ELECTROCHEMICAL STUDY OF ISOPOLY

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Electrochimico
Acto, Vol. 41, No. 6, pp. 895-902. 19%
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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.
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