594
Anal. Chem. 1092, 6 4 , 594-598
(4) Mefford, I . N. J . Chrometogr. 1988, 368, 31-37. (5) Borgerdlng, M. F.; Hinze, W. L. Anal. Chem. 1985, 57, 2183-2190. (8) Borgerdlng, M. F.; Quina, F. H.; Hinze, W. L.; Bowemaster, J.; McNair, H. M. Anal. Chem. 1988. 60, 2520-2527. (7) Berry, J. P.; Weber, S. G. J . Chromatogr. Sci. 1987, 25, 307-312. (8) Arunyanart, M.; Cline Love, L. J. Anal. Chem. 1984, 56, 1557-1561. (9) Hernandez-Torres, M.; Landy, J. S.; Dorsey, J. G. Anal. Chem. 1988, 58. 744-747. Khaledi, M. G. AM. chem. i m , 60, 876-887. Khaiedl, M. G.; Strasters, J. K.; Rodgers. A. H.; Breyer, E. D. Anal. Chem. 1990, 62, 130-136. Strasters, J. K.; Breyer, E. D.; Rodgers, A. H.; Khaledl, M. G. J . ChromtitOQf. 1990, 577; 17-33. Pramauro, E.; Minero, C.; Saini, G.; Gragli, R . ; Pelizzetti, E. Anal. Chim. Acta 1988, 212, 171-180. Lavine, B. K.; White, A. J.; Han, J. H. J . Chromatcgr. 1991, 542, 29-40. Cline Love, L. J.; Habata, J. G.; Dorsey, J. G. Anal. Chem. 1984, 58, 1132A- 1148A. Kandori, K.; McGreevy, R. J.; Schechter, R. S. J . Colloid Interface Scl. 1989, 732, 395-402. Gao, 2.; Wasyllshen, R. E.; Kwak, J. C. T. J . Phys. Chem. 1991, 95, 482-467. Mulllns, F. G. P.; Kirkbright, 0.F. Anakst 1984, 709, 1217-1221. Mullins, F. G. P. In Recent Developments in Ion-Exchange; Williams, P. A., Hudson, M. J., Eds.; Elsevler Applied Science: London, 1987; pp 87-97. Okada, T. Anal. Chem. 1988, 60, 1551-1516. Okada, T. J . Chromatogr. 1991, 538, 341-354. Okada, T. Anal. Chem. 1988, 60, 2116-2119. Mullins, F. G. P.; Kirkbright, G. F. Anakst 1987, 772, 701-703. Mullins. F. G. P. ACS Symp. Ser. 1987, 342, 115-129. Rosen, M. J. Surfactants and Interfa&/ Phenomena; Wiiey: New York, 1978.
(26) Cassldy. R. M.; Elchuck, S. Anal. Chem. 1982. 54, 1558-1583. (27) Cassidy, R. M.; Elchuk, S.; Joe, K. S. In Recent Devekpmsnts in IonExchange; Wiiilams, P. A., Hudson, M. J., Eds.; Elsevler Applied Science: London, 1987; pp 40-48. (28) Cassldy, R. M.; Elchuk, S. J . Chrmatogr. Scl. 1981. 79, 503-507. (29) Sevenich, 0. J.; Frltz, J. S. Anal. Chem. 1983, 55, 12-16. (30) Uden, P. C. Trends Anal. Chem. 1987, 6 , 238-246 and references therein. (31) Takata, Y.; Arkawa, K. Bunsekl KaQaku 1975. 24, 762-767. (32) Story, J. N.; Frltz, J. S. Talenta 1974, 27, 892-894. (33) Quina, F. H.;Chaimovich, H. J . phys. Chem. 1979, 83. 1644-1650. (34) M o l , Y. J . CoIM Interface Scl. 1988, 722, 308-314. (35) Conin, M. L.; Harkhs, W. D. J . Am. Chem. S0c. 1947, 69, 683-888. (36) Charblt, G.; Dorbn, F.; Gaborlaud, R. J . CdlOM Interface Scl. 1985. 706, 265-268. (37) Martell. A. E. Stabllm Constant; The Chemical Society: London, 1964; pp 412-413. (38) Fletcher, P. D. 1.; Robinson, B. H. J . Chem. Soc.,Faraday Trans. 7 1984, 80, 2417-2437. (39) Arunyanart, M.; Cline Love, L. J. Anal. Chem. 1966, 57, 2837-2643. (40) Khaiedl, M. G.; Rodgers, A. H. Anal. % ( T.I Acta 1990, 239, 121-128. (41) Gaborlaud, R.; Charbit. G.; Dorbn, F. In Surfactants In SoluMOn; M h i , K. L., Lindman, E., Eds.; Plenum: New York, 1984: Vd. 2, pp 119 1- 1206. (42) Fernhdez, M. S.; Fromherz, P. J . Phys. Chem. 1977, 8 7 , 1755-1761. (43) El Seoud. 0. A. A&. ColbU Interface Scl. 1989, 30, 1-30. (44) Peliuetti, E.; Pramauro, E. Anal. Chlm. Acta 1980, 777, 403-406. (45) Deming, S. N.; Morgan, S. L. Anal. Chem. 1973, 45. 278A-283A.
RECEIVED for review September 23,1991. Accepted December 10, 1991.
Surface Studies of the Copper/Silver Sulfide Based Ion-Selective Electrode Membrane Roland De Marco, Robert W.Cattrall,* John Lieeegang, and Graeme L.Nyberg School of Physical Sciences, La Trobe University, Melbourne, Victoria 3083, Australia Ian C. Hamilton BHP Research, Newcastle Laboratories, Newcastle, New South Wales 2287, Australia
The offed d oxygenated solutlom on the surface composltkn d the Jalpalte-based (Ag,.,Cu,,,S) copper lon-selectlve electrode membrane k examlned, and It Is demonstrated that oxidation d the surface occurs wlth a consequent shtft In the E o of the system. I t Is also establlshed that only the out(Hmolf surface layer (-2 nm) Is hydrated and that the oxldatlon process extendvely corrodes the surface d the membrane, causing crystals of Insoluble salts to grow on the membrane surface. There Is strong evidence that the severe Interference wlth the functlon of the copper-selectlve electrod. by both Hg(I1) and Fe( 111) h due to a photooxidation reactbn In ambient light which leads to severe corroslon of the mcwnkane surface and the fonnatlon of surface layers of mercury(1) and silver sulfates, respectively. I n the dark, corrosion of the surface lo less extensive although oxldatlon of the surface still occurs through reactlon with dissolved oxygen to form mercury(I1) sulfate on the surface In the presence of Hg( I I ) and sllver sulfate In the presence of Fe(III).The strong Interference from halides In amblent light k not due to photooxldatlon but to coupling of oxldatlon Invdvlng dissolved oxygen wlth lon-exchange to produce surface deposits of the sliver halide. A slmllar reactlon takes place In the dark.
INTRODUCTION One of the important precipitate-based copper(II)-sensitive ion-selective electrodes uses a membrane prepared by presaing a disk of coprecipitated copper and silver sulfides. Good mechanical and electrochemical properties have been correlated with disks containing significant quantities of jalpaite, A~,,,CU,,~S, which implies that copper is present as the Cu(1) ion.’ This has been confirmed by Coetzee et aL2using X-ray photoelectron spectroscopy (XPS). Jalpaite is found as a major component in the membranes of several commercial copper ion-selective electrode^.^ The presence of Cu(1) in the membrane provides a similarity between this membrane and the other types used in copper ion-selective electrodes which are based on copper(1) sulfide or selenide. The copper(1) sulfide and selenide membranes are Cu(1) ion conductors, and it has been suggested that they respond to Cu(II) by simple ion-exchange which releasea Cu(D ions to the diffusion layer! It could be presumed that the jalpaite electrode responds by a similar mechanism; however, an X P S study of the membrane surface after soaking in 1M copper(II) nitrate revealed that copper was still present mostly as the Cu(1) ion.2 Further work is required to fully elucidate the response mechanism of the Cu(I1)-sensitive electrodes. Recent work by the authors5 involving surface analysis of
0003-2700/92/0364-0594$03.00/00 1992 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 64, NO. 6, MARCH 15, 1992
595
Table I. XPS Binding Energies (eV) after Treatment with Hg(I1) and Fe(II1) levels
clean"
Hg(I1) hvb
Hg(IUb
368.0 374.1
nd nd nd nd 100.4 104.8 358.9 378.4
nd nd nd 101.1 105.7 360.0 379.4
933.0
953.0
______ ___- _ _ ---e
161.9
-__
162.2
___ ___
167.9
nd
___ 162.1 168.0 ___ ___
Hg(I1) h#
Hg(W
368.0
367.9
368.0
932.9
932.9 952.9 101.2 105.5 359.9
933.1
__-
953.0 100.6 104.9 359.0 378.4
___
162.5 168.0
___
379.1
_--
162.5 168.0
______
Fe(II1) hd
Fe(III)d
374.0 952.9
___
_--
____ _ nd 162.0
___ ___
168.1 533.1 284.7 "Polished membrane; water-treated membrane gave identical peaks. bTreated in 0.1 M Hg(NO3I2for 7 days in ambient light and in darkness, respectively. 'Treated in deoxygenated0.1 M Hg(NOJ2 for 7 days in ambient light and in darkness, respectively. dTreated in 0.2 M Fe(NOg),for 7 days in ambient light and in darkness, respectively. e - - -,not measured. nd, not detected. the silver sulfide electrode membrane using XPS and scanning electron microscopy (SEM) demonstrated that photoinduced redox reactions play an important role in the response mechanism of the silver sulfide electrode, particularly in the presence of interfering ions. The presence of Cu(1) in the jalpaite membrane suggested that redox reactions might also be important in the response mechanism of the copper(I1) electrode, and 80 a similar study of the jalpaite membrane was deemed appropriate.
__ __ __
line intensitiesllusing empirical atomic sensitivity factors reported by Wagner et al.12 SEM studies were carried out using a Siemens autoscan microscope. This instrument incorporates an EDAX International Inc. accessory. The microscope chamber was maintained at a pressure of Pa. This instrument permitted attainment of images at a maximum magnification of 1OOOO.
RESULTS AND DISCUSSION Response Characteristics of the Cu(I1) Electrode. The response of the Orion 9429 copper(I1)-selective electrode was EXPERIMENTAL SECTION measured in copper(I1) nitrate solutions in the concentration Membranes were prepared by coprecipitationof copper/silver range to lo-' M in the presence and absence of light. sulfides by addition of sodium sulfide solution to a solution of Nernstian response was obtained in each case, but the calicopper(I1) and silver nitrates according to the method of Heijne bration linea were separated by 2.5 f 0.2 mV (average of three and Van Der Linden.lV6 Membranes were prepared by pressing the dry powder in a Specac punch and die at 10 tons to give disks determinations each), with the measurements in light giving 13 mm in diameter and 1-3 mm thick. the higher potential values. This photosensitivity of the Potentiometric measurements were made with an Orion ionjalpaite membrane raises the possibility of redox processes alyzer (Model 901) in the following cell using an Orion 94-29 occurring at the surface. copper ion-selective electrode and an Orion double-junction Further evidence for this is seen in a study of the potensleeve-type reference electrode. tiometric response curves obtained after soaking a polished AglAgClIKCl(satd)lll.3 M NH4N031(samplelmembrane~Cu(II) (fine alumina on a felt pad) membrane in water for various reference solutionlAgCllAg time intervale in the light. After soaking for 1,2, and 3 days the response was still Nernstian but the response curves were X-ray diffraction studies have shown that the membrane of the about 30, 80, and 100 mV, respectively, higher than for the Orion copper ion-selective electrode consists mainly of jalpaite? freshly polished membrane. Repolishing the membrane reThe surface of the electrode was polished with fine emery paper stored the original response characteristics of the electrode. followed by moist alumina on a felt pad. These results suggest a modifcation of the membrane surface. Copper(II), mercury(II), and iron(II1) nitrate salts were disMembrane Surface Studies. An XPS study was made solved in 0.01 M nitric acid to prevent hydrolysis. All chemicals of the surface of a jalpaite membrane after polishing and after were of analytical reagent grade. Copper ion activities were soaking in water for even longer periods of time than used in calculated using the Debye-Htickel equation. Experiments were performed in deoxygenated Hg(N03)2and the response study. This was necessary in order to form NaCl solutions to explore the effect of the absence of dissolved detectable amounts of the surface degradation producta. The oxygen on the photooxidation of jalpaite. The solutions were peaks obtained for a polished and a water-treated membrane initially purged with high-purity nitrogen for 1h (50 mL/min). had identical binding energies. The surface stoichiometry was The membranes were placed in the solution, and the memdeduced from the intensities of the relevant XPS peaks for brane/solution system was purged with nitrogen (50 mL/min) Ag, Cu, S, and 0. The results confirmed the jalpaite stoifor 7 days. chiometry (Ag1.5C~.5S) for a clean membrane, but soaking in The XPS study was made using a high-vacuum spectrometer and techniques described in previous papers by the author~.~J~* water led to a change in surface composition. The Cu/S ratio decreased with time of soaking to a value of -0.2 after 14 days This instrument has an Al Ka X-ray source, and the spectrometer chamber is maintained at a pressure of -106 Pa. The resolution whereas the Ag/S ratio increased to -1.8. The ratio of O / S or full width at half-maximum is 1 2 eV. Binding energies were remained reasonably constant at 3. Also, the single oxygen determined using the Cu (2pSf2)peak at 932.67 eV (referenced peak at a binding energy of 533.1 eV is consistent with the to the Fermi level) as the calibrant? Chemical depth profiles were presence of water13J4and not metal oxides.'"17 measured by sputtering the sample in argon at Pa using an Additional information about the surface was obtained by ion gun (Varian, Model 981-1045). The sample was bombarded a closer examination of the XPS binding energies given in by 600-eV ions at a current density of -5 MAcm-2. The depth Table I. The single S (2p) peak at 161.9 eV is indicative of scale was calibrated by calculating the sputter rate, using the sulfur as the sulfide ion. The Ag (3d) spectrum also shows equation given by Hofmannloas has been used previously by the the characteristic 3d5f2and 3d3/2spin-rbit split components. authors.78 The sputter rate was calculated to be -0.7 nm mi&. The binding energy of the Ag (3d) level is insensitive to the Atomic concentration ratios were calculated from photoelectron
-
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 6, MARCH 15, 1992
Table 11. Surface Atomic Ratios"after Treatment with Hg(I1) and Fe(II1) ratio S2-/Hg SOd2-/Hg S2-/Ag SO:-/Ag
s2-/cu
cleanb ..f
_-0.63 nd 2.0
Hg(I1) hv'
Hg(W
Hg(I1) hvd
Hg(IUd
Fe(II1) hve
0.15 0.45 nd nd nd
0.49 0.49 nd nd nd
0.42 0.21 2.0
0.77 0.46 2.4 1.2 4.2
--_-_
--_ --_
0.20 0.55 1.0
0.32 0.29 2.2
1.0 3.0
Fe(III)e
"Calculated from relative XPS peak intensities. *Polished membrane. 'Treated in 0.1 M Hg(NO& for 7 days in ambient light and in darkness, respectively. dTreated in deoxygenated 0.1 M Hg(NO& for 7 days in ambient light and in darkness, respectively. eTreated in 0.2 M Fe(NO& for 7 days in ambient light and in darkness, respectively. f - - -, not measured. nd, ion not detected.
oxidation state,17 but the Auger parameter (a + hv = EK[Ag(M4N45)]+ Eb[Ag (3d5/&])of 724.3 f 0.3 eV is consistent with Ag+ rather than A$.17 The Cu (2p) level shows two peaks corresponding to the 2p312and 2plj2spin-orbit split components. The binding energy of the Cu (2p3/,) peak at 933.0 eV is consistent with both the Cu(I1) and Cu(1) ions,17 but the absence of shake-up satellite peaks a t 6-9 eV higher binding energy eliminates the presence of Cu(II)'* (at least in the polished membrane). Adventitious hydrocarbons are evident from the single C (1s) peak at 284.7 eV. This cannot originate from carbonate since this would require an additional peak at 289.3 eV.I7 Summarizing this evidence leads to the conclusion that a jalpaite membrane, on soaking in water, undergoes decomposition which alters the surface stoichiometry. This change in the surface stoichiometry also affects the Eo of the copper electrode but does not affect the response to copper(I1). The surface composition after soaking in water is best represented by the formula Agl,5Cu,,62rS1-x.3H20,where x increases with the time of soaking from 0 to -0.17 after 14 days. The most plausible mechanism for the decomposition of jalpaite involves the loss of Cu(1) by oxidation to soluble Cu(II), the oxidant being dissolved oxygen. The mechanism can be represented by the following equation. A ~ ~ , ~ C U+O2.5XO2 . ~ S + 2xH+ = Agl.5C~.5-2xS1-x + X S O ~+~ 2xCu2+ + xH20 (1) A necessary consequence of this mechanism is the appearance of Cu(I1) in the water phase. Analysis by atomic absorption spectrometry (AAS) of the water after 14 days soaking did indeed confirm the presence of quite high concentrations of copper (0.06 mM in 50 mL of water). No silver was detected in the solution. Heijne and Van Der Linden1 have suggested from X-ray diffraction evidence that jalpaite decomposes after 14 days in solution to covellite (CuS) and acanthite (Ag2S). Cu(I1) was not observed, although it should be pointed out that the low amount of copper for the water-treated membrane (- 7 mol % ) is close to the detection limit of XPS ( 1-2 mol % 1, making the detection of the Cu(I1) satellites very difficult. A chemical depth profile was obtained for the water-treated membrane, and it was found that only the outermost surface layer (-2 nm) is hydrated and that the decomposition products are confined to this region. The XPS and AAS results suggest that the surface monolayer of the membrane corrodes to form aqueous Cu(I1) and metal-deficient sulfide (i.e. Agl.5C~.5-2xS1-x). This is in contrast to the lanthanum fluoride electrode membrane, which previous work by the authors7v8has shown to possess a hydrated gel layer which is about 20 nm thick, consisting of insoluble lanthanum hydroxy-fluoride salts. Effect of Interfering Ions. (a) Mercury(1l). The Hg(I1) ion interferes strongly with the response of the copper ionselective electrode based on a jalpaite membrane.lg This interference effect was studied by conditioning a membrane in 0.1 M mercury(I1) nitrate solution for 7 days in both the presence and absence of light and examining the surface by
-
XPS. The membrane was rinsed in distilled water and blotted dry prior to XPS analysis. The binding energies of the XPS peaks are given in Table I, and the relative intensities of these peaks have been used to obtain the atomic ratios given in Table 11. The major metal ion peaks observed for the membrane conditioned in light are for mercury. Copper and silver were detected for the membrane soaked in a deoxygenated solution but were absent for the membrane exposed to an oxygenated solution. The Hg (40 and Hg (4d) levels show the characteristic 7/2-5/2 and 5/2-3/2 spin-orbit split components, respectively. The binding energies of the 4f7/2(100.4 eV) and 4d5/2(358.9 eV) levels are 1eV higher than for the metal (99.7 and 357.7 eV, re~pectively'~) which might be expected for the Hg(1) ion. It is suggested that Hg(II) ions can undergo reductive ion-exchange with the jalpaite membrane surface in a way similar to that found for the silver sulfide membrane.5 Both copper(1) and silver(1) sulfides are semiconductors possessing band gaps of 1.21 and 1.03 eV, respectively,mand become photoconductive when exposed to ambient light. It is proposed that this photoconductivity accelerates the electron-transfer process. Two peaks are obtained for the S (2p) spectrum for the Hg(I1)-treated membrane. The binding energy of the main peak at 168eV is consistent with sulfur in the sulfate ion, while that of the minor peak a t 162 eV occurs in the region for sulfide.17 Thus it appears that conditioning of the membrane in an oxygenated Hg(II) solution in light leads to the oxidation of a considerable proportion of the jalpaite sulfide to sulfate. The atomic ratios in Table I1 show values of -0.15 for S2-/Hg and -0.45 for S042-/Hg,which suggest the surface consists of a mixture of mercury(1) sulfate and mercury(1) sulfide in the ratio of about 3:l. The oxidation of the surface sulfide is more extensive than was found to be the case for the silver sulfide membrane in the presence of Hg(I1) in light5 since, in that system, the product was elemental sulfur. This suggests that dissolved oxygen may also be involved in the oxidation of jalpaite. The atomic ratios for the membrane exposed to a deoxygenated solution reveal that the oxidation is less extensive; some copper and silver still remain (i.e. S2-/Cu -3 and S2-/Ag -2), and the mercury(1) sulfate to sulfide ratio is about 1:2. The small amount of sulfate formed on exposure to the deoxygenated solution may be due to the oxidation of sulfide by nitrate ions in the solution. The XPS peak binding energies for the membrane treated in the dark are shown in Table I. It may be seen that the binding energies of the mercury levels exhibit about a 1-eV shift to higher energies and are thus consistent with mercury as Hg(II). The S (2p) level, however, still shows the presence of both sulfide and sulfate. In the oxygenated solution case, the peak intensities are almost equal and give atomic ratios consistent with the presence of about 50% of both species (refer to Table 11). Exposure to the deoxygenated solution produced less sulfate (Le. S042-/S2- -0.5), and significant quantities of copper and silver (i.e. S2-/Cu -4.2 and S2-/Ag -2.4) were retained on the membrane surface. Once again, the small amount of sulfate formed on conditioning in the
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 6, MARCH 15, 1992
deoxygenated solution is attributable to the oxidation of sulfide by nitrate ions. In the dark, it is evident that the jalpaite membrane is less easily oxidized and does not undergo electron transfer with Hg(I1) ions; however, oxidation of sulfide on the surface still occurs. This suggests the involvement of dissolved oxygen which, in the presence of Hg(II), leads to the formation of considerable amohts of mercury(I1) sulfate on the surface in addition to mercury(II)sulfide. As was the case previously, the exchange of Cu(1) and Ag(1) by Hg(I1) is incomplete in a deoxygenated solution. SEM studies of the membrane after conditioning in 0.1M Hg(I1) for 7 days in both light and darkness revealed bulky, needlelike crystals (-7 pm long) on the surface of the light-treated membrane and clusters of small and thinner needles (-2 pm long) for the membrane treated in the dark. EDAX spectra confirmed the crystals in each case to contain mercury and sulfur. Summarizing the above results, it is possible to suggest the following equations for the surface reactions between Hg(I1) and the jalpaite membrane in ambient light and in the dark. light (2)
+
+
~A~,.,CU,,~S 8xHg2+ 10x02 8xH+ = 8Ag1.5Ch.k15xS1-x 4xHgSO4 + 4xHgS + ~ ~ x C U ~++ , , 4XH@ (3)
+
levels
clean"
C1-b
Cl-'
Brd
I-"
Ag (3d5/2) Ag (3d,/2) Cu ( 2 p 4 Cu (2p,/J s (2P) c1 (2P) Br (3d) I (3d5/J I (3d3,d
368.0 374.1 933.0 953.0 161.9 nd
368.0 374.1 933.0 953.0 162.1 198.4
367.8 374.0 932.8 952.9 162.1 nd
368.0 374.0 933.2 953.0 161.9
368.1 374.0 nd nd nd
--_
_____ _
68.8
nd nd nd
___
___
-__
..f
___ ___
___
--619.5 631.0
"Polished membrane. bTreated in 0.4 M NaCl for 7 days in ambient light. 'Treated in deoxygenated 0.4 M NaCl for 7 days in ambient light. dTreated in 0.4 M NaBr for 7 days in ambient light. "Treated in 0.4 M NaI for 7 days in ambient light. f - - - , not measured. nd, not detected.
instrument and so it is not possible to say categorically that Cu(1) and/or Cu(I1) is present. A feasible equation for the reaction is the following.
+
dark
+
Table 111. XPS Binding Energies (eV) after Treatment with C1-, Br-, and I-
4Ag1.5Ch.5S + 2xFe3+ + 6 x 0 2 = 4Ag1.k1.5,Ch.&0.5~Si-x. 3xAg2S04+ XCUS+ xCu2+ 2xFe2+ (4)
+
4 A g , . , c ~ , ~ S 8xHg2++ 6x02 = 4Ag1&~,5-&S1-~+ 3xHgzSO4 + xHg2S +
597
The metal-deficient sulfide species (e.g. Ag15Ch.+2xS11 and Ag1.5C~g.51.kSl~ for membranes exposed to oxygenated Hg(II) solutions are probably beneath the mercury sulfide/mercury sulfate layer at the Ag1.5Ch.5S interface and are not detected by XPS. In a deoxygenated solution, however, the corrosive action of Hg(I1) solutions is less extensive and significant amounts of Cu(1) and Ag(1) are retained on the membrane surface. The exchange mechanisms represented by eqs 2 and 3 require Cu2+ions to be transferred to the aqueous solution, and this was tested by AAS. A high concentration of copper (0.05 M in 25 mL solution) correspondingto dissolution of 19% of the total copper content of the membrane (3 g of jalpaite) was observed in the light, with a lower concentration (0.003 M) being found in the dark. The concentrations of silver were very low in each case (3.5 X and 7.0 X M, respectively). This demonstrates the considerable corrosion of the surface which takes place in the presence of Hg(II), particularly in light. The high concentration of copper in the aqueous solution suggests that the copper sulfide component of jalpaite exhibits greater susceptibility to attack by Hg(I1) than does the silver. (b) Iron(IZ0. Iron(1II) exhibits a strong interference on the response of the jalpaite-based copper electrode.21 This was examined by XPS by conditioning a jalpaite membrane in 0.2 M ferric nitrate for 7 days in ambient light and in the dark. The XPS binding energies are shown in Table I. As was the case for the mercury-treated membrane, the S (2p) spectrum for the membrane treated in the light shows the presence of both sulfate and sulfide, with the relative peak intensities suggesting a predominance of sulfate. No iron was detected on the surface, and the Cu (2p) region is consistent with both Cu(1) and/or Cu(II) although again no shake-up satellites due to Cu(I1) were observed. The atomic ratios shown in Table 11can be interpreted in terms of a surface consisting of AgaO, and CuS in the ratio of about 3:l. However, the amount of copper detected was close to the detection limit of the XPS
+
The formation of Ag2S04is surprising in view of the high stability of silver sulfide species. The XPS, SEM, and AAS results for jalpaite membranes exposed to Hg(I1) and Fe(II1) imply that the photooxidation of jalpaite occurs by at least two different mechanisms and demonshate that a considerable portion of the sulfide in jalpaite is photooxidized to sulfate. The metal-deficient compounds (e.g. Agl.5Ch.6&?r for Hg2+ in ambient light) are byproducts of this reaction. The XPS spectrum for the membrane treated in the dark shows peaks at the same binding energies as in the light (Table I), but the relative peak intensities were consistent with almost equal amounts of sulfate and sulfide, as was the case with mercury. The atomic ratios shown in Table I1 are consistent with a surface composed of a mixture of approximately 55% jalpaite and 45% Ag2S04. SEM examination of the surface in each case revealed amorphous deposits which by EDAX were shown to contain copper, silver, and sulfur (and a trace of iron). Once again quite high concentrations of copper were found in solution after conditioning the membrane in the Fe(II1) solution in the light (0.015 M in 25 mL of solution) with a smaller concentration in the dark (0.002 M). As was the case for Hg(II), the concentrations of silver in the solutions were also lower than for copper (0.002 M in light and 0.001 M in the dark) but are of the order expected for an equilibrium with Ag2S04on the surface (log Ksp = -4.7722). (c) Halides. The halides are also strong interferents with the copper electrode.21 Thus, a study was made of the effect of chloride, bromide, and iodide on the surface of the jalpaite membrane. Membranes were conditioned in 0.4M solutions of the halides for 7 days in the light and in darkness, and the surfaces were examined by XPS and SEM. The XPS peak binding energies are given in Table 111. The table shows only the results for membranes treated in ambient light since similar results were obtained for those treated in the dark. For the iodide case, the only peaks obtained of sufficient intensity to be measured were those of silver and iodine. The I (3d5,z) and I (3d3,2)binding energies at 619.5 and 631.0 eV, respectively, are characteristic of the iodide ion,17 which suggests the surface layer is silver iodide. The atomic ratios shown in Table IV support this conclusion. For chloride and bromide treatments, the C1 (2p) and Br (3d) peaks at 198.4 and 68.8 eV, respectively, are characteristic of the chloride and bromide ions,17which is consistent with the presence of
598
ANALYTICAL CHEMISTRY, VOL. 64, NO. 6, MARCH 15, 1992
Table IV. Surface Atomic Ratios"after Treatment with C1-, Br-, and Iratio
cleanb
C1-'
C1-d
Br-e
I-'
halide/Ag S2-/Ag SZ-/Cu
ndg 0.63 2.0
0.40 0.16 2.5
nd 0.36 2.5
0.40 0.21 1.0
0.9 nd nd
Calculated from relative XPS peak intensities. Polished membrane. 'Treated in 0.4 M NaCl for 7 days in ambient light. dTreated in deoxygenated 0.4 M NaCl for 7 days in ambient light. 'Treated in 0.4 M NaBr for 7 days in ambient light. fTreated in 0.4 M NaI for 7 days in ambient light. gnd, ion not detected.
silver chloride or bromide on the surface. However, copper and sulfur were also detected in addition to silver. While the binding energy for the Cu (2p) peak is consistent with either Cu(1) and/or Cu(II),17there is no evidence for the presence of the characteristic Cu(I1) shake-up satellites (althoughagain the small amount of copper present restricts their detection). The S (2p) binding energy is characteristic of the sulfide and there is no evidence for the presence of sulfate (as was found with Hg(II) and Fe(III)). Thus it appears that, although the halide ions severely corrode the jalpaite membrane surface, the mechanism is not the same as that proposed for Hg(I1) and Fe(II1). In the case of the halides there is no evidence for a photooxidation process on the surface. This is supported by the fact that the surface reaction products are the same in the dark as in ambient light. This is surprising in view of our work with the silver sulfide electrode? which showed that treatment of the membrane with halides in ambient light led to photooxidation of the surface and that no reaction occurred in the dark. The atomic ratios for the chloride- and bromide-treated cases shown in Table IV suggest mixtures of the silver halide and copper and silver sulfides. A SEM examination of the membrane surface for the chloride-, bromide-, and iodide-treated cases, respectively, revealed cubic crystals (-6 bm in size), amorphous deposita, and large hexagonal crystals ( 70 pm). EDAX spectra conf m e d these deposita to be the appropriate silver halide. Each of the treated membranes, particularly the chloride- and bromide-treated cases, showed regions in which there were no deposita. EDAX spectra of these regions showed they contained mostly copper and sulfur (with traces of silver and halide). The mechanism which is proposed to explain the formation of silver halides on the membrane surface first involves the oxidation of jalpaite in the presence of dissolved oxygen, as represented previously by eq 1. In the presence of halide, the XPS resulta suggest that the resulting acanthite and Cu(I1) ions exchange with the halide ion according to eq 5. This
-
Ag,S
+ Cu2+aq+ 2C1-,,
= 2AgC1 + CuS
(5)
reaction is thermodynamically feasible in terms of solubility product calculations. This proposal is strengthened by the observation that no copper was detected in the solution using atomic absorption spectrometry although some silver was detected. The silver is thought to be due to the formation of soluble anionic haloargentate(1) complexes as suggested by Lamasa The absence of silver chloride on the surface of the jalpaite membrane soaked in deoxygenated NaCl solution (see Tables I11 and IV) also supports the above mechanism for halide interference.
CONCLUSION This work has further elucidated the effects of dissolved oxygen and interfering ions such as Hg(II),Fe(IIB, and halidea on the surface of the jalpaite-based copper ion-selective electrode. The surface is severely corroded in a relatively short period of time in the presence of these interferenta, and in the case of Hg(I1) and Fe(III), the effects are more extensive in ambient light because of photooxidation. These reaulta also confirm previous observations%Bthat the copper ion-selective membrane is not particularly stable in the presence of oxygenated solutions, and that oxidation of the surface occurs with a consequent shift in the Eo of the system. The electrode is probably beat used in deoxygenated solutions, in the absence of light. It has been shown that halide ions react with the silver sulfide component of the membrane and so the use of pure copper sulfide membranes may provide electrodes which are less susceptible to halide ion interference. This study also demonstrates the importance of regular polishing of the membrane to preserve acceptable response. ACKNOWLEDGMENT We thank F. Daniels of the Botany Department, La Trobe University, for assistance with the SEM studies. R.D.M. is grateful for the receipt of an Australian Postgraduate Research Award. We also thank the Australian Research Council for financial support. REFERENCES Heljne. G. J. M.;Van Der Linden, W. E. Anal. Chlm. Acta 1977, 93, 99-110. Coetzee, J. F.; Isotone, W. K.; Carvalho, M. Anal. Chem. 1980, 52. 2353-2355. Slemroth, J.; Hennlg. I . Anal. Chem. Symp. Ser. 1981, 8 , 339-348. Hulanlckl, A.; Lewenstam, A. Talenta 1978, 23, 661-665. De Marco, R.; Cattrall, R. W.; Llesegang, J.; Nyberg, G. L.; Hamhon, 1. C. Anal. Chem. 1890, 62, 2339-2346. Heijne, G. J. M.; Van Der Llnden, W. E.; Den Boef, G. Anal. chkn. Acta 1977, 89, 287-296. De Marco, R.; Cattrail, R. W.; Llesegang, J.; Nyberg, G. L.; Hamilton, I. C. Surf. Interface Anal. 1989, 14, 457-462. De Marco, R.; Hauser, P. C.; Cattrall, R. W.; Llesegang, J.; Nyberg, G. L.; Hamilton, 1. C. Surf. Intefface Anal. 1988, 74, 463-468. Anthony, M. T. I n Practfcel &face Ana&& by Auger and X-Ray photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; Wiley: Chichester, U.K., 1983 pp 429-435. Hofmann, S. Reference 9, pp 141-179. Ebel, M. F.; Ebel, H.; Hlrokawa, K. Specirochlm. Acta 1982, 378, 461-471. Wagner, C. D.; Davls, L. E.; Zeller, M. V.; Taylor. J. A.; Raymond, R. M.; Gale, L. H. SWt. Interface Anal. 1981, 3 , 211-225. Handbook of Spectroscopy; Robinson, J. W., Ed.; CRC Press: Cleveland, OH, 1974; p 674. Wagner, C. D.; Zatko, D. A.; Raymond, R. H. Anal. Chem. 1980, 52, 1445-145 1. Schon, G. Acta Chem. Sand. 1973, 27, 2623-2633. Hammond, J. S.; Garrenstroom, S. W.; Wlnograd, N. Anal. Chem. 1975, 47, 2193-2199. Hendbodr of X-ray photoektrrm Spectroswpy; Wagner, C. D., R b 3 . W. M.,Davis, L. E., Moulder. J. F., Mullenburg, 0. E., E d . ; Perkln-Elmer: Eden Pralrle, MN, 1979. Wallbenk, B.; Main, I. 0.; Johnson, C. E. J . Electron Specbpsc. Relet. PhenOm. 1874, 5 , 259-266. Hoyer, B.; Loftager, M. Anal. Chem. 1988, 60. 1235-1237. Strehlow, W. H.; Cook, E. L. J . Fhys. Chem. Ref. Data 1973, 2, 163-200. Cammann, K. Wcnklng WHh IOn-SektrVe Ektrcdes: ChemlCalLaboratory h c t f c e ; Springer-Verlag: Berlin, 1979 p 70. King, E. J. QuallteHve Ana!~sLsand E k & m SdUHons; Harcourt, Brace and Co.: New York, 1959 p 607. Lana, P. Anal. Chlm. Acta 1978, 105, 53-65. Blaedel, W. J.; Dlnwlddle, D. E. Anal. Chem. 1974, 46, 873-877. Midgely. D. Anal. Chlm. Acta 1878, 87, 7-17.
RECEIVED for review December 19,1990. Revised manuscript received June 24,1991. Accepted November 1,1991,
