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Glassy carbon surface effects on the electroreduction of aromatic carbonyl compounds
185
J. Electroanal. Chem., 303 (1991) 185-197 Elsevier Sequoia S.A., Lausanne
Glassy carbon surface effects on the electroreduction of aromatic carbonyl compounds Part 1. Benzaldehyde M. Chandrasekaran, Central Electrochemical
M. Noel and V. Krishnan Research Institute, Karaikudi
623006 (Indra)
(Received 7 June 1990; in revised form 15 October 1990)
Abstract
The voltammetric behaviour of benzaldehyde on a glassy carbon electrode (GCE) in DMF and aqueous media is reported and the distinct electrode effect of the GCE when compared with a mercury electrode under identical conditions is discussed. In DMF, on the GCE also, benzaldehyde undergoes one-electron reductive dimerization as in the case of mercury. But even in the presence of protonating agents such as o-cresol and benzoic acid, one-electron dimerization predominates on the GCE whereas two-electron reduction is noticed on mercury. The interaction of benzaldehyde radical anions with acidic functional groups on the GCE leads to a higher surface concentration of radical anions and hence faster dimerization. On a partially polished GCE in DMF as well as on a well-polished GCE in neutral aqueous media, a new time-dependent surface pre-wave is noticed. Evidence indicates that this pre-wave is due to the strong interaction of acidic functional groups with the reactant, benzaldehyde. Constant potential electrolysis indicated the formation of a dimer even in aqueous media. Very strong blocking effects were also noticed on the GCE in alkaline media.
(I) INTRODUCTION
In recent times extensive investigations on the activation of glassy carbon electrodes (GCEs) for the enhancement of electron-transfer rates have been reported [l-5]. Although the formation of oxidized surface functional groups on glassy carbon during the activation-oriented pretreatment has now been confirmed by electrochemical as well as by other surface analytical techniques [6,7], the actual chemical interactions involved in the electron transfer through activated glassy carbon have not yet received any careful treatment. GC is more active in acidic media than in alkaline media [8,9]. The involvement of acidic functional groups such as -COOH and phenolic -OH groups on a GCE is probably connected with this behaviour. The surface acid-base properties have acquired greater importance in 0022.0728/91/$03.50
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recent times in understanding the overall electrocatalytic effects of non-metallic electrode surfaces [lO,ll]. The interesting catalytic effect of a Nafion-coated GCE, which implies enhanced surface acidity [12], has been reported recently. It was hence felt desirable to investigate systematically the electrode effect on a series of aromatic carbonyl compounds of varying basicity on a GC electrode to find out whether the surface acidity of GC has any influence on these carbonyl compounds. In this paper, the electrochemical reduction of benzaldehyde in non-aqueous and aqueous media is reported. Since the electrochemistry of benzaldehyde on a mercury electrode is well known [13-17 and references cited therein] and only one brief report on the voltammetric behaviour on a GCE is available in the literature [18], the present experiments were confined mainly to the GCE although comparison is often attempted with literature data on mercury. (II) EXPERIMENTAL
The glassy carbon electrode (5 mm diameter, Tokai GCA) was fabricated, polished, cleaned and activated through electrochemical cycling procedures as described elsewhere [8,19]. The electrode activity in aqueous solution was evaluated using the cyclic voltammetric response of the ferricyanide-ferrocyanide redox couple in 0.1 M KC1 media [8]. In non-aqueous solvents, the voltammetric response depended greatly on the level of polishing and pretreatment. From a number of experiments it was found that a “perfectly polished” GCE surface for non-aqueous studies was obtained by polishing the GCE with from l/O to 4/O emery paper for 10 n-tin each; washing it with water, trichloroethylene and the solvent supporting electrolyte (SSE) solution; and introducing it into the cell in a wet condition and activating it electrochemically by cycling in the potential region of interest at 10 mV/s for 15 min without adding the compound. An electrode activated according to this procedure produces the voltammetric response of anthracene (An) reported in the literature for mercury [20] and Pt 1211 electrodes in the same potential region. Any electrode not polished to this level (i.e. 40 min) of polishing gives rise to a small decrease in the peak current for the An reduction and a AE, value of the An/An’couple greater than 59 mV. Such electrodes are referred to as “partially polished” electrodes in the present work. An H-type cell with platinum counter-electrode was used for the voltammetric studies. A saturated calomel electrode (SCE) was connected to the working electrode compartment through a KCl-agar Luggin capillary. For cyclic voltammetry in DMF, the Luggin capillary was equilibrated for 1 h in the SSE mixture just before use. The SSE itself was kept in suspended alumina for 24 h and filtered just before use. The working electrode compartment was deaerated with purified nitrogen until dissolved oxygen was completely removed (indicated by the absence of a reduction peak around - 0.8 V). All experiments were carried out at 25 t_ 1” C. 50% ethanolic solutions were used in the aqueous studies. All chemicals were of AR grade. After constant potential electrolysis in aqueous solutions, the electrolyte was ether-extracted and evaporated when unreacted benzaldehyde was either evaporated
187
or converted to benzoic acid. This product was again washed with sodium bicarbonate until benzoic acid was completely removed as indicated by the absence of a carbonyl frequency in the infrared (IR) spectra. The melting point of the resulting sample was 132 o C, which is close to the literature value for the hydrobenzoin of the meso-isomer. The product hydrobenzoin was further confirmed from the IR spectra. (III) RESULTS
AND
DISCUSSION
(III. I) Electroreduction
of benzaldehyde
in DMF
In aprotic solvents like DMF, benzaldehyde is generally reduced in a one-electron step and the product dimerizes, leading to the formation of hydrobenzoin on a mercury electrode [15]: RCHO+e-
RCHO --
TABLE
e
R-CHO’-
rds
-
(1)
ooI I R-CH-CH-R
1
Electrochemical
behaviour
of benzaldehyde
Medium
in aprotic
GCE a - Ep,z /V
I. 0.1 M TBAI/DMF DMF Additives (a) o-Cresol (10 mM) (b) Benzoic acid (9.7 mM) II. 0.1 MH,SO,/SO% III. 0.1 M TEATS/SO’%
and protic
EtOH EtOH
$,/PA
media Hg - El,2
Other solid electrodes -
4,2/V
1.790 2.650
225 _
1.78 b 2.67
1.857 = (Pt) _
1.700 1.500
192 141
-
_ _
1.090
162
0.96 d
0.900 e (Au/H&
1.505 1.155
140 33
1.45 -
f
1.600 e (Au/H@ _
212
1.592 s
(pre-wave) IV. 0.1 M TBAH,’ 50% EtOH
1.535
a Benzaldehyde concentration: 4.7 mM; sweep rate: 40 mV s-l. sulphonate; TBAH: tetra-n-butyl ammonium hydroxide. b DMF+TBAI [14]. ’ TBAP+ sulfolane 1161. d 1 M H,SO, [22]. e 1 M H,SO,, pH 6.8, phosphate buffer [25]. ’ 48% EtOH, pH 8.6 [23]. g TBAH+50% EtOH [24]. ’ 0.5 M NaOH [25].
TEATS:
1.500 ’ (Au/H& tetraethylammomium
p-toluene
188
Also on a glassy carbon electrode we observe a single irreversible one-electron wave. The peak potential of this wave is quite close to the reduction potential on a mercury electrode (Table 1) [14,16,22-251. The assumption of a one-electron reduction on the GCE, as in the case of mercury, is further supported by the appearance of a second reduction wave at more negative potentials as well as by the appearance of an anodic peak related to the first cathodic peak at high sweep rates. In the voltammogram on the GCE, the blocking/adsorption effects show up in the form of a continuously decreasing peak current function (~,/Acv’/~) with the sweep rate, a negative shift of E, with increasing concentration of benzaldehyde, and unusual % - %/z values as well as d E,/d log v values. Such effects are indeed predicted for blocking effects on solid electrodes [26,27]. Because of these features, simple qualitative methods developed for analysing the voltammetric response of an EC reaction scheme could not, strictly speaking, be employed for determining the “n ” value and other relevant parameters. Controlled potential coulometry could not be employed either, primarily because of the passivation effect of adsorbed films on the GC surface. In spite of all these difficulties, if we assume that the adsorption effects are absent or at least marginal at very low benzaldehyde concentrations, some quantitative calculations can be made. Using the peak current constant values obtained at low concentrations and a diffusion coefficient value of 1.44 X lo-’ cm* s-i [28], the n values were found to be 1.2-1.3 or approximately 1, suggesting a dimerization process. The dE,/d log v value at low benzaldehyde concentrations is found to be 22 mV/decade, which is close to the value 19.4 mV predicted for the radical-radical dimerization mechanism represented by eqn. (2) [17]. If this mechanism is assumed, the ratio of the anodic to the cathodic peak current can be used to evaluate the
O-8
1.0
1.2
1.L
1.6
1.8
2.0
2.2
24
2-5
-E/V vs SCE Fig. 1. Effect of o-cresol as the proton donor on the reduction of benzaldehyde on a GCE in 0.1 M TBAI + DMF at 40 mV s-‘. 4.7 mM benzaldehyde+ x mM o-cresol. x: (a) 0; (b) 2.7; (c) 5.3; (d) 7.8; (e) 10.
189
dimerization constant [29]. The average dimerization rate constant value of 4.4 x lo3 mol-’ s-i obtained is quite comparable to the value obtained on a mercury electrode under similar conditions. All these facts indeed support the view that the electrochemical process on a “perfectly” cleaned GCE is the same as that on a mercury electrode, except for the fact that adsorption and blocking effects are more prevalent on the solid (GCE) electrode, as would be expected. However, in contrast it is noted that protonating agents have very different effects on a GCE when compared with a mercury electrode. Neither reaction (1) nor reaction (2) seems to be affected by the addition of small quantities (say, mM) of water, as shown by the insensitivity of the voltammetric response to the addition of water as in the case of mercury [13]. In the presence of o-cresol, however, the reduction potential on the GCE is shifted positively (Fig. l), probably due to simultaneous protonation together with electron transfer (eqn. 3, where BH+ refers to o-cresol). RCHO+e-+BH++RdHOH+B
(3)
The peak current in the presence of o-cresol actually decreases slightly, suggesting that the protonation in the present case on the GCE does not enhance further reduction of the radical anion leading to the formation of benzyl alcohol through an overall two-electron ECE mechanism. This is in contrast to the electroreduction on mercury, where the peak current increases with the o-cresol concentration, suggesting benzyl alcohol formation [13,30]. Benzoic acid (BH+), being a stronger protonating agent, actually protonates the reactant molecule itself through direct protonation. RCHO+BH+=RdHOH+B
(4)
Since the protonated species is easily reducible compared with benzaldehyde itself, a separate reduction wave at more positive potentials (Fig. 2) is noticed. The wave height due to the protonated species increases with the benzoic acid concentration, as would be expected. The total wave height of the two waves does not increase but rather decreases slightly with the benzoic acid concentration. This again is in contrast to the behaviour on mercury, where the total wave height increases due to simultaneous two-electron ECE reduction leading to partial formation of alcohol [30]. The protonating agents thus do show their effect on the GCE as in the case of mercury. However, on the GCE, the dimerization process of both the protonated and the unprotonated intermediate proceeds much faster than further reduction to alcohol. Hence no protonating agent is able to change the dimerization pathway to the ECE reaction pathway. A similar faster reductive dimerization of diacetyl on a GCE when compared with mercury was reported earlier [31]. In the case of the oxidation of azide anions as well, a faster dimerization on graphite, when compared with a Pt electrode, was reported [32]. At this stage, it is quite difficult to pin-point the cause of such observations. However, one would conclude from these observations that the radical
190
04
1.0
1.2 -E/V
1.4 vs
1.6
1.6
2.0
2.2
SCE
Fig. 2. Effect of benzoic acid as the proton donor on the reduction of benzaldehyde on a GCE in 0.1 M TBAI + DMF at 40 mV s-‘. 9 mM benzaldehyde+ x mM benzoic acid. x: (a) 0; (b) 1.8; (c) 3.5; (d) 5.1; (e) 6.7; (f) 8.2.
species is held loosely on the carbon surface for sufficiently long time intervals so that they can interact mutually and form the dimer product. The interaction in this case should be confined to the reactive carbonyl centre and not to the aromatic ring, since this type of faster dimerization when compared with the ECE pathway is noticed for benzaldehyde (this work) as well as for diacetyl [31], which is an aliphatic compound. Since carbonyl compounds are generally basic in nature, the acidic functional groups present on the GCE are likely centres for the radical ,adsorption and stability. (III.2)
Surface effecti in DMF
Even a “perfectly” polished GCE can still contain some small concentrations of surface functional groups [5]. In the previous section, we were dealing with such a low level concentration of surface functional groups. The concentration of these surface functional groups may be increased by partial or incomplete polishing, or by chemical or electrochemical activation. Let us denote the surface functional groups by a common formula, namely S-COOH, always remembering that this can denote any other functional group (such as the quinonoic or phenolic group) which can show acid-base properties. These surface functional groups can serve as protonating agents for benzaldehyde. S-COOH
+ RCHO + S-COO.
. . . ZH(OH)R
(5)
This type of surface protonated species can be reduced easily when compared with RCHO itself. Hence whenever such a surface protonation becomes possible, one should notice a pre-wave before the main reduction peak.
191
0.6
1.0
1.2
1.L
-E/V
vs
1.6
1.8
2.0
2.2
SCE
Fig. 3. Cyclic voltammograms for the reduction of benzaldehyde on a GCE (partially polished) in 0.1 M TBAI + DMF at various sweep rates in 4.7 mM benzaldehyde. u/mV s-‘: (a) 10; (b) 20; (c) 40; (d) 80; (e) 160; (f) 320.
Since a “perfectly” polished GCE contains a very low concentration of surface functional groups, such pre-waves are not seen on these electrodes. On a partially treated GCE, additional S-COOH groups lead to pre-waves (Fig. 3). The similarity between the pre-wave on a partially cleaned GCE (Fig. 3) and the pre-wave in the presence of benzoic acid on a clean GCE (Fig. 2) would further substantiate this viewpoint. To our knowledge, such a type of pre-wave has not been reported so far for carbonyl compounds for mercury or any other electrode in the absence of protonating agents. If very low concentrations are employed, the entire reduction process can proceed essentially through the surface protonation route, giving rise to a single wave in the positive potential region alone. Below the 1 mM concentration range of benzaldehyde, such a situation is indeed observed on the GCE (Fig. 4). In this case, one notices a reduction wave around - 1.4 V (close to the pre-wave in Fig.
192
0.8
1.0
1.2
14
-E/V
vs
1.6
1.8
2.0
2.2
SCE
Fig. 4. Cyclic voltammograms for the reduction of benzaldehyde on a GCE (partially polished in 0.1 M TBAI+DMF at a low concentration at various sweep rates. Benzaldehyde concentration: 1.0 mM. u/mV s-l): (a) 20; (b) 30; (c) 40; (d) 50; (e) 60; (f) 80; (g) 100; (h) 120; (i) 140; (j) 175; (k) 220; (I) 250; (m) 350; (n) 450; (0) 550; (p) 650.
3) and the main reduction wave around - 1.6 V (in Fig. 3) is totally absent. The peak current also increases steadily with the sweep rate. The surface protonation model presented here is quite different from the surface protonation reactions by water molecules and weak acids suggested earlier for benzaldehyde [33] and other electro-organic [34] reductions. Protonation by surface functional groups was suggested in recent work from this laboratory [35] pertaining to the reduction of benzophenone on a GCE. This surface protonation model is also quite similar to the one proposed to explain similar pre-waves in Nafion-coated GCEs for different organic molecules of lesser basicity [12]. (III.3)
Electroreduction
of benzaldehyde
in aqueous media
Benzaldehyde shows a reduction peak around - 1.0 V in aqueous acids and around - 1.5 V in neutral and alkaline pH regions on both GCEs and mercury electrodes (Table 1). However, in neutral pH regions, the GCE alone shows an additional surface wave at more positive potentials (see Section 111.4). Although the main reduction peak on the GCE as well as on Hg is noticed in the same potential region as reported earlier [18], the voltammetric responses on a GCE in acid medium show strong blocking effects [26,27]. The peak current constant of the reduction wave decreases quite substantially with increasing sweep rate and concentration. The peak potential shifts negatively with increasing concentration of benzaldehyde. The E, - Ep,2 and dE,/d log v values vary with the sweep rate and concentration of benzaldehyde. The blocking effect is noticed even more predominantly on a GCE in TBAH media (Fig. 5). At fairly high sweep rates and at high concentrations (Fig. 5), the
193
1-o
1.1
1.2
1.3
1.L
1.5
1.6
1.7
-E/V vs SCE Fig. 5. Cyclic voltammograms for the reduction of benzaldehyde on a GCE in 0.1 M TBAH + 50% EtOH at 40 mV s-‘. Benzaldehyde conzentration: (a) 1.0 mM; (b) 2.0 mM; (c) 2.9 mM; (d) 3.8 mM; (e) 4.7 mM; (f) 5.6 mM; (g) 6.5 mM; (h) 7.3 mM; (i) 8.2 mM.
voltammetric peak turns into a wave and the peak current becomes almost independent of the sweep rate as well as of the benzaldehyde concentration. Since this effect was noticed only on a GCE and not in the earlier studies on mercury [17], as well as in some specific experiments carried out during the present work on Hg-coated Pt electrodes, this type of behaviour should be due to the surface blocking effect and not to the slow preceding chemical reaction. Because of the interference due to the blocking effect discussed above, it was impossible to determine the value of n by voltammetric methods. However, constant potential electrolysis in acid, neutral and alkaline media indicated clearly that around the peak potential corresponding to the main wave, the one-electron reduction product hydrobenzoin alone was formed. The more or less identical i,/Acv’12 value obtained at low concentrations and sweep rates in all of these media also suggests that the reduction proceeds by a one-electron route. (III.4)
Surface effects in aqueous media
As noted in the previous section, benzaldehyde reduction on a GCE in neutral aqueous media is always associated with a pre-wave (Fig. 6A). Such a pre-wave again has neither been reported earlier on Hg, nor observed under identical conditions in the present work on a Hg-plated Pt electrode (Fig. 6B). Unlike the surface-sensitive pre-wave observed in DMF media (see Section 111.2), the surface wave observed in neutral aqueous systems is highly reproducible. These waves cannot be eliminated by any amount of surface polishing and pretreatment. The
1.2
14
-E/V
I.0
1-2
14 -E/V
1.6
Vs SCE
1.6
Vs SCE
Fig. 6. Cyclic voltammograms for the reduction of benzaldehyde on a GCE (A) in 0.1 M TEATS + 50% EtOH at various sweep rates. Concentration: 4.7 mM. v/mV sell: (a) 20; (b) 40; (c) 80; (d) 160; (e) 320. (B) On mercury-platelet platinum. u/mV s-‘: (a) 10; (b) 20; (c) 40; (d) 80; (e) 160.
voltammograms taken at different concentrations and sweep rates (Fig. 6) are quite reproducible with respect to the main wave and pre-wave. However, one particular condition should always be satisfied to obtain highly reproducible results. Between each voltammetric sweep there must be a sufficient time gap (say 3-4 min). This is probably due to the long time-scales required for the surface process causing this effect to attain a steady state or equilibrium (see later). This slow nature of the surface process involved is also evident from the multi-sweep cyclic voltammogram shown in Fig. 7. In this figure, the pre-wave is noticed only in the first sweep and is practically absent in all the subsequent sweeps. Some detailed investigations on this
195
1.0
1.1
1.2 -E/V
1.3 vs
1.4
1.5
1.6
1.7
SCE
Fig. 7. Cyclic voltammograms for the reduction of benzaldehyde on a GCE in 0.1 M TEATS + 50% EtOH in a multi-sweep at 40 mV s-l. Concentration: 2.9 mM. The numbers on the curves indicate the sweeps.
type of time effect have been reported for the voltammetric behaviour of benzophenone under identical conditions [35]. The most striking feature about the pre-wave is the fact that these waves appear only in aprotic solvents and in neutral pH regions in aqueous solvents. It is this fact which lends strong support to the view that the surface acid-base groups are the most probable catalytic centres leading to the observation of the pre-wave. At acidic pHs, the carbonyl compound can undergo protonation in the bulk and hence the surface protonating agents will not have any effect. Under alkaline pH conditions, the surface groups would also exist in the basic form (-COOinstead of -COOH, say) and so again no catalytic effect would be noticed. Only in aprotic solvents and neutral aqueous media are both criteria of the presence of acidic groups on the electrode surface and of unprotonated carbonyl groups in the solution satisfied, and hence the appearance of surface catalytic pre-waves in these two conditions alone. The acidity of the functional groups present on the GCE can also depend on the solvent employed. This is probably responsible for the sensitivity of the pre-wave to the surface pretreatment in DMF alone (see Section 111.2). Within the concentration ranges investigated, the pre-wave is independent of the rather than i,/v is constant. In fact, concentration. It is also noticed that i,/v”*
196
for the surface process involving a submonolayer level of acid-base interactions considered here, i,/v would be expected to be a constant. The cause of this unexpected behaviour is not quite clear. Although i,/v ‘/* is constant, this process is not a diffusion-controlled process. If this were the case, one would expect a pre-wave in all of the sweeps of the multi-sweep experiment (Fig. 7). The peak current would also be expected to increase with the concentration of benzaldehyde. Finally, this pre-wave also cannot be due to the “product adsorption” waves discussed extensively in the polarographic literature [36]. Such adsorption-desorption processes are quite fast equilibrium processes. In contrast, the surface protonation reaction (eqn. 5) suggested here is highly time-dependent. (IV) CONCLUSIONS
On a “perfectly” polished GC electrode, benzaldehyde in DMF is reduced in the same potential region where this process takes place on a mercury electrode. However, some blocking effects quite common to solid electrodes are also noticed. Protonating agents interact in the usual way with the radical anions that are formed on the GCE. However, the protonating agents do not enhance the process of alcohol formation (two-electron process) on the GCE, as in the case of a mercury electrode. A partially treated GCE shows an additional pre-wave for the reduction of benzaldehyde. The pre-wave is quite sensitive to the pretreatment. The available evidence suggests that the surface acid functional groups are responsible for this pre-wave. In aqueous media also, the main reduction peaks on the GCE as well as on mercury are observed in the same potential region. However, the blocking effects on the GCE are even more predominant in aqueous media when compared with non-aqueous media. The voltammetric response in TBAH in particular is quite different from a diffusion-controlled process. Constant potential electrolysis around the main peak potential confirms the formation of hydrobenzoin. In non-aqueous media, the pre-wave is sensitive to the surface pretreatment procedures. In aqueous solutions, this pre-wave is observed only in neutral media. This fact together with all the other experimental evidence suggests that a slow time-dependent surface protonation process is responsible for the pre-wave formation on a glassy carbon electrode. REFERENCES 1 2 3 4 5 6 7
1.F. Hu, D.H. Kanveik and T. Kuwana, J. Electroanal. Chem., 188 (1985) 59. J. Wang and L.D. Hutchins, Anal. Chim. Acta, 167 (1985) 325. T. Nagaoka and T. Yoshino, Anal. Chem., 58 (1986) 1037. M. Poon and R.L. McCreery, Anal. Chem., 58 (1986) 2745. G.N. Kaman, Anal. Chim. Acta, 207 (1988) 1. R. Engstrom and A.V. Strasser, Anal. Chem., 56 (1984) 136. L.J. Kepley and A.J. Bard, Anal. Chem., 60 (1988) 1459.
197 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
M. Noel and P.N. Anantharaman, Analyst, 110 (1985) 1095. M.R. Deakin, K.J. Stutts and R.W. Wightman, J. Electroanal. Chem. 182 (1985) 113. M. Noel, Bull. Electrochem., 5 (1989) 393. M. Noel and K.I. Vasu, Cyclic Voltammetry and Frontiers of Electrochemistry, Aspect Publ. Ltd., London, 1990. MS. Mubarak and D.G. Peters, J. Electroanal. Chem., 273 (1989) 283. P.H. Given and M.E. Peover, J. Chem. Sot., (1960) 385. S. Wawzonek and A. Gundersen, J. Electrochem. Sot., 107 (1960) 537. W.R. Fawcett and A. Lasia, Can. J. Chem., 59 (1981) 3256. N.R. Armstrong, R.K. Guinn and N.E. Vanderborgh, Anal. Chem., 46 (1974) 1759. L. Nadjo and J.M. Saveant, J. Electroanal. Chem., 33 (1971) 419. J. Weber and J. Volke, Electrochim. Acta, 24 (1979) 113. M. Chandrasekaran, Ph.D. thesis, Madurai-Kamaraj University, Madurai, 1989. H.O. House, D. Koepsell and W. Jaeger, J. Org. Chem., 38 (1973) 1167. D.J. Freed and L.R. Faulkner, J. Am. Chem. Sot., 93 (1971) 2097. R. Kalvoda and G.K. Budnikov, Collect. Czech. Chem. Commun., 28 (1963) 838. R. Pastemak, Helv. Chim. Acta, 31 (1948) 753. J.W. Hayes, I. Ruzic, D.E. Smith, G.L. Booman and J.R. Delmastro, J. Electroanal. Chem., 51 (1974) 269. L.N. Nekrasov and A.D. Korsun, Elektrokhimiya, 4 (1968) 539, 996. C. Amatore, J.M. Saveant and D. Tessier, J. Electroanal. Chem., 146 (1983) 37. C. Amatore, J.M. Savtant and D. Tessier, J. Electroanal, Chem., 147 (1983) 39. L. Meites, Polarographic Techniques, Wiley, New York, 1965. M.L. Olmstead, R.G. Hamilton and R.S. Nicholson, Anal. Chem., 41 (1969) 261. V.P. Gultyai, I.V. Proskrovskaya and T.Ya. Rubinskaya, Tezisy Dokl. Seveshch. Polyarogr., 7 (1978) 14. S. Thangavelu, M. Noel and KS. Udupa, Electrochim. Acta, 31 (1986) 1563. V. Plazek and H. Wendt, J. Electroanal. Chem., 180 (1984) 185. S.G. Mairanovskii, Sov. Electrochem., 19 (1983) 383. S.G. Mairanovskii, Catalytic and Kinetic Waves in Polarography, Plenum Press, New York, 1968. M. Chandrasekaran, M. Noel and V. Krishnan, Bull. Electrochem, 2 (1986) 469. E. Laviron, J. Electroanal. Chem., 35 (1972) 333.
J. Electroanal. Chem., 303 (1991) 185-197 Elsevier Sequoia S.A., Lausanne
Glassy carbon surface effects on the electroreduction of aromatic carbonyl compounds Part 1. Benzaldehyde M. Chandrasekaran, Central Electrochemical
M. Noel and V. Krishnan Research Institute, Karaikudi
623006 (Indra)
(Received 7 June 1990; in revised form 15 October 1990)
Abstract
The voltammetric behaviour of benzaldehyde on a glassy carbon electrode (GCE) in DMF and aqueous media is reported and the distinct electrode effect of the GCE when compared with a mercury electrode under identical conditions is discussed. In DMF, on the GCE also, benzaldehyde undergoes one-electron reductive dimerization as in the case of mercury. But even in the presence of protonating agents such as o-cresol and benzoic acid, one-electron dimerization predominates on the GCE whereas two-electron reduction is noticed on mercury. The interaction of benzaldehyde radical anions with acidic functional groups on the GCE leads to a higher surface concentration of radical anions and hence faster dimerization. On a partially polished GCE in DMF as well as on a well-polished GCE in neutral aqueous media, a new time-dependent surface pre-wave is noticed. Evidence indicates that this pre-wave is due to the strong interaction of acidic functional groups with the reactant, benzaldehyde. Constant potential electrolysis indicated the formation of a dimer even in aqueous media. Very strong blocking effects were also noticed on the GCE in alkaline media.
(I) INTRODUCTION
In recent times extensive investigations on the activation of glassy carbon electrodes (GCEs) for the enhancement of electron-transfer rates have been reported [l-5]. Although the formation of oxidized surface functional groups on glassy carbon during the activation-oriented pretreatment has now been confirmed by electrochemical as well as by other surface analytical techniques [6,7], the actual chemical interactions involved in the electron transfer through activated glassy carbon have not yet received any careful treatment. GC is more active in acidic media than in alkaline media [8,9]. The involvement of acidic functional groups such as -COOH and phenolic -OH groups on a GCE is probably connected with this behaviour. The surface acid-base properties have acquired greater importance in 0022.0728/91/$03.50
0 1991 - Elsevier Sequoia S.A
186
recent times in understanding the overall electrocatalytic effects of non-metallic electrode surfaces [lO,ll]. The interesting catalytic effect of a Nafion-coated GCE, which implies enhanced surface acidity [12], has been reported recently. It was hence felt desirable to investigate systematically the electrode effect on a series of aromatic carbonyl compounds of varying basicity on a GC electrode to find out whether the surface acidity of GC has any influence on these carbonyl compounds. In this paper, the electrochemical reduction of benzaldehyde in non-aqueous and aqueous media is reported. Since the electrochemistry of benzaldehyde on a mercury electrode is well known [13-17 and references cited therein] and only one brief report on the voltammetric behaviour on a GCE is available in the literature [18], the present experiments were confined mainly to the GCE although comparison is often attempted with literature data on mercury. (II) EXPERIMENTAL
The glassy carbon electrode (5 mm diameter, Tokai GCA) was fabricated, polished, cleaned and activated through electrochemical cycling procedures as described elsewhere [8,19]. The electrode activity in aqueous solution was evaluated using the cyclic voltammetric response of the ferricyanide-ferrocyanide redox couple in 0.1 M KC1 media [8]. In non-aqueous solvents, the voltammetric response depended greatly on the level of polishing and pretreatment. From a number of experiments it was found that a “perfectly polished” GCE surface for non-aqueous studies was obtained by polishing the GCE with from l/O to 4/O emery paper for 10 n-tin each; washing it with water, trichloroethylene and the solvent supporting electrolyte (SSE) solution; and introducing it into the cell in a wet condition and activating it electrochemically by cycling in the potential region of interest at 10 mV/s for 15 min without adding the compound. An electrode activated according to this procedure produces the voltammetric response of anthracene (An) reported in the literature for mercury [20] and Pt 1211 electrodes in the same potential region. Any electrode not polished to this level (i.e. 40 min) of polishing gives rise to a small decrease in the peak current for the An reduction and a AE, value of the An/An’couple greater than 59 mV. Such electrodes are referred to as “partially polished” electrodes in the present work. An H-type cell with platinum counter-electrode was used for the voltammetric studies. A saturated calomel electrode (SCE) was connected to the working electrode compartment through a KCl-agar Luggin capillary. For cyclic voltammetry in DMF, the Luggin capillary was equilibrated for 1 h in the SSE mixture just before use. The SSE itself was kept in suspended alumina for 24 h and filtered just before use. The working electrode compartment was deaerated with purified nitrogen until dissolved oxygen was completely removed (indicated by the absence of a reduction peak around - 0.8 V). All experiments were carried out at 25 t_ 1” C. 50% ethanolic solutions were used in the aqueous studies. All chemicals were of AR grade. After constant potential electrolysis in aqueous solutions, the electrolyte was ether-extracted and evaporated when unreacted benzaldehyde was either evaporated
187
or converted to benzoic acid. This product was again washed with sodium bicarbonate until benzoic acid was completely removed as indicated by the absence of a carbonyl frequency in the infrared (IR) spectra. The melting point of the resulting sample was 132 o C, which is close to the literature value for the hydrobenzoin of the meso-isomer. The product hydrobenzoin was further confirmed from the IR spectra. (III) RESULTS
AND
DISCUSSION
(III. I) Electroreduction
of benzaldehyde
in DMF
In aprotic solvents like DMF, benzaldehyde is generally reduced in a one-electron step and the product dimerizes, leading to the formation of hydrobenzoin on a mercury electrode [15]: RCHO+e-
RCHO --
TABLE
e
R-CHO’-
rds
-
(1)
ooI I R-CH-CH-R
1
Electrochemical
behaviour
of benzaldehyde
Medium
in aprotic
GCE a - Ep,z /V
I. 0.1 M TBAI/DMF DMF Additives (a) o-Cresol (10 mM) (b) Benzoic acid (9.7 mM) II. 0.1 MH,SO,/SO% III. 0.1 M TEATS/SO’%
and protic
EtOH EtOH
$,/PA
media Hg - El,2
Other solid electrodes -
4,2/V
1.790 2.650
225 _
1.78 b 2.67
1.857 = (Pt) _
1.700 1.500
192 141
-
_ _
1.090
162
0.96 d
0.900 e (Au/H&
1.505 1.155
140 33
1.45 -
f
1.600 e (Au/H@ _
212
1.592 s
(pre-wave) IV. 0.1 M TBAH,’ 50% EtOH
1.535
a Benzaldehyde concentration: 4.7 mM; sweep rate: 40 mV s-l. sulphonate; TBAH: tetra-n-butyl ammonium hydroxide. b DMF+TBAI [14]. ’ TBAP+ sulfolane 1161. d 1 M H,SO, [22]. e 1 M H,SO,, pH 6.8, phosphate buffer [25]. ’ 48% EtOH, pH 8.6 [23]. g TBAH+50% EtOH [24]. ’ 0.5 M NaOH [25].
TEATS:
1.500 ’ (Au/H& tetraethylammomium
p-toluene
188
Also on a glassy carbon electrode we observe a single irreversible one-electron wave. The peak potential of this wave is quite close to the reduction potential on a mercury electrode (Table 1) [14,16,22-251. The assumption of a one-electron reduction on the GCE, as in the case of mercury, is further supported by the appearance of a second reduction wave at more negative potentials as well as by the appearance of an anodic peak related to the first cathodic peak at high sweep rates. In the voltammogram on the GCE, the blocking/adsorption effects show up in the form of a continuously decreasing peak current function (~,/Acv’/~) with the sweep rate, a negative shift of E, with increasing concentration of benzaldehyde, and unusual % - %/z values as well as d E,/d log v values. Such effects are indeed predicted for blocking effects on solid electrodes [26,27]. Because of these features, simple qualitative methods developed for analysing the voltammetric response of an EC reaction scheme could not, strictly speaking, be employed for determining the “n ” value and other relevant parameters. Controlled potential coulometry could not be employed either, primarily because of the passivation effect of adsorbed films on the GC surface. In spite of all these difficulties, if we assume that the adsorption effects are absent or at least marginal at very low benzaldehyde concentrations, some quantitative calculations can be made. Using the peak current constant values obtained at low concentrations and a diffusion coefficient value of 1.44 X lo-’ cm* s-i [28], the n values were found to be 1.2-1.3 or approximately 1, suggesting a dimerization process. The dE,/d log v value at low benzaldehyde concentrations is found to be 22 mV/decade, which is close to the value 19.4 mV predicted for the radical-radical dimerization mechanism represented by eqn. (2) [17]. If this mechanism is assumed, the ratio of the anodic to the cathodic peak current can be used to evaluate the
O-8
1.0
1.2
1.L
1.6
1.8
2.0
2.2
24
2-5
-E/V vs SCE Fig. 1. Effect of o-cresol as the proton donor on the reduction of benzaldehyde on a GCE in 0.1 M TBAI + DMF at 40 mV s-‘. 4.7 mM benzaldehyde+ x mM o-cresol. x: (a) 0; (b) 2.7; (c) 5.3; (d) 7.8; (e) 10.
189
dimerization constant [29]. The average dimerization rate constant value of 4.4 x lo3 mol-’ s-i obtained is quite comparable to the value obtained on a mercury electrode under similar conditions. All these facts indeed support the view that the electrochemical process on a “perfectly” cleaned GCE is the same as that on a mercury electrode, except for the fact that adsorption and blocking effects are more prevalent on the solid (GCE) electrode, as would be expected. However, in contrast it is noted that protonating agents have very different effects on a GCE when compared with a mercury electrode. Neither reaction (1) nor reaction (2) seems to be affected by the addition of small quantities (say, mM) of water, as shown by the insensitivity of the voltammetric response to the addition of water as in the case of mercury [13]. In the presence of o-cresol, however, the reduction potential on the GCE is shifted positively (Fig. l), probably due to simultaneous protonation together with electron transfer (eqn. 3, where BH+ refers to o-cresol). RCHO+e-+BH++RdHOH+B
(3)
The peak current in the presence of o-cresol actually decreases slightly, suggesting that the protonation in the present case on the GCE does not enhance further reduction of the radical anion leading to the formation of benzyl alcohol through an overall two-electron ECE mechanism. This is in contrast to the electroreduction on mercury, where the peak current increases with the o-cresol concentration, suggesting benzyl alcohol formation [13,30]. Benzoic acid (BH+), being a stronger protonating agent, actually protonates the reactant molecule itself through direct protonation. RCHO+BH+=RdHOH+B
(4)
Since the protonated species is easily reducible compared with benzaldehyde itself, a separate reduction wave at more positive potentials (Fig. 2) is noticed. The wave height due to the protonated species increases with the benzoic acid concentration, as would be expected. The total wave height of the two waves does not increase but rather decreases slightly with the benzoic acid concentration. This again is in contrast to the behaviour on mercury, where the total wave height increases due to simultaneous two-electron ECE reduction leading to partial formation of alcohol [30]. The protonating agents thus do show their effect on the GCE as in the case of mercury. However, on the GCE, the dimerization process of both the protonated and the unprotonated intermediate proceeds much faster than further reduction to alcohol. Hence no protonating agent is able to change the dimerization pathway to the ECE reaction pathway. A similar faster reductive dimerization of diacetyl on a GCE when compared with mercury was reported earlier [31]. In the case of the oxidation of azide anions as well, a faster dimerization on graphite, when compared with a Pt electrode, was reported [32]. At this stage, it is quite difficult to pin-point the cause of such observations. However, one would conclude from these observations that the radical
190
04
1.0
1.2 -E/V
1.4 vs
1.6
1.6
2.0
2.2
SCE
Fig. 2. Effect of benzoic acid as the proton donor on the reduction of benzaldehyde on a GCE in 0.1 M TBAI + DMF at 40 mV s-‘. 9 mM benzaldehyde+ x mM benzoic acid. x: (a) 0; (b) 1.8; (c) 3.5; (d) 5.1; (e) 6.7; (f) 8.2.
species is held loosely on the carbon surface for sufficiently long time intervals so that they can interact mutually and form the dimer product. The interaction in this case should be confined to the reactive carbonyl centre and not to the aromatic ring, since this type of faster dimerization when compared with the ECE pathway is noticed for benzaldehyde (this work) as well as for diacetyl [31], which is an aliphatic compound. Since carbonyl compounds are generally basic in nature, the acidic functional groups present on the GCE are likely centres for the radical ,adsorption and stability. (III.2)
Surface effecti in DMF
Even a “perfectly” polished GCE can still contain some small concentrations of surface functional groups [5]. In the previous section, we were dealing with such a low level concentration of surface functional groups. The concentration of these surface functional groups may be increased by partial or incomplete polishing, or by chemical or electrochemical activation. Let us denote the surface functional groups by a common formula, namely S-COOH, always remembering that this can denote any other functional group (such as the quinonoic or phenolic group) which can show acid-base properties. These surface functional groups can serve as protonating agents for benzaldehyde. S-COOH
+ RCHO + S-COO.
. . . ZH(OH)R
(5)
This type of surface protonated species can be reduced easily when compared with RCHO itself. Hence whenever such a surface protonation becomes possible, one should notice a pre-wave before the main reduction peak.
191
0.6
1.0
1.2
1.L
-E/V
vs
1.6
1.8
2.0
2.2
SCE
Fig. 3. Cyclic voltammograms for the reduction of benzaldehyde on a GCE (partially polished) in 0.1 M TBAI + DMF at various sweep rates in 4.7 mM benzaldehyde. u/mV s-‘: (a) 10; (b) 20; (c) 40; (d) 80; (e) 160; (f) 320.
Since a “perfectly” polished GCE contains a very low concentration of surface functional groups, such pre-waves are not seen on these electrodes. On a partially treated GCE, additional S-COOH groups lead to pre-waves (Fig. 3). The similarity between the pre-wave on a partially cleaned GCE (Fig. 3) and the pre-wave in the presence of benzoic acid on a clean GCE (Fig. 2) would further substantiate this viewpoint. To our knowledge, such a type of pre-wave has not been reported so far for carbonyl compounds for mercury or any other electrode in the absence of protonating agents. If very low concentrations are employed, the entire reduction process can proceed essentially through the surface protonation route, giving rise to a single wave in the positive potential region alone. Below the 1 mM concentration range of benzaldehyde, such a situation is indeed observed on the GCE (Fig. 4). In this case, one notices a reduction wave around - 1.4 V (close to the pre-wave in Fig.
192
0.8
1.0
1.2
14
-E/V
vs
1.6
1.8
2.0
2.2
SCE
Fig. 4. Cyclic voltammograms for the reduction of benzaldehyde on a GCE (partially polished in 0.1 M TBAI+DMF at a low concentration at various sweep rates. Benzaldehyde concentration: 1.0 mM. u/mV s-l): (a) 20; (b) 30; (c) 40; (d) 50; (e) 60; (f) 80; (g) 100; (h) 120; (i) 140; (j) 175; (k) 220; (I) 250; (m) 350; (n) 450; (0) 550; (p) 650.
3) and the main reduction wave around - 1.6 V (in Fig. 3) is totally absent. The peak current also increases steadily with the sweep rate. The surface protonation model presented here is quite different from the surface protonation reactions by water molecules and weak acids suggested earlier for benzaldehyde [33] and other electro-organic [34] reductions. Protonation by surface functional groups was suggested in recent work from this laboratory [35] pertaining to the reduction of benzophenone on a GCE. This surface protonation model is also quite similar to the one proposed to explain similar pre-waves in Nafion-coated GCEs for different organic molecules of lesser basicity [12]. (III.3)
Electroreduction
of benzaldehyde
in aqueous media
Benzaldehyde shows a reduction peak around - 1.0 V in aqueous acids and around - 1.5 V in neutral and alkaline pH regions on both GCEs and mercury electrodes (Table 1). However, in neutral pH regions, the GCE alone shows an additional surface wave at more positive potentials (see Section 111.4). Although the main reduction peak on the GCE as well as on Hg is noticed in the same potential region as reported earlier [18], the voltammetric responses on a GCE in acid medium show strong blocking effects [26,27]. The peak current constant of the reduction wave decreases quite substantially with increasing sweep rate and concentration. The peak potential shifts negatively with increasing concentration of benzaldehyde. The E, - Ep,2 and dE,/d log v values vary with the sweep rate and concentration of benzaldehyde. The blocking effect is noticed even more predominantly on a GCE in TBAH media (Fig. 5). At fairly high sweep rates and at high concentrations (Fig. 5), the
193
1-o
1.1
1.2
1.3
1.L
1.5
1.6
1.7
-E/V vs SCE Fig. 5. Cyclic voltammograms for the reduction of benzaldehyde on a GCE in 0.1 M TBAH + 50% EtOH at 40 mV s-‘. Benzaldehyde conzentration: (a) 1.0 mM; (b) 2.0 mM; (c) 2.9 mM; (d) 3.8 mM; (e) 4.7 mM; (f) 5.6 mM; (g) 6.5 mM; (h) 7.3 mM; (i) 8.2 mM.
voltammetric peak turns into a wave and the peak current becomes almost independent of the sweep rate as well as of the benzaldehyde concentration. Since this effect was noticed only on a GCE and not in the earlier studies on mercury [17], as well as in some specific experiments carried out during the present work on Hg-coated Pt electrodes, this type of behaviour should be due to the surface blocking effect and not to the slow preceding chemical reaction. Because of the interference due to the blocking effect discussed above, it was impossible to determine the value of n by voltammetric methods. However, constant potential electrolysis in acid, neutral and alkaline media indicated clearly that around the peak potential corresponding to the main wave, the one-electron reduction product hydrobenzoin alone was formed. The more or less identical i,/Acv’12 value obtained at low concentrations and sweep rates in all of these media also suggests that the reduction proceeds by a one-electron route. (III.4)
Surface effects in aqueous media
As noted in the previous section, benzaldehyde reduction on a GCE in neutral aqueous media is always associated with a pre-wave (Fig. 6A). Such a pre-wave again has neither been reported earlier on Hg, nor observed under identical conditions in the present work on a Hg-plated Pt electrode (Fig. 6B). Unlike the surface-sensitive pre-wave observed in DMF media (see Section 111.2), the surface wave observed in neutral aqueous systems is highly reproducible. These waves cannot be eliminated by any amount of surface polishing and pretreatment. The
1.2
14
-E/V
I.0
1-2
14 -E/V
1.6
Vs SCE
1.6
Vs SCE
Fig. 6. Cyclic voltammograms for the reduction of benzaldehyde on a GCE (A) in 0.1 M TEATS + 50% EtOH at various sweep rates. Concentration: 4.7 mM. v/mV sell: (a) 20; (b) 40; (c) 80; (d) 160; (e) 320. (B) On mercury-platelet platinum. u/mV s-‘: (a) 10; (b) 20; (c) 40; (d) 80; (e) 160.
voltammograms taken at different concentrations and sweep rates (Fig. 6) are quite reproducible with respect to the main wave and pre-wave. However, one particular condition should always be satisfied to obtain highly reproducible results. Between each voltammetric sweep there must be a sufficient time gap (say 3-4 min). This is probably due to the long time-scales required for the surface process causing this effect to attain a steady state or equilibrium (see later). This slow nature of the surface process involved is also evident from the multi-sweep cyclic voltammogram shown in Fig. 7. In this figure, the pre-wave is noticed only in the first sweep and is practically absent in all the subsequent sweeps. Some detailed investigations on this
195
1.0
1.1
1.2 -E/V
1.3 vs
1.4
1.5
1.6
1.7
SCE
Fig. 7. Cyclic voltammograms for the reduction of benzaldehyde on a GCE in 0.1 M TEATS + 50% EtOH in a multi-sweep at 40 mV s-l. Concentration: 2.9 mM. The numbers on the curves indicate the sweeps.
type of time effect have been reported for the voltammetric behaviour of benzophenone under identical conditions [35]. The most striking feature about the pre-wave is the fact that these waves appear only in aprotic solvents and in neutral pH regions in aqueous solvents. It is this fact which lends strong support to the view that the surface acid-base groups are the most probable catalytic centres leading to the observation of the pre-wave. At acidic pHs, the carbonyl compound can undergo protonation in the bulk and hence the surface protonating agents will not have any effect. Under alkaline pH conditions, the surface groups would also exist in the basic form (-COOinstead of -COOH, say) and so again no catalytic effect would be noticed. Only in aprotic solvents and neutral aqueous media are both criteria of the presence of acidic groups on the electrode surface and of unprotonated carbonyl groups in the solution satisfied, and hence the appearance of surface catalytic pre-waves in these two conditions alone. The acidity of the functional groups present on the GCE can also depend on the solvent employed. This is probably responsible for the sensitivity of the pre-wave to the surface pretreatment in DMF alone (see Section 111.2). Within the concentration ranges investigated, the pre-wave is independent of the rather than i,/v is constant. In fact, concentration. It is also noticed that i,/v”*
196
for the surface process involving a submonolayer level of acid-base interactions considered here, i,/v would be expected to be a constant. The cause of this unexpected behaviour is not quite clear. Although i,/v ‘/* is constant, this process is not a diffusion-controlled process. If this were the case, one would expect a pre-wave in all of the sweeps of the multi-sweep experiment (Fig. 7). The peak current would also be expected to increase with the concentration of benzaldehyde. Finally, this pre-wave also cannot be due to the “product adsorption” waves discussed extensively in the polarographic literature [36]. Such adsorption-desorption processes are quite fast equilibrium processes. In contrast, the surface protonation reaction (eqn. 5) suggested here is highly time-dependent. (IV) CONCLUSIONS
On a “perfectly” polished GC electrode, benzaldehyde in DMF is reduced in the same potential region where this process takes place on a mercury electrode. However, some blocking effects quite common to solid electrodes are also noticed. Protonating agents interact in the usual way with the radical anions that are formed on the GCE. However, the protonating agents do not enhance the process of alcohol formation (two-electron process) on the GCE, as in the case of a mercury electrode. A partially treated GCE shows an additional pre-wave for the reduction of benzaldehyde. The pre-wave is quite sensitive to the pretreatment. The available evidence suggests that the surface acid functional groups are responsible for this pre-wave. In aqueous media also, the main reduction peaks on the GCE as well as on mercury are observed in the same potential region. However, the blocking effects on the GCE are even more predominant in aqueous media when compared with non-aqueous media. The voltammetric response in TBAH in particular is quite different from a diffusion-controlled process. Constant potential electrolysis around the main peak potential confirms the formation of hydrobenzoin. In non-aqueous media, the pre-wave is sensitive to the surface pretreatment procedures. In aqueous solutions, this pre-wave is observed only in neutral media. This fact together with all the other experimental evidence suggests that a slow time-dependent surface protonation process is responsible for the pre-wave formation on a glassy carbon electrode. REFERENCES 1 2 3 4 5 6 7
1.F. Hu, D.H. Kanveik and T. Kuwana, J. Electroanal. Chem., 188 (1985) 59. J. Wang and L.D. Hutchins, Anal. Chim. Acta, 167 (1985) 325. T. Nagaoka and T. Yoshino, Anal. Chem., 58 (1986) 1037. M. Poon and R.L. McCreery, Anal. Chem., 58 (1986) 2745. G.N. Kaman, Anal. Chim. Acta, 207 (1988) 1. R. Engstrom and A.V. Strasser, Anal. Chem., 56 (1984) 136. L.J. Kepley and A.J. Bard, Anal. Chem., 60 (1988) 1459.
197 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
M. Noel and P.N. Anantharaman, Analyst, 110 (1985) 1095. M.R. Deakin, K.J. Stutts and R.W. Wightman, J. Electroanal. Chem. 182 (1985) 113. M. Noel, Bull. Electrochem., 5 (1989) 393. M. Noel and K.I. Vasu, Cyclic Voltammetry and Frontiers of Electrochemistry, Aspect Publ. Ltd., London, 1990. MS. Mubarak and D.G. Peters, J. Electroanal. Chem., 273 (1989) 283. P.H. Given and M.E. Peover, J. Chem. Sot., (1960) 385. S. Wawzonek and A. Gundersen, J. Electrochem. Sot., 107 (1960) 537. W.R. Fawcett and A. Lasia, Can. J. Chem., 59 (1981) 3256. N.R. Armstrong, R.K. Guinn and N.E. Vanderborgh, Anal. Chem., 46 (1974) 1759. L. Nadjo and J.M. Saveant, J. Electroanal. Chem., 33 (1971) 419. J. Weber and J. Volke, Electrochim. Acta, 24 (1979) 113. M. Chandrasekaran, Ph.D. thesis, Madurai-Kamaraj University, Madurai, 1989. H.O. House, D. Koepsell and W. Jaeger, J. Org. Chem., 38 (1973) 1167. D.J. Freed and L.R. Faulkner, J. Am. Chem. Sot., 93 (1971) 2097. R. Kalvoda and G.K. Budnikov, Collect. Czech. Chem. Commun., 28 (1963) 838. R. Pastemak, Helv. Chim. Acta, 31 (1948) 753. J.W. Hayes, I. Ruzic, D.E. Smith, G.L. Booman and J.R. Delmastro, J. Electroanal. Chem., 51 (1974) 269. L.N. Nekrasov and A.D. Korsun, Elektrokhimiya, 4 (1968) 539, 996. C. Amatore, J.M. Saveant and D. Tessier, J. Electroanal. Chem., 146 (1983) 37. C. Amatore, J.M. Savtant and D. Tessier, J. Electroanal, Chem., 147 (1983) 39. L. Meites, Polarographic Techniques, Wiley, New York, 1965. M.L. Olmstead, R.G. Hamilton and R.S. Nicholson, Anal. Chem., 41 (1969) 261. V.P. Gultyai, I.V. Proskrovskaya and T.Ya. Rubinskaya, Tezisy Dokl. Seveshch. Polyarogr., 7 (1978) 14. S. Thangavelu, M. Noel and KS. Udupa, Electrochim. Acta, 31 (1986) 1563. V. Plazek and H. Wendt, J. Electroanal. Chem., 180 (1984) 185. S.G. Mairanovskii, Sov. Electrochem., 19 (1983) 383. S.G. Mairanovskii, Catalytic and Kinetic Waves in Polarography, Plenum Press, New York, 1968. M. Chandrasekaran, M. Noel and V. Krishnan, Bull. Electrochem, 2 (1986) 469. E. Laviron, J. Electroanal. Chem., 35 (1972) 333.