Anal. Chem. 1994, 66,3202-3207
Response of Copper( I I ) Ion-Selective Electrodes in Seawater Roland De Marco Department of Physical Sciences, University of Tasmania, Launceston, Tasmania, 7250, Australia
A comparison of the behavior of three types of Cu(I1) ionselective electrode (Le., copper sulfide, copper selenide, and copper/silver sulfide) in seawater has been undertaken. X-ray photoelectron spectroscopy and X-ray diffraction have shown that the unacceptablyhigh detection limit of the CuS electrode ( lo4 M Cu2+)is due to membrane oxidation to CuS04 and Cu3(SOd)(OH)4. Bare Cul.&e and CuS/Ag2S electrodes displayed Nernstian response (Le., 100% Nemstian slope) in the range 10-16-10-8 M free Cu(I1) with Cu(II)-ethylenediamine buffers also containing 0.6 M NaCl. It is proposed that amelioration of the chloride interference at low levels of free Cu(I1) (i.e,, M) is due to kinetic limitations of the membrane reaction that is responsible for the chloride interference. Corrosionof the Cul& electrodecontaminated seawater with a high level of Cu(I1) ( 100 nM), while the CuS/Ag2S electrode released a much lower amount of Cu(I1) (-2.4 nM). Electrode carryover and contaminationof seawater by adsorbed free Cu(I1) is minimized by equilibration of the electrode in a sacrificial Cu(I1) buffer (Le., pCufree= 15) before analysis. The behavior of Cu(I1) electrodes in seawater has been interpreted in relation to free Cu(I1) levels, and results indicate a proportionality between free Cu(I1) and the electrode potential. N
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techniques (e.g., ligand competition and bacterial bioassay). Ex situ analysis of free Cu(I1) is prone to experimental error as the removal of seawater from the ocean can change the speciation of Cu(I1). Potentially, a Cu(1I) ion-selective electrode (ISE) is capable of rapid in situ monitoring of environmental free Cu(I1). Unfortunately, Cu(I1) ISEs have not been used widely for the analysis of seawater due to the chloride interference that is alleged to render the electrode nonfunctional in this matrix.6 Westall et al.' and Lewenstam et aL8proposed a diffusion layer mechanism to account for the chloride interference on the CuS electrode, viz.
+
CUS(S) Cu2+(aq)= S(S)
-
+~u+(aq)
Cu+(aq) + nCl-(aq) = [CUCI,]'"(as)
(1)
(2)
Normally, the potential of a CuS electrode is due to detection of Cu+ released by exchange of Cu2+with CuS at the electrode diffusion layer. In the presence of chloride, however, the formation of copper(I)+hloro complexes (i.e., [CuCl,] decreases the level of Cu+ and the electrode potential. Neshkova9demonstrated that the same mechanism applies to the Cu1,gSe electrode except that Cu3Se2 is formed on the membrane instead of S. Copper(I1) is present in natural waters in a variety of Seawater is buffered with respect to free Cu(I1) because chemical forms. Pagenkopf et a1.l and Sylva2 indicated that the total concentration of Cu(I1) is M, while the the following species are found in freshwater systems: Cu2+; concentration of free Cu(I1) is much lower (i.e., 10-13-10-11 CUCO~C ; U ( C O ~ ) ~CUHCO~+; ~-; CuOH+; C U ~ ( O H ) ~ ~ + ; M).3 The detection limit of the Cu(I1) electrode in unbuffered CuCl+. It was also found that Cu2+canbe removed completely -lo-' M6. Avdeef et al.1° have shown that the solutions is from aquatic systems by precipitation as Cu(OH)2, CuCO3, detection limit of a CuS/Ag2S electrode can be extended to and Cu(OH)n(C03)1-n/2. lO-I9M through the use of chloride-free Cu(II)-ethyleneSunda and Hanson3 have used ligand competition techdiamine (en) buffers. niques for the analysis of free Cu(I1) in seawater. This work Jasinski et al." noted Nernstian response in the range 6-79 demonstrated that only 0.02-2% of dissolved Cu(I1) is nM Cu2+ with a Agl,sCu& electrode whileanalyzing acidified accounted for by inorganic species (Le., Cu2+,CuCO3, Cuseawater-at pH = 3-using a standard addition technique. (OH)+, CuCl+,etc.); the remainder is associated with organic Conversely, a negative shift in potential and super-Nernstian complexes. Clearly, the speciation of Cu(I1) in seawater is response was observed with raw seawater (at pH = 8). These markedly different from that in freshwater. authors attributed the electrode behavior in raw seawater to Importantly, Sunda and c o - w o r k e r ~demonstrated ~.~ that chelation of Cu(I1) by organic ligands in seawater; acidification free Cu(I1)-not total Cu(I1)-is responsible for Cu(I1) releases bound Cu(II), giving Nernstian response. toxicity. Consequently, the impact of Cu(I1) on the marine More recently, Hoyer12 and Belli and ZirinoI3 obtained environment can be ascertained only by measurement of free near-Nernstian response with Cul.&e and Agl.&uo.sS ISEs Cu(I1) levels.
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Presently, in situ monitoring of free Cu(I1) in seawater is not possible due to the practical limitations of existing (1) Pagenkopf, G. K.; Russo,
R.C.; Thurston, R. V. J . Fish. Res. Board Can.
1974,31,462-5. (2) Sylva, R.N. Water Res. 1976, 10, 789-92. (3) Sunda, W. G.; Hanson, A. K. Limnol. Oceanogr. 1987,32,537-51. (4) Sunda, W. G.; Lewis, J. M. Limnol. Oceanogr. 1978,23, 87C-76. ( 5 ) Sunda, W. G.; Gillespie, P. A. J . Mar. Res. 1979,37,761-77.
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(6) Gulens, J. Ion-Sel. Electrode Reo. 1987, 9, 127-7 1. (7) Westall, J. C.; Morel, F. M. M.; Hume, D. N. Anal. Chem. 1979,51,1792-8. ( 8 ) Lewenstam, A.; Hulanicki, A.; Ghali, E. In Contemporary Electroanalytical Chemistry;Ivaska, I., et al., Eds.;Plenum Press: N e w York, 1990;pp 213-22. (9) Neshkova, M. T. Anal. Chim. Acta 1993,273, 25565. (10) Avdeef, A.; Zabronsky, J.; Stuting, H. H. Anal. Chem. 1983, 55, 298-304. (1 1) Jasinski, R.; Trachtenberg, I.; Andrychuk, D. Anal. Chem. 1974.46,364-9. (12) Hoyer, B. Talanra 1991, 38, 115-8. (13) Belli, S . L.; Zirino, A. Anal. Chem. 1993,65, 2583-9. 0003-2700/94/036&3202$04.50/0
0 1994 American Chemical Society
in Cu(I1) buffers containing 0-1 M NaC1. The Cu&e electrode12displayed Nernstian response in the range 1O-l6M Cu2+for buffers containing 0 or 0.5 M NaCl, while theAgl.5Cu0.5Selectrode13exhibited collinear Nemstian plots in the range 10-11-10-2 M Cu2+for solutions comprising 0-1 M NaC1. These authors did not explain the suppression of chloride interference at low levels of free Cu(I1) (i.e., M), and no attempt was made to interpret the electrode response in natural seawater (i.e., unspiked at pH = 8) in terms of free Cu(I1) levels. It is not possible to judge which Cu(1I) electrode is best suited to an investigation of Cu(I1) complexation in seawater from these isolated studies. Consequently, a comparative examination of the response of CuS, Cu&e, and Agl.sCuo.sS electrodes in artificial and real seawater was deemed appropriate. In particular, the dynamic ranges, extents of membrane corrosion, and response times of electrodes have been compared. Alleviation of the chloride interference in Cu(I1) buffers is consistent with kinetic suppression of the chloride interference reaction mechanism proposed by Westall et al.7 and Lewenstam et a1.8 Additionally, the response characteristics of Cu(I1) electrodes in artificial and real seawater have been interpreted in terms of concentrations of free Cu(I1).
EXPERIMENTAL SECTION Two commercial Cu(I1) ISEs were used: a Radiometer F1112 electrode that employs a single crystal of Cul.gSe as the membrane and a Orion Model 94-29 electrode utilizing a pressed disk of Ag1.sCuo.5S as its membrane. An Orion doublejunction sleeve-type reference electrode (Model 900200) was employed. The Cu(I1) electrodes were cleaned before analysis by polishing the membrane on alumina grit. Potentials were recorded on an Orion expandable ion analyzer (Model EA 940) using a stability criterion of 0.2 mV/min. The total Cu(1I) content of seawater was determined by differential pulse anodic stripping voltammetry (DPASV) using the method described by Scarano et al.14 Voltammetry was performed using a Metrohm 646 VA processor connected to a Metrohm 647 VA stand equipped with a Metrohm multimode electrode. Copper sulfide ISEs were prepared using the methods described by Pungor et al.I5 The CuS powder was characterized by use of a X-ray photoelectron spectrometer (AEI ES100) and a X-ray diffractometer (Philips Model PW17). Analytical grade reagents (Ajax Chemicals) and ultrahigh-purity water (Elgastat UHQ) were employed in this study. Copper(I1) standards in the range 10-7-10-1M were prepared by serial dilution of a 10-l M C u ( N 0 3 ) ~solution. Copper(I1) ion activities were calculated using the ion-association model of Dickson and Whitfield16 in conjunction with the data provided in ref 17. Standards containing M Cu2+were prepared fresh daily. Copper(I1) ion buffers (refer to Appendix, section A. 1 for calculations of PCUfre) were prepared by pH adjustment of M en, and solutions containing M Cu(N03)2, 1.5 X
0.6 M NaCl using concentrated HN03; pHs in the range 6.0-7.5 were measured using a combination glass electrode in conjunction with standard buffers. The high stability of Cu(II)-en buffers was demonstrated by a constancy of electrode calibration data over several months of continuous use. Two types of artificial seawater samples were employed in this study: (i) Cu(I1)-glycine buffers comprising 2 X 10-4 M total Cu(II), M glycine, and 0.1 or 0.6 M NaC1; (ii) Cu(I1)-glycine buffers comprising 3 or 36 nM total Cu(I1) (determined by DPASV), 10-6 M glycine, and 0 or 0.3 M NaCl. For type i solutions, the equilibrium concentrations of free Cu(I1) were calculated using the method outlined in the Appendix, section A.2. At low levels of total Cu(I1) in type ii solutions, however, it was also necessary to account for complexationof Cu2+by OH-, HC03-, and C o g . For reasons of predictability, type ii solutions were equilibrated with atmospheric CO2 before analysis. First, the concentration of C o t was calculated using the solubility parameters given by Weiss,l* and the concentrations of HC03- and co32-were estimated at the equilibrium pH by use of the H2CO3 and HC03- ionization constants reported by Edmond and Gieskes.19 Free Cu(I1) concentrations were calculated by use of the ionic strength dependent equilibrium constants for Cu(OH)+, Cu(OH)2, Cu(HCO3)+, CuCO3, and C U ( C O ~ ) ~ ~ reported by Turner and Whitfield17along with Cu(I1)-glycine equilibrium constants calculated using the data provided in the Appendix, section A.2. Seawater was obtained from the Key Centre for Teaching and Research in Aquaculture at the University of Tasmania. Electrodes were equilibrated with a sacrificial Cu(I1) buffer (pCufr, = 15) before each seawater analysis, minimizing electrode carryover of adsorbed Cu2+. An ethanolic Nafion solution (5% (w/v) Nafion, DuPont) was applied to a polished electrode membrane. A small volume (50 pL) was applied to the Radiometer electrode (membrane diameter 5 mm) and 150pL to the Orion electrode (membrane diameter 8 mm). After evaporation of the solvent, the Nafion film was dried for 60 s with a hair drier. The heat treatment improves the stability of the Nafion film.20 All solutions were stored in polyethylene containers that had been soaked in 1 M H N 0 3 for 48 h and rinsed with copious quantities of ultra-high-purity water. Duplicate analyses were carried out in all cases, except for the determination of free Cu(I1) in seawater, which was performed on four separate occasions. Note that 100-200 mL of seawater was used in all potentiometric studies.
RESULTS AND DISCUSSION CIS Electrode Behavior. The potentiometric response of a CuS electrode (not shown) revealed a detection limit (Le., lo4 M Cu2+)that is too high for the determination of free Cu(I1) in seawater. Characterization of CuS by X-ray photoelectron spectroscopy and X-ray diffraction has shown that the high detection limit is due to leaching of soluble Cu(11) salts formed by atmospheric oxidation of the CuS
~~
(14) Scarano, G.; Morelli, E.; Seritti, A.; Zirino, A. A w l . Chem. 1990,62,943-8. (15) Pungor, E.; Toth, K.; Papay, M. K.; Polos, L.; Malissa, H.; Grasserbauer, M.; Hoke, E.; Ebel, M. F.; Persy, K. Anal. Chim. Acta 1979, 109, 279-90. (16) Dickson, A. G.; Whitfield, M. Mar. Chem. 1981, 10, 315-33. (17) Turner, D. R.; Whitfield, M. Geochim. Cosmochim. Acra 1987,51,3231-9.
(18) Weiss, R. F. Mor. Chem. 1974, 2, 203-315. (19) Edmond, J. M.; Gicskcs, J. M. T. M. Geochim. Cosmochim. Acra 1970.34, 1261-9 1. (20) Hoyer, B.; Florencc, T. M.; Batley, G. E. And. Chem. 1987,59, 1608-14.
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possible explanation for the shift in Eo of the Cul.gSeelectrode is that the membrane composition changes as the electrode soaks in Cu(II)-en buffers containing 0.6 M NaC1. The subNernstian responseof the Cul.gSeelectrode in the range 10-loE 10-8 M Cu2+ ( 13.9 mV per decade change in aCu2+)is probably linked to this shift in E O . Figure l b depicts the potentiometric behavior of a Ag1.5Cu0.5Selectrode in Cu(II)-en buffers comprising 0.6 M NaCl (A) and chloride-free standards in the range 10-7-10-1 M -200 ! I Cu2+ (0). Significantly, Nernstian response was obtained -1 5 -10 -5 0 within the range 10-15-10-1 M Cu2+, spanning a Cu2+ Iogacu2+ concentration range of 14 decades. The response curve of a Ag1.5Cuo.5S electrode in chloride-free standards (> M Cu2+)displayed two key features; namely, Nernstian response between 10-6 and 10-l M Cu2+and a deviation from Nernstian 200 response below lo4 M Cu2+due to Cu(I1) contamination by > E reagent impurities and/or dissolution of the membrane. An 100-m .extension of the Nernstian response for chloride-free 5 0standards-above 10-6 M Cu2+-falls on the calibration data a obtained with Cu(II)-en buffers containing 0.6 M NaCl. -100 Clearly, the chloride interference is negligible at free Cu(1I) levels of < 1 P M. . -200 -15 -10 -5 0 In summary, the chloride ion interference on Cul.gSe and logaCu2+ Ag1.5Cuo.5Selectrodes is alleviated in the range 10-16-10-8 M Figure 1. Potentiometricresponse curves of Cu(I1) electrodes In CuCu2+due to an absence of copper(I)-chloro complexes in the (II)-en buffers: (a) Cu,.&ie; (b) Ag,.5Cuo.5S. The graphical marks electrode diffusion layer at very low levels of free Cu(I1). This indlcate the following: (0)bare electrode in chloride-free standards above lo-' M Cu2+;(A)bare electrodein Cu(I1)-en buffers containing is not unexpected as the formation of copper(I)-chloro 0.6 M NaCI. complexes by reductive ion exchange of Cu(1I) is severely limited, kinetically, at very low levels of free Cu(I1) (i.e., M). The wide dynamic range of the Agl.5Cuo.sS membrane (viz., CuS044H20 and Cu3(S04)(0H)4). Adelectrode (Le., 10-15-10-1M Cu2+)indicates that it might be ditionally, it was necessary to rinse the electrode with 0.02 M possible to use this electrode in a manner similar to a pH HNO3 when changing from concentrated to dilute solution. electrode; Le., pCu in buffered solutions may be measured if Presumably, this is associated with electrode carryover arising the electrode is calibrated in standard pCu buffers (e.g., Cufrom chemisorption of Cu2+ onto the membrane surface by (II)-en solutions). equilibration with the sparingly soluble salt C U ~ ( S O ~ ) ( O H ) ~ . Responseof Nafion-Coated Electrodes in Cu(II)-en Buffers Consequently, the CuS electrode was deemed unsuitable for Containing Chloride. Previous work by Hoyer and Loftage92 the analysis of free Cu(I1) in seawater. has shown that the chloride interference experienced by the Response of Bare Electrodes in Cu(II)-en Buffers ContainCu&e electrode at high levels of Cu2+ (i.e., >10-6 M) is ing Chloride. This work uses equilibrium constants derived suppressed by coating the electrode membrane with the cation by Avdeef et al.1° with a CuS/Ag2S electrode in chloride-free exchanger Nafion. More recently, Hoyer12demonstrated that Cu(II)-en buffers to assign levels of free Cu(I1) to saline a Nafion-coated Cul.gSe electrode exhibited sluggish response buffers. The response of Cul.gSe and Ag&uo.sS electrodes, to Cu2+in Cu(I1) buffers, and this precluded its use in Cu(I1) in these saline buffers, has been tested under identical binding studies. It is crucial that the Nafion coating allows conditions. a proportionate permeation of species in buffered Cu(I1) Figure l a shows the potentiometric response of a Cul.gSe systems (e.g., Cu2+,Cu(en)2+,Cu(en)z2+,enH+, enHz2+and electrode in Cu(II)-en buffers containing 0.6 M NaCl (A). en) in order for Cu(I1) speciation equilibria and electrode Also included in Figure l a is the response of the electrode in sensitivity to Cu2+to be maintained. This is not necessarily chloride-free Cu(N03)2 standards in the range 10-6-10-1 M thecase, and this work explores the feasibility of using NafionCu2+(0). It can be seen that Nernstian response (i.e., -30 coated electrodes in Cu(I1) complexation studies. mV per decade change in aCu2+)was obtained with a Cu1,gSe The responseof a Nafion-coated Ag1.5Cu0.5Selectrode (not electrode in the ranges 10-16-10-10 and 10-5-10-1 M Cu2+; shown) yielded Nernstian response (i.e., 29.5 mV per decade however, the linear segments do not converge. This divergence change in aCu2+) in the range 10-16-10-8 M Cu2+,while a is symptomatic of an alteration in the standard potential (EO) Nafion-coated Cu1,gSeelectrodeexhibited a somewhat reduced of the Cu1,gSe electrode; it has not been investigated further. sensitivity, i.e., 20.9 mV per decade change in aCu2+. It is It should be noted, however, that De Marcoet a1.2l haveshown important to note that Nafion-coated electrodes had long that the Eo of a Ag1.5Cu0.5Selectrode varies as the surface equilibration times in Cu(II)-en buffers (i,e., -30 min for composition of the membrane is altered by oxidation. A Ag1.5Cuo.sS and -3 h for Cul.gSe). The sub-Nernstian cuu
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I
n
l
'"i
I
I
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-
I
(21)De Marw, R.;Cattrall, R. W.; Liesegang, J.; Nyberg, G. L.; Hamilton, I. C. Anal. Chem. 1992, 64, 594-8.
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(22) Hoyer, B.;Loftager, M. Anal. Chem. 1988, 60,1235-7.
response of a Nafion-coated Cul.gSe electrode is due to a steady-state potential not being attained within the time-frame of measurement. Belli and Zirino13 noted membrane tarnishing and deterioration of potentiometric response for a Agl.sCu& electrode after prolonged exposure to saline solution (e.g., artificial seawater for several days), and De Marco et al.21have shown that this is due to electrode fouling by AgCl. Subsequently, a Nafion-coated A~~.sCUO.SS ISE is recommended for investigation of Cu(I1) complexation in seawater as the coating protects the membrane against sluggish chloride-induced poisoning (i.e., AgCl) that eventually renders the electrode unusable in seawater. Dissolution of Cu(I1) ISE Membranes. Copper(I1) contamination of samples arising from membrane corrosion is a serious problem with measurements of free Cu(I1) using IS&. HoyerZ3demonstrated that polished Cul.gSe and Agl.sCuo.sS electrodes released, respectively, 200 and 60 nM total Cu(I1) when soaked in 10 mL of 0.5 M NaCl for 5 min. Obviously, these levels of Cu(I1) release would contaminatesignificantly seawater containing 1-10 nM total Cu(I1). The corrosion of Cu(I1) ISEs has been reexamined by measuring total Cu(I1)-in duplicate by DPASV-released after immersion of both the Cu ISE and reference electrode in 100 mL of 0.6 M NaCl for 15 min. A Cu&e electrode released 100 nM total Cu(I1). Clearly, this is a serious level of contamination that precludes the use of a Cul.gSe electrode for examination of Cu(I1) complexation in seawater. By contrast, the Agl,5Cuo.sSelectrode released a much lower amount of Cu(I1) (Le., 2.4 nM). It should be noted that equilibrium calculations based on conditional stability constants cited by HiroseZ4for seawater ligands have shown that this level of contamination will give an electrode error of 0.20.7 pCufr, unit in seawater containing 1-10 nM total Cu(I1). Obviously, the higher concentration of Cu(I1) in the electrode diffusion layer will increase the magnitude of this electrode error. The lower level of Cu(I1) released from the Agl.~Cu0.$3 electrode noted in this work is probably due to minimization of electrode carryover of adsorbed Cu2+ resulting from cleansing of the electrode in a sacrificial Cu(I1) buffer (PCUfre = 15) before analysis. It is important to note that a X-ray photoelectron spectroscopicstudy of theAgl,~Cuo.sS membrane by De Marco et al.2' has shown that the membrane photooxidizes in the presence of dissolved oxygen, viz. Ag,,,Cuo,5S+ 2 . 5 ~ 0 + , 2xH' = Ag,,,Cuo,,-$l~x + xS0;-
+ 2xCu2++ xH20 (3)
Copper(I1) contamination of seawater by membrane corrosion might be alleviated by using the electrode in the absence of light and dissolved oxygen. Analysis of Artificial Seawater. The feasibility of using Cu(I1) ISEs in studies of Cu(I1) speciation in seawater has been studied by analyzing several saline Cu(I1)-glycine buffers, i.e., artificial Seawater. In the following sections, the (23) Hoyer, B. Talanta 1992, 39, 1669-14. (24) Hirose, K. AMI. Chim. Acta 1994,284,621-34.
Table 1. Values ol pCu, In Cu( II)-Glydne Buffers.
Pcur~
[NaCl], M
CalCd
Cu1.8Se
Agi.sCw.B
0.1 (pH = 7.43) 0.6 (pH = 7.46) 0.1 (pH = 9.91) 0.6 (pH = 9.87)
8.04 8.09 11.99 11.93
11.30 11.54 12.10 12.07
8.14 8.13 11.86 11.87
0 Comprising 2 X 1 Ptotal Cu(II), 10-3 M glycine, and 0.1or 0.6 M NaCI; determined using bare Cul.8Se and Ag&t& ISES. The calculated equilibrium values are included for comparison.
Table 2. Values
of pCu In Cu( 1I)-Glyclne Bufferse PCU DPASVb calcdC Agl.&u&d
water (pH = 9.46) 0.3 M NaCl (pH = 9.31)
8.53 7.44
13.30 12.88
13.60 12.64
Cul~Sed
12.74 11.77
a Comprising 3 or 36 nM total Cu(II), 10-6 M glycine, and 0 or 0.3 M NaC1; determined using DPASV [Le., total Cu(II)] and ISE
potentiometry [Le., free Cu(II)]. The calculated equilibrium values are included for comparison. Total Cu(I1). CalculatedequilibriumpCum. Bare electrode [free Cu(I1)I.
*
Table 3. Valuer ol A, B, and C (after Avdeel et al.lO)for Cu( 1 I ) a n Equlllbrla. A B C log K (I = 0.6M)
log KlH logKZH log Kl log K2
9.930 6.924 10.366 8.963
0.0550 0.7297 0.6375 0.2071
0.1535 0.1734 0.0739 0.2480
10.046 7.347 10.688 9.202
Also included is log K for I = 0.6 M. Table 4. Parameters Used In the Calculation ol log K(I) for Cu(Gly)+ and c ~ ( G l y ) ~ ~ "
species cu2+ GlYCu(Gly)+ Cu(Gly)2
log K
8.53 15.60
B
C
1.718 1.670 1.5
0.083 0.095 -0.447 -0.744
D
log K 0.1 M 0.6M
-0.186 8.17 15.06
8.15 15.01
Also included are log K values for I = 0.1 and 0.6 M.
electrode response has been assigned to pCuf,, using the calibration curves shown in Figure 1, parts a and b. Table 1 shows results for buffers comprising 2 X 10-4 M total Cu(II), M glycine, and 0.1 or 0.6 M NaCl. It can be seen that the values of PCUfr, determined using Cul.8Se and Agl.~Cuo.sSelectrodes-at pH = 9.9 and pCuf, = 12-are similar to the calculated equilibrium values. At pH = 7.45 and pCufrcc = 8, however, the Cul.gSe sensor displays a significant electrode error @e., ApCuf,, = 3). This error is most likely ascribed to the large variation in Eo experienced by a Cu1,gSe electrode above M Cu2+ (refer to Figure la). By contrast, the Agl,~Cuo.$electrode provided pCufm values within fO. 1 unit of the calculated equilibrium values. Two samples with Cu(1I) complexation capacity similar toseawater (i.e., total Cu(I1) of 3 and 36 nM, 10-6 M glycine, and 0 or 0.3 M NaCl) were also analyzed by ISE potentiometry (refer to Table 2). Results for PCUfr, determined with a Ag&uo.5S electrode fall within 0.24-0.30 unit of calculated equilibrium values. As expected, the Cul.gSeelectrode yielded Analytical Chemistry, Vol. 66, No. 19, October 1, 1994
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erroneous results (Le., ApCuf,, = 1) due to severe Cu(I1) contamination of samples by membrane corrosion (Le., 100 nM). Additionally, a Ag1.5Cuo.5Selectrode provided a pCuf,, of 8.8 f 0.2 for four samples of straight ultra-high-purity water; this is similar to the value of total Cu(I1) determined by DPASV (Le., pCutotal= 8.53). It should be noted that an error is expected when assigning the Agl,5Cuo.5Selectrode's response in artificial seawater to pCufr, because membrane corrosion releases a much higher level of Cu(I1) contamination in the electrode diffusion layer than detected in the bulk solution [i.e., 2.4 nM total Cu(II)]. Nevertheless, the agreement between calculated and measured pCuf,, values infers a proportionality between electrode response and levels of free Cu(I1). This was confirmed by the near-Nernstian behavior of a Agl.sCuo.sSelectrode (i.e., 27.1 mV per decade change in aCu2+ at four pCuf,, values with a correlation coefficient of 0.975) in the artificial seawater sample containing 36 nM total Cu(I1) adjusted to different pHs. Analysisof Seawater. Copper(I1) in seawater was analyzed by DPASV and ISE potentiometry; the results conveyed several key points. First, the value of pCutOtaldetermined by DPASV (i.e,, 7.767) was similar to that obtained for acidified seawater (pH = 2.0) using a Agl,sCuo.sSelectrode (Le., 7.80). This is expected as all Cu(I1) species are converted to free Cu(I1) on acidification of seawater.13 The pCufr, values obtained in this study (Le., 12.6 f 0.1) are comparable to those reported by Sunda and co-workers in seawater,4q5 viz. pCurr, 11.012.3. Finally, the value of pCuf,, determined using a Nafioncoated Ag1.5Cuo.5S electrode (Le., 12.4) was similar to that obtained with a bare electrode (Le,, 12.6). It is known that the Nafion coating alleviates the chloride interference on Cu(11) ISEs.Z2 Additionally, it was shown that the chloride interference on the Agl,$ho.sS electrode is suppressed at free Cu(I1) levels of < l e 8 M (refer to Figure lb). Clearly, the response of a Agl.sCu0.5Selectrode in seawater is proportional to the level of free Cu(I1); it is not affected by chloride. Amelioration of the problems of high detection limit in unbuffered solutions (Le., -10-6 M Cu2+) and chloride interference for a Agl.~Cuo.sSelectrode is attainable through electrode calibration in Cu(II)-en buffers. Presumably, the ligands in seawater (mainly humic and fulvic substance^^^) buffer, to some extent, the Cu2+released by dissolution of the Agl.sCuo.sS membrane [Le., 2.4 nM total Cu(II)]. Two uncertainties still remain with interpretation of the Ag1.5Cu0.sS electrode's response in seawater: (i) corrosion of the membrane generates a higher level of Cu(I1) contamination in the electrode diffusion layer than the bulk solution; (ii) it is known that the response of Cu(I1) electrodes in Cu(I1) buffers is dependent on the nature of the ligand,6 and the ligands in seawater (mainly humic and fulvic substances) may affect deleteriously the electrode behavior.
CONCLUSIONS This study demonstrates that the response of an Orion 94-29 (i.e., Agl,&uo,5S) Cu(I1) ISE is proportional to the level of free Cu(1I) in artificial and real seawater. A Radiometer F1112 electrode (i.e,, Cul,sSe), however, is ~~
~~~
~~
~~
( 2 5 ) Francois, R. Reu. Aquaf. Sci. 1990, 3, 41-80.
3200 Analytical Chemistry, Vol. 66, No. 19, October 1, 1994
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PC" 10
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0 2
I
I
8
4
6
I
0
10
12
PH Flgwo 2. Calculated values of pCu in a solution comprislng 10" M en and 0.6 M NaCl versus pH. Cu(NO&, 1.5 X
M
unsuitable for studies of Cu(I1) complexation in seawater due to severe Cu(I1) contamination arising from corrosion of the membrane. In situ environmental monitoring of Cu(I1) during a sea voyage is possible by incorporation of a Cu(I1) ISE in a specially designed flow analyzer (i.e., continuous flow or flow injection analyzer). Copper(I1) contamination of seawater by photooxidation of the Agl,~Cuo.sSmembrane might be alleviated by carrying out analyses in the absence of light and dissolved oxygen. It is anticipated that ablation of the membrane by a continuous flow of solution-in a flow analyzer- will further alleviate the problem of Cu(I1) contamination of samples. Work is in progress to develop a flow analysis technique for mapping of free Cu(I1) in various Tasmanian waters (e.g., Derwent River, Bathurst Harbour, Macquarie Harbour, etc.).
ACKNOWLEDGMENT The author thanks Dr. D. Mackey (CSIRO Marine Laboratories) for helpful advice. Thanks are due to Mr. M. Hughes (Chemistry Department, University of Tasmania) for assistance with the X-ray photoelectron spectroscopywork. The author gratefully acknowledges the University of Tasmania for financial support. I thank twoanonymous reviewers for their useful suggestions that have helped to strengthen the paper. APPENDIX. CALCULATION OF EOUILIBRIUM CONCENTRATIONS OF FREE CU( 11) A.l. Cu-en Buffers. Avdeef et al.1° have shown that the predominant equilibria in solutions containing Cu2+and en are en(aq)
+ H+(aq) = enH+(aq) K,H= [enH+]/([en][H+])
enH+(aq) + H+(aq) = e n H F ( a q )
K F = [enHF]/([enH+][H+])
+
Cu2+(aq) en(aq) = Cu(en)'+(aq) K, = [ ~ u ( e n ) ~ + ] / ( [ ~[en]) u~+] Cu(en)'+(aq)
+ en(aq) = Cu(en),z'(aq) K , = [~u(en)22+]/([~u(en)*+] [en])
where KIH, KzH, K1, and K2 are the respective equilibrium constants.
The following mass balances apply:
[enIT = [en]
+ [enH+] + [ e n H y ] + [Cu(en)’+] + 2[~u(en),z+]
where the subscript, T, denotesthe total concentrationof Cuz+ and en. Solving the above-mentioned equations simultaneously yields the parameters [Cuz+],[Cu(en)’+], [Cu(en)zz+],[en], [enH+], and [enHz2+]. Avdeef et al.1° cited ionic strength dependent expressions for the equilibrium constants, viz. log K = A
+ B I ~ / ’ / ( +I P ) + CI
where I is the ionic strength, and A, B, and C are constants. These constants along with log K values for I = 0.6 M are provided in Table 3. M and Values of pCurrw calculated for [ C u 2 + ]= ~ en]^ = 1.5 X IO-’ M using the equilibrium constants at I = 0.6 M are presented in Figure 2. A.2. Cu(II)-Glycine Buffers. Turner and Whitfield17 indicated that the following reactions are most prominent in Cu(I1)-glycine buffers containing high levels of total Cu(I1) and glycine (i.e., -lV-lO-’ M): H+(aq)
+ Gly-(aq) = HGly(aq) KH = [HGlYl/([H+l[GlY-l)
Cu’+(aq)
+ Gly-(aq) = Cu(Gly)+(aq) K, = [Cu(Gly)+]([Cu’+][Gly-1)
+
Solving mass balances for Cu(I1) and glycine simultaneously enables a calculation of the concentration of free Cu(11). For the protonation of Gly-, Turner and Whitfield17 calculated the equilibrium constant using the following formulas: log p(I)= 9.55- [11/’/(1 (0.25)’/’/(1
+ 1.511/’) -
+ 1.5(0.25’/’))] + 0.49(1- 0.25) 0.27(13/’ - 0.253/2)
where 0.25 M is the reference ionic strength and log P(I) at ionic strengths of 0.1 and 0.6 M are 9.57 and 9.56, respectively. By contrast, Turner and Whitfield” calculated the equilibrium constants for Cu( 11)-glycine complexes using their so-called ion-association model, which accounts for the variation of activity coefficients, 7,with ionic strength, viz. log K(I) = log K +
log y where K is the thermodynamic stability constant, Zv log y = (sum of the multiples, Y log y, for reactants) minus (sum of the multiples, Y log 7,for products), and u denotes the stoichiometric coefficients of species. The ionic strength dependency of activity coefficients is given by
+
u
+ +
log = -0.51 i z z ~ 1 / 2 / ( i BI’/’) CI D I ~ / * where Z is the charge on the ion; B, C, and D are constants. The parameters for calculation of stability constants along with values of log K a t I = 0.1 and 0.6 M are given in Table 4.
Cu2+(aq) 2Gly-(aq) = Cu(Gly),(aq) K2 = ~ c ~ ( ~ l Y ) 2 1 / ( ~[Gly-l’) c~z+l where Gly- is an abbreviation for the deprotonated form of glycine and K are equilibrium constants.
Received for revlew April 5, 1994. Accepted June 21, 1994.* *Abstract published in Advance ACS Abstracts, August 1, 1994.
AnelyNcal Chemktry, Vol. 66, No. 19. October 1, 1994
520’1
