Anal. Chem. 1999, 71, 4056-4060
Hydrodynamic Chronocoulometric Determination of Diffusion Coefficients and Concentrations of Dioxygen in Media Containing Quinoline, Isoquinoline, and Methylquinolines Jian Fei Wu, Yong Che, Takeyoshi Okajima, Futoshi Matsumoto, Koichi Tokuda, and Takeo Ohsaka*
Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
The diffusion coefficients (DO2) and concentrations (CO2) of O2 in O2-saturated solutions (containing 0.1 M tetra(n-butyl)ammonium perchlorate as supporting electrolyte) of quinoline (Q), isoquinoline (IQ), and 2-, 3-, 4-, 6-, 7-, and 8-methylquinolines (2-Q, 3-Q, 4-Q, 6-Q, 7-Q, 8-Q) have been determined using a hydrodynamic chronocoulometry at a rotating disk gold electrode. Significant solvent effects on the values of DO2 and CO2 have been observed. A good linear correlation between the logarithm of mole fraction solubility of O2 (fO2) and the square of the Hildebrand solubility parameter (δH2) was obtained, indicating that O2 is more soluble in a solvent with a smaller solvent-solvent interaction (i.e., smaller δH2). A much higher solubility than expected from the obtained linear relation between log(fO2) and δH2 was obtained for 4-Q, which was explained on the basis of a relatively strong charge-transfer interaction between the O2 molecule and 4-Q solvent. DO2 decreases with increase in the solvent viscosity. In the course of a series of recent studies concerning the electrogeneration, electrochemical detection, and reaction control of reactive oxygen species (ROS) in aqueous and nonaqueous media, we1,2 found that quinoline and methylquinolines are very useful as aprotic nonaqueous solvents, especially in examining the electrode reaction of the O2/O2- (superoxide ion) couple and the electrochemical and physicochemical reactivities of O2-; that is, in these media the one-electron electroreduction of O2 to O2is possible and O2- is chemically stable on the time scale of the usual electrochemical measurements. The solvents usually available for such purposes are few (typically, acetonitrile, dimethyl sulfoxide, N,N-dimethylformamide, and pyridine). To our knowledge, these heterocyclic compounds have not been employed as a solvent for electrolysis except for their use as surfactant molecules on the mercury electrode surface,3-11 and thus, no data concerning the diffusion coefficients (DO2) and concentrations * Corresponding author: (e-mail)
[email protected]; (fax): +81-45924-5489; (tel): +81-45-924-5404. (1) Matsumoto, F.; Tokuda, K.; Ohsaka, T. Electroanalysis 1996, 8, 648. (2) Wu, J. F.; Che, Y.; Okajima, T.; Tokuda, K.; Ohsaka, T., unpublished. (3) Chevalet, J.; Rouelle, F.; Gierst, L.; Lambert, J. P. J. Electroanal. Chem. 1972, 39, 201.
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(CO2) of O2 in these media have been reported yet. A knowledge of DO2 and CO2 is essential in the quantitative analysis of the electrode reaction itself of O2 as well as its electroanalytical application. The aim of the present study is to determine the values of DO2 and CO2 in media containing quinoline, isoquinoline, and methylquinolines. For this purpose, we have successfully applied hydrodynamic chronocoulometry (HCC) at a rotating disk electrode, which allows simultaneous determination of DO2 and CO2 without previous knowledge of either. The theory of HCC and a typical example of its application have been reported by us.12,13 Based on the obtained data of DO2 and CO2, a brief discussion concerning the solvent dependence of solubility and diffusion of O2 is given. EXPERIMENTAL SECTION Materials. The solvents used in this study(quinoline, isoquinoline, 2-, 3-, 4-, 6-, 7-, and 8-methylquinolines, shown in Figure 1) were of reagent grade and were obtained from Tokyo Kasei Co. Ltd. They were purified by distillation when necessary. All the solutions contained 0.1 M tetra(n-butyl)ammonium perchlorate (TBAP) (reagent grade, Tokyo Kasei) as supporting electrolyte. The conductivities of these solutions were estimated to be 2.4 × 10-4, 2.5 × 10-4, 8.2 × 10-5, 8.9 × 10-5, 7.9 × 10-5, 1.2 × 10-4, 2.1 × 10-4, and 5.0 × 10-5 S cm-1 for Q, IQ, 2-Q, 3-Q, 4-Q, 6-Q, 7-Q, and 8-Q, respectively, at 25 °C. The Au working electrodes were prepared by sealing 1.0-mm-diameter Au rod in an insulating tube. The exposed gold surface was polished with 0.3-µm alumina (4) Lipkowski, J.; Buess-Herman, C.; Lambert, J. P.; Gierst, L. J. Electroanal. Chem. 1986, 202, 169. (5) Buess-Herman, C.; Gierst, L. Electrochim. Acta 1984, 29, 303. (6) Buess-Herman, C.; Gierst, L.; Vanlathem-Meuree, N. J. Electroanal. Chem. 1981, 123, 1. (7) Buess-Herman, C.; Franck, C.; Gierst, L. Electrochim. Acta 1986, 31, 965. (8) Quarin, G.; Buess-Herman, C.; Gierst, L. J. Electroanal. Chem. 1983, 148, 79. (9) Ohsaka, T.; Matsumoto, F.; Kitamura, F.; Tokuda, K. Denki Kagaku (Electrochem.) 1993, 61, 763. (10) Matsumoto, F.; Tokuda, K.; Ohsaka, T. J. Deuterium Sci. 1996, 5, 23. (11) Ohsaka, T.; Matsumoto, F.; Tokuda, K. In Frontiers of Reactive Oxygen Species in Biology and Medicine; Asada, K., Yoshikawa, T., Eds.; Excerpta Medica: Amsterdam, 1994; p 91. (12) Tsushima, M.; Tokuda, K.; Ohsaka, T. Anal. Chem. 1994, 66, 4551. (13) Ohsaka, T.; Tsushima, M.; Okajima, T.; Tokuda, K. Denki Kagaku (Electrochem.) 1994, 62, 1300. 10.1021/ac9901992 CCC: $18.00
© 1999 American Chemical Society Published on Web 08/12/1999
Table 1. Kinematic Viscosity Data of Quinoline, Isoquinoline, and Methylquinoline Solutions Measured by Ubbelohde Viscometer at 25.0 ( 0.1 °C solutiona
γ/cm2 s-1 b
solutiona
γ/cm2 s-1 b
Q IQ 2-Q 3-Q
0.033 25 0.036 04 0.042 01 0.038 70
4-Q 6-Q 7-Q 8-Q
0.045 05 0.046 69 0.039 05 0.049 73
a Containing 0.1 M TBAP. b For each solution, the measurements were repeated 3-5 times and the errors were less than 1%.
Figure 1. Quinoline and methylquinolines used in this study: (1) quinoline (Q), (2) isoquinoline (IQ), (3) 2-methylquinoline (2-Q), (4) 3-methylquinoline (3-Q), (5) 4-methylquinoline (4-Q), (6) 6-methylquinoline (6-Q), (7) 7-methylquinoline (7-Q), and (8) 8-methylquinoline (8-Q).
powder (Marumoto Kogyo Co. Ltd.) on a polishing microcloth wetted with Milli-Q water before use. The surface was then carefully sonicated in water, and it was rinsed successively with water, acetone, and finally with the solvent being used. Other chemicals were of reagent grade and were used as received. Apparatus and Procedures. A computer-controlled electrochemical system (BAS 100 B/W) and a rotating electrode system (Nikko Keisoku Co.) were employed in a manner similar to that previously12 used for hydrodynamic chronocoulometric experiments. The electrolysis was carried out in a potential-step mode. The potential was stepped from an initial value, where no current flows, to a value that causes the electrode reaction product to be generated at a mass-transfer-controlled rate. The electrochemical cell was a conventional two-compartment Pyrex glass container with an Au working electrode, a spiral Pt wire auxiliary electrode, and an Ag/AgClO4 (0.01 M) reference electrode. In the measurements in O2-saturated media, O2 gas (99.98%) was bubbled directly into the cell in order to obtain a saturated solution, and during the measurements, O2 gas was flushed over the cell solution. All the electrochemical measurements were performed at room temperature (25 ( 1 °C) The kinematic viscosities (γ) of the solutions containing 0.1 M TBAP were measured using an Ubbelohde viscometer in a thermostated water bath (Coolnics model CTR/CTE-120, Komatsu-Yamato, Japan). The temperature of each solution was kept at 25.0 ( 0.1 °C. The γ values determined for the electrolyte solutions used in this study are summarized in Table 1. RESULTS AND DISCUSSION Determination of DO2 and CO2. Figure 2 shows the typical cyclic voltammograms obtained for the redox reaction of the O2/ O2- couple in each of O2-saturated 0.1 M TBAP solutions. The formal potentials (estimated as the average of the anodic and cathodic peak potentials) for the O2 /O2- couple are in the range of about 0.8-1.0 V. From the separation between the anodic and cathodic peak potentials and the ratio of the anodic peak current to cathodic one, it is obvious that the redox reaction of the O2 /O2- couple is simple and almost quasi-reversible in Q, IQ, 2-Q, 3-Q, and 4-Q media, while in 6-Q, 7-Q, and 8-Q media it is not
Figure 2. Typical cyclic voltammograms obtained for the redox reaction of the O2 /O2- couple at a Au electrode (φ ) 1 mm) in O2saturated solutions (containing 0.1 M TBAP) (A) Q, (B) IQ, (C) 2-Q, (D) 3-Q, (E) 4-Q, (F) 6-Q, (G) 7-Q, and (H) 8-Q. Potential scan rate, 100 mV s-1. The voltammograms are shown in the same potential range of -1.4 to +0.4 V with respect to Ag/AgClO4 (0.01 M).
simple. In the latter case, a few redox reactions other than the redox reaction of the O2 /O2- couple were observed during the potential scan to the positive direction after the reduction of O2 to O2-. The overall electrode reactions may involve the follow-up chemical reaction of the reaction product O2- with the solvent molecules.14 However, from the Levich plots of the steady-state voltammograms for the O2 reduction at a rotating disk electrode in O2-saturated solutions, we confirmed that the reduction process of O2 to O2- is diffusion-controlled in all the media examined. For example, typical steady-state voltammograms for the O2 reduction in the O2-saturated 2-Q solution containing 0.1 M TBAP are shown in Figure 3. As expected, the linear Levich plot passing through the origin was obtained, indicating that the limiting current is controlled by mass transfer of O2 from the bulk of solution to the electrode surface. The typical example of hydrodynamic chronocoulometric curves obtained for the reduction of O2 to O2- in the 2-Q media is shown in Figure 4. A linear regression for each curve was carried out over the electrolysis time domain between 1.2 and 1.5 s, where a steady-state current condition was reached. The (14) Wu, J. F.; Okajima, T.; Tokuda, K.; Ohsaka, T., manuscript in preparation.
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Figure 3. Typical steady-state voltammograms for the reduction of O2 to O2- at a rotating Au disk electrode (φ ) 1 mm) in O2-saturated 2-Q solution containing 0.1 M TBAP. Electrode rotation rate: (1) 200, (2) 400, and (3) 600 rpm. The voltammograms were recorded while the potential was scanned in a negative direction at 5 mV s-1.
Figure 5. (A) iL vs ω1/2 plots and (B) Qδ vs ω-1/2 plots for the reduction of O2 to O2- in various O2-saturated solutions containing 0.1 M TBAP: (3) Q, (×) IQ, (b) 2-Q, (O) 3-Q, (9) 4-Q, (0) 6-Q, ([) 7-Q, and (∆) 8-Q. Figure 4. Typical hydrodynamic chronocoulometric data for the reduction of O2 to O2- at a rotating Au disk electrode (φ ) 1 mm) in O2-saturated 2-Q solution containing 0.1 M TBAP. Electrode rotation rate: (1) 400, (2) 600, (3) 800, (4) 1000, (5) 1200, and (6) 1400 rpm. The potential of the working electrode was stepped from -0.30 to -1.50 V vs Ag/AgClO4 (0.01 M).
is given by
δ ) 1.610 DO21/3γ1/6ω-1/2(1 + 0.2980Sc-1/3 + 0.14514Sc-2/3) (4)
straight line obtained by such a linear regression can be expressed by the following charge (Q)-time (t) relationship:12
Q ) Qδ + iLt
(1)
Qδ ) 0.3764nFACO2δ
(2)
iL ) nFADO2CO2/δ
(3)
with
where δ is the thickness of the hydrodynamic boundary layer and 4058 Analytical Chemistry, Vol. 71, No. 18, September 15, 1999
n is the number of electrons involved in the electrode reaction (in this case n ) 1), F is the Faraday constant, A is the electrode surface area, γ is the kinematic viscosity of the solution, Sc is the Schmidt number (γ/DO2), and ω is the electrode angular velocity (ω ) 2πf, where f is the frequency of the electrode rotation). The slope (iL) and intercept (Qδ) of the Q vs t plots were plotted against ω1/2 and ω-1/2, respectively, as shown in Figure 5A and B, where the data obtained for the media other than 2-Q are also summarized. For every case, both iL vs ω1/2 and Qδ vs ω-1/2 plots were found to be satisfactorily linear and to pass through the origin, as expected from eqs 2 and 3. Therefore, according to the same procedure as described previously,12 the values of DO2 and CO2 (in this study, saturated concentrations) of O2 for all of the
Table 2. Simultaneous Determination of Concentrations and Diffusion Coefficients of O2 in Various O2-Saturated Solutions by Hydrodynamic Chronocoulometry at 25 °C
a
solutiona
CO2/mM
105 DO2/cm2 s-1
Q IQ 2-Q 3-Q 4-Q 6-Q 7-Q 8-Q
1.49 ( 0.13 1.20 ( 0.19 6.52 ( 0.39 5.10 ( 0.56 10.6 ( 0.60 4.60 ( 0.13 4.35 ( 0.24 3.40 ( 0.28
1.71 ( 0.21 0.81 ( 0.05 0.11 ( 0.05 0.28 ( 0.04 0.097 ( 0.007 0.20 ( 0.02 0.31 ( 0.05 0.98 ( 0.19
Table 3. Mole Fraction Solubility of O2 in Various Solutions and Solvent Characteristics
Containing 0.1 M TBAP.
media examined could be estimated and are summarized in Table 2. Solvent Dependence of CO2 and DO2. From the data in Table 2, at first glance we can see that both CO2 and DO2 are highly influenced by the solvent even in a homologous series of methylquinolines. It can be observed, for instance, that CO2 in 4-Q is ∼3 times its value in 8-Q and DO2 in 8-Q is ∼10 times larger than that in 4-Q. The observed solvent dependence of CO2 and DO2 will be briefly discussed below. According to a scaled particle theory,15 the process of dissolution of a gas in a liquid is taken to consist of two hypothetical stages: The first stage consists of the creation of a cavity suitably sized to accommodate a gas in the solvent and the second stage consists of the filling of the cavity with the gas molecules. Thus, the effect of solvents on the solubility may be rationalized by correlating it to parameters characteristic of the solvents. Taft and co-workers16,17 successfully employed a simple two-parameter equation, which relates the Gibbs energy of solution (∆G°) to the Hildebrand solubility parameter (δH)18 and the dipolarity parameter (π*),19,20 to interpret the solvent effect on the free energies of solution of a very wide range of solutes in a conceptually simple manner. Recently, we have also found solvatochromic linear solvation energy relationships for the solubility of O2 in various solvents.21,22 The mole fraction solubility of O2, fO2 which is correlated to ∆G° by the equation ∆G° ) -2.303RT log(fO2) (where R is the gas constant), can be correlated with the solvent-solvent interaction energy required for the formation of the cavity, i.e., the energy required to separate solvent molecules from one another:16,17,19,21-23
log(fO2) ) fO2° + hδH2
(5)
where fO2° and h are constants that characterize O2 solute. The (15) Pierotti, R. A. Chem. Rev. 1976, 76, 717. (16) Abraham, M. H.; Kamlet, M. J.; Taft, R. W. J. Chem. Soc., Perkin Trans. 2 1982, 923. (17) Taft, R. W.; Abraham, M. H.; Doherty, R. M.; Kamlet, M. J. J. Am. Chem. Soc. 1985, 107, 3105. (18) Barton, A. F. M. Chem. Rev. 1975, 75, 731. (19) Kamlet, M. J.; Abboud, J. L. M.; Abraham, M. H.; Taft, R. W. J. Org. Chem. 1983, 48, 2877. (20) Abboud, J. L.; Kamlet, M. J.; Taft, R. W. J. Am. Chem. Soc. 1977, 99, 8325. (21) Che, Y.; Tsushima, M.; Okajima, T.; Tokuda, K.; Ohsaka, T. Anal. Sci. 1997, 13, 1039. (22) Che, Y.; Tokuda, K.; Ohsaka, T. Bull. Chem. Soc. Jpn. 1998, 71, 651.
solutiona
log(fO2)b
Tb/Kc
F/g cm-3 d
V/cm3 mol-1 e
δH2/J cm-3 f
Q IQ 2-Q 3-Q 4-Q 6-Q 7-Q 8-Q
-3.76 -3.85 -3.06 -3.17 -2.85 -3.21 -3.24 -3.34
512.15 515.15 519.15 525.15 533.15 526.15 527.15 524.15
1.095 1.098 1.060 1.070 1.083 1.068 1.072 1.073
117.8 117.5 134.9 133.6 132.0 133.6 133.4 133.3
493 498 440 453 470 454 455 452
a Containing 0.1 M TBAP. b The values of f O2 were calculated from the obtained data (Table 2) of CO2 in each solution (O2-saturated at 1 atm and 25 °C) containing 0.1 M TBAP using the equation relating fO2 to CO2 , fO2 ≈ CO2MW/103F, where MW is the molecular weight of solvent molecule and F is the density of solvent. c Boiling point of pure solvent.27 d From ref 27. e Molar volume of pure solvent. f The square of the Hildebrand solubility parameter, δH ) (-E/V)1/2; E is the molar cohesive energy and is given by -E ) ∆H298 - RT, where R is the gas constant, T is the absolute temperature, and ∆H298 is the molar enthalpy of vaporization at 298 K; ∆H298 (J mol-1) ) -12340 + 99.2Tb + 0.084Tb2.18
Figure 6. Plot of log(fO2) vs δH2 at 25 °C.
data of fO2 and δH2 are summarized in Table 3, together with other physicochemical data of the solvents used. From Figure 6, it is obvious that log(fO2) is very well correlated with δH2 (the correlation coefficient is 0.98 except for 4-Q). That is, fO2 tends to decrease with an increase in δH2 (i.e., the sign of the h in eq 5 is negative), indicating that the solubility of O2 is lower in a solvent with a larger solvent-solvent interaction (i.e., larger δH2), as expected.18,23 The unexpectedly high solubility of O2 in 4-Q is considered to result from the so-called solute-solvent interaction. According to the scaled partial theory,15 this interaction is associated with the second stage in the dissolution of O2 in this solvent, i.e., the filling of the cavity, which is created in the first stage, with O2 molecules. Recently, several groups16,17,21,22,28,29 have successfully interpreted the solvent effect (23) Kamlet, M. J.; Abboud, J. L. M.; Taft, R. W. In Progress in Physical Organic Chemistry; Taft, R. W., Ed.; Wiley: New York, 1980; Vol. 13, p 485. (24) Wu, J. F.; Ohsaka, T., manuscript in prepareation. (25) Robinson, R. A.; Stokes, R. H. Electrolyte Solutions, 2nd ed. revised; Butterworths: London, 1965. (26) Bader, R. F. W.; Henneker, W. H. J. Chem. Phys. 1967, 46, 3341. (27) Scheflan, L.; Jacobs, M. B. The Handbook of Solvents; R. E. Krieger Press: Huntington, NY, 1953.
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on the dissolution of various solutes including O2 by taking into account the exoergic second-stage effects described by some solvatochromic parameters (e.g., π*, a measure of dipolarity/ polarity, R, a scale of hydrogen bond donor acidity, and β, a scale of hydrogen bond acceptor basicity), in addition to the first stage, i.e., the endoergic cavity formation. Unfortunately, such solvatochromic parameters are not available for the present solvents. However, we have found that the charge-transfer interaction between O2 as an electron acceptor and 4-Q solvent as an electron donor is much stronger than that in other solvents. That is, for 4-Q the maximum absorption wavelength (λmax) and the molar absorption coefficient at λmax (max) were obtained as 428 nm and 1.4 M-1 cm-1, respectively, while for other solvents, λmax ) 348372 nm and max ) 0.2-0.4 M-1 cm-1. Thus, such a strong shortrange interaction between the O2 molecule and 4-Q solvent is considered to significantly contribute to the unexpectedly high solubility of O2 in this solvent. The detail will be reported elsewhere.24 The obtained values of DO2 are in a relatively wide range of ∼(0.1-1.7) × 10-5 cm2 s-1 irrespective of the limited number (eight) of the solvents examined. Assuming that the StokesEinstein equation (eq 6) is applicable to the present case, DO2’s were plotted against η.
D ) kBT/cπηrs
(6)
where kB is the Boltzmann constant, c is constant (4 and 6 for the slip and stick boundary conditions, respectively), η is the solvent viscosity, and rs is the radius of diffusing species modeled as a sphere. The decrease in DO2 with increasing η was observed, but (28) Rutan, S. C.; Carr, P. W.; Taft, R. W. J. Phys. Chem. 1989, 93, 4292. (29) Kamlet, M. J.; Carr, P. W.; Taft, R. W.; Abraham, M. H. J. Am. Chem. Soc. 1981, 103, 6062.
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the obtained dependence of DO2 on η was not clear (possibly owing to a limited range of η of the solvents examined). A further quantitative discussion based on eq 6 is limited, because the assumption for the adaptation of eq 6,25 i.e., the diffusing species are big (compare with solvent liquid molecules) spherical objects suspended in liquid, does not apply for the present case. The solvent molecules used (the molecular size of Q, which is the smallest in solvent molecules examined is ∼7 Å × 8.7 Å × 3 Å6) are much larger than O2 molecule (3.18 Å × 3.18 Å × 4.18 Å).26 CONCLUSIONS Hydrodynamic chronocoulometry has been successfully applied to determine DO2 and CO2 in 0.1M TBAP solution of quinoline, isoquinoline, and a series of methylquinolines. Both DO2 and CO2 are found to be largely influenced by the solvent. Especially, the mole fraction solubility of O2, which is estimated from the CO2 data, is in an expected manner very well correlated with the Hildebrand solubility parameter. Based on the present data, further study concerning the electrode reaction of the O2/O2(superoxide ion) couple and the follow-up reaction in the quinoline and methylquinoline media is now under way. ACKNOWLEDGMENT The present work was financially supported by Grant-in-Aids for Scientific Research on Priority Areas, “New Polymers and Their Nano-Organized Systems” (277/1012619) and “Scientific Research (A)” (10305064) from the Ministry of Education, Science, Sports and Culture, Japan and the Katoh Science Foundation, Japan. J.F.W. gratefully acknowledges the Government of Japan for the Monbusho Fellowship. Received for review February 18, 1999. Accepted June 15, 1999. AC9901992
