1526
Anal. Chem. 1985,57,1526-1532
Table 111. Comparison of the Results of the Determination of Sodium Carbonate in Commercial Heavv Powder ” Dutv .” - --Detergents or Calcium Carbonate in Commercial Toothpastes by the Proposed Method with Those of Established Methods (5, 6 )
sample detergent A detergent B detergent C detergent D detergent E
sodium carbonate, % proposed method established method” 22.4
11.6 0.8 19.0 5.4
21.2 11.3
0.9 19.8 4.9
toothpaste F toothpaste G
37.1 37.1 45.7 44.3 “Detergentsare analyzed by the automated analyzer method (5). Toothpastes are analyzed by the titration method (6). determined by adding 350 mL of water followed by 10 mL of potassium chloride-hydrochloric acid buffer solution. A calibration curve of calcium carbonate agrees well with that of sodium carbonate. Table I11 shows the determination of calcium carbonate blended as an abrasive in commercial toothpastes, the results are in good agreement with those of the titration method (6). Heavy duty powder detergent can also be analyzed by a direct sampling method with satisfadory repeatability (mean 8.49%, coefficient of variation 1.89% for n = 9). Therefore, the proposed method should be useful and applicable for the determination of various carbonates in detergents as a rapid and convenient method.
ACKNOWLEDGMENT The authors are grateful to Y. Ohhira of Toa Electronics, L a . , for his kind advice and valuable discussions. We are also indebted to R. Azuma and H. Ichikawa for their technical assistance. Registry No. Sodium carbonate, 497-19-8;calcium carbonate, 471-34-1; poly(oxyethy1eneglycol), 25322-68-3; sodium sulfate, 7757-82-6;sodium silicate, 1344-09-8;sodium tripolyphosphate, 7758-29-4; sodium pyrophosphate, 7722-88-5;sodium nitrilotriacetate, 10042-84-9;sodium perborate, 7632-04-4. LITERATURE CITED (1) (2) (3) (4) (5) (6) (7)
Japanese Industrial Standard K3362 (1974).
Nagai, T.; Nihongi, T. Yukagaku 1971, 20, 816. Fujlta, M.; Asarni, A.; Inoue, S.; Hayashi, N. Yukagaku 1074, 23, 188. Zeman, I. J . Chromatogr. 1084, 286,311. Kawase. J.; Yamanaka, M. Yukagaku 1978, 27, 44. Hasegawa, A.; Ohtsuka, H.; Tsuji, K. Yukagaku 1984, 33, 380. Milwidsky, 9. M.; Gabriel, D. M. “Detergent Analysis”; Halsted-Wlley: New York, 1982. (8) Llenado, R. A.; Neubecker, T. A. Anal. Chem. 1983, 5 5 , 93R. (9) Tsuji, K. Bunsekl 1084, 178. (10) Pungor, E.; TBth, K. Analyst (London) 1970, 95, 625. (11) Llenado, R. A. Anal. Chem. 1975, 47,2234. (12) Craggs, A.; Moody, G. J.; Thomas, J. D. R.; Birch, B. J. Analyst (Lon-
don) 1980, 705, 426. (13) Frend, A. J.; Moody, G. J.; Thomas, J. D. R.; Birch, B. J. Analyst(London) 1983, 708,1072. (14) Huianicki, A.; Trojanowicz, M.; Pobozy, E. Analyst (London) 1982, 707, 1356. (15) Severinghaus, W.; Bradley, A. F. J . App. Physiol. 1058, 73, 515. (16) Keeiy, D. F.; Walters, F. H. Anal. Lett. 1983, 76(A20), 1581.
RECEIVED for review February 11, 1985. Accepted April 11, 1985.
Electrochemically Polymerized N ,N-Dimethylaniline Film with Ion-Exchange Properties as an Electrode Modifier Noboru Oyama,* Takeo Ohsaka, and Takashi Shimizu
Department of Applied Chemistry for Resources, Tokyo University of Agriculture and Technology, Koganei, Tokyo 184, Japan
Poly(N,Ndlmethylanlllne) (PDMA) was prepared by electrochemical polymerlratlonof the correspondlng monomer and was found to have the structure of an lonene polymer wlth positively charged sltes as quaternary ammonlum groups In the polymeric backbone. I t Is demonstrated that the PDMA fllm has an anion-exchange character lrrespectlve of the pH of a solution, and coatlng electrodes wlth PDMA produces surfaces whlch strongly blnd multlply charged negatlve Ions.
Electrochemically initiated polymerization (sometimes referred to as electropolymerization or electrochemical polymerization) has recently received great attention in the modification of electrode surfaces, because of the potential applications (electrocatalysis (I, 2),protection of metals from corrosion (antiphotocorrosion) ( 3 4 , electrochromic display (7,8),energy storage (9),“ion gate” membrane (IO),etc.) of the resulting modified electrodes. This procedure also holds great promise for the synthesis of new organic conducting (metallic), semiconducting, or nonconducting polymers (e.g.,
poly(pyrrole),poly(thiophene), poly(aniline),and poly(pheno1)) (2,6, 10-20). The electrochemical, electrical, and physicochemical properties of the films prepared by electropolymerization of aniline, phenol and their derivatives, vinyl monomers, aromatic heterocyclic compounds, etc. have been extensively examined, together with the mechanism of electropolymerization for individual cases and the structures of the prepared films. It has become obvious that the properties and structures of the prepared films depend on the kind of the monomer used for polymerization as well as the experimental conditions used in their preparations (e.g., solvent, electrode material, supporting electrolyte, pH of electrolytic solution, and temperature). By the appropriate choice of a monomer and the experimental conditions for preparation, polymer films with a particular desired property can be prepared. In the present paper, we wish to report the preparation of poly(N,N-dimethylaniline)(PDMA) film by the anodic oxidation of NJ-dimethylaniline (DMA) and its electrochemical properties and structure. The PDMA is shown to be an “ionene polymer” with positively charged sites as quaternized
0003-2700/85/0357-1526$01.50/00 1985 American Chemlcal Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
ammonium groups in the polymeric backbone. In addition, the stability and utility of the PDMA film as an anion-exchange film will be demonstrated on the basis of the electrostatic incorporation of highly charged anionic redox species into the PDMA film and the kinetic considerations of the electrode reaction of the redox species thus confined in the PDMA film on a pyrolytic graphite electrode. EXPERIMENTAL SECTION N,N-Dimethylaniline (DMA) of reagent grade was obtained from Wako Pure Chemical Industries and was purified by distillation using ordinary techniques. Sodium sulfate as a supporting electrolyte was reagent grade and was used without further purification. Other chemicals were reagent grade and were used as received. A standard three-electrode electrochemical cell was used for all electrochemicalexperiments. Poly(N,N-dimethylaniline) (PDMA) was synthesized by the same procedure as previously described (18-20). Typically, 0.1 M DMA was oxidized at 0.8 or 1.0 V vs. a sodium chloride saturated calomel electrode (SSCE) on a basal-plane pyrolytic graphite (BPG, Union Carbide Co.) (surface area 0.17 cm2)or an Inz03(1-2 cm2)electrode in 0.5 M NazS04solutions (pH 1.0 and pH 13). The film prepared on the electrodeswas washed with HzSO4 solution (pH 1.0) and then with distilled water and dried under vacuum. The thickness of the film prepared on electrodes was controlled by the charge passed during the anodic oxidation of DMA, and typically the films of PDMA (4.8 km thick) were grown on an electrode by passing 10 C cm-2 of charge. The film thicknesses were measured with a Surfcom 550A (Surface Texture Measuring Instrument, Tokyo Seimitsu Co.). The amount (r)of the Fe(CN)2-l4- species incorporated electrostatically into the films deposited on electrodes was estimated in units of mol cm-2 by measuring the area of cyclic voltammograms (for the oxidation-reduction reaction of the incorporated redox couple) obtained at slow potential scan rate (2 mV 9-l) in 0.2 M CF3COONa aqueous solution (adjusted to pH 1.0 with CF3COOH)and measuring the charge required in a potential-step experiment to quantitatively oxidize or reduce the redox species confined in the films. The r values estimated by the two procedures were the same within experimental error. The molar concentration (in units of mol ~ m - of~ the ) incorporated redox species was calculated from the r thus obtained using the thickness of the film under the swelling state once the film is placed in the electrolyte solution. The film thickness under the swelling state was also measured with the Surfcom 550A. IR absorption spectrometry (A302 Infrared Spectrometer, Japan SpectroscopicCo.) was used to identify the films formed on Inz03 and BPG electrodes. For cyclic voltammetry, normal pulse voltammetry, potential step chronoamperometry, and potential step chronocoulometry, a homemade instrument (21) was employed along with an X-Y recorder (Watanabe Co.). All experiments were run under nitrogen atmosphere at laboratory temperature (25 h 1 "C). Potentials were measured and are quoted with respect to SSCE.
RESULTS AND DISCUSSION Cyclic Voltammetry of NJV-Dimethylaniline (DMA). The cyclic voltammograms shown in Figure 1 are typical of those obtained for DMA at a BPG electrode in acidic and alkaline aqueous solutions. The voltammograms in acidic media are consistent with those reported previously by other investigators (22-24). The observed anodic and cathodic peaks have been ascribed to the sequential one-electron oxidation reactions. The anodic peaks at ca. 0.8 V and 1.1V vs. SSCE correspond to the oxidation of DMA to the cation radical, DMA', and that of DMA+ to the dication, DMA2+,respectively. The corresponding cathodic peaks are reductions of DMA2+ and DMA+, though the peak currents are much smaller than the anodic ones due to chemical reactions of generated oxidized species. Furthermore, the cathodic peak at 0.5 V may correspond to the reduction of the oxidized form (TMBOx) of tetramethylbenzidine (TMB)that can be formed via two cation radicals (DMA+)coupling with the loss of two protons (24)and the anodic peak at 0.6 V is the corresponding oxidation of TMB to TMBOx.
1527
/I
-0
05
10
15
0
05
10
SSCE E / V Y S SSCE Flgure 1. Cyclic voltammograms of DMA at a BPG electrode in an acidic and an alkaline media: solution composition,(A) 0.1 M DMA 0.5 M Na,SO, + H,S04 (pH l.O), (B) 3.5 mM DMA + 0.5 M Na,SO, NaOH (pH 13); scan rate, 50 mV s-': electrode area, 0.17 cm2. The arrow indicates the direction of scan. E/ V
YS
+
+
On the other hand, the voltammograms in alkaline media are significantly different from those in acidic media. Only one anodic peak, being due to the oxidation of DMA to DMA+, is observed a t ca. 0.8 V in the first anodic scan of potential. This anodic current is overlapped with the background current due to O2 evolution reaction. The cathodic peak corresponding to the anodic peak at 0.8 V is not observed. The oxidationreduction wave corresponding to the TMB-TMBOx redox couple is not observed. Such differences between the cyclic voltammograms of DMA in acidic and alkaline media are probably due to the different stabilities of DMA+ and DMA2+ in both media. The stability of DMA' and DMA2+increases in more acidic media (22, 24). In fact, the reversibility of DMA/DMA+ and DMA+/DMA2+are greatly improved in very acidic media (e.g., in 3 M H2S04solution). It is common to the cyclic voltammograms in both media that the anodic peak currents corresponding to the oxidation of DMA+ and/or DMA decrease with the succeeding potential scans. This is due to the formation of the electroinactive polymer films on electrode surfaces. The controlled-potential electrolysis was used to prepare the PDMA films. The electrolyses were mainly conducted at 1.0 V in acidic media for 15 min, since the film prepared from alkaline media was very thin, compared with that obtained in acidic media. As the electrolysis proceeded, the acidic electrolytic solutions changed from colorless to brown or deep blue. The colors of TMBOx and the monocation radical of TMBOx, which are soluble in the electrolytic solutions used, are yellow and green, respectively (23). The film-coated electrodes were completely washed with a H2S04 solution and then with distilled water, until the solutions into which the fiim-coated electrodes were rinsed become colorless. When the thickness of PDMA film was above several tens of micrometers, it was possible to peel the film with tweezers from an electrode surface without tearing. The color of the resulting polymeric films was purplish brown. They were insoluble in all the solvents examined (i.e., DMF, Me2S0, toluene, methanol (MeOH), benzene, CHC13, 1,2-dichloroethane, CF3COOH, H2S04, H20, DMF-MeOH, Me2SOMeOH). Thus, the procedures for characterization of the prepared films were limited. The color and the thickness of the prepared films were found to change with the electrolysis procedures (e.g., controlled-potential electrolysis or potential sweep electrolysis), the concentration of DMA, the pH of the electrolytic solutions, the value of an applied potential during electrolysis, etc. For example, when the anodic electrolysis
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ANALYTICAL CHEMISTRY, VOL. 57,
NO. 8, JULY 1985
Table I. IR Spectra of DMA and of PDMA Film Resulting from the Electrochemical Oxidation of DMA (Fundamental Vibrations, Frequencies in cm-l)n vibration compd
u(C-C)
dC-C)
u(C-H)
dC-H)
v(C-N)
DMA
1510 1600
670
3050 3025 2930 2870
690 750 860
1190 1220 1340
PDMA
1510 1600
700
3050 2940
840
1250 1340
v(C-H)~
so:-
2790
620 1120
OThe PDMA film was formed on an InpOs electrode by the anodic oxidation of DMA in 0.5 M Na2S04solution (pH 1.0) containing 0.1 M DMA a t 1.0 V vs. SSCE. u, stretching vibration. Oy, 6, out-of-plane bending vibration. C-H stretching vibration of the N-CHs bond of tertiary amines.
in acidic media was carried out at 0.8 V, the electrolytic solution changed from colorless to purplish. In this case, the color of the resulting film was black and the thickness was much thinner than that of the film prepared at 1.0 V under the same experimental conditions. These results may be useful for the elucidation of the mechanism of the formation of PDMA. The more detailed experiments concerning this are currently under way. IR Absorption Spectrometric Identification of PDMA Films. The IR spectral data of the polymer film formed on the In203electrodes during the anodic oxidation of DMA in acidic media (pH 1.0) are summarized in Table I in which the IR spectral data of DMA itself is also shown for comparison. The absorption peaks at 3050,1510,1600, and 670-700 cm-l, which are common to the PDMA films and DMA, are characteristic of the various vibration modes of the C-H and C-C bonds of the aromatic nuclei (25,26).The peaks at 1340-1350, 1220-1250, and 1190-1200 cm-l correspond to the stretching of the C-N bonds on the aromatic amines and the aliphatic amines (25, 26). In this case, the absorption peaks corresponding to the stretching of the C-N bonds on the aromatic amines were observed at higher wavenumber than those on the aliphatic amines. Moreover, relatively strong absorption peaks between 900 and 700 cm-l which arise from C-H outof-plane bending modes were observed (25, 27). The absorption peaks in this region are diagnostically very useful in the correct assignment of the substitution pattern of the aromatic ring. In the spectrum of DMA, one weak absorption peak was observed at 860 cm-l and two strong absorption peaks were observed at 750 and 690 cm-’, as expected for the monosubstituted benzene. In the spectrum of the PDMA film, one relatively strong peak was found at 840 cm-*, indicating that the PDMA film possesses the benzene-structure disubstituted at the 1- and 4-positions with respect to the dimethylamino group as a unit structure; that is, in this case, the electropolymerizationproceeds via coupling to the phenyl ring in the para position. The same type of electropolymerization has previously been found for the case of aniline and its derivatives in acidic aqueous solutions (19,20, 28). The peaks at 2700-2800 cm-l, which are due to the C-H stretching vibration of the N-CH3 bond generally appearing in tertiary amines, were observed for DMA monomer but not done for the PDMA film, indicating that there are few tertiary amines in this PDMA film. The absorption peaks which are characteristic of the stretching vibration of the C=N bond and the N-H bond are not observed in the absorption regions expected for their vibration modes. This indicates that in the course of the formation of PDMA films the bond cleavage of the N-CH3 bonds does not occur. Peaks at 1120 and 620 cm-l observed in the spectra of the PDMA films are assigned to the presence of SO:-, which were used as an electrolytic anion during the film formation. The IR spectra of the PDMA films prepared by the potential scan mode were essentially the same
I
1
i/l
0
5
10
15
Q/Ccm2
Flgure 2. Correlation between the thickness of films and the amount of the charge passed during the anodic oxidation of DMA. The films were prepared by the anodic oxidation of DMA at 1.0 V vs. SSCE in a 0.5 M Na,SO, solution (pH 1.O) containing 0.1 M DMA. Widths of error bars indicate uncertainties in thickness measurements.
as those of the PDMA films prepared by controlled-potential electrolysis. The PDMA films were prepared on both InzOs and BPG electrodes. The IR spectra of these films were almost the same, indicating that in this case the PDMA formation was essentially independent of the electrode material, in contrast with the case of the ring-substituted aniline derivatives (20, 28). The above-mentionedspectrometric results along with the electrochemical data mentioned below indicate that the PDMA prepared is a polymer with quaternary ammonium groups in the polymeric backbone, that is, a kind of “ionene polymer” (29,30). Assuming this structure of the PDMA, the formation of PDMA is considered to proceed via the “head to tail” coupling between monocation radicals of DMA’s, their dications, the monocation and the parent DMA, and/or the dication and the parent DMA, as previously suggested for the formation of some kinds of the dimeric compounds (22-24, 32-34), the octamer (i.e., emeraldine) (31),and the polymeric compounds (2,19,20,28,31)by the anodic oxidation of aniline and its derivatives. Thickness Control of PDMA Film. Figure 2 shows the correlation between the thickness (4) of the film prepared and the amount (Q) of charge passed during the anodic oxidation of DMA. In this case, the films were prepared by the anodic oxidation of DMA at 1.0 V in a 0.5 M Na2S04solution (pH 1.0) containing 0.1 M DMA. The 4 is proportional to Q with the proportionality constant ca. 0.48 pm C-’ cm-2. This proportionality is particularly surprising in view of the fact that most of the electrolysis products were soluble in the electrolytic solutions used and diffuse from the electrode surface to the bulk of the solution, as can readily be seen from the colored electrolytic solution. The value of the proportionality constant is about one-tenth of that (4 pm C-’ cm-2)
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8,JULY 1985
1529
e - 50 r n n
2
4
6
8 1 0 1 2
9H
Incorporation of Fe(CN)z- into the PDMA film on a BPG electrode as a function of pH. The PDMA film (thickness,3.7 pm) was prepared on a BPG electrode surface by the anodic oxidation of DMA at 1.0 V vs. SSCE in a 0.5 M Na,SO, solution (pH 1.0) containing 0.1 M DMA. Electrode was equilibrated for several hours with a 0.2 mM solution of Fe(CN)z-at each pH before transfer to the solution containing only supporting electrolyte for measurement of the quantity incorporated. The supporting electrolyte was as follows: pH 1.O, 0.2 M CF,COONa CF,COOH; pH 3.0,0.2 M CF,COONe 50 mM ckrate buffer;pH 4.0,0.2 M CF,COONa 50 mM acetate buffer; pH 6.0 and 7.0, 0.2 M CF,COONa 50 mM citrate/phosphatebuffer; pH 8.0, 0.2 M CF,COONa 50 mM borate buffer; pH 10.0 and 11.0, 0.2 M CF,COONa + 50 mM phosphate buffer: pH 12.0, 0.2 M CF,COONa NaOH. Figure 4.
+
+
+
+
+
+
I
0
04 E/V vs SSCE
08
0
,
l
06
,
I
08
E I V vs SSCE
(A) Cyclic voltammograms for Fe(CN)63-14recorded continuously at a PDMA filmdeposited BPG electrode in a 0.2 M CF,COONa solution (pH 1.0) containing 0.2 mM Fe(CN):-. (B) Cyclic voltammograms obtained when the electrode used in A was washed with water and transferred to a 0.2 M CF,COONa solution (pH 1.0). (C) Cyclic voltammogram obtained at a bare BPG electrode in the same solution as used in A. (D) Cyclic voltammograms for Fe(CN)e3-/4recorded continuously at a PDMA film-deposited BPG electrode in a 0.2 M CF,COONa solution (pH 8.7)containing 0.2 mM Fe(CN),,- and 50 mM borate buffer. (E) Cyclic voltammograms obtained when the electrode used in D was washed with water and transferred to a 0.2 M CF,COONa solution (pH 8.7).(F) Cyclic voltammogram obtained at a bare BPG electrode in the same solution as used in D. The PDMA filmdeposited electrodes (film thickness, 3.4 pm) were prepared by the anodic oxidation of DMA at 1.0 V vs. SSCE in a 0,5 M Na2S0, solution (pH 1.O) containing 0.1 M DMA. Immersion time is indicated on the vokammograms. The potential was scanned between -0.2 and 0.8 V vs. SSCE at 50 mV s-’. Figure 3.
obtained for the electropolymerization of aniline in 0.5 M Na2S04solution (pH 1.0) (19). Thus, in the case of the PDMA formation, the charge passed during the anodic oxidation of DMA is less effectively used for the PDMA formation, compared with poly(ani1ine)formation. This may be ascribable to the fact that the poly(ani1ine) film is electroactive, but the PDMA film is electroinactive. In other words, the poly(aniline) film prepared on electrode surfaces can mediate the successive oxidation of the aniline monomers diffusing from the bulk of the solution to the electrode surface, while the PDMA film does not have the ability of such mediation. At any rate, the data shown in Figure 2 should be noted, because the film thickness can be arbitrarily controlled by the charge passed during the electropolymerization. Incorporation of Multiply Charged Anionic Redox Species into PDMA Films. A series of cyclic voltammograms demonstrating the incorporation of Fe(CN)63-into a film of PDMA are shown in Figure 3A,D. The incorporation of the multiply charged anion (Le., Fe(CN)63-)by anion exchange with the supporting electrolytic anions (i.e., Sod2-
and/or CF3COO-)initially present in the film continued for about 50 min at pH 8.7 and pH 1.0. No further increases in peak currents were observed for longer potential scanning times. Figure 3B,E shows that much of the Fe(CN):- remains in the films for long periods if the electrodes are removed from the incorporating solutions, washed, and replaced in pure supporting electrolytic solutions. The initial decay in the peak currents results from the loss of the incorporated Fe(CN)63most accessible to ion exchange. The voltammograms reached steady states after ca. 15 and 80 min of scanning in the cases of the transfer into solutions of pH 1.0 and pH 8.7, respectively. These behaviors are the same as those found for the incorporation of the multiply charged anionic and cationic redox species into cationic and anionic polymer domains on electrodes by the electrostatic interactions between the polyelectrolyte layer and the redox species carrying opposite charge (35-50). The incorporated Fe(CN)63-is less stably retained in the PDMA film immersed in pH 8.7 solution, compared with in pH 1.0 solution, as seen from Figure 3B,E. Concerning the behavior of cyclic voltammograms obtained with the PDMA film-coated electrode in pH 8.7 solution, there was not any dependence on buffer ion concentration (0-50 mM) of a soaking solution at the constant pH. Therefore, it can be considered that such a rapid loss of the Fe(CN)63shown in Figure 3E results from the content of a mixture of amine types in the PDMA which responds differently to pH. Figure 3B,E demonstrates that the peak current obtained in an incorporating solution of pH 1.0 is larger than that obtained in an incorporating solution of pH 8.7. This difference may be caused by the difference of charge transfer rates of the Fe(CN)63-/4-couple at electrode/film interfaces and in films in the soaking solution of the different pHs. These peak currents in Figure 3A,D are about 60 times larger than those (shown in Figure 3C,F) obtained with a bare electrode in the same solutions as those used to obtain Figure 3A,D. Figure 4 shows the effect of pH on the incorporation of Fe(CN)63-by the PDMA film prepared. Experiments of this type were performed by exposing a coating of PDMA to a solution of Fe(CN):- that was adjusted to pH values between 1.0 and 13 and by scanning the electrode potential between -0.20 and +0.80V vs. SSCE. After the incorporation equilibrium had been attained (about several hours of exposure proved adequate) the film-coated electrode was transferred to a pure supporting electrolyte solution at the same pH and then the amount l?Fe(CNp of Fe(CN)63-retained by the PDMA
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
v I
I
0
I
0.4
1 0.8
I
E / V vs. SSCE Flgure 5. Cyclic voltammograms for the oxidation-reduction of the
Fe(CN),3-'' redox couple confined in the PDMA fllm on a B f f i electrode in a 0.2 M CF,COONa solution (pH 1.0). The PDMA film-deposited electrode, which was prepared in the same manner as in Figure 4, was soaked in a 0.2 M CF, COONa solution (pH 1.0) containing 0.2 mM Fe(CN)B3-and then the electrode potential was scanned between -0.2 and 0.8 V vs. SSCE at 50 mV s-' for ca. 60 min. After that, the electrode was washed with water and then transferred to a 0.2 M CF,COONa solution (pH 1.0) for measurement of the potential scan rate dependence of the cyclic voltammogram. The inset shows the dependence of the anodic and cathodic peak currents upon (scan rate)'I2: (0)anodic peak currents; (0)cathodic peak currents.
film was evaluated by integration of the current of the voltammogram obtained with the potential scan rate of 2 mV s-l for the oxidation-reduction of the Fe(CN)63-/4-redox couple (35). An incorporation experiment similar to that shown in Figure 4 was also conducted with the PDMA film-coated electrode (the thickness of PDMA is ca. 4 pm) by changing the bulk concentration of Fe(CN)63-from 0.002 mM to 200 mM in 0.2 M CF3COONasolution (pH 1.0). The results are that rFe(CNp increases with an increase in the bulk concentration of Fe(CN)63-and approaches a constant value (ca. 2 X lo-' mol cm-2)when the bulk concentration of Fe(CN)63is above 0.2 mM. In this case, the partition coefficient of Fe(CN):-, which is expressed as the ratio of equilibrium concentration of Fe(CN)63-in the PDMA film to that in the bulk of the solution, was estimated to be larger than lo4. It is apparent from Figure 4 that rFe(CN)6tis independent of pH in the range of pH 1.0 to 13.0. Further, it was found that other highly charged anionic redox species besides Fe(CN):- (e.g., M O ( C N ) ~ Fe(CN)64-, ~-, W(CN)84-,F e ( b ~ ) , ~(bp: - bathopenanthroline disulfonate, ClzH6Nz(C6H4S03-)2))are also incorporated into the PDMA film and confined stably in it. Homogeneous Charge-Transport Process within the PDMA Films Incorporating Fe(CN):- and Heterogeneous Electron-Transfer Process between Electrode and Fe(CN)63-Incorporated into the PDMA Films. Figure 5 shows cyclic voltammograms for the oxidation-reduction of the Fe(CN):-I4- redox couple incorporated into the PDMA film on BPG electrodes in a 0.2 M CF3COONasolution (pH 1.0). The cyclic voltammogramsqualitatively resemble those of solution-dissolved reactants diffusing to the electrode surface. Plots of the cathodic and anodic peak currents for the reduction and oxidation of the incorporated Fe(CN):-/4couple vs. the square root of the potential scan rate, u1/2,were nearly linear from 5 to 200 mV s-l, indicating that the charge-transport through the PDMA film is "apparently"
Figure 6. Typical normal pulse voltammograms for the reduction of the Fe(CN)," confined in the PDMA film on a BPG electrode at various sampling times In a 0.2 M CF,COONa solution (pH 1.0). The concentratlon of Fe(CN):confined in PDMA film (thickness, 2.75 pm): mol om-,. Sampling times, r, (1) 10, (2) 6, (3) 4, and (4) 5.0 X 3 ms.
diffusional and obeys Fick's law. The separation between the anodic and cathodic peak potentials, AE,,varies with u. At slow scan rates (15mV s-l), the value of AE, is close to that expected for thin-layer behavior (51-53), and at higher scan rates the value of AE, exceeds that (0.059 mV) expected for diffusion-controlled,reversible waves. For example, AE, = 80 mV at u = 200 mV s-l. This is attributed to slow heterogeneous electron transfer between the electrode and the redox centers in the film and uncompensated film resistance which is the resistance to ion flow through the film caused by low ambient ion populations and/or sluggish ion mobility (46, 53-58). Normal pulse voltammetric experiments were carried out to more closely examine the charge diffusion behavior within the PDMA film incorporating Fe(CN):- and the heterogeneous electron-transfer reaction between the electrode and Fe(CN)63-confined in the PDMA film, since normal pulse voltammetry has proved to be useful for the quantitative examination of the overall charge transport process at polymer-coated electrodes (59-63). Figure 6 shows typical normal pulse voltammograms for the reduction of the Fe(CN)63confined in the PDMA film on a BPG electrode at various sampling times. It is obvious from this figure that these S-shaped voltammograms are similar to those observed for solution-phase redox species at an uncoated electrode. Plots of the cathodic limiting currents (iIim)of these normal pulse voltammograms against the inverse square root of the sampling time (7) were found to be linear. This means that the limiting current is diffusion controlled. Thus, the apparent diffusion coefficient, Dapp,for the process of the charge transport within the film can be obtained from the slope of ilimvs. T - ~ /plot ~ by using the normal pulse voltammetric Cottrell equation (59-64)
(id)cott= nFACpDappl/2(~~)-1/2 (1) where (id)cottdenotes the limiting diffusion current, n is the number of electrons involved in the heterogeneous electrontransfer reaction between the electrode and the incorporated electroactive species, C, is the volume concentration of the electroactive species incorporated into the film, A is the electrode area, and F is the Faraday constant. The value of Dappwas estimated to be (1.8 0.2) x cm2s-l which was in good agreement with that ((1.8 0.1) X lo-' cm2 s-') obtained by potential step chronoamperometry and chronocoulometry. The kinetic parameters (i.e., the standard rate constant, k", and the transfer coefficient, a) of the heterogeneous electron-transfer reaction between the electrode and the incorporated Fe(CN):- were evaluated from the analysis of the
*
*
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
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2.0
,
1531
(CN)63-I4-couple confined electrostatically in the protonated poly(4-vinylpyridine) (PVP) (61). In this case, it has been found that D,,, k", and a depend on the volume concentration (C,) of the Fe(CN)$- incorporated into the PVP film, so that the values cited for comparison are ones a t the same concentration of Fe(CN)$- as that (C,,, 1.0 X mol ~ m - in ~) the present work. Registry No. PDMA (homopolymer),54140-98-6;Fe(CN)l-,
1
13408-62-3. L."
0.10
E/ V
03 5 0.20 SSCE
LITERATURE CITED
vs.
Flgure 7. Representative results for the plots of current vs. potential for the reduction of Fe(CNI2- confined in PDMA film on a BPG electrode. Sampling times, T , were (1) IO,(2) 6, (3)4, and (4) 3 ms. Other experimental conditions are the same as those given in Figure 6.
rising part of the current-potential curves shown in Figure 6 by using (59-63, 65)
E = E*
In ( X [ 1.75
-
+ X2(1 + exp([))2]1'z)
1 - X(1 + exp(5))
anF with
(
RT In -E* = ElI2' + 4 ko anF 3112 D1/2
5
= (nF/RT)(E- Eip?
D =
(Dcathodic)a(Danodic)l-a
(3)
(5) (6)
where E is the electrode potential, i is the normal pulse voltammetric current, Ellz*is the reversible half-wave potential of the redox couple (confined in the film, at the present case), Dcathdic and Dmdicare the apparent diffusion coefficients for the cathodic process and the anodic process, respectively, and R , T, and F have their usual meanings. Equations 2 and 3 are represented for the case that the overall electrode process is cathodic, and the equations corresponding to the anodic process are obtained by changing the plus or minus sign between the first term and the second term on the right-hand side of eq 2 and 3 and changing the exponential term of eq 2 from exp(C;)to exp(-l). Plots of the second term on the right-hand side of eq 2 against E gave the straight lines the slopes of which were constant at the different sampling times within experimental error (see Figure 7). The cathodic transfer coefficient was evaluated from their reciprocal slopes. The values of E* were obtained from the intercepts of the lines shown in Figure 7 with the zero line. The plot of E* vs. In 7,also gave a straight line, the slope allowing us to evaluate the transfer coefficient, as can be seen from eq 3. The cathodic transfer coefficients evaluated were 0.31 f 0.02. The standard rate constant, ko, was evaluated by means of eq 3. For doing this evaluation, the value of El$ was estimated as the average of the anodic and cathodic peak potentials of the cyclic voltammogram for the oxidation-reduction of the Fe(CN)B3-/4-redox couple confined in the PDMA film on electrodes in a solution containing only supporting electrolyte. By assuming that Dcathodic = Dmdc,the value of D was estimated as that of Dcathdic.The value of k" thus obtained was (1.3 f 0.2) X cm s-l. The obtained values of Dapp,k", and a are almost same as those (1.9 X lo* cm2 s-l, 1.4 X cm s-l, and 0.30 for Dapp, k", and a, respectively) previously obtained for the Fe-
Bull, R. A.; Fan, FA.; Bard, A. J. J. Electrochem. Soc. 1983, 130, 1636- 1638. Volkov, A.; Tourillon, G.; Lacaze, P.-C.; Dubois, J.-E. J . Electroanal. Chem. 1980, 115, 279-291. Skothelm, T.; Lundstrom, I.; PraJza, J. J. Electrochem. Soc. 1981, 128, 1625-1626. Noufi, R.; Nozik, A. J.; Whlte, J.; Warren, L. F. J. Electrochem. Soc. 1982, 129, 2261-2265. Frank, A. J.; Honda, K. J. fhys. Chem. 1982, 86, 1933-1935. Dubois, J.-E.; Lacaze, P.-C.; Pham, M. C. J. Electroanal. Chem. 1981, 117, 233-241, and references therein. Garnier, F.; Tourillon, G.; Gazard, M.; Dubois, J. C. J. Electroanal. Chem. 1983, 148, 299-303. Kobayashi, T.; Yoneyama, H.; Tamura, H. J . Electroanal. Chem. 1984; 161, 419-423; Heerger. A. J. I n "Eiectro-active Polymers"; Ecole d'Hiver: Font-Romeu, France, 1982; Part 1, p 3. Burgmayer, P.; Murray, R. W. J. Nectroanal. Chem. 1983, 147, 330-344. ... Waltman, R. J.; Bargon, J.; Diaz, A. F. J. Phys. Chem. 1983, 87, 1459- 1463, and references therein. Tourillon, 0.; Garnier. F. J. Elecfroanal. Chem. 1982, 135, 173-178. Mourcel, P.; Pham, M.-C.; Lacaze, PA.; Dubois, J.-E. J . Elecfroanal. Chem. 1982, 145, 467-472. Plckup, P. G.; Osteryoung, R. A. J. Am. Chem. Soc. 1984, 106. 2294-2299. Ghosh, P. K.; Spiro, T. G. J. Electrochem. Soc. 1981, 128, 1281-1287. Caivert, J. M.; Schmehl, R. H.; Sullivan, B. P.; Facci, J. S.; Meyer, T. J.; Murray, R. W. Inorg. Chem. 1983, 22, 2151-2162. Ellis, C. D.; Margerum, L. D.; Murray, R. W.; Meyer, T. J. Inorg. Chem. 1983, 22, 1283-1291. Ohnuki, Y.; Matsuda. H.; Ohsaka, T.; Oyama, N. J. Electroanal. Chem. 1983, 158, 55-67. Oyama, N.; Ohnukl, Y.; Chlba, K.; Ohsaka, T. Chem. Left. 1983, 1759-1762. Ohsaka, T.; Ohnukl, Y.; Oyama, N.; Katagiri, G.; Kamisako, K. J . Nectroanal. Chem. 1984, 161, 399-405. Oyama, N.; Ohsaka, T.; Ushirogouchi, T. J. fhys. Chem. 1984, 88, 5274-5280. Mizoguchl, T.; Adams, R. N. J. Am. Chem. SOC. 1962, 84, 2058-2061. Galus, Z.; Adams, N. R. J. Am. Chem. Soc. 1962, 84, 2061-2065. Neubert, G.; Prater, K. B. J. Electrochem. Soc. 1974, 121, 745-749. Dolphin, D.; Wick, A. I n "Tabulatlon of Infrared Spectral Data"; Wiiey: New York, London, Sydney, Toronto, 1977. Pouchert, C. J. I n "The Aldrlch Library of Infrared Spectra", 2nd ed.; Aldrich Chemical Company: Milwaukee, WI, 1975. Cross, A. D.; Jones, R. A. I n "An Introduction to Practical Infrared Spectroscopy", 3rd ed.; Butterworth: London, 1969. Ohnukl, Y.; Matsuda, H.; Oyama, N. Nippon Kagaku Kaishi 1984, 1801-1809. Rembaum, A.; Noguchi, H. Macromolecules 1972, 5 , 261-269. Casson, D.; Rembaum, A. Macromolecules 1972, 5 , 75-81. Mohilner, D. M.; Adams, R. N.; Argersingers, W. J., Jr. J. Am. Chem. Soc.1962, 84,3618-3622. Breitenback, M.; Heckner, H. H. J. Electroanal. Chem. 1971, 29, 309-323. Hand, R. L.; Nelson, R. F. J. Am. Chem. Soc. 1974, 96, 850-861. Sharma, L. R.; Manchanda, A. K.; Slngh, G.; Verma, R. S. Electrochim. Acta 1982. 27, 223-233. Oyama, N.; Anson, F. C. J . Electrochem. Soc. 1980, 127, 640-647. Oyama, N.; Anson, F. C. J. Electroanal. Chem. 1980, 127, 247-250. Oyama, N.; Shimcnura, T.; Shigehara, K.; Anson, F. C. J. Electroanal. Chem. 1980, 112, 271-280. Oyama, N.; Anson. F. C. Anal. Chem. 1980, 52, 1192-1198. Shlgehara. K.; Oyama, N.; Anson, F. C. Inorg. Chem. 1981, 20, 518-522. Facci, J.; Murray, R. W. J . Elecfroanal. Chem. 1981, 124, 339-343. Kuo, K. N.; Murray, R. W. J. Electroanal. Chem. 1982, 131, 37-60. Martin, C. R.; Dollard, K. A. J. Electroanal. Chem. 1983, 159, 127-135. Majda, M.; Faulkner, L. R. J. Electroanal. Chem. 1984, 169, 77-95. Majda, M., Faulkner, L. R. J . Elecfroanal. Chem. 1984, 169, 97-112. White, H. S.; Leddy, J.; Bard, A. J. J. Am. Chem. SOC. 1982, 104, 481 1-48 17. Martin, C. R.; Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1982. 104. 4817-4824. Oyama, N.; Yamaguchl, S.; Nlshlki, Y.; Tokuda, K.; Matsuda, H.; Anson, F. C. J. Electroanal. Chem. 1982, 139, 371-382. Buttry, D. A.; Anson, F. C. J. Am. Chem. SOC. 1983, 105, 685-689.
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Anal. Chem. 1085, 57, 1532-1536
(49) Anson, F. C.; Ohsaka, T.; Saveant, J. M. J . Phys. Chem. 1983, 87, 640-647. (50) Anson, F. C.: Ohsaka, T.; Saveant, J. M. J . Am. Chem. SOC.1983, 705, 4883-4890. (51) Laviron, E. J . Nectroanal. Chem. 1972, 39, 1-23. (52) Laviron, E. J . Nectroanal. Chem. 1974, 52, 395-402. (53) Lavlron, E. J . Nectroanal. Chem. 1980, 772, 1-9. (54) Daum, P.; Murray, R. W. J . Phys. Chem. 1981, 85, 389-396. (55) Nowak, R. J.; Schuiz, F. A.; Umana, M.; Law, R.; Murray, R. W. Anal. Chem. 1980, 52, 315-321. (56) Peerce, P. J.; Bard, A. J. J . Nectroanal. Chem. 1980, 774, 89-115. (57) Van De Mark, M. R.; Miller, L. L. J . Am. Chem. SOC. 1978, 700, 3223-3224. (58) Rouiiier, L.; Lavlron, E. J . Nectroanal. Chem. 1983, 157, 193-203. (59) Oyama, N.; Sato, K.; Yamaguchi, S.; Matsuda. H. Denki Kagaku Oyobi Kogyo Butsurl Kagaku 1983, 51, 91-92. (60) Sato, K.; Yamaguchi, S.; Matsuda, H.; Ohsaka, T.; Oyama, N. Bull.
Chem. Soc. Jpn. 1983, 56, 2004-2008. (61) Oyama, N.; Ohsaka, T.; Kaneko, M.; Sato, K.; Matsuda, H. J . Am. Chem. SOC. 1983, 705, 6003-6008. (62) Ohsaka, T.; Yamamoto, H.; Kaneko, M.; Yamada, A.; Nakamura, M.; Nakamura, S.; Oyama, N. Bull. Chem. SOC. Jpn. 1984, 57, 1844-1849. (63) Ohsaka, T.; Sato, K.; Matsuda, H.; Oyama, N. J . flectrochem. Soc., in press. (64) Cottreli, F. G. Z . Phys. Chem. 1903, 42, 385-431. (65) Matsuda, H. Bull. Chem. SOC.Jpn. 1980, 53, 3439-3446.
RECEIVED for review December 6,1984. Accepted March 27, 1985. The present work was partially supported by Grantin-Aid for Scientific Research No. 59219008 for N. Oyama, from the Ministry of Education, Science, and Culture, Japan.
Direct Determination of Molybdenum in Seawater by Adsorption Voltammetry Constant M. G. van den Berg
Department of Oceanography, University of Liverpool, Liverpool L69 3BX, England
Complex ions of moiybdenum(V1) with 8-hydroxyqulnoilne (oxine) are shown to adsorb onto the hanging mercury drop electrode. This property forms the basls of a sensitive electrochemical technique by which dissolved molybdenum in seawater can be determined directly. The reduction current of adsorbed complex ions Is measured by differential pulse adsorption voltammetry, preceded by a period of 1 or 2 min of unstlrred collection at an adsorption potential of -0.2 V. I n the presence of 2 X lo-' M oxlne and at pH 2.5 the potential of the main reduction peak is located at -0.59 V. The peak current-molybdenum concentratlon relationship is linear up to 3 X lo-' M; the detection limit is 4 nM. Greater sensitivity is obtained after stlrred collection at pH 3.0 and with lo4 M oxine, but the cailbration curve is nonilnear. I n these conditions the limit of detectlon lies at lo-'' M after 10 min stlrred collection.
The concentration of dissolved molybdenum in seawater typically lies in the range of (1.0-1.3) X M (I). The predominant oxidation state is molybdenum(V1) and at neutral pH it occurs as the anion Mood2-(2,3). Its concentration has variously been determined by using spectrophotometry of its dithiol complex or by atomic absorption spectrophotometry (I) of by adsorption onto Chelex-100 ion exchange resin (4)or Sephadex gel (5). Another technique is based on the catalytic effect of molybdenum on the oxidation of iodide by hydrogen peroxide (6). Electrochemical methods were hitherto not sufficiently sensitive for a direct determination of molybdenum in natural waters, one voltammetric method was based on the formation of mercurous molybdate crystals on a mercury electrode, followed by the reduction of the mercurous ions by a potential scan in negative direction (7); another made use of accumulation by reduction of molybdate to Mooz (solid) onto a mercury electrode, followed by oxidation of the deposit using chronopotentiometric stripping (8). The limits of detection of these techniques were and 5 X lo4 M, respectively.
Recently procedures have been developed to determine the concentrations of various metal ions using adsorption voltammetry (AV) preceded by adsorptive collection of their surface active complexes on a hanging mercury drop. Thus uranium, iron, copper, and vanadium can be determined after addition of catechol (9-12), zinc with aminopyrrolidinecarbodithioate (13), and nickel with dimethylglyoxime as added chelator (14).This technique has alternatively been called adsorption voltammetry (14)and film stripping voltammetry (15),indicating the type of collection, and cathodic stripping voltammetry (9-12) which describes the direction of the potential scan. Preliminary experiments were performed with various complexing ligands in order to investigate their use for the determination of molybdenum using AV. Two reduction peaks were obtained in the presence of gallic acid at pH 4, but it was not possible to separate the two peaks and the peak height concentration relationship was generally nonlinear. Analyticallymore useful results were obtained with the compound 8-hydroxyquinoline(oxine), and these will be reported in this paper. Oxine does not form complexes with molybdenum selectively. It is in fact used as a general complexing agent to preconcentrate metals by extraction procedures. A recent polarographic study has indicated that the reduction wave of molybdenum(V1) as well as of uranium(V1) and bismuth(II1) in an oxine extract in dichloromethane is catalytic in nature (16). It will be shown here that this was caused by adsorption of the Mo(V1)-oxine complex onto the surface of the mercury electrode. This effect will be used to preconcentrate the element onto the electrode followed by its determination using a reductive potential scan. EXPERIMENTAL SECTION
Equipment was the same as before (9-11).The surface area of the HMDE was 2.92 mm2. Potentials are quoted relative to the Ag/AgCl, saturated KC1 reference electrode. A stock aqueous solution of Mo(V1) was prepared by dilution of a BDH standard solution for atomic absorption spectrometry. A stock solution of 0.2 M oxine (BDH, AnalaR) was prepared in 0.45 M HC1 (Aristar) and used without purification. Aristar HCl, 1:l diluted
0003-2700/85/0357-1532$01.50/00 I985 American Chemical Society
