1429
Anal. Chem. 1983, 55, 1429-1431
curves should be run in the presence of the same large concentration of the primary anion present in the unknown sample. 750
Registry No. Sodium hydroxide, 1310-73-2; sulfuric acid, 7664-93-9. LITERATURE CITED
CI
500
CI-
a.
*
w
(1) Small, H.; Stevens, T. S.; Bauman, W. C. Anal. Chern. 1975, 4 7 , 1801-1809. (2) Sawickl, E., Mulik, J. D., Wlttgensteln, E. W., Eds. "Ion Chrornatographlc Analysls of Environmental Pollutants"; Ann Arbor Science: Ann Arbor, MI, 1078-1979; Vols. I and 11. (3) Pohl, C. A.; Johnson, E. L. J . Chromatogr. Scl. 1980, 78, 442-452. (4) Bynum, M. A. 0.;Tyree, S. Y., Jr.; Welser, W. E. Anal. Chern. W81, 53, 1935-1936. (5) Jenke, D. Anal. Chern. 1881, 5 3 , 1536-1538.
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TIME (mm)
Flgure 5. Chromatograms of 0.8 N NaOH (samples C and D) using 10 mM borate eluent.
In view of previously published data on the effect of high concentrakions of one anion on the analysis of a second anion (4,5), along with the data presented here, it can be concluded that these phenomena are general. As a result, calibration
The Bendix Corporation Kansas City Division D/816, SA-1 P.O. Box 1159 Kansas City, Missouri 64141 RECEIVED for review September 22,1982. Resubmitted March 17,1983. Accepted March 28,1983. The Bendix Corporation, Kansas City Division, is operated for the U.S. Department of Energy under Contract No. DE-AC04-76DP00613.
Incorporation of Redox Polymers to Polyelectrolyte-Coated Electrode Surfaces Sir: Much research effort has been directed in recent years to the preparation, characterization, and applications of polymer-coated modified electrodes (1-6). Polymeric ligands (4), polyellectrolytes ( 1 , 2 ) ,and various forms of polymerized redox compounds (7-9) have proved to be greatly useful in the modification of electrode surface properties. Of these polymers studied to date, Nafion, a perfluorinated ion exchange polymer has proved to be one of the most useful for preparing the polymer-coated electrodes of this type (6, 10). Special attention has been given to the nature of the charge and mass transfer through Nafion films (II), the possible catalytic properties of Nafion-coated electrodes (6), and the electrogenerated chemiluminescence of the bound species (6, 12). This 11sbecause Nafion polymers are very chemically inert and thermally stable and they strongly retain certain electroactive ions even in the presence of large excesses of supporting electrolyte. So far, the electroactive species incorporated into the domain of Nafion film have been limited to the relatively srnall and simple species which are usually cationic ions and neutral species in special cases (6). Nafion membranes were shown to be more selective for large organic cations like tetrabutylammonium than for small cations like Na+ (13,14). If the electroactive polymer can be inserted into Nafion film by a simple dip method, Nafion film possesses more potentialities for their various application. In this loaper, we report that the polymer pendants Ru(bpy)32+(where bpy = 2,2'-bipyridine) complex and viologen (15,16) are incorporated into Nafion membrane 125 and its thinner film coated on pyrolytic graphite electrodes in nonaqueous solutions by electrostatic binding. EXPERIMENTAL SECTION Electrochemical studies were performed as previously described (15). Nafion membrane 125,which was obtained from Du Pont
(Wilmington,DE), was boiled in 0.2 M NaOH solution for 30 min, washed with the distilled water, and dried under the reduced pressure. Finely cut particles of the membrane were soaked in dimethyl sulfoxide (Me2SO)and the resulting solution was boiled for 3 h and then filtered through a glass filter under reduced pressure. The filtrate was used as a stock solution of Nafion, after adding ethanol (EtOH) to the filtrate at the volume ratio of 1/1 for EtOH/Me2S0 in order to make easily the coating. The concentration of Nafion was 5.6 mg/mL. Aliquots of the stock solution were spread by a microsyringe on the freshly cleaved disk surface of basal plane pyrolytic graphite (BPG) (Union Carbide Co., Parma, OH), being sealed to the end of glass tube with heat-shrinkable polyolefin tube. All electrodes employed had geometric area of 0.1'7 cm2. The polymer pendants Ru(bpy),2+ complex and viologeni with average molecular weight 2900 and 9800, respectively, shclwn in Figure 1were prepared as described elsewhere (15,16). N,N'-DimethyL4,4'-bipyridinium dichloride (methylviologen,MVCCl2)(Wako Chemical Co.) was recrystallizc?d twice from methanol--acetone. Ru(bpy),C12 was obtained from G. F. Smith Chemical Co. and was recrystallized twice according to the standard procedure. All other chemicals were reagent grade. Solutions were freed of air with prepurified argon prior to the electrochemical measurements. The concentrations of electroactive species incorporated into a Nafon membrane (thicknew, 0.0127 cm) were determined spectrophotometrically. On the othier hand, the concentrations of these species incorporated into the Nafion film coated on BPG electrodes were determined in the following manner. The quantities of electroactive species present in the coatings were measured in units of mol cm-2by integrating the current from cyclic voltammograms which were observed in aqueous solutions at a low scan rate of 1-4 mV s-l(17), and then their molar concentratlions were calculated by using the thickness of the coating film used here which was estimated to be 6.6 X cm, assuming that the density of Nafion film is about 2 g ~ r n - ~ (6). Potentials were measured and are reported with respect to a sodium chloride saturated calomel electrode (SSCE). Experiments were conducted at 25 "C.
0003-2700/83/0355-1429$01.50/0 -..... . 0 1983 American Chemlcal Soclety
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ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1963
1
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Figure 1. Polymer pendants R~(bpy),~+and viologen.
RESULTS AND DISCUSSION Incorporation of Redox Polymer into Nafion Membrane. In previous papers (15),we have reported that Nafion membrane 125 dissolves in 5050 EtOH-Me2S0 mixture solution. Martin et al. haye also found that 1100 and 1200 equiv wt Nafions dissolve in 50:50 propanol-water and 50:50 EtOH-water, when heated to 250 "C under a high pressure (IO). Nafion membrane 125 is little swollen in water, but well swollen in some nonaqueous solvents, e.g., Me2S0 and methanol-MezSO mixture, in which the polymer pendants Ru(bpy),2+ and viologen, shown in Figure 1, are well soluble. Thus, in solvents which can swell Nafion membrane 125, we examined whether p0ly(4-MV)~+is incorporated into Nafion 125 or not. The evidence of incorporation of the viologen polymer into Nafion membrane was confirmed by spectrophotometry. The methylviologen groups of p0ly(4-MV)~+were reduced by Na2S204and converted to the monocation radical which is purplish blue as previously reported (18). The concentration of the methylviologen groups was evaluated by using the molar extinction coefficient reported previously (19). Nafion membrane was immersed in a 50:50 weight percent methanol-MezSO mixture solution in which p0ly(4-MV)~+ of 1.8 X M as methylviologen unit was dissolved. After soaking, the membrane was washed with the methanol-MezSO mixture solvent and then the quantity of the incorporated p0ly(4-MV)~+ was measured spectrophotometrically. At a soaking time of 10 min, 1.8 X M p0ly(4-MV)~+was incorporated into Nafion and at a longer soaking time, e.g., 36 h, the concentration of the incorporated p01y(4-MV)~+could be estimated to be 1.9 X M. Further, when Nafion membrane was soakedrin a MezSO solution containing 1.8 X lo-, M p0ly(4-MV)~+for 10 min, the concentration of the incorporated p0ly(4-MV)~+ was 5.1 X M. The incorporation of p01y(4-MV)~+into Nafion in an aqueous solution could not be performed because of the water insolubility of p0ly(4-MV)~+. Methylviologen monomer was found to be incorporated into Nafion from nonaqueous solutions (Le., methanol-Me2S0 mixture and MezSO) as well as from an aqueous solution. For example, when Nafion was soaked for 10 min in a water, a 50:50 weight percent methanol-Me2SO mixture solution, and a Me2S0 solution which contained 1.8 X lo-, M methylviologen monomer, the concentrations of the incorporated methylviologen were 1.0 X 1.2 X and 6.2 X M, respectively. The distribution of methylviologen
YO.
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Flgure 2. (A) Cyclic voltammograms for p0ly(4-MV)~+recorded continuously at a pyrolytic graphite electrode coated with Nafion of = 1.3 X g cm-* in a Me,SO solution of 1.8 X lo-, M of viologen unit as poly(4-MV)*+. (e) Cyclic voltammogram obtained at an uncoated graphite electrode in the same solution as used in A. (C) Cyclic voltammograms obtained when the electrode used in A was washed with a supporting electrolyte solution and replaced in a Me,SO solution. (D) Cyclic voltammograms obtained when the electrode used in A was washed and transferred to an aqueous solution (pH 3.0). Immersion time is indicated on the voltammograms. Supporting electrolyte was 0.2 M NaCIO,. Scan rate was 100 mV s-I.
monomer in Nafion was homogeneous, but in the case of polymer we observed an inhomogeneous distribution of p0ly(4-MV)~+, which looked like a number of spots composed of aggregates of giant molecules. Even after 10 days since the membrane into which methylviologen monomer or poly(4MVl2+was incorporated was immersed in water, their quantities in Nafion were almost the same as the original ones. Ru(poly(St-Vbpy))(bpy)2+ of 1.4 X M as Ru unit was also spectrophotometrically found to be incorporated into Nafion membrane 125, when Nafion was soaked for 10 min in a MezSO solution containing the Ru polymer of 6.4 X M. Further, it was confirmed that the R ~ ( b p y ) , ~complex + can be incorporated and confined in Nafion in MezSO. The incorporation of the monomer R u ( b p ~ ) , ~into + Nafion was faster than that of Ru(poly(St-Vbpy))(bpy)2z+. At the same soaking time and in the same concentration of the bathing solution the quantity of the incorporated monomer was about 20-30 times that of the incorporated polymer, in analogy with the above results obtained for the incorporation of methylviologen monomer and p0ly(4-MV)~+. Incorporation of Redox Polymer into Nafion Film Coated on an Electrode. Figure 2A shows a series of cyclic voltammograms demonstrating the incorporation of poly(4MV)2+into Nafion coated on a pyrolytic graphite electrode in a MezSO solution containing 1.8 X lo-, M p0ly(4-MV)~+ and 0.2 M NaC104. The incorporation of this cationic polymer by cation exchange with the sodium ions initially present in the film was continued for about 30 min. No further increases in peak current were observed with scanning time longer than 30 min. The peak current at the coated electrode was about three times that at an uncoated electrode (Figure 2B) and the concentration o f the incorporated poly(4-MV)'+ was ca. 7.5 x M. When the Nafion-coated electrode was removed from the incorporating solution, washed, and replaced in a MezSO solution containing only supporting electrolyte (0.2 M NaC104), much of the p0ly(4-MV)~+leached quickly from the Nafion film and the peak current reached a steady state after ca. 5 min (Figure 2C). However, when the electrode was transferred to an aqueous solution, even after 10 h the quantity of p0ly(4-MV)~+ leaching from Nafion was less than 5% of its initial value (Figure 2D). In this case, it was also observed
ANALYTICAL CHEMISTRY, VOL. 55, NO. 8, JULY 1983 10.0
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Flgure 3. Equilibrium distrlbutlon curve of p0ly(4-MV)~+In the bulk of Me,SO solutions and in the Nafion film coated on electrodes. Supporting electrolyte was 0.2 M NaCiO,. cdmcMv+ and cWw(-p+ refer to the concentrations of p0ly(4-MV)~+as viologen unit in the bulk of Me2S0solution and In the Naflon film coated on a graphRe eieclrcde, respectivleiy. Quantity of Naflon coated on electrode is the same as in Figure 2.
that the magnitude of peak currents and the position of peak potentials are dependent on the electrolyte anion used (20). Figure 3 shows the equilibrium distribution curve of the p0ly(4-MV)~+concentration in the bulk of MezSO solutions containing the various concentrations of p0ly(4-MV)~+and in the Nafion film coated on graphite electrodes, as the plot of c~1Ypoly(4Mv)~+ against where C ~ ~ Y ~ ~ ~ ( ~ Mand V ) Z + pol poly (4.MV)~+are the concentrations of poly(4-MVP2+partitioned into the Nafion film and in the bathing solution, respectively. From the observation of the inhomogeneous distribution of p0ly(4-MV)~+in Nafion membrane (see above), it is thought that p0ly(4-MV)~+ partitions inhomogeneously intothe swollen Ndion f i i on electrodes. It is apparent from Figure 3 that p0ly(4-MV)~+ is incorporated and concentrated into Nafion film on electrodes. For example, when the concentration of a bulk solution of p0ly(4-MV)~+ was above ca. 2X M, the equilibrium concentration of p0ly(4-MV)~+ incorporated into Nafion film was about 7.5 X M. However, the value wa5 lower than that (70 X M) for the methylviologen monomer obtained in MezSO solution and much lower compared with the concentration (- 1 M) for the methylviologen monomer incorporated into the same coating film in an aqueous solution. Note that such a large difference between the quantity of the incorporated monomer in an aqueous solution or a MezSO solution and that of the incorporated polymer in a MezSO solution was observed. In both cases of the incorporation of methylviologen monomer into Nafion film in an aqueous solution or a MezSO solution and that of p0ly(4-MV)~+ in a MezSO solution, the electrostatic interactions (1-6) between the cationic methylviologens and anionic sulfonyl groups of Nafion polymer are the basis of the incorporation. However, in the case of p0ly(4-MV)~+, the electrostatic interaction will be limited by the geometry which is the intrinsic property of the polymer. As a result, the quantity d the incorporated viologen polymer is less than that of the corresponding monomer. It was also observed that Ru(poly(tSt-Vbpy))(bpy)z2+is confined in the swollen Nafion film. However, the quantity of the incorporated Uu(poly(St-Vbpy))(bpy)?+was much smaller compared with 0.1 M Ru(bpy)zZ+(15,16). Further, the hydrophobic interaction (6, 21-23) between the hydrophobic fluorocarbon backbone materials of Ndion polymer and the hydrophobic main chains of p0ly(4-MV)~+ should be considered as an additional factor
1431
contributing to the incorporation of p0ly(4-MV)~+ into Ndion polymer, as the hyldrophobic interaction has been extensively recognized on the basis of the “ion-cluster model” (21-23) which characterizeri the special morphology of Ndion polymer. It is worth noting that giant molecules such as p0ly(4-MV)~+ and Ru(poly(St-Vbpy))(bpy)zz+ can be incorporated by electrostatic force and hydrophobic interaction into Nafion membrane and its thinner film coated on electrode in nonaqueous solutions. l[n previous papers (6,10,12) regarding;the incorporation of various electroactive species into Nafion film on electrode surfaces, the incorporated electroactive speties have been limited to relatively small and simple cations such as Ru(bpy)32+and methylviologen monomer (6, 10, 12),and neutral species, e.!;., ferrocene (12). ACKNOWLEDGMENT We are thankful to Masao Kaneko and Akira Yamada at The Institute of Physical and Chemical Research, Saitama, Japan, for kindly providing the viologen polymer and the polymer pendant R ~ ( b p y ) , ~complex. + Several perceptive comments and criticisms by the reviewers of an earlier version of this manuscript helped us to formulate our speculatiions more clearly. Registry No. Ru(bpy)?+, 15158-62-0;p0ly(4-MV)~+, 85:37003-2;graphite, 7782-42-5;Ndion 125,65506-90-3;methyl viologen, 1910-42-5, LITERATURE CITED Oyama, N.; Anson, F. C. J. Elecfrochem. SOC.1980, 127,247-250. Oyama, N.; Shimomura, T.; Shigehara, K.; Anson, F. C. J. Electroanal. Chern. 1988, 112,271-280. Schnelder, J. R.; Murray, R. W. Anal. Chem. 1982, 54, 1508-1515. Oyama, N.; Anson, F. C. J. Am. Chem. SOC.1979, 101, 3450-3456. Oyama, N.; Anson, F. C. Anal. Chem. 1980, 52,1192-1198. Rublnsteln, I.; Bard, A. J. J. A m . Chem. SOC. 1981, 103, 5007-5013.
Merz, A.; Bard, A. J. J. Am. Chem. SOC. 1979, 100, 3222-3223. Wrighton, M. S.; Austin, R. 0.;Bocarsly, A. B.; Bolts, J. M.; Hass, N.; Lean. K. D.: Nadio. L.: Palazzotto, M. C. J . Am. Chem. SOC. 1978,
1Oz: 1602-1603 Kaufman, F. B.; Enaler, E. M. J. Am. Chem. SOC.1979, 101, 547-549.
Martin, C. R.; Rhoades, T. A.; Ferguson, J. A. Anal. Chem. 1982, 54, 1639.... 164 . .. 1..
Buttry, D. A,; Anson, F. C. J. Elecfroanal. Chem. 1981, 130, 333-338.
Rubinstein, I.; Baird, A. J. J. Am. Chem. SOC. 1981, 103, 512-516. Lee, P. C.; Meisel, D. J. Am. Chem. SOC. 1980, 102,5477-5481. Martin, C. R.; Frelser, H. Anal. Chem. 1981, 53,902-904. Oyama, N.; Yamnguchi, S.; Kaneko, M.; Yamada, A. J. Electrosinal. Chem. 1982, 13i?,215-222. Kaneko, M.; Yamada, A,; Oyama, N.; Yamaguchi, S. Makrornol. Chem. Rapid Common. 1982, 3 769-772. I
Oyama, N.; Anson, F. C. J. Necfrochem. SOC.1980, 127,640-1547. Factor, A,; Heinsohn, G. E. folym. Lett. 1971, 9 , 289-295. Steckhan, E.;Kuwana, T. Ber. Bunsenges. fhys. Chem. 1974, 78, 253-259.
Oyama, N.; Ohsalta, T., unpublished results. Lowry, S. R.; Mauritz, K. A. J. A m . Chem. SOC. 1980, if02, 4665-4667. Falk, M. Can. J. Chem. 1980, 58, 1495-1501. Yeager, H. L.; Steck, A. J. Elecfrochem. SOC. 1981, if28, 1880- 1884,
Noboru Oyama* Takeo Ohsaka Keiichi Sato Hisao Yamamoto Department of Applied Chemistry for Resources Tokyo University of Agriculture and Technology Koganei, Tokyo 184, Japan RECEIVED for review December 20,1982. Accepted March 22, 1983.
