1Pergnmon Electrochimico Acto, Vol. 41, No. 6, pp. 895-902. 19% Copyright 0 19% El.&ier sciena Ltd. Printed in Great Britain. All rights rcwwd OOW4686/96 SIS.00 + 0.00 001~4686(95)00383-5 ELECTROCHEMICAL STUDY OF ISOPOLY- AND HETEROPOLY-OXOMETALLATES FILM MODIFIED MICROELECTRODES-VI. PREPARATION AND REDOX PROPERTIES OF 12-MOLYBDOPHOSPHORIC ACID AND 12-MOLYBDOSILICIC ACID MODIFIED CARBON FIBER MICROELECTRODES BAOXING WANG and SHAOJUN DONC* Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Academia Sinica, Changchun, Jilin 130022, People’s Republic of China (Received 23 January 1995; in reoisedform 25 September 1995) Abstract-The redox behaviours of 12-molybdophosphoric acid (12-MPA) and 12-molybdosilicic acid (12-MSA) in aqueous acid media are characterized at the carbon fiber (CF) microelectrode. The preparation of CF microelectrode modified with 12-MPA or 12-MSA monolayer and the oxidation-reduction properties of the modified electrode in aqueous acid media or 50% (v/v) water-organic media containing some inorganic acids are studied by cyclic voltammetry. 12-MPA or 12-MSA monolayer modified CF microelectrode with high stability and redox reversibility in aqueous acidic media can be prepared by simple dip coating. The cyclic voltammograms of 1ZMPA and I2-MSA and their modified CF microelectrodes in aqueous acid solution exhibit three two-electron reversible waves with the same half-wave potentials, which defines that the species adsorbed on the CF electrode surface are 12-MPA and l2-MSA themselves. The acidity of electrolyte solution, the organic solvents in the electrolyte solution, and the scanning potential range strongly influence on the redox behaviours and stability of 1ZMPA or l2-MSA monolayer modified electrodes. On the other hand, the catalytic effects of the 12-MPA and IZMSA and chlorate anions in aqueous acidic solution on the electrode reaction processes of 12-MPA or 12-MSA are described. Key words: 1Zmolybdophosphoric fied electrode. _ _ acid, 12-molybdosilicic acid, carbon fiber electrode, chemically modi- INTRODUCtiON 12-Heteropolymolybdates have been known to be effective catalysts for many heterogeneous and homogeneous oxidations. It is well known that the reduction products of 1Zheteropoly molybdenum blue compounds and that many workers applied the formation of heteropoly molybdenum blue compounds to the determination of traces of P(V), As(V), Si(IV) or related elements without the clarification of the properties of the heteropoly molybdenum blue compounds in detail. Heteropolymolybdates are very suitable compounds for electrochemical studies since several of them are known to undergo reversible oxidation-reduction in both aqueous and mixed solvents. The information obtained from such an investigation may lead not only to useful preparatives for new heteropoly compounds, but such data may also aid their application to homogeneous catalysis. Many electrochemical studies of 12-MPA and related 12-heteropolymolybdates have been carried out in order to make clear the redox properties of a series of mixed valence Mo(V, VI) complexes known as a Keggin structure. The previous voltammetric * Author to whom correspondence should be addressed. and poiarographic investigations have shown that 12-MPA as well as 12-MSA in acidic solution containing some organic solvents undergoes a series of consecutive two-electron reductions to yield various mixed valence Mo(V, VI) complexes[l]. It seems likely that the first three two-electron redox couples of 12-heteropolymolybdates in acidic solutions containing dioxane or ethanol are remarkably fast and reversible[2-61. On further reduction above six electrons, however, different results have been reported concerning the fourth and subsequent redox reactions of 12-heteropolymolybdates in acidic solutions containing dioxane or ethanol[2,3]. Tsigdinos and Hallada[Z] have shown that IZMSA in 50% (v/v) water-dioxane solutions containing hydrochloric acid exhibited five two-electron reduction waves. On the other hand, Souchay and his colleagues[3] have reported that the reduction processes of both IZMPA and 12-MSA in 50% (v/v) water-ethanol media containing hydrochloric acid displayed three two-electron waves, and 12heteropoly acids were not further reduced above six electrons in acidic media containing ethanol. These results suggest that the redox processes of 12heteropolymolybdates in acidic solutions are influenced by the nature of organic solvent. Itabashi[7] presented some unusual solvent effects on the BAOXING WANGand SHAOJUN DUNG 896 gradual reduction processes of 12-heteropolymolybdates in acidic solutions with or without some organic solvents at a glassy carbon electrode in detail. Generally speaking, the electrode reactions of 1ZMPA and 12-MSA are very simple and very similar to each other. Therefore, it was thought that the role of central atoms such as P, Si and As in the electrode reactions of 12-heteropolymolybdates is not so significantC8, 93. However, our experimental results also indicate that the influence of a central atom on the electrode reactions of 12-MPA and 12-MSA cannot be ignored, which is agreement with the conclusion reported by K. Unoura and N. Tanaka[6]. In our previous papers[lO-121, we have described the preparation and the electrochemical behaviours of CF microelectrodes modified with isopolymolybdate and 2 : 18-molybdodiphosphate monolayer. The present paper presents the preparation of CF microelectrodes modified with 12-MPA or 1ZMSA monolayer and their electrochemical properties in electrolyte solutions with or without organic solvents, and studied the catalytic effects of CF microelectrodes modified with 12-MPA and 12-MSA. EXPERIMENTAL SECTION Chemicals Ammonium 1Zmolybdophosphate was obtained from Beijing Chem. Reagent Factory, ammonium 12-molybdosilicate, from our Institute. Other chemicals were of reagent grade and used as received, and all chemicals were used without further purification. All solution were prepared with doubly distilled water. Apparatus A conventional single-compartment cell with standard three-electrode configuration was used. Electrochemical experiments were performed on a PARC Model 370 electrochemical system equipped with a Model 173 Potentiostate and monitored by a Model 175 Universal Programmer. Cyclic voltammograms were recorded on a Gould Series 60000 X-Y recorder. Electrodes The reference electrode was a saturated calomel electrode (see), the counter electrode was a platinum disk with larger surface areas and the working electrode was a carbon fiber (CF) microelectrode (0 = 30pm, L = 0.5 cm, preparation procedure is the same as previous paper[lO].) Before each experiment, the CF electrode was ultrasonically by washed in 2 M NaOH, 98% H,SO, and doubly distilled water bathes, respectively, then was rinsed with doubly distilled water and ethanol. The CF electrode was again electrochemically pretreated by the cathodic polarization at - 3.OV for two minutes in 2 M H2SOo, then cycled potential between +0.8 and -0.3 V until the background current stabilized. In this paper, all potentials were measured and reported vs the saturated calomel electrode (see). RESULTS AND DISCUSSION 1. Voltammetric behaviours of 12-MPA and 1ZMSA in aqueous acidic solution Figure 1 shows cyclic voltammograms of 5.0 x 10m3M 12-MSA in 2 M H,SO, solution in different potential scan ranges at different scan rates. In the range from +0.8 to -0.1 V at the rate of 50 mV/s, three reversible redox waves of equal height appear at the potentials of +0.35, +0.24 and +0.06 V, and their peak potential differences AS, ( = E, - Epcl)are - 35, - 30 and - 30 mV, respectively. The potential separations of the first three redox waves correspond closely to the theoretical value of -29mV for a Nernstian two-electron wave at 25°C. This is consis-’ tent with dc polarographic, coulometric and cyclic voltammetric data obtained in water-ethanol (or dioxane) solutions or aqueous acid solutionsC2, 6, 7, 13, 141. In the potential scan range from +0.80 to -04OV, 1ZMSA gives out four well-defined cathodic peaks. The fourth wave is less reversible than the first three cathodic ones, but its peak height is approximately 5.3 times the height of the first cathodic peak. The peak current on the stationary electrode voltammetry is proportional to n”‘, namely 3/z nl 3/2=. n4 !t!L ‘P4 where n is the number of electron involved in the electrode process. Because the first redox wave is a two-electron reversible one (ni = 2), n4 = 6.08 x 6, namely the fourth step is attributable to a sixelectron process. The peak potentials of four steps are invariant with the scan rate (v) for v between 10 and lOOOmV/s, and the peak currents are proportional to the square root of the scan rate. So the electrode process of 12-MSA at CF electrode mainly manifests a diffusion-controlled one. However, when the concentration of 12-MSA in electrolyte solution is lower than 1.0 x lo-‘M, the redox waves of the 1ZMSA are very symmetrical, and the potential differences between the oxidation and corresponding reduction peaks of the first three redox couples are less than 20mV. Their waveforms are characteristic for a diffusionless-limited process (Fig. 2(A)). The peak currents of the first three redox waves are proportional to scan rates, and the ratios of the reduction and oxidation currents are equal to unity. The potential separations and the peak potential don’t almost shift with the scan rates. These results indicate that 12-MSA can be adsorbed on CF electrode surface and its electrode process is characteristic of a reversible surface wave. The above experimental results illustrate the electrode process of 1ZMSA in aqueous acidic solution are controlled by simultaneous diffusion and adsorption processes. At low concentration of lZMSA, the currents of 12-MSA are mainly produced by the adsorbed monolayer. Whereas, at high concentra- Electrochemical study of isopoly- and heteropoly-oxometallates-VI to.8 10.4 to.6 E , v YJ. to.2 0 i0.6 to. & to 4 SCE E / +0.2 v *. 0 -0. 2 897 -0 4 SCE Fig. 1. Cyclic voltammograms of 5.0 x 10e3 M 1ZMSA in 2M H,SO, at different scan rates. Scan potential range: (A)+0.80 to -0.20V;(B) +0.80to -0.4OV. tion, the currents result from the adsorbed monolayer covered by the larger diffusion current of 1ZMSA in the solution, so its electrode process is mainly characteristic of a diffusion-controlled one. Since the (NH,),PMo,, . xH,O has low solubility in aqueous acidic solution, the concentration of 1ZMPA in the solution is difficultly defined. Thus, the electrochemical experiment proceeds in 12-MPAsaturated H2S0, (2M) solution. Figure 3 presents the cyclic voltammogram of lZ-MPA in the 12-MPA-saturated 2M H,SO, solution at CF microelectrode. From Fig. 3, we observed that the cathodic peaks of LZMPA are symmetrical with the corresponding anodic peaks and the waveform is characteristic of a diffusionless-limited process. The potential separations of the three redox waves are all less than 20 mV. 2. Preparation of 12-MPA and 12-MSA monolayer modified CF microelectrode The CF microelectrodes in the above experiments were taken out and rinsed thoroughly with 2M H,S04, then immersed in 2 M H,SO, solution and potential cycle at scan rate of ZOOmV/s was begun. There appeared three distinct reversible redox couples between +0.80 and -0.10 V in the cyclic voltammograms, which are quite similar to those of B I 4Q.6 40.4 E I V to.2 VLSCE 0 to 6 60 nA 10.6 +o. 4 E I V a.2 0 VLSCE Fig. 2. Cyclic voltammograms of 1.0 x 10m5M 1%MSA (A) and the 12-MSA monolayer modified CF microelectrode at scan rate 2OOmV/s (B) in 2 M H,SO,. (A) scan rate: (l)lOOO, (2)500, (3200, (4)100, (950, (6)20mV/s; (B) the second cycle (-); after reaching steady state (---). BAOXING WANG 898 to. 6 to. 4 to. 2 and SHAOJUN DUNG 0 E / V vs.SCE Fig. 3. Cyclic voltammograms of the 12-MPA in 12-MPA- saturated 2 M H,SO, at scan rate 200mV/s. 12-MPA or 12-MSA in solution, indicating the CF microelectrode to be modified with a film of this species (1ZMPA or 12-MSA). The modified CF electrode obtained by this method have good stability in 2 M H,SO,. A CF electrode was immersed in 2 M H,SO, solution containing 1.0 x 10m4M IZMPA or 12-MSA and stood for 10s 3Os, 60 s, 5 min and lOmin, respectively, then taken out and rinsed, and the resulting electrode was transferred to 2M H,SO,. The cyclic voltammograms obtained by this method E/ v W.scE are the same as that obtained by the above method. Figure 2(B) represents the cyclic voltammograms of 12-MSA monolayer modified CF electrode in 2M H,SO,. Initially, the peak currents (i,) decrease slightly with potential scans, then quickly to a steady state after a few cycles. However, the cyclic voltammograms (Fig. 4(A)) of 12-MPA modified CF microelectrode in 2M H,SO, are different from those of 12-MSA modified electrode. Initially, the peak currents continuously increase with potential scans, and the cathodic peak potentials shift to positive potential but the anodic peak potentials to negative; namely the peak potential separations reduce with potential scans. Finally, the peak currents reach a steady state (Fig. 4(B)) after a few minutes. Owing to the monolayer modification, it is reasonable to use the surface area A as the same as the naked CF microelectrode for calculation of F’. In the present experiments, from i, = (n2F2/4RT)uA, where n is the number of electron, u is the scan rate, the for obtained values are 5.3 x lo-” mol/cm* 1ZMPA and 4.5 x lo-“mol/cm2 for 12-MSA, respectively. On the other hand, the surface concentration can be also achieved by the size of the adsorbing molecule or ion. The crystal structure of H,PMo,~O,,~(H,O),,_,, has been determined from three-dimensional X-ray diffraction data collected with a PAILRED diffractometer using MoKradiation by Strandbreg[lS]. The cell dimensions of the tetragonal (14Jamd) unit cell are a = 16.473(5)A and c = 23.336(7)& and it contains four formula units. The maximum value of the cross-section obtained by these crystal parameters is approximately 384.414 A*, ie the cross-section(s) of each unit is 96.104A2. So the surface coverage (r’) is: r’ = (No.s)- ’ = 17.33 x lo-” mol/cm* (where E/\‘vrS(‘E. Fig. 4. Cyclic voltammograms of the 12-MPA monolayer modified CF microelectrode in 2 M H,SO, at scan rate 2OOmV/s. (A) before the steady state; (B) after the steady state. Electrochemical study of isopoly- and heteropoly-oxometallates-VI No = 6.02 x 10z3). This value is about 3 times the measured coverage, which may be due to the formation of larger hydrated anions in aqueous acidic solution. The above results prove that the 12-MPA or 12-MSA thin film on a CF microelectrode adsorbed monolayer. is an 3. Characterization of both 12-MPA and 12-MSA monolayer modtjied CF microelectrodes in aqueous acidic solution Figure 5 shows the typical cyclic voltammograms of 12-MSA and 12-MPA monolayer modified CF electrodes prepared by simple adsorption in 2 M H,SO, at different potential scan ranges, respectively. In +0.70 and -O.lOV, the film electrodes present three couples of reversible redox waves in 2 M H,SO, . For IZMSA, the reduction waves were observed at peak potentials (E,s) of +0.33, +0.22, -0.01 V; while anodic peaks occurred at +0.34; +0.23 and +O.Ol V with the peak potential separations (A.E,s) of 10, 10 and 20mV. For IZMPA, reduction waves appeared at $0.35, +0.21 and -O.O2V, while the anodic peaks took place at +0.36, +0.20 and 0.0 V with the peak potential separations of 10, 10 and 20mV. When the scan potential range was maintained between $0.80 and 4.6 t0.6 +0.4 0 to.2 899 -0.2OV, the fourth redox waves appeared at -0.15 V and gave rise to the complex of the anodic process of 12-MSA (Fig. 5(A)) and resulted in the instability of the modified electrode. For 12-MPA, the fourth redox couple appeared at -0.17 V, but this redox couple was reversible and did not affect the redox processes of the first three waves (Fig. 5(C)). If the potential was scanned to -0.3OV, the cyclic voltammograms of 1ZMSA modified electrode (Fig. 5(B)) were very different from those of 12-MPA modified electrode (Fig. 5(D)). the fourth step of 12-MPA is also a reversible wave which should be a four-electron process according to the conclusion obtained by Itabashi et aI.[7]. Its peak height is approximately 2.65 times but not 4 times (theoretically, i,, = (n.&,)' . i,, = 4i,,) the height of the first two-electron reversible wave, which is probably related to the instability of eight-electron reduction product of 12-MPA. Although the fifth cathodic wave is less reversible than the first cathodic one, the first and fifth cathodic steps have nearly equal height. It is reasonable to assign that the fifth cathodic step also corresponds to a twoelectron process. The relationship of the peak potential, peak current and peak potential separation of the first three redox waves of the 1ZMPA and 12-MSA with -0.2 E I V vs.SCE +0.6+0.6+0.4 iO.2 E / V vr.BCE 0 -0.2 to. d to.4 +“. 2 0 -0.2 -0.4 E / v vs.SC& Fig. 5. Cyclic voltammograms of the 12-MSA (A and B) and 12-MPA (C and D) monolayer modified CF microelectrodes in 2 M H,SO, in the different scan potential range at scan rate 200 mV/s. !m BAOXING WANG and SHAOJUN DONG the scan rate (u) indicate that with increased scan rate the reduction potential E, slightly shifts towards positive, while the oxidation potential E, shifts towards negative, resulting in a slight decrease of AE,, and the peak current (i,) is linearly proportional to u up to lCOOmV/s and AE, is less than 25mV, as expected for a surface process. In general, the peak potential separation increases with the increase of the scan rate. Why was the opposite result obtained in our experiment? We suppose that this unusual phenomenon results from the characteristics of both structure and special redox properties of IZMPA and 12-MSA. Many results[16] have proved the bridging oxygen atoms (MO-O,-MO) are exclusively reactive and consumed in the early stages of the reduction of 1ZMPA and 12-MSA. The addition of electrons will result in a weakening of the bridge-oxygen bond, O,-MO, and the increase of the basicity of the anions, which is accompanied by a decrease in the strength of 12-MPA and 12MSA[l6]. This is harmful to electron transfer in the LZMPA and 12-MSA. At low scan rate, it has enough time to weaken the bridge-oxygen bond of 1ZMPA and lZMSA, while at a faster sweep rate, it has not enough time to weaken the bond. This is the reason why the redox reversibility of 12-MPA and 12-MSA anion thin film at fast scan is better than that at lower one. On the other hand, the rate constants of a heterogeneous electron-transfer process of both IZMPA and 1ZMSA are relatively large (around 10-l cm/s)[6]. At a large sweep rate, therefore, the affect of the sweep rate itself on the electron-transfer of 1ZMPA and 1ZMSA can be ignored. 4. Self-electrocatalytic e&cc of 12-MPA and 1ZMSA and electro-catalytic reduction of chlorate ions on * to.8 t0.6 , 40. 4 to. 2 U E / v vs.SCE Fig. 6. Cyclic voltammograms of the 12-MPA monolayer modified CF microelectrode in the 12-MPA-saturated 2 M H,SO, at the different scan rates. Scan rate: (1)50, (2)20, (3)10, (4)5 mV/s. 1ZMPA and 12-MSA monolayer modified CF microelectrode The experimental results obtained by Unoura et al.[6, 173 indicate that 12-MPA in solution exhibited the autocatalysis and 1ZMPA and 12-MSA in solution could catalyze the reduction of chlorate ion. Do the CF microelectrodes directly modified with 12-MPA or 1ZMSA monolayer maintain the same catalytic properties as in solution? Figure 6 shows that the cyclic voltammograms of 1ZMPA at a CF microelectrode under different scan rate in 12-MPA-saturated-2M H,SO,. At a large scan rate (> 50mV/sf, the 12-MPA displays three reversible peaks on both cathodic and anodic scans, and the peak currents are proportional to scan rate. The first three processes of both 12-MPA and 12-MSA are not different from each other. However, at a smaller scan rate ( < 50 mV/s), the cyclic voltammograms of 12-MPA and 12-MSA are clearly different. For IZMSA, the peak currents of the first three couples of redox waves are linear to scan rate. For 12-MPA, although the peak currents of the first two couples of redox waves are proportional to the scan rate, the third cathodic peak of 1ZMPA becomes a plateau with the decrease of scan rate and the corresponding anodic peak disappears. The third cathodic wave of 1ZMPA seems to be heterogeneous autocatalytic, which was observed at a small scan rate. The cyclic voltammograms of 12-MPA at 20mV/s obtained after the electrolyte solution was stirred and statically placed for 5 seconds and 5 minutes, are respectively shown in Fig. 7 (curve a and curve b). Figure 7 further defines the autocatalysis which occurs at the third cathodic wave of 12-MPA. In the case of lZMSA, the autocatalytic effect was not observed at the third cathodic wave of I2-MSA. The above results show that the six-electronreduction product of 12-MPA is more reactive for a heterogeneous electron-transfer reaction than that of 1ZMSA. In the presence of chlorate ions, a typical cyclic voltammogram of 12-MPA modified CF microelectrode is given in Fig. 7 (curve c). Chlorate ions produce a remarkable influence on the third cathodic wave, whereas the first two remain almost unvaried upon the addition of chlorate ions. The third cathodic wave shows a plateau rather than a peak and increases in height with increasing chlorate ion concentrations. Correspondingly, the third anodic peak related to the reoxidation of the product by six-electron reduction decreases and disappears completely; finally the cathodic and anodic curves almost overlap. These indicate that the third Electrochemical study of isopoly- and heteropoly-oxometallates-VI 1 to.8 tO.6 to.4 to.8 +0.6 +o.4 E/ v 10.2 0 V vs.SCE E / I to.2 20 nA 0 vs. SCE Fig. 7. Cyclic voltammograms of the 12-MPA monolayer modified CF microelectrode in the 12-MPA-saturated 2 M H,SO,(a and b) or the lZMPA-saturated 2M H$O, + 0.05M CIO; (c) at scan rate 20mV/s. The solutions were stirred and statically placed for 5 seconds(a) and 5 minutes(b), respectively. reduction wave of 1ZMPA anions is catalytic in nature in the presence of chlorate ions. 5. Medium effects on the redox properties of 1ZMPA and 1ZMSA The redox processes of 1ZMPA and IZMSA in acidic solutions are influenced by the nature of organic solvents such as ethanol, acetone and acetonitrile. In this experiment, the medium effects on the redox properties of IZMPA and IZMSA are differed from that previously reported by E. Itabashi[7]. Figure 8(A) is the cyclic voltammograms of 12-MPA film modified CF microelectrode in 50% I to.a I to. 6 to. 4 * to.2 0 E / V vs.SCE Fig. 8. (A) Cyclic voltammograms of the I2-MPA monolayer modified CF microelectrode in 50% (v/v) waterethanol solution containing 2M H,SO, and (B) cyclic voltammograms obtaining after the 12-MPA monolayer modified CF microelectrode is transfered from 50% (v/v) water-ethanol solution containing 2M H,SO, 2M into 2 M H&SO,. Scan rate: 2OOmV/s. (v/v ) water-ethanol media containing sulfuric acid. The peak currents of the first three redox waves are very small and reduced a little with the potential scan. This is different from the redox behavior of 12-MPA film modified CF microelectrode in 2M H,SO, solution without organic solvents (as see Fig. 4). However, if the CF microelectrode was taken out and removed into 2M H,SO, solution in the absence of organic solvents, we observed the peak 902 BAOXINGWANGand SHAOJUNDONG currents of the first three redox waves of 12-MPA increase with the potential scan (see Fig. S(B)) and reach a ready state after 5 minutes. The same case as Fig. 8(A) was observed if this film CF microelectrode was removed into the 50% (v/v) water-ethanol solution containing 2 M H,SO, again. These results show the 12-MPA film absorbed on the CF microelectrode surface wasn’t destroyed or didn’t move away from the electrode surface in the 2M H,SO, solution containing organic solvents. However, the experimental results of IZMSA film modified CF microelectrode obtained in the 50% (v/v) water-ethanol solution containing 2 M H,SO, are completely different from those of 1ZMPA film modified electrode. Why can the peak currents of 12-MPA monolayer modified microelectrode decrease in the solution with organic solvent but increase in the solution without organic solvents? If the 1ZMSA film electrode was transferred into the 2M H,S04 solution in the absence of organic solvents from the electrolytic solution in the presence of organic solvents again, the peak currents at the steady state are only one third of the peak currents, which defined that the IZMSA film modified on electrode may remove away from the electrode surface in the acidic media containing organic solvents. In our experiments, in the aqueous acidic solution, some protons are attached to the 0, atoms of 1ZMPA by bonding and this easily results in the redox reaction in the presence of protons on the CF microelectrode, while another protons may adsorbed around LZMPA by some effect. However, in the presence of organic solvents, these organic solvent molecules may be adsorbed around the 1ZMPA and this adsorption effect is stronger than that of the protons. This may be reason why the peak currents of 12-MPA film modified CF microelectrode are very small in the acidic solutions with organic solvents. On the other hand, when the 12-MPA modified CF microelectrode was removed into the acidic solution without organic solvents and started to cycle potential, the protons in the electrolytic solution gradually diffused into the IZMPA film on the CF microelectrode and gradually replaced the organic solvent molecules around the 12-MPA because of the destruction of the following balance equation between the protons and organic solvent molecules : film modified CF microelectrode 12-MPA.xH,O+ + XL= lZMPA*xL + xH,O* unelectroactive electroactive Here, L represents ethanol or acetone or acetonitrile. The balance moves to the left of the equation, which results in the electroactivity of the 1ZMPA film continuously increasing with the potential cycling and the peak currents of the redox waves of the 12-MPA also continuously increasing and reaching a maximum value. These experimental phenomenon remain to be further studied. CONCLUSION The preparation of CF microelectrode modified with 1ZMPA or IZMSA monolayer is very simple and the modified electrodes exhibit well-defined oxidation-reduction and electrocatalytical properties in aqueous acid media. On the other hand, the redox processes of 1ZMPA and IZMSA in acidic soiutions are influenced by the presence of organic solvents. REFERENCES 1. M. T. Pope, Heteropoly and Isopoly Oxometallates, Chap. 4. Springer (1983). 2. G. A. Tsigdinos and C. J. Haiiada, J. Less-Comm. Met. 36,79 (1974). 3. J. P. Launay, R. Massart and P. Souchay, J. LessComm. Met. 36, 139(1974). 4. N. Tanaka, K. Unoura and E. Itabashi, Inorg. Chem. 21,2087 (1982). 5. C. 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