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Electrocatalytic Oxidation and Liquid Chromatographic Detection of Aliphatic Alcohols at a Nickel-Based Glassy Carbon Modified Electrode Innocenzo G. Casella,’ Tommaso R. I. Cataldi, Anna M. Salvi, and Eli0 Desimoni Dipartimento di Chimica, Uniuersitt5 degli Studi della Basilicata, Via N . Sauro 85, 85100 Potenza, Italy
The electrocatalyticoxidationof aliphaticalcohols in alkaline solutions at a nickel-based chemically modified glassy carbon electrode (Ni-CME) was investigated. The electrode was characterized by voltammetry and flow injection and applied for amperometric detection of mono- and polyhydric alcohols separated by liquid chromatography. A mechanistic electrooxidation model is proposed that involves a Langmuir-typeadsorption process on different sites: NiO and NiOOH for polyhydric and monohydridic compounds, respectively. The process is followed by a hydrogen abstractionfrom the carbon a to the OH group through a radical pathway in the rate-determiningstep. Constantpotential amperometric detection was applied either after an anion-exchange chromatography with alkaline mobile phase or a size exclusion separation with a postcolumn addition of alkaline solution. Examples of applications,which include the separation and detection of simple alcohols present in real samples, are given. INTRODUCTION Although high-performanceliquid chromatography (HPLC) provides efficientseparations of alcoholscontained in complex mixtures, standard refractive index or UV-visible detectors do not allow sufficiently low detection limita. Occasionally, the absence of chromophore and/or fluorophore groups can be overcomeby pre- or postcolumn derivatizationeven though such applications are not absolute and are relatively time consuming.14 Indirect detection methods have been also proposed,B. but problems of baseline instability and the appearance of extraneous peaks, so-called “system peaks”, can seriously affect quantitation. Electrochemicaldetection (ED) in liquid chromatography (LC-ED) is very attractive owing to ita high sensitivity and wide dynamic range. However, the actual analytical performances strongly depend on the electrode material, ita physicochemical state, and the substrates involved. Glassy carbon (GC) is widely used in electroanalyticalapplications.7-8
It is characterized by several interesting features such as good electrical conductivity, large anodic window in several solvents, very low surface porosity and, finally, low and constant residual current. Ita major drawback is due to a rather large overpotential for the oxidation of many organic compounds, which affecta both selectivity and detection limita. Transition metale have been extensively wed aa electrode material for the electrochemicaldetection of many aliphatic compounds.l”l7 Noble metals such as Au and Pt are able to decrease the activation barrier of scarcely electroactive substrate by stabilizationof intermediate free radicals through partially unsaturated surface d-orbitale. However, the strong adsorption that usually occurs on these electrodes leads to fouling of the electrode surface by accumulation of reaction products. In this respect, pulsed amperometric detection techniquea,17-19 which combine cleaning, activation, and detection steps, have been developed to overcome much of the problems. Electrochemical catalysis is very appealing for lowering the potential required to oxidize aliphatic alcohols. Nickel, copper, and cobalt can catalyzethe electrode oxidation procegs through the formation of a high-valent, oxyhydroxide species (NiOOH) which is commonly believed to act as a redox mediator between substrate and electrode. An alkaline medium representa the main requirementto form and stabilize the catalytic species on the electrode surface. Efforta to devise chemically modified electrodes (CMEs) have been a major research goal of recent years, greatly improving the detector capabilities in flow-through analysis.*28 The use of polymeric membranes which operate as size or charge exclueionbarriers has enhanced both selectivity and stability of CMEs.l*12C26 Interesting designs of CME obtained by modifying glassy carbon or carbon powders with metal or metal oxide electrocatalyst have been applied to detect scarcely electroactive analytes such as carbohydrates,
(10)Luo, P.;Zhang, F.; Baldwin, R. P. Anal. Chim. Acta 1991,224, 169-178. (11)Van Effen, R.M.; Evans, D. H. J. Electroanul. Chem. 1979,103, 383-397. (12)Reim, R.E.; Van Effen, R. M. Anal. Chem. 1986,68,3203-3207. (13)Yuan, C. J.; Huber, C. 0. Anal. Chem. 1986,67,180-186. (14)Morrison, T. N.;Schick, K. G.; Huber, C. 0. Anal. Chim. Acta 1980,120,75-80. (16)Neuburger, G. G.; Johnson, D. C. Anal. Chim. Acto 1987,192, 205-213. (16)Vwilyev, Yu. B.; Khazova,0.A,;Nikolaeva, N.N.J. Electroanal. * To whom compondence ahodd be addrcnured. Chem. 1985,196,127-144. (1) Mopper, K. Anal. Chem. 1980,62,2018-2023. (17)Johneon, D.C.;Lacourse, W. R. Anal. Chem. 1990,62,689A(2)Kiba, N.;Mntauhita, R.; Oyamn, Y.; Furusnwa, M. Anal. Chim. 697A. Acta 1991,248,367-370. (18)Bindra, D.5.;Wileon, G. S. Anal. Chem. 1989,61,2588-2670. (3)Berthold, A.;Glick, M.; Winefordner, J. J.Chromatogr. 1990,602, (19)Neuburger, G. G.;Johnson, D. C. Anal. Chem. 1987,69,204-207. 303-316. (20)Prabhu, 5. V.; Baldwin, R. P. Anal. Chem. 1989,61,862-866. (4)Gao, C. X.;Krull, I. S. J. Chromtogr. 1990,616,337-366. (6)h , A .W.M.;Chan,W.H.;Lee,K.W.;Au,L.S.;Choi,W.K.Anal.(21)Murrny, R.W.; Ewing, A. Go;Durst, R.A. Anal. Chem. 1987,69, 379A-390A. Chim. Acta 1990,230,203-206. (22)Wang, J.; Golden, T.; Li, R. Anal. Chem. 1988,60,1642-1646. (6) Schill, 0.;Crommen, J. Trends Anal. Chem. 1987,6,111-116. (23)Santoe, L. M.; Baldwin, R. P. Anal. Chim. Acta 1988,206,86-96. (7)Van Der Linden, W. E.; Dieker, J. W. Anal. Chim. Acta 1980,119, (24)Yamamoto, K.;Park,Y. 5.;Takeoka, S.; Teuchida, E. J. Elec1-24. troanal. Chem. 1991,318,171-181. (8)Deknnaki, A.;Marinkovic, N. S.;Stevanovic, J.; Jnvanovic, V. M.; (26)Witkowski, A,;Brejter-Toth, A. Anal. Chem. 1992,64,636-641. L a w v i c , 2. Vacuum 1990,41,1772-1776. (26)Wang, E.; Liu, A. J. Electroonal. Chem. 1991,319,217-226. (9) Gunaeingham, H.;Tan, C. B. Analyet 1989,114,695-698. 0003-2700/93/03663143$04.0010
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amino acids, oxydryl, and sulfhydryl compounds20PnaO using constant-potential amperometric operation. Electrodes having deposita of high surface area, prepared by electrodeposition of transition metals on graphitic substrates, provide greatly enhanced currents for alcohol oxidation.31 Recently, we found that incorporation of Ni and Co species into the glassy carbon surface makes possible the electrocatalytic detection of several classes of compounds. The use of an inert glassy carbon substrate, on which transition metals can be entrapped by droplet evaporation, offers a simple way to study the catalytic properties of these systems at several stoichiometric ratios, making the field of CMEs very attractive. In this respect, nickel- and cobalt-based CMEs were successfully applied to the determination of mono-, di-, and trisaccharides in real samples.32-34 The electrocatalyticbehavior of Ni- and Co-CMEswas investigated by electrochemical techniques, X-ray photoelectron spectroscopy, and scanning electron microscopy. Continuing the investigation on the possible applications of the Ni-CME, here we report the electrocatalytic detection of aliphatic alcohols. Particular attention was devoted to the elucidation of electrooxidation mechanisms, and some mechanistic models are proposed. Chromatographic separations by either an anion-exchange column with alkaline mobile phase or a size exclusion column with water as mobile phase followed by postcolumn pH adjustment are reported. EXPERIMENTAL SECTION Reagents. Methanol, methan-ds-ol (CDaOH), ethanol, 1,2ethanediol (ethylene glycol), 1,2,&propanetriol (glycerol), 1,2propanediol, 1-butanol,1-pentanol,1,2-propanediol,2,3-butanediol, 2-methyl-2-propanol xylitol, sorbitol, mannitol (Aldrich), and sodium hydroxide (Carlo Erba, Italy) were used as received. All solutions were prepared just prior to use with deionized and double distilled water. Alkaline solutions of sodium hydroxide were protected from carbon dioxide and oxygen by purging with high-puritynitrogen. Experiments were performed by using 0.2 M sodium hydroxide as background electrolyte unless otherwise specified. Apparatus. A Princeton Applied Research (PAR EG&G) potentiostat/galvanostat, Model 273, controlled by the 270 ElectrochemicalAnalysis Software was used for electrochemical measurements. Cyclic voltammetry (CV) was done in a threeelectrode cell using a Ni-CME working electrode, an Ag/AgCl reference electrode (4 M KC1) and a platinum foil counterelectrode. The glassy carbon electrode (4 mm in diameter) used in CV was purchased from PAR. Amperometric measurements in flowing stream were performed using a PAR Model 400 electrochemical detector, and a flow-through thin-layer electrochemical cell consisting of dual glassy carbon working electrodes in serial configuration (MP 1304),Ag/AgC1(4 M KCl) reference electrode, and stainlesssteel auxiliary electrode. The output signal was recorded by an X-t Amel Model 868 recorder. Flow injection and liquid chromatography experimentswere carried out with a Varian 2510 pump equipped with a Rheodyne (Berkeley, CA) Model 7125 injector using a 10-rL sample loop. Postcolumn addition of 0.2 M NaOH was accomplished by a peristaltic pump (Watson-Marlow Model 502 S) and a pulse dampener (SSIModel LP-21) at 0.7 mL/min flow rate. A mixing tee was connected to the exit of the analytical column, and a 5 (27) Prabhu, S. V.; Baldwin, R. P. Anal. Chem. 1989,61,2258-2263. (28) Albert, M. K.; Baldwin, R. P. Anal. Chem. 1985,57, 591-595. (29) Xie, Y.; Huber, C. 0. Anal. Chem. 1991,63, 1714-1719. (30) Cox, J. A.;Dabek-Zlotorzyneka,E. J. Chromatogr. 1991,543,226232. (31) O’Sullivan,E.J. M.;Calvo,E.J. InElectrodeKinetics: Reactiom; Compton, R. G., Ed.;Elsevier: Oxford, UK, 1987; Vol. 27, Chapter 3. (32) Casella, I. G.; Deeimoni, E.; Salvi, A. M. Anal. Chim. Acta 1991, 243.61-63. (33) Casella, I. G.; Desimoni, E.; Cataldi, T. R. I. Anal. Chim. Acta 1991,248, 117-125. (34) Cataldi, T. R. I.; Casella, I. G.; Desimoni, E.; Rotunno, T. Anal. Chim. Acta 1992,270, 161-172.
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POTENTIAL (V vs. Ag/AgCI) Flgurr 1. Cyclic voltammograms of ethanol at NCCME In 0.20 M N a W (-) blank soluton, (- -) 10.0 mM, and (-) 20 mM. Each voltammogram was recorded at 100 mV/s.
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cm x 4.6 mm mixing column fiied with 250-rm silanized beads (Supelco Inc., Bellafonte PA) was inserted between mixing tee and electrochemicaldetectorto enhance the mixingof thealkaline solution with eluent streams with minimal band broadening. Chromatographicseparationsof aliphaticalcoholscompounds were effected either with alkaline mobile phase using an anionexchange column Carbopac PA1 250 X 4 mm i.d. (Dionex), or neutral mobile phase by a size exclusion column Polipore H 220 X 4.6 mm i.d. (Applied Biosystems). A Timberline Instruments Inc. Model H-500 (Boulder, CO) was used as column heater, while thermostatsd electrochemical experimentswere carried out with a circulatingwater bath (Colora Model WK 4 DS, Colora Messtechnik GmbH). Temperature dependencemeasurements were carried out between 0 and 40 O C *0.5 OC. Electrode Preparation. The Ni-CME was prepared, as previouslyreportd,m by depositing 10p L of 50 mM nickel nitrate solution on the surface of the glassy carbon electrode. The electrode was dried in an air oven at 35 OC for 30 min and washed with water; then it was conditioned at 500 mV w AgIAgC1 for the time necessary to obtain a steady-state value of background current. Typical current values after 1 h of conditioning step ranged between 0.2 and 1.0 pA. RESULTS A N D DISCUSSION Electrochemical Measurements. Representative cyclic voltammograms obtainedat the Ni-CME in alkaline solution are reported in Figures 1-3. Well-reproducible cathanodic waves were obtained in blank 0.2 M NaOH solution. As previouslyreported,~~~ the anodic (all and cathodic (c)peaks a t about 0.45 and 0.35 V vs Ag/AgCl, respectively, can be ascribed to the Ni(II)/Ni(III) couple. The anodic peak potential (al) shifted toward the positive direction with
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increasing scan rate, while the cathodic peak was slightly shifted toward a more negative potential. For example, peak potential separations were 85 and 200 mV at 10 and 300 mV/ s, respectively. Moreover, peak width at half-maximum and the charge associated with anodic and cathodic waves relevant to the Ni(II)/Ni(III)redox couple are dependent on scan rate. Notably, the observed separations between anodic and cathodic peaks markedly deviated from a surface-confined electrode process with fast electron-transfer behavior.s*w Additional experiments were performed to evaluate the dependence of the anodic peak current (Inl) on scan rate at modified electrodes having different film thicknesses. Different amounts of nickel oxide were obtained by depositing on the glassy carbon surface 10 p L of 0.25 pM,250 pM, and 50 mM Ni(NO& solution, respectively. Integration of the anodic peak gives the charge associated with the catalytic nickel oxide loading. The three modified electrodes exhibited 6,115, and 300pC of total charge, respectively, in 0.2 M NaOH solution and 10 mV/s scan rate. The results of these experiments are illustrated in Figure 4. For relatively low nickel oxide loadings (curve a, 6 pC of associated charge), the anodic peak current was proportional to the first power of the scan rate, as expected for surface-confined redox species; while at higher catalytic loadings (curve b, 115pC of associated charge), a marked deviation was observed. The same data of curve b of Figure 4A are compared with those obtained at the highest electrode loadings considered here (curve c, 300 pC) in Figure 4B,where they are plotted against the square root of the scan rate. The linear relationship observed for relatively high surface loadings (curves b and c of Figure 4B) suggests a mass-transfer control through nickel oxide film. A diffusion-limited current was also observeds7*afor hydrated nickel electrodes in alkaline media. Ni-CMEs utilized in the present work were always prepared with relativelyhighnickel(11)concentrations. When Ni-CMEs were used in solutions containing a monohydric alcohol such as ethanol (see Figure 11, a new oxidation peak (Epr,)appeared at about 0.55 V while the peak due to Ni(II1) formation (Epr,)was almost unaffected and remained under diffusion control. The magnitude of peak a2 increased on increasingthe alcohol concentration. In contrast to peak al, the anodic peak a2 was proportional to the first power of the scan rate up to 100mV/s. CVs were quite similar for all examined monohydric alcohols, with indication of a (36)Rubimtein, I.; Bard, A. J. J. Am. Chem. SOC.1980,102,68416642. (36)Murray, R. W.In Chemically Modified Electrode; Bard, A. J., Ed.;M. Dekkrr, Inc.: New York, 1984. (37)Robertson, P.M.J. Electroaal. Chem. 1980,Ill, 97-104. (38)Arthur, D.M. J. Electrochem. SOC.1970,117,422-426.
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POTENTIAL (V v s . AgIAgCI) Fl~urr 9. Cyclic vottammograms of NECME in the presence of xylitol: bhnk solutlon (-), (- -) 2.1 mM, and (-e) 4.1 mM. Other experimental condttions as in Figure 1.
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significant reaction only after peak al. Probably the fact that Epnn > Epnl(Figure 1)implies the requisite catalytic state is not fully generated throughout the entirety of peak al. A different behavior was observed for di- and polyhydric alcohols. Figure 2 displays CVs for ethylene glycol at different millimolar concentrations. The anodic peak a1 was slightly shifted to more positive potentials whereas peak a2 exhibited a broader shape. This trend is more evident in the case of xylitol (Figure 3) and other polyhydric compounds where a single and broad oxidation wave was observed. The anodic peak potential, EWl,in the presence of increasing concentrations of ethylene glycol shifted by 2.5 mV/mM, whereas a 16.0 mV/mM slope was evaluated for xylitol. Such a parameter also moved toward positive potential values on increasing the scan rate, while the magnitude of the anodic peak current increased linearly with increasing concentration of analyte up to 12mM. In the case of ethanol, the relationship between anodic peak current and concentration was linear up to 20 mM. This different voltammetric profile between monohydric and polyhydric alcohols is probably associated with differenttransition states involving reactant and catalytic sites. Interestingly, the current observed in CV between 0.7 and 0.8 V vs Ag/AgCl, in the presence of monohydric alcohols (Figure 1)was lower than that observed in blank electrolyte solution,while the opposite situation was observed with xylitol and other polyhydric alcohols (Figure 3). Perhaps two different catalytic sites may be involved with the oxidation of mono- and polyhydric alcohols. Particularly, we suggest that in the case of simple alcohols the catalytic sites are highvalent nickel species (e.g., NiOOH) to great extent covered by adsorbed monohydrid alcohol molecules and/or their reaction products which prevent the adsorption of hydroxyl ions, so a lower concentration of dioxygen is formed at a potential higher than 0.65 V vs Ag/AgCl (see Figures 1-3). This aspect will be considered in more detail below.
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For all aliphatic alcohols investigated, the magnitude of cathodic peak current (Zw) became less pronounced and practically disappeared at relatively high analyte concentrations. Such an effect was even more noticeable when polyhydric alcohols were used as substrate. This behavior likely suggests a slow chemical oxidation of aliphatic alcohols by NiOOH electrochemically generated onto the electrode surface.*e Previously, similar results were obtained for the electrooxidation of reducing and nonreducing sugars in alkaline media.89 No response was observed in CV for all alcohols investigated when an unmodified glassy carbon electrode was employed in place of the Ni-CME. Therefore, the electrode oxidation that occured at the nickel-basedglassy carbon electrode can be attributed to the catalytic activity of Ni(II1) oxyhydroxidegenerated at potentials more positive than 0.4 V vs Ag/AgCl in alkaline solution (0.2 M NaOH). These results indicate that the Ni-CMEs are very efficient as electrocatalysts for the oxidation of aliphatic alcohols and this can be exploited to detect such compounds in flowthrough analysis. Effect of Temperature. As already observed in the case of carbohydrates at Ni and Pt electrodes123@and at a NiCME,93 temperature significantly affects the overall electrooxidation rate. Thus, catalytic oxidation of aliphatic alcohols at a Ni-CME was examined as a function of temperature. It was assumed that the mechanism of electrochemical oxidation involves mainly the following steps: (i) electron transfer at the electrode surface, Ni(II)/Ni(III); (ii) adsorption-desorption on the electrode surface of reactants and products; (iii) chemical oxidation of reactants adsorbed at the catalytic sites. If the adsorption step, which reasonably precedes the chemical oxidation, follows the Langmuir isotherm behavior (seedetails below), the peak current is related to temperature by the equation4@41 d ln(Zp)/d(l/Z')= -(AG,d/R)
- (AGah/R) + &(E
- E o ) / R (1)
where Zp is the anodic peak current evaluated in CV experiments at the Ni-CME, AG,d is the virtual activation energy of the anodic process at E = E O , AGabis the activation energy of the adsorption step, and the third term represents the temperature contribution to the electron-transfer process at the applied electrode potential (E). Other symbols have their usual electrochemical meaning. The (-AG,dR) term, referring to chemical oxidation of reactants by Ni(II1) species present on the electrode surface, is obviously negative in value because the electrochemical reactions are not a spontaneous process (AGmd > 0). The second term, (-AGadJR), which is relative to a spontaneous adsorption process (AG,h < 01, should be positive in value,@since the adsorption activation energy usually has an inverse relationship with temperature.4 Figure 5 illustrates three representative examples of ln Zp vs. 1 / Tplots with sorbitol, ethanol, and glycerol, curves b-d, respectively. Curve a in the same figure represents the plot obtained in blank 0.20 M sodium hydroxide solution. As can be seen, the experimental slope of these plots is negativewithin the temperature range 273-313 K. According to literature information4*the temperature dependence is negligible for (39)Fleischmann,M.;Korinex, K.; Pletcher, D. J.Electroanal.Chem. 1971,31,39-49. (40)Hughes, 5.;Johnson, D. C. Anal. Chim. Acta 1983,149,1-10, (41)Bockris, J. OM.;Conway, B. E. Modern Aspect of Elect~ochemistry; Butterewortha Scientific Pbl.: London, 1954;Chapter 4. (42)Bockris, J. OM.;Reddy, A. K. N. Modern Electrochemistry; Plenum: New York, 1970;Vol. 2, Chapter 8. (43)Bockrie, J. OM.;Conway, B. E.; Yeager, E. Comprehensiue Treatise of Electrochemiatry; Plenum: New York, 1980,Chapter 4.
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Table I. Estimated d ln(Zp)/d(l/T) for Ethanol, Glycerol, and Sorbitol from the Slope of Plots in Figure 8. d Wp)ld(llT) I.b blank electrolyte ethanol (10mM) glycerol (10mM) sorbitol (10mM)
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an electrochemical process under diffusion control. This is proved by the slope of curve a in Figure 5. Taking into account the response obtained for monohydric (ethanol), trihydric (glycerol),and oligohydric (sorbitol)compounds, it seema that the first term of eq 1prevails over the others. These results likely suggest that the electrochemical oxidation of aliphatic alcohols at Ni-CMEs is under chemical control. Furthermore, because experimental values of the slope are not very different between mono- and polyhydric compounds, it is also likely that aliphatic alcoholsreact by similur mechanismsregardless of OH groups present. Results are summarized in Table I. FIA Measurements. Hydrodynamic voltammograms (HVs) of glycerol, ethylene glycol, sorbitol, and ethanol, obtained under flow injection conditions using a thin-layer electrochemical cell, are shown in Figure 6. In 0.2 M sodium hydroxide solution plus and 0.2 mM each compound, the maximum response of the limiting current was found at about +0.5 V vs Ag/AgCl. In the same experimental conditions, similar behavior was reported for mono-, di-, and trisaccharides.33 Note that the catalytic response of ethanol was very low at potential values less positive than 0.4 V. This result is in agreement with CV behavior (see Figure l),where the peak due to ethanol oxidation (a2) appeared only after Ni(111)species were produced, i.e., at potentials more positive than 0.5 V vs Ag/AgCl. In contrast to a simple diffusionlimited process, the HVs at potential values higher than 0.5 V exhibit a decreased electrode response which is similar for all investigated analytea. Since the response is rapidly restored as the potential is lowered, such as effect might be due to an increase of population in the active oxyhydroxide layer of hydroxide radicals (i.e., 'OH), which gives rise to oxygen gas evolution (vide infra). The effect of flow rate on the response of Ni-CME for ethanol, ethylene glycol,and sorbitol, curves a-c, respectively, is illustrated in Figure 7. At an applied potential of +0.50 V, current was found to be inversely related to flow rate.
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Table 11. Molar and Mass Response Factors of Mono- and Polyhydric Alcohole at a Ni-CMEin FIA*
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methanol methan-da-ol ethanol 2-propanol 1-butanol 2-methyl-2-propanol I-pentanol
1.30 0.21 1.01 0.34 1.04 0.21 0.92
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ethyleneglycol 5.23 84.0 1,2-propanediol 3.48 46.0 2,3-butanediol 2.13 24.0 glycerol 14.18 14.99 163.0 93.2 xylitol sorbitol 15.32 84.1 mannitol 14.69 81.3
Applied potential, 0.50 V VB Ag/AgC1; flow rate, 1.0 mL/min; carrier electrolyte, 0.2 M NaOH. 10 ppm injected.
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ethanol, (b) ethyleneglycol, (c)sorbitol, and (d) glycerol. The Injection volume was 10 pL and the electrolyte/carrlerwas 0.20 M NaOH flow rate, 1.O mL/mln; analyte Concentration, 10 ppm. Thin-layer cell with serial electrodes conflguratlon. 4.2
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FLOW RATE, mL/min Figure 7. Dependence of the peak current upon the carrier flow rate In FIA of (a) sorbitol, (b) ethylene glycol, (c)ethanol. Concentrations, 0.20 mM of each compound; applled potential, 0.50 V vs Ag/AgCI; carrier, 0.20 M NaOH.
These plots indicate a stronger dependence between limiting current and flow rate of carrier electrolyte up to about 2.0 mL/min. Such dependences were also directly related to alcohol concentration. Moreover, the flow rate dependence of methanol, ethanol, and 1-pentanol at the same molar concentration (not shown) provides the suggestion of responses related to molecular size. In fact, alcohols having the same number of OH groups but increasing molecular size always show a marked flow rate dependence. These results are in contrast with theoretical expectations based on laminar flow assumptions and may be interpreted in terms of decreased residence time of the analyte on the electrode surface on increasing the flow rate. Compounds having low electrooxidation kinetics need a longer residence time at the electrode surface. Thus, different responses on molecular size for peak current vs flow rate plots are likely associated to adsorption-orientation and/or slow desorption processes. In fact, pronounced peak current broadening is observed at very low flow rates. Considering an adsorption process that takes place prior to the electrochemicaloxidation step, and invoking a Langmuir-isotherm type, the relationship between peak current (Ip)and concentration (C) is
Ip= C/(b + aC) (2) where a and b are specific constants of the electroactive
substrate under investigation.39 Equation 2 applies in the case of relatively low surface coverages where lateral interaction of adsorbed molecules is negligible and there is a fully equivalence of the adsorbing sites. For all alcohols under study, the plots l/Ipvs l/C (not shown) exhibited linear analytical dependence in the concentration range between 0.01 and 3.0 mM with correlation coefficients better than 0.99 in all cases. The relationship may also be used to extend the analytical response range in similar electrodic processes. These results indicate that the assumption of a Langmuirtype alcohol adsorption on the electrode surface is actually a good approximation. In order to provide a clear overview of the electrode capability toward catalytic oxidation of aliphatic alcohols, the molar and mass response factors obtained in FIA of some representative compounds were evaluated. The results are reported in Table 11. In first approximation, the responses are related to the number of OH groups per molecule, reaching a maximum value for glycerol. Response remains nearly constant for glycerol,xylitol, sorbitol, and mannitol,suggesting the onset of some steric hindrance. At 1.0 mL/min flow rate similar molar responses were obtained for monohydric alcohols such as methanol, ethanol, 1-butanol, and 1-pentanol (see Table 11). This is in contrast with the noticeable decrease of response observed at metallic electrodes such as Pt" and Ni.6 For example, the molar response ratio between methanol and 1-pentanolis 1.4and 6.7at the Ni-CME and Pt electrode,M respectively. This different behavior may be explained in terms of (i) greater density of the catalytic sites and (ii) more uniform distribution of the active sites on bulk metal electrodescompared with the Ni-based surface modified glassy carbon electrodes. It appears that a low number of active catalytic sites and a random distribution are features which prevail on an electrode prepared by entrapping catalytic species on the glassy carbon surface. Interestingly, a fractal-like distribution of metal oxide/hydroxide was observed at the Ni-CME by scanning electron microscopy.33 We suggest that on bulk metal electrodes the increase of molecular size may give rise to an increase of electrode area occupied by each compound, so that a greater number of catalytic sites are buried and remain inactive. Consistent with this fact is that the anodic oxidation takes place only on those catalytic sites with favorable chemisorption, while the other ones are poisoned by steric hindrance. This might be the reason for the drastic decrease in response observed with bulk metal electrodes. Previous r e ~ u l t s ~ * @showed ' ~ ~ . ~ ~that the rate-determining step in alcohol oxidation is the hydrogen abstraction from (44)LaCourse, W. R.; Johnson, D. C.; Rey, M. A.; Slingsby, R. W. Anal. Chem. 1991, 63, 134-139. (45) Hui, B. S.; Huber, C. 0. Anal. Chim. Acta 1982, 134, 211-218. (46)Koaka, R.; Terabe, S.; Kuruma, K. J. Org. Chem. 1969,34,13341337. (47) Ocon, P.; Alonso, C.; Celdran, R. Gonzales-Velasco, J. J. Electroanul. Chem. 1986,206,179-196.
the carbon in the a-position with respect to the alcoholgroup. This may suggest strong kinetics effects induced by isotopic substitution of hydrogen by deuterium. Accordingly, the experimental ratio between the catalytic oxidation currents observed for CH30H and CD30H was 6.3 at ambient temperature (see Table 11). Presumably, deuterated methanol undergoes a much slower deuterium abstraction compared to methanol in accordance with the well-recognized isotopic effect. The oxidation currents of some secondary and primary alcohols are also reported in Table 11. It is well-known that for a simple hydrogen abstraction in homogeneous phase the oxidation rate of secondary alcohols is greater than primary alcohols, according to carbon radical stability. Our experimental data show a reversed behavior and suggest the involvement of additional and more important factors which govern the adsorption and the radical dehydrogenation steps. It is also interesting to note that similar results were obtained for diols, so that the electrooxidation rate of 12propanediol is higher compared to 2,3-butanediol. These compoundsdiffer only in a methyl group in place of a hydrogen bound to the a-carbon. Once again, it is suggested that steric hindrance, preferred orientation, molecular dimension, and structure most likely determine the overall oxidation process. Since no hydrogens in the a-position are present for 2-methyl-2-propano1,a different oxidation mechanism needs to be considered. Probably the abstraction of the alcoholic hydrogen (O-H) leads to breaking of the a- and 8-carbon (aC-Cj3) bond and this is made possible by a decrease of the O-H bond dissociation energy following adsorption on the electrode surface.a Alternatively, the oxidation current of tert-alcohols might have a tensammetric nature. Electrocatalytic Oxidation Mechanisms. The overall electrooxidation mechanism of aliphatic alcohols at the NiCME surface has to be consistent with the following experimental evidence: (a) alcohols adsorption onto the electrode surface occurs before electrochemical oxidation and such a process obeys the Langmuir isotherm; (b) in the presence of polyhydric compounds, the single anodic wave is shifted toward more positive values with increasing concentration, while peak potential (Epl))in the case of monohydric alcohols does not change and peak current remains diffusion controlled; (c) monohydric alcohols cause poisoning of the catalytic sites at potentials more positive than 0.65 V vs Ag/AgCl,provoking a more restrained dioxygen evolution; (d) the comparison between current responses of CH3OH and CD3OH is typical of a system under isotopic control; (e)temperature dependence suggests that a chemical process is the rate-determining step; (f) for all tested mono- and polyhydric alcohols the response in FIA decreases at potential values higher than 0.5 V. Therefore, taking into account all these observations and assuming that the mechanism of electrochemical oxidation does not change with temperature increase, within the explored temperature range, the following model can be proposed: Scheme I NiO + OH- + NiOOH + eRCH20H + NiO + NiO(RCH,OH),,
-+
(3) (4)
NiO(RCH,OH),,
+ NiOOH NiO + NiO(R’CHOH),, + H 2 0 (5)
NiO(R’CHOH),,
+ NiOOH
NiO
NiO(RCOH),
+ H 2 0 (6)
At potential values more positive than those relevant to Ni(111) formation, the following process most likely became
predominant: 2NiOOH
-
2Ni0
+ 2.W
-
’/*02 + H20
(7)
in which the amount of adsorption of alcoholic species is less dependent than the density of NiOOH catalytic sites upon applied potential. This means that at 0.6-0.8 V the surface density of ‘OH is relatively high so that a massive oxygen evolution takes place by reaction 7. Our experimental data suggest that the rate-determining step is reaction 5, in general agreement with previous results.% However, Scheme I does not fully explain differencesbetween mono- and polyhydric alcohols;therefore, a different pathway needs to be considered. For example, during positive scan potential in CV (see Figure l), the catalytic oxidation of monohydric alcohols clearly occurs after Ni(II1) formation. Additionally, more reactive catalytic sites are involved for the adsorption of simple alcohols, so a plausible oxidation mechanism may be the following: Scheme I1 NiO + OH- + NiOOH + eR’CH20H + NiOOH NiOOH(R’CH,OH),
+ + + +
(3) (8)
NiOOH(R’CH,OH),, NiOOH NiO NiOOH(R’’CHOH)a~+ H20 (9) NiOOH(R”CHOH),,
NiOOH NiO NiOOH(R’COH),,
+ H20 (10)
in which the monohydric alcohols (R’CH20H) are preferentially and favorably adsorbed onto the NiOOH catalytic site instead of NiO. Although the adsorption processes occur on different catalytic sites (steps 4 and 81, in both schemes the reaction is followed by the cooperative involvement of neighboring NiOOH active sites to dehydrogenate the a-carbon. Alternatively, the observed differences may reflect a different ability of mono- and polyhydric alcohols to replace adsorbed hydroxide radicals from the electrode surface. The rate-determining steps, eqs 5 and 9 for Schemes I and 11, respectively, likely involve the concerted formation of a bridged cyclic intermediate and the abstraction of a hydrogen from the carbon in a-position with respect to the alcoholgroup. Cyclicintermediates were already suggested for the oxidation of l-propanol on Au47 and polyhydric compounds on carbon paste modified with copper oxide.% Primary and secondary aldehydic and a-hydroxyaldehydic compounds, once formed (eqs6 and lo), are further oxidized. At potential values higher than 0.5 V, the high concentration of ‘OH radicals yields a large 02 evolution via eq 7. Calibration, Repeatability, and Detection Limits. Analytical results of some aliphatic alcohols in FIA are summarized in Table 111. Calibration curves are typically nonlinear, but plots (peak current vs concentration) were found linear up to about 100 ppm levels with correlation coefficients of at least 0.996. The precision, estimated in terms of relative standard deviation (5% RSD) by five repetitive injections of a solution containing 10 ppm tested analyte, ranged between 1.6 and 5.0%. The limits of detection (LOD) were evaluated from the slope of the calibration plots and based on the signalto-noise level of 3. Low LOD were obtained for all analytss at an applied potential of +0.50 V vs Ag/AgCl. The detector response, after 50 repetitive injections of a 10 ppm, 2,3butanediol solution over 24 h, exhibited a decreased current response of 2.3 % .
ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1, 1993 S149 d c
Table 111. Flow Injection Analyeie of Mono- and Polyhydric Alcohols at the Ni-CME. LOD linearrange (ppm)* (ppm) P
methanol ethanol %propanol 1-butanol 1-pentanol ethylene glycol 1,2-propanediol 23-butanediol glycerol sorbitol xylitol mannitol
0.01 0.01
100-0.05 100.06 100-1.0
0.50
0.998 0.996
100-1.0 50-0.2
100-0.05 100-0.05
100-0.05 100-0.25 200-0.5
0.50
2.8 2.9 6.6
0.996 0.999 0.999 0.999 0.998 0.996 0.999 0.998 0.999 0.999
100-0.05
0.02 0.02 0.02 0.02 0.02 0.02 0.10 0.20
%RsP
200-1.0
2.5 5.6
4.0
5.0 1.8 1.9
5.1
1.6 1.8
0 Conatant potential amperometric detection at +O.W V vs Ag/ AgCI;flow rate, 1.0 mL/min and canier/electrolyte 0.20 M NaOH. Limit of detection. Relativestandard deviationobtained from five repetitive injections (10 ppm) of samples.
I
2 ELUTION TIME (min)
( 6
4
0
Flgure 9. Liquid chromatogram for a mixture of mono- and polyhydric compounds: (a) ethanol, (b) xylitol, (c) sorbitol, (d) mannitol, and (e) glucose at a NCCME (€& +0.50 V vs Ag/AgCI). Column, Dlonex Carbopac PA1 (250 X 4 mm); lsocratlcelution with 0.20 M NaOH; flow rate 0.5 mL/mln; column temperature, 20 O C .
C
I
1
,
I
1
20
15
10
5
0
ELUTION TIME ( m i n ) I 35
I
I
I
I
I
I
I
30
25
20
15
10
5
0
ELUTION TIME ( m i n ) Flgure 8. Liquid chromatogram of a mixture containing (a) glucose,
(b)sorMtol,(c)xylitol,(d)glycerol,(c)ethyleneglycol,(1)1,2-propanedId, (9)methanol, (h) ethanol,(I) 2-propanol,(I) 1-butanol,and (m) 1-pentanol at NCCME (€*, +OS0 V vs Ag/AgCI): column, Pollpore H (220 X 4.6 mm 1.d.): lsocratlc elution with water as mobile phase; flow rate, 0.3 mL/mln; postcolumn addition of 0.2 M NaOH at 0.7 mL/min; column temperature, 60 O C . The effect of hydroxide concentration on the electrode performances was evaluated by repetitive injections of 10 ppm ethanol, glycerol, and sorbitol. A constant 1.0 M ionic strength medium containing hydroxide ions concentrations ranging between 0.1 and 0.5 M was employed. The peak current increased by a factor of about 1.6 and 2.3 for ethanol and glycerol or sorbitol, respectively. At the same time the background current increased by a factor of 3.3. Since high background currents are generally responsible for less stable electrode signals (i.e., larger oxygen gas evolution occurs) and a high hydroxide concentration requires very efficient mixing in postcolumn operations, 0.2 M NaOH was used to obtain reasonable electrode response and good stability and sensibility. Chromatographic Separations. Figure 8 shows the chromatogram obtained with a standard mixture of aliphatic alcohols by using a size exclusion Polipore H column with water as mobile phase and postcolumn addition of sodium hydroxide, as required according to electrochemical results. Since a t ambient temperature ethanol and ethylene glycol were not well separated from 2-propanol and 1,2-propanediol, respectively,and the polyhydric alcohols sorbitol, xylitol, and
Flguro 10. Liquid chromatogram of commercial brandy dlluted 1 5 0 with water: (a)sugar compounds, (b) glycerol, (c)ethylene glycol, (d) 2,?bbutanedlol,(e) methanol, and (f) ethanol. Experimental conditions as In Flgure 8. mannitol were partially overlapped, the separation was also accomplished at 60 "C. The separation of polyhydric compounds is not appreciably influenced by temperature, likely because these analytes do not have appreciable interactions with the stationary phase. However, Figure 9 shows that their separation can be achieved by an anion-exchange column at ambient temperature with 0.2 M NaOH as mobile phase. AnalyticalApplications. As an example of the analytical performance of the Ni-CME and to show ita simple operation, some alcoholic beverages were analyzed. The only required preliminary treatment was an adequate dilution with the mobile phase. Liquid chromatography of a brandy sample obtained in size exclusion mode with postcolumn addition of NaOH, and a commercial red wine accomplished by anion exchange separation, are shown in Figures 10 and 11, respectively. In all cases original samples were diluted 1:50. Both figures clearly illustrate the performance of the electrode in detecting complex mixtures of alcohols and carbohydrates present in a real matrix. In conclusion, the ability of glassy carbon to incorporate nickel species and the catalytic activity of these species have been exploited for electrochemical detection of aliphatic alcohols in basic media. The Ni-CME seems to offer a promising compromise between background current stability, catalytic activity, low detections limits, and reproducibility. Moreover, the procedure adopted here to modify the electrode surface seems to give rise to a less uniform distribution of
3150
ANALYTICAL CHEMISTRY, VOL. 65, NO. 21, NOVEMBER 1, 1993
n
of catalytic sites caused by steric hindrance of adsorbed molecules. In conjunction with liquid chromatography, the Ni-CME offers good responses and sufficient selectivity. In particular, it seems to represent a convenient sensor for many medium-high molecular weight compounds containing OH groups. Some aspects of the electrooxidation mechanismand the role of stereochemicaleffects are still not fully understood and work is in progress in these directions.
ACKNOWLEDGMENT
I
12
10
8
6
4
2
1
0
This work was carried out with the financial assistance of the Italian National Research Council (C.N.R., Rome) and of Minister0 dell'Universit4 e della Ricerca Scientifica (M.U.R.S.T., Rome). Some results have been obtained with the help of Dr. A. Carlucci, during his undergraduate research training.
ELUTION TIME ( m i n ) Figure 11. Liquid chromatogram of red wlne diluted 1:50 wlth 0.20 M alkaline solution: (a) ethanol, (b) sorbltol, (c) fnannltol, (d) gluocse, and (e) fructose. Experlmental condltlons as In Figure 9.
RECEIVED for review January 25, 1993. Accepted July 19, 1993.'
catalytic sites compared to bulk metal electrodes or massive metal electrodeposition,thereby providing reduced poisoning
* Abstract published in Advance ACS Abstracts, September 1,1993.
