Permselectivity of films prepared by electropolymerization of 2,6

Jul 15, 1987 - Xiaohong Chen, Norio Matsumoto, Yibai Hu, and George S. Wilson .... Joseph Wang , Teddy Martinez , Daphna R. Yaniv , Larry D. McCormick...
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Anal. Chem. 1987, 59, 1758-1761

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Permselectivity of Films Prepared by Electropolymerization of 2,6-Dimethylphenol Takeo Ohsaka, Tomoaki Hirokawa, Hirohisa Miyamoto, and Noboru Oyama* Department of Applied Chemistry for Resources, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan A seiectlve permeability of the poiy(2,6-dlmethyi-l,4phenylene oxide) (PPO) films being prepared by electropolymerizatlon of 2,6-dlmethylphenoi to the various redox specles dissolved in the solution was investigated by means of hydrodynamic voltammetry at a rotatlng disk electrode. The selective permeability was found to be different for the PPO flimr prepared from the different electrolytic solutions. The PPO films, prepared from the basic methanoilc solution, had a hlgh selective pemwaMHty to hydrogen ion, but Mocked the access of larger redox Ions such as Fe(CN)z-, Fez+, and (ethylenedlaminetetraacetato)lron( I I I ) (Fe( edta)-) to the electrode surface. Further, the PPO films whlch were in situ deposited on electrode surfaces during the electropolymerlration of 2,&dlmethylphenol in the basic methanolic solution were found to be superior In terms of a seiectlve ion permeablilty to those prepared by spreading aliquots of the dlchloromethane solution of the chemically prepared PPO by a microsyrhge on electrode surfaces and then by evaporatlng the solvent. On the bask of the obtalned results, the signlficance of such PPO film coated electrodes for the deveiopment In ion sensors Is dlscussed.

A number of papers concerning the preparation of the poly(pheny1eneoxide)s by chemical oxidative coupling polymerization (1-6) as well as electropolymerization (7-24) of phenolic compounds and the mechanism of the polymerization reactions have been reported since the oxidative coupling of 2,6-disubstituted phenols to poly(l,4-phenyleneoxide)swas first reported by Hay et al. in 1959 (25). From the viewpoint of polymer formation, there seems no essential difference between the chemical oxidative polymerization and the electropolymerization of phenolic compounds. However, in the case of the electropolymerization, the polymerization reaction occurs at and/or in the vicinity of electrode surface and in some cases the resulting polymers deposit on electrode surfaces, and consequently ”thin-film-coated electrodes” can be obtained in situ. Some interesting functions such as pH response (20), protection of metal corrosion (10-13) and permselectivity (19) have been reported for the film-coated electrodes prepared by electropolymerization of a variety of phenolic compounds. In a previous paper (19),we have reported that the electroinactive polyphenol film deposited on a Pt electrode by electropolymerization of phenol possesses the selective permeability to dissolved redox species such as H+, Br-, Cr3+, Eu3+,Fe(CN)63-,and iron(II1)-ethylenediaminetetraacetate complex. More recently, we (26)have found that the poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) films prepared from the electropolymerization of 2,6-dimethylphenol in different electrolytic solutions have different permselectivities to dissolved redox species and have reported the preliminary results concerning this.

PPO 0003-2700/87/0359-1758$01 SO10

In the present work, we examine in more detail the selective ion permeabilities of the PPO films, prepared from different electrolytic solutions, by means of hydrodynamic voltammetry at a rotating disk electrode. The ion permeability of the PPO films, which were prepared by spreading aliquots of the dichloromethane of the chemically prepared PPO on electrode surfaces and then by evaporating the solvent, was also examined. The results and the comparison with the previous data (19) for the polyphenol film are presented. In addition, the significance of these thin-film-coated electrodes in regard to sensor development is described.

EXPERIMENTAL SECTION Materials. 2,6-Dimethylphenol (C6H3(CH3),(OH),Tokyo Kasei Co., Ltd.) of reagent grade was used without further purification. Sodium trifluoroacetate (CF,COONa) and sodium perchlorate (NaC104)(Aldrich) of reagent grade were employed as a supporting electrolyte. Acetonitrile and methanol (Kanto Kagaku Co.) were purified by distillation. Diethylamine ( (CzH5),NH),added to the electrolytic solutions as a basic additive, was purified by distillation. Iron(II1)-ethylenediaminetetraacetate complex (Fe(edta)-)was prepared according to ref 27. All other chemicals were reagent grade and were used as received. Apparatus and Procedures. A standard three-electrode electrochemicalcell was used for all electrochemicalexperiments. The electrode assembly consisted of a bare or film-coated Pt cm2)as working electrode, electrode (geometric area, 7.9 X a spiral Pt wire as counter electrode, and a sodium chloride saturated calomel electrode (SSCE) as reference electrode. Cyclic and rotating-disk voltammograms were obtained with standard, previously described procedures and apparatus (19, 28). Rotating-disk current-potential curves were recorded by scanning the electrode potential at 5 mV s-’. The pretreatment of Pt electrode surfaces was conducted as described previously (19). The pH measurements were performed on a digital pH-meter (Denki Kagaku Keiki Co., Model HG-3). According to the previously described procedure (26),the PPO film coated Pt electrodes were prepared by in situ electropolymerization of 2,6-dimethylphenol (50 mM) in (i) an acetonitrile solution containing 0.3 M diethylamine + 0.2 M NaClO, and (ii) a methanol solution containing 0.3 M NaOH. Then, the thicknesses of the PPO films were controlled by the amount of the charge passed during the electrolysis (26),and the PPO films with various thicknesses were employed in the permselectivity measurements. The film thicknesses were measured with a Surfcom 550 A (surface texture measuring instrument, Tokyo Seimitsu). Scanning electron micrographs of the PPO films were obtained on a JEOL JSM-T100 scanning electron microscope (SEM) at an accelerating voltage of 25 kV. In this case, the PPO films deposited on Pt electrodes were sputter-coated with ca. 100 A of gold or aluminum. RESULTS AND DISCUSSION Permeability of the Swollen PPO Films to Hydrogen Ion. As described previously (26), the PPO films show no electronic conductivity in dry condition, but in a supporting electrolytic solution the swelling occurs and electrolytic ions can penetrate selectively into the swollen PPO films. Typical steady-state current-potential curves for the reduction of hydrogen ion a t both bare and PPO-coated rotating Pt disk electrodes in a solution of 0.2 M CF3COONa+ CF3COOH (pH 2.7) are shown in Figure 1,where the PPO film coated electrodes were prepared from the methanol solution containing 0 1987 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987

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r

-

c

___-’ -0.5

YVVS.

ssc E

1.0 2.0 i06cb/mol cm-3

0

Figure 1. Typical steady-state current-potential curves for the reduction of hydrogen ion at bare and PPO-coated rotating Pt disk electrodes: solution composition, 0.2 M CF,COONa CF,COOH (pH 2.7); PPO thickness (4), 500 A; electrode rotation rate, (A) 400, (B) 1600, (C) 3600 rpm; potential scan rate, 5 mV s-’;electrode area, 7.9 X cm2; (-) at PPO-coated Pt electrode, which was prepared from the basic methanolic solution (see Experimental Section), (- -) at bare Pt electrode.

+

-

I

Figure 3. Dependences of the current iFon the PPO film thickness bulk concentration of hydrogen ion (Cb). The i , is the current represented as the reciprocal of the intercept of the plots such as those shown in Figure 28 (see text).

(4) and the

geneous electron-transfer reaction of hydrogen ion ( n = I), F is the Faraday constant, A is the electrode area, Dsolnis the diffusion coefficient of hydrogen ion in a bulk solution, v is the kinematic viscosity of the solution, Cb is the bulk concentration of hydrogen ion, and o is the rotation rate of the disk electrode. Figure 3 shows the dependences of iF on the PPO film thickness (4) and Cb:iF can be considered to be approximately proportional to $-l, though Figure 3A shows some scatter. We can also see that iF is proportional to Cb. On the basis of these facts, the current iF can be identified with the current is (29-31), which means the rate of permeation of hydrogen ion through the PPO film

i, = nFAKCbD,4-’ Flgure 2. (A) Levich plots of limiting cwent, i, vs. (rotation rate, w)‘” for the reduction of hydrogen ion at bare and PPO-coated rotating Pt disk electrodes. (B) Kouteckv-Levich plots of ill,,,-’ vs. w-’” for the data used in A. Experimental conditions were as follows: solution CF,COOH (pH 2.7); thicknesses of composition, 0.2 M CF,COONa PPO films, (0)0 (bare electrode), (A)230 and (0) 500 A. Other experimental conditions are the same as in Figure 1.

+

0.3 M NaOH + 50 mM 2,6-dimethylphenol. The shapes of the curves obtained at both electrodes are almost the same except that the limiting currents at a PPO-coated electrode are largely smaller than those at a bare Pt electrode. Such a difference between the limiting currents is considered to reflect the different diffusion rates of hydrogen ion in the bulk of the solution and the swollen PPO film. The quantitative analysis of this problem can be made according to the previously described procedure (20, 28), as mentioned below. Figure 2A shows the Levich plots of limiting current, ilim, vs. (rotation rate, co)lI2 obtained from the steady-state current-potential curves for the reduction of hydrogen ion such as those shown in Figure 1. The deviations of the limiting currents from the straight line obtained with a bare Pt electrode were observed with a PPO-coated electrode, and their degree increased with film thickness at a given w and with w for a given film. This can be expected for a reactant that must penetrate the polymer film to reach the electrode surface (19, 28-31). Figure 2B shows the Kouteckj-Levich plots of ilim-l vs. w-ll2 for the data of Figure 2A. The linearity of these plots with slopes which match that at a bare Pt electrode demonstrates that they obey the Kouteckg-Levich equation (32)

l/ilim = l/iLeV

+ l/iF

(1)

where ihv is the Levich current (33) (expressed by eq 2) and iF is the current (19,28-31) represented as the reciprocal of the intercept of the plots shown in Figure 2B

iLev= 0.62nFAD,,1,2/3v-1/6Cb~1/2 (2) where n is the number of electrons involved in the hetero-

(3)

where D, is the diffusion coefficient of hydrogen ion within the swollen PPO film and K is the partition coefficient of hydrogen ion between the PPO film and the bulk of the solution. Thus, from the data shown in Figures 2 and 3, the values of D, and DWhwere estimated to be (1.3 f 0.3) X and (1.0 f 0.2) X lo4 cm2s-l, respectively. One can see that the diffusion rate of hydrogen ion in the swollen PPO film is by about 3 orders of magnitude slower than that in the solution. The KDS for the PPO film is about one-third of that ((4.1f 0.5) X lo-’ cm2 s-l) reported for the polyphenol film (19). For the PPO films that were prepared from the acetonitrile solution containing 50 mM 2,6-dimethylphenol+ 0.3 M diethylamine + 0.2 M NaC104,the value of K Dwas ~ estimated to be (3.5 f 0.4) X lo4 cm2 s-l. Such a difference in KDis for the PPO films prepared from the different electrolytic solutions was also observed in their permselectivity to dissolved redox species other than hydrogen ion, as shown below. The permeability of the “chemicallyprepared” PPO films, which were prepared by spreading aliquots of the dichloromethane solution of the chemically prepared PPO on electrode surfaces and then by evaporating the solvent, to hydrogen ion was also examined for the comparison with those of the electropolymerizedPPO films: The dependences of the limiting currents of the hydrodynamic voltammograms for the reduction of hydrogen ion at the chemically prepared PPO film coated electrodes upon the rotation rate of electrode, the film thickness, and the concentration of hydrogen ion were analyzed in a similar manner as above. The value of KD,was estimated to be (1.6 f 0.3) X lo4 cm2s-l, which was almost the same as that of the PPO film electrochemicallyprepared from the diethylamine-containingacetonitrile solution. Thus, the porosities of both these films are expected to be almost the same. Selective Permeability of t h e Swollen P P O Films to Various Solution-Phase Redox Species. Figure 4 shows typical steady-state current-potential curves for the reduction of hydrogen ion, Fe(CN)63-and Fe(edta)- ions and for the oxidation of Fe2+ and Br- ions at rotating disk bare and

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ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987

Table I. Ion-Selective Permeability of PPO Films Prepared by Electropolymerization of 2,6-Dimethylphenoln~*

film

PPOd PPO' polyphenol' stokes radius/Af

H+ reduction

Broxidation

0.96 f 0.10 0.62 f 0.06 0.78

0.96 f 0.10 0.25 f 0.02 0.68 1.18

dissolved redox species Fe(CN):reduction

Fez+ oxidation

Fe(edta)reduction

0.93 f 0.09

0.95 f 0.09

0.75 f 0.08

C

C

C

C

0.17 3.43

C

2.73

"Ratio of ibfh to ilimbars at a rotation rate of 470 rpm, where irmfh and ilimbare are the limiting currents obtained with polymer-coatedand bare rotating disk electrodes, respectively. *@: (5.0 f 1.0) X lo4 cm. cilimfilm/ilimbara < 0.0001. dPrepared from the diethylamine-containing acetonitrile solution. e Prenared from the basic methanolic solution. f From ref 19.

Figure 4. Typical steady-state current-potential curves for the reduction and oxidation of various dissolved redox specles at both bare and Pw-coated rotating Pt disk electrodes: reduction of (A) H+ at pH 2.7, (B) Fe(edta)- (2.0 mM) at pH 3.0, (C) Fe(CN)B' (2.0 mM) at pH 7.0; oxidation of (D) Fe2+(2.0 mM) at pH 1.5 and (E) Br- (2.0 mM) at pH 3.0; supporting electrolyte, 0.2 M CF,COONa; electrode rotation rate, 470 rpm; (-) at PPO-coated Pt electrode,which was prepared from the basic methanolic solution, (- -) at Ppo-Coated Pt electrode,which was prepared from the diethylamine-containing acetonitrile solution, (---) at bare Pt electrode. Other experimental conditions are the same as in Figure 2.

PPO-coated Pt electrodes, where the data for the PPO films (whose thicknesses are 500 A) prepared from the different electrolytic solutions (see Experimental Section) are compared to each other. At first, consider the permselectivity of the PPO films prepared from the basic methanolic solution. The reduction of Fe(edta)- and Fe(CN)63-ions and the oxidation of Fe2+ion were not observed a t the PPO-coated electrode, indicating that the films do not permit the penetration of these ions. On the contrary, hydrogen ion and Br- ion can, though partially, penetrate the PPO films. On the other hand, in the case of the PPO films prepared from the diethylamine-containing acetonitrile solution, the permeability to the above-mentioned ions is apparently different from that obtained for the PPO films prepared from the basic methanolic solution. All the ions examined can considerablypenetrate the PPO films. As can be readily seen from Figure 4, irrespective of the kind of the redox species, the limiting currents at the PPO film-coated electrode are only slightly smaller than those at a bare electrode. This indicates that the PPO films just described rarely possess the selective permeability such as that of the PPO films prepared from the basic methanolic solution. The data on the permeability of the PPO films, being prepared from the above-mentioned two kinds of the electrolytic solutions, to the various ions are summarized in Table I. From this table it is apparent that (i) the ion-selective permeabilities of the films for a given ion are in the following order: PPO f i i (prepared from the basic methanolic solution) > polyphenol film (19) > PPO film (prepared from the diethylamine-containingacetonitrile solution), and (ii) the redox ions, the Stokes radii (19) of which are larger than ca. 2.7 A, cannot essentially penetrate into the swollen PPO films

prepared from the basic methanolic solution, while all the redox ions examined are allowed to permeate into the swollen PPO films prepared from the diethylamine-containing solution. For the former PPO films the thicknesses of which are ca. 150 A, we also confirmed the excellent ion-selective permeability almost similar to that of the films of 4 = 500 A (shown in Table I). As can be readily seen from the comparison of the iEmfilm/ilimbare values for H+and Br- ions, the PPO film prepared from the basic methanolic solution has the significantly different permeabilities for H+ and Br- ions, while the PPO film prepared from the diethylamine-containing acetonitrile solution and the polyphenol film do not possess such a selective permeability for both these ions. Further, it should be noted that the Stokes radius of the Fe(CN)B3-ion is smaller than that of the Fez+ion, but the Fe(CN)63-ion cannot penetrate into the polyphenol film which is apparently permeable for Fez+ion. As mentioned previously (19),this is due to the fact that Fe2+ion is significantlytrapped in the polyphenol film, but Fe(CN)z- ion is not trapped in it. Thus, the partition coefficient K of Fe2+ion in the polyphenol film is considered to be larger than that of the Fe(CN)63-ion. The details have been represented previously (19). The K Dvalues ~ for Fe3+and Fe(CN)69-ions within the PPO film electrochemicallyprepared from the diethylamine-containing acetonitrile solution were estimated to be (1.1 0.3) x and (3.4 f 1.0) X cm2 s-l, respectively and those for Fe3+and Fe(CN)63-ions within the chemically prepared PPO film were estimated to be (2.1 f 0.5) X and (1.3 f 0.3) x cm2s-l, respectively. These K Dvalues ~ are much smaller than the D,h values estimated ((7.8 f 0.8) x lo4 and (7.6 f 0.8) X lo4 cm2 s-l for Fe3+ and Fe(CN)63-ions, respectively) as well as the K Dvalues ~ for hydrogen ion (see the previous section). The scanning electron micrographs (SEM) of the PPO films at a resolution of 0.1 Mm, which were prepared on Pt electrodes from the basic methanolic solution and the diethylaminecontaining acetonitrile solution, showed that the films are smooth and featureless. That is, any essential differences in the morphology, porosity, and homogeneity of both these films were not found out in the SEMs, although the different ion permeabilities of these PPO films could be easily realized by the hydrodynamic voltammetric experiments as mentioned above. These results seem to be expected, because the different ion permeabilities of these PPO films are actually reflected on the permeation of the redox ions of several angstroms. Application of PPO-Coated Electrodes to pH Sensors. The PPO film is insulating, hydrophobic, and strongly adhesive to metal surfaces. Further, it is stable in acids as well as mineral bases and is insoluble in water and alcohols (IO). As suggested by Bruno et al. (IO),these properties of the PPO films allowed them to be used as a protective coating of metal surfaces. In addition to such unique physical properties of the PPO films, another interesting property is the ion-selective permeability which has been demonstrated at the present

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ANALYTICAL CHEMISTRY, VOL. 59, NO. 14, JULY 15, 1987

study and in recent papers (19,20,26). The property seems

to be of great usefulness in the applications of the PPO-coated electrodes to ion sensors. For example, the PPO-coated Pt electrode responds to protons as the bare Pt electrode does. In this case, the PPO-coated Pt electrode can be considered to respond to protons that reach the electrode surface through the polymer layer. As shown in this study, this is due to the small size of protons. We (19, 34) and other (20, 35) have recently demonstrated the improved pH-responses of Pt, vitreous carbon, and basal-plane pyrolytic graphite electrodes, which themselves respond to protons, by the f i coating using electropolymerization. It goes without saying that such an improvement of the pH-response performance can be ascribed to the excellent ion-selective permeability of the coating films. Thus, the PPO film coated Pt electrodes could be expected as excellent pH-response electrodes, since the PPO films (especially prepared from the basic methanolic solution) have a higher selectivity for ion permeability than previously reported electropolymerized films (polyphenol,polyaniline, etc.) (19, 20, 35). Further, it should be noted that the PPO film electrochemically prepared in situ from the basic methanolic solution is considerably superior to the chemically prepared one in terms of a selective ion permeability. In this case, the latter film can be prepared by spreading aliquots of the chemically prepared PPO solution (solvent, dichIoromethane) on electrode surfaces and by evaporating the solvent. This fact encourages us to employ the in situ electropolymerization method in the preparation of the thin-film-coated electrodes with various functions such as selective ion permeability (19), concentration property (36), protection of metal corrosion (10-13), etc. Registry No. PPO, 24938-67-8; C,H,(CH,),(OH), 576-26-1; (C2H&NH, 109-89-7;H', 12408-02-5;Br-, 24959-67-9;Fe(CN)6", 13408-62-3;Fe(edta)-, 15275-07-7;Fe2+,15438-31-0;Pt, 7440-06-4; NaOH, 1310-73-2; NaClO,, 7601-89-0.

LITERATURE CITED Hay, A. S. J. Polym. Sci. 1962, 58,581. Hay, A. S. Adv. Polym. Sci. 1987, 4 , 496. Cooper, G. D.; Katchman, A. Adv. Chem. Ser. 1969, No. 97,660. Cooper G. D.; Bennett, J. G. J. Org. Chem. 1972, 37,441, and references therein. Van Dort, H. M.; De Jonge, C. R . H. I.; Mijs, W. J. J. Polym. Sci., Part C 1988, No. 22,431. Endres, G. F.; Hay, A. S.; Eustance, J. W. J. Org. Chem. 1963, 28, 1300.

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Ronlan, A. Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Lund, H., Eds.; Marcel Dekker: New York, 1978; Vol. XI, p 242. Mengoli, G. Adv. folym. Sci. 1979, 33,26. Subramanlan, R. V. Adv. folym. Sci. 1979, 33,43. Bruno, F.; Pham. M. C.; Dubois, J. E. Electrochim. Acta 1977, 22, 451. Pham, M. C.; Lacaze, P. C.; Dubois, J. E. J. Nectroanal. Chem. 1978, 86, 147. Dubois, J. E.; Lacaze, P. C.; Pham, M. C. J. Electroanal. Chem. 1981, 777,233. Pham, M. C.; Dubols, J. E.; Lacaze, P. C. J. Electroanal Chem. 1979, 99,331. Iwakura, C.; Tsunaga, M.; Tamura, H. Electrochim. Acta 1972, 77, 1391. Sasaki, K.; Nanao, S.; Kunai, A. Denki Kagaku Oyobi Kogyo Butsuri Kagaku 1977, 45, 130. Mengoli, G.; Daolio, s.; Giulio, V.; Folonari, C. J. Appl. Electrochem. 1979, 9 , 483. Mengoli, G.; Daolio, S.;Musiani, M. M. J. Appl. Electrochem. 1980, 10, 459. Tsuchida, E.; Nishide, H.; Maekawa, T. J. Macromol. Sci., Chem. 1984, A21 1081. Ohnuki, Y.; Ohsaka, T.; Matsuda, H.; Oyama, N. J. Electroanal. Chem. 1983, 758,55. Cheek, G.;Wales, C. P.; Nowak, R. J. Anal. Chem. 1963, 55,380. Evans, D. H.; Jirnenez. P. J.; Kelly, M. J. J. Electroanal. Chem. 1984, 763,145, and references therein. Speiser, B.; Rieker, A. J. Chem. Res., Synop. 1977, 314. Taylor, W. 1.; Battersby, A. R. Oxidative Coupling of Phenols; Marcel Dekker: New York, 1967; p 54. Bejerano, T.; Forgacs, C.; Gileadi, E. J. Electroanal. Chem. 1970, 27, 69. Hay, A. S.; Blanchard, H. S.; Endres, G. F.; Enstance, J. W. J. Am. Chem. SOC. 1959, 87, 6335. Oyama, N.; Ohsaka, T.; Ohnuki, Y.; T. J. Electrochem. Soc., in press. Chaberek, S.; Martell, A. E. J. Am. Chem. SOC.1955, 77, 1477. Oyama, N.; Ohsaka, T.; Okajima, T.; Hirokawa, T.; Maruyama, T.; Ohnuki, Y. J. Elechoanal. Chem. 1985, 79,187. Andrieux, C. P.;SaQeant, J. M. J. Electroanal. Chem. 1982, 742,1, and references therein. Gough, D. A.; Leypoldts, J. K. Anal. Chem. 1979, 57,439. Oyama, N.; Ohnuki, Y.; Ohsaka, T.; Matsuda, H. Nlppon Kagako Kaishi 1983, 949. Kouteckv, J.; Levich, V. G. Zh. Fir. Khim. 1956, 32, 1565. Levich, V. G. Physicochemical Hydrodynamics ; Prentice-Hail, Englewood Cliffs, NJ, 1962. Oyama, N.; Hirokawa, T.; Yamaguchi, S.; Ushizawa, T.; Shimomura. T. Anal. Chem. 1967, 59,258. Heinemann, W. R.; Wieck, H. J.; Yacynych, A. M. Anal. Chem. 1980, 52,345. Oyama, N.; Ohsaka, T.; Nakanishi, M. J. Macromol. Sci., Chem. 1987, A24, 375. ~

RECEIVED for review October 21, 1986. Accepted March 1, 1987. The present work was partially supported by Grantin-Aid for Scientific Research No. 61103002, for N. Oyama, from the Ministry of Education, Science, and Culture, Japan, and the Nissan Science Foundation.