done with a 50 mV pulse amplitude and a sweep rate of 10 mV/sec. All potentials are referred to the sodium chloride saturated calomel electrode (SSCE) without regard to the liquid-junction potential. Procedures Electrodes were modified with a polymeric film of [Ru(v-bpy),12+ by reductively initiated *v-bpy = 4-vinyl-4’-methyl-2,2’-bipyridyl.
al.
polymerization from acetonitrile/O. lit4 TBAP solution as previously described.” The exact coverage was determined from the charge under the voltammetric wave for the Ru(III/II) process at about + 120 V. Typical coverages were 2-3 equivalent monolayers. The modified electrodes were immersed in a stirred 1OmM aqueous solution of Chromotrope 2B for 15 min. After rinsing with water, the electrodes were placed in contact (for 15 min, with stirring) with aqueous solutions of Cu(1) [obtained by the addition of a fivefold excess of hydroxylamine hydrochloride to Cu(I1) chloride solutions] at various concentrations and in the presence (or absence) of other metal cations or ligands, also at various concentrations. Afterwards, the electrodes were rinsed with water. Determinations (by differential pulse voltammetry) were performed in either aqueous trifluoroacetate buffer (pH 5.25) or in acetonitrile/O.lM TBAP. The currents were normalized with respect to the surface coverage, which was determined as described above. At least five replicate determinations were performed.
RESULTS AND DISCUSSION
Preliminary voltammetric characterization In the absence of copper, C2B did not exhibit any redox response in trifluoroacetate buffer at a glassy carbon electrode in the potential range between -0.5 and + 1.0 V. Upon addition of copper ions, a well defined wave was observed, which had a conditional potential of -0.05 V and that we ascribe to formation of a stable C2B/Cu complex. The electrochemical behavior of electrodes modified with poly-[Ru(v-bpy)$+ and loaded with C2B was also investigated before and after its exposure to an aqueous copper solution. Prior to exposure (Fig. l,A), the only electrochemical response observed between - 0.50 and + 1.40 V was that of the poly-[Ru(v-bpy),12+ at about + 1.20 V. After exposure, however, an additional redox response with a formal potential of - 0.05 V was observed (Fig. 1,B); consistent with the results in solution. When the electrochemical response of modified electrodes (again prior to and after exposure to copper-containing solution) was measured in acetonitrile medium, analogous results were obtained except that the wave for the C2B/Cu complex was at a formal potential of +0.45 v.
Amperometric determination
of copper
91
I +1.6
Fig. 1. Differential pulse voltamperograms for a glassy-carbon electrode modified with poly[Ru(v-bpy)p]2+ loaded with C2B prior to (A) and after (B) exposure to a SOpM solution of copper. Inset: structure of C2B.
Copper determination studies Electrodes modified with C2B were exposed to aqueous copper solutions (pH 5.25) at various concentrations, as described in the experimental section. Subsequently, the electrochemical response of the surface-immobilized C2B/Cu complex in either aqueous pH 5.25 buffer or acetonitrile/O. IM TBAP was obtained, with the waves at formal potentials of -0.05 and +0.45 V respectively, as the analytical signal. Figure 2 shows a plot of the logarithm of the normalized current (for the determination in acetonitrile medium) vs. the logarithm of the copper concentration in solution. A fairly good correlation (r = 0.98) was obtained over the concentration range 7.2 x lo-‘-1.1 x 1O-4M. This illustrates the sensitivity of the method and its wide dynamic range. A saturation response is obtained as shown by the levelling of the curve at the higher concentrations,
-5.0
-7.0
-6.0
-5.0 log
-4.0
-3.0
-2.0
PJI
Fig. 2. Calibration curve for the determination of copper with electrodes modified with poly-[Ru(v-bpy)$+ loaded with C2B.
because of coordination of all the available surface sites. This was corroborated by the fact that the current for the immobilized C2B/Cu complex did not increase with further increase in the solution concentration of copper. In addition, the observed currents correlated very well with our estimates for a completely metalated film, calculated from the experimentally determined surface coverage of the polymer on the electrode surface an suming complete neutralization of the charge due to the pendant [Ru(v-bpy),]‘+ groups, by the sulphonate side-chains in C2B. This is based on the fact that we could precisely determine the amount of surface-immobilized [Ru(v-bpy)J*+ (from its voltammetric response) and from this, determine the maximum amount of C2B that could be incorporated by ion-exchange. Since the Cu/C2B stoichiometry is known, the measurement of the redox response due to the immobilized metal/ligand complex for an ostensibly metal-saturated film yields the amount of C2B incorporated. The experimental and calculated values were always in good agreement. We have also previously showni that for similar ligands there is virtually no loss of ligand or complex from the electrode surface upon potential cycling or in the presence of high (1 .OM) electrolyte concentrations for extended time periods. At the low-concentration end, the response appears to level off at a copper concentration of about 5.4 x 10m8M. This, however, appears to be due not to the limit of detection of the technique, but to background levels of copper in the reagents employed.
SEGNG K. ctu et al.
92
Competitive binding studies
We have studied the effects of competitive binding of copper ions by other ligands or of other metals by the surface-immobilized ligand (C2B). These studies will be helpful in trying to apply the approaches described here to speciation studies since speciation fundamentally involves competitive equilibria. Competitive binding of copper by other ligands.
In the investigation of the effects of competing ligands on the determination of copper, the modified electrode was exposed to solutions of copper at a fixed concentration of 1 x lo-‘M, which also contained a competing ligand at various concentrations. In this manner, we studied the effects of chloride, bromide, oxalate, ammonia, acetate, citrate, borate, humic and fulvic acids as competing ligands. Figure 3 shows representative results obtained when the competing ligands were chloride, bromide, oxalate and hurnic acid. In all cases there was a diminution in the response for the surfaceimmobilized C2B/Cu complex and the magnitude of this effect was proportional to the solution concentration of the competing ligand. The linearity of the log(i/r) vs. log[competitive ligand] plots strongly suggests that indeed the observed effects are due to competitive binding between the surface-immobilized C2B and the
ligand in solution. In addition, for the cases where there are reliable values for the conditional stability constants (b’) of the copper complexes with the competitive ligands, we find that a plot of log(i/r) vs. log /?’ is also linear. In other words, the response depends on the conditional formation constants for the copper complexes under the specific conditions used.28 This supports our assertion that competitive binding effects are responsible for the observed diminution in the analytical signal, and more establishes that the relative importantly, strengths of coordination of the various ligands are maintained under the experimental conditions employed. This implies that the coordination properties of an interface (a modified electrode in this case) can be systematically and deliberately controlled by choice of the immobilized ligand as well as by the presence of other competitive ligands in solution. This should have significant implications for speciation studies, which we are currently exploring. Competitive binding of other metals by surface-immobilized Chromotrope 2B. We have also
performed effects of nation of C2B. The containing
1.10 F”“‘“““‘“-l
1.00
h
some preliminary studies on the other metal cations on the determicopper by electrodes modified with electrodes were exposed to solutions copper at a concentration of 5OpM
0.90
.-
$ 0.80 r - 0.0950
0.70
t
I.
*
1.0
*
I
I
I.
2.0 -log [CT]
I
-
r - 0.9762
I
O.Q50.01.0
3.0
3.0
-log
[Bi]
1.oo 0.90 4
0.30
4.0
-log
[Oxalate’4
-h
4.2
4.4 -log
4.6
4.8
[Humic Acid]
Fig. 3. Effects of competing ligands at various concentrations on the determination of copper at a solution concentration of 1 x 10-5M at electrodes modified with poly-(Ru(v-bpy),]*+ loaded with C2B.
5.0
Amperometric determination Table 1. Effects of competing metal ions on the determination of copper* at electrodes modified with C2B
M’ = cobalt M’ = nickel
[M’l/]Cul*
ir, arbitrary units
0.0 1.0 5.0 0.0 0.2 1.0
5.55 4.08 3.90 5.55 2.73 1.05
*[Cu] held constant at 50@4.
as well as either Co(I1) or Ni(I1) at various concentrations. The signal due to the immobilized C2B/Cu complex was then obtained. The results (Table 1) show that Ni(I1) had a more dramatic effect on the signal than Co(I1) did, which is consistent with the higher formation constant for the nickel complex. (The formation constants obtained in the literature were for Chromotrope 2Rz9 and we have assumed that the trends in coordination strength are the same for the closely related C2B.) The lack of additional data on formation constants precludes any further analysis, but the trends are consistent with differences in the strength of coordination. The presence of nickel gave rise to an additional redox response at -0.29 V. Thus, electrodes modified with C2B could be employed in the determination of nickel in solution.
CONCLUSIONS
We have demonstrated that electrodes modified with C2B can be employed in the determination of copper in solution at concentrations down to 7 x lo-*M, with a wide dynamic range. The presence of competing ligands or other metal ions affects the analytical signal in proportion to the strength of coordination of the other ligands with copper ions or of other ions with the surface-immobilized C2B. The presence of competitive equilibria suggests that this analytical approach might be applicable to speciation studies, and this is currently under investigation. Acknowledgements-This work was supported in part by the National Science Foundation. HDA is a recipient of a Presidential Young Investigator Award (1984-1989) and an A. P. Sloan Fellowship (1987-1991). SKC acknowledges support by the Korean Ministry of Education.
of copper
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