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Turbulent hydrodynamic voltammetry. part i. the distribution of voltammetric current on electrode surfaces
Analytica 0 Elsevier
Chimica Acta, 80(1976)31-37 Scientific Publishing Company,
TURBULENT TRIBUTION
M. VARADI* Institute (Received
HYDRODYNAMIC OF VOLTAMMETRIC
Amsterdam
-
Printed in The Netherlands
VOLTAMMETRY. PART I. THE DISCURRENT ON ELECTRODE SURFACES
and E. PUNGOR
for General
and Analytical
Chemistry,
Technical
University
Budapest
(Hungary)
14th May 1975)
The effects of hydrodynamic parameters in flowing systems on voltammetric currents have been thoroughly studied by several workers. For the case of an electrode surface perpendicular to the direction of laminar flow, Matsuda [l] gave the following correlation between the diffusion current and the various hydrodynamic parameters v-116 (U/L)112 (1) Id = const. n FACD2R where la is the diffusion limiting current, n the number of electrons taking part in the’electrochemical reaction, F the Faraday constant, A the electrode surface area, C the concentration of the electroactive component, D the diffusion constant, Y the kinematic viscosity, U the linear flow rate, and L the characteristic size of the electrode (e.g. in the case of disc-shaped electrodes L = nr/2, where r is the radius of the disc).. The validity of the above equation was also confirmed by the experimental data of Dikusar and Bardin [2,3] who studied on cone and disc-shaped electrodes, the effects of shape and size on the formation of mass transport. In further investigations, Matsuda and Yarnada [4] determined under well defined geometrical conditions the value of the constants shown in eqn. (1) and proved experimentally the correctness of their equations. They found that when the electrode is arranged so that the electrolyte flows through a jet onto the surface of the disc electrode, the minimum electrode radius which can still be used for the measurements, is 1.5 mm. In the work described here the optimal size of the electrode surface area and the distribution of the mass transport on the electrode surface were investigated empirically for a system in which the electrolyte flows directly from a nozzle onto the electrode surface. In front of the electrode is placed a jet which causes an increase in the linear flow rate of the liquid; because the flow is perpendicular to the surface, a turbulent liquid flow is ensured in the vicinity of the electrode. The construction of the cell and its application to liquid chromatography, have been described in an earlier paper [5]. Since in the case of a single electrode,
* Permnnent Budapest,
address: Hungary.
LABOR
MIM Laboratory
Instruments
and Equipment
Works,
32
the current densities originating from electrochemical reactions taking place on different parts of the electrode surface could not be established, the single electrode was replaced by a multichannel electrode consisting of several symmetrically placed electrodes. In this way it was possible to study the mass transport defined at the various points of the electrode by the flow conditions. EXPERIMENTAL
Reagents The solution examined was a 10-j M potassium hexacyanoferrate(I1) solution in 10” M KCl. The chemicals used were of analytical grade (Reanal, Budapest). Equipment The experimental arrangement is shown in Fig. 1. To transport the liquid, an LMIM LS-204 type micropump with piston was used, thus ensuring a precision of + 0.5% in the liquid flow rate in the range of 20-250 ml h-l to a pressure of 50 atm.
Fig. 1. Schematic diagram of the experimental device: 1, electrolyte reservoir; 2, pump; 3, jet; 4, indicator electrode; 5, reference electrode; 6, electric unit; 7, calibrated receiver.
The electric unit contained the equipment necessary to apply a voltage to the cell and to measure the current. Initially, a Radelkis type OH 102 polsrograph was used for this purpose; the sensitivity of the polarograph conld be adjusted between 8 lo- ’ 1 and 6.4 lo-’ A mm- ‘. Later, the polarizing voltage was obtained from a 3-V battery, and the signal was recorded by means of a Philips oscilloscope type PM 3251. The sensitivity of the oscilloscope and its resolution in time could be adjusted between 2 mV/div-20 V/div and 0.05 ys/div-1 s/div. For the fitting of the oscilloscope, a Keithley differential electrometer type 604 was connected into the circuit. The construction of the measuring cell was identical to that described earlier l
l
33
[5] ; the diameter of the jet opposite the indicator electrode was 0.5 mm or 0.3 mm. Silicone rubber-based graphite electrodes of various sizes served as indicator electrodes 163 for practical purposes, but for theoretical purposes a multichannel electrode was used. A Ag/AgCl reference electrode was employed. The construction of the multichannel electrode is shown in Fig. 2. The body of the electrode is a teflon rod of g-mm diameter which is supplied with five bores, each 1.2 mm in diameter. One of these bores is placed centrally while the other four are made at identical distances approximately 3.5 mm from the central electrode. The sensing surface of the electrode consists of silicone rubber-based graphite polymerized into the bores.
K ti -i-
--o
---CD
--a 42
I
!
J-
!
II
-___
5
I
b_ I
k I
Fig. 2. Conetruction of the multichannel electrode: 1, jet; 2, direction of the fluid streaming; 3-7, indicator electrode; 8, toflon body; 9, silicone rubbor-based graphite; 10, graphite powder; 11, electric connexions; 12, glass tube.
34
..
RESULTS
Effect
AND DISCUSSION
of the surface
area of the electrode
on the voltammetric
current
The effect of the sensing area on the voltammetric signal was studied. Graphite electrodes of different sizes were used with the arrangement shown, in Fig. 1, and the current intensities were measured in both stationary and flowing solutions by means of a polarograph. The linear flow rate of the electrolyte was 6.6 cm s-’ . In the flowing system the measurements were made at 0.76 V vs. the Ag/AgCl electrode. The results obtained are given in Table 1. The maximum values of current intensities measured on the electrode in the stationary solution served as a basis of comparison. As the data in Table 1 show, the maximum increase in current intensity in the flowing system was observed for the electrode with the smallest surface area. In the case of larger surface areas, only a small part of the electrode comes in contact with the fresh solution streaming at high speed from the 0.5-mm jet, and most of the other parts of the electrode receive the liquid transport through a Prandtl layer, the thickness of which depends on the distance from the point where the flow stream hits the electrode. Consequently, increasing the surface of the electrode decreases the current intensity using the same jet system. TABLE
1
Relationship Electrode
between
diameter
the geometrical
(mm)
i fl0wlng
size of the electrode
and the voltammetric -
2
6
7
9
2.31
1.79
0.68
0.50
current
istat.
Investigation
of current
distribution
with a multichannel
electrode
For the examination of the current distribution on the electrode surface, the multichannel electrode shown in Fig. 2 was used. The electrode was connected to the system shown in Fig. 1. First, the values of the current intensities were examined by means of a polarograph. The current intensity on each of the microelectrodes was measured separately, and after the five electrodes had been connected, the total current intensity was established. The applied flow rate was 13.2 cm s-l. The results obtained are summarized in Table 2. As in the earlier experiments, the tabulated data prove that the current intensities of the electrodes placed on the periphery are in fact lower than that on the electrode placed opposite the jet. The current intensities for the peripheral electrodes are nearly equivalent because of their almost symmetrical positions; the small differences
35
occurred because the positioning was not exactly symmetrical. When the flow rate was reduced to 6.6 cm s-l, the results obtained became less reproducible, and the differences found between the current intensities measured on the electrodes at different positions became significantly smaller. TABLE
2
The values of the current Position of electrode*
istat. (10-O
A)
intensities
Total Connected electrodes *Positions
_
204.0 189.3 238.0 181.0 216.0
190.8 198.8 249.9 180.0 334.8
1028.3
1154.3
935.6
1043.1
as indicated
i =
;:?‘“i)
.----
3 4 5 6 7
on electrodes
in Fig, 2. Position
in different i
iflowhp,
’ mcan ___--
istnt. _-.------.
1043.1 p= 935.6 7 is central,
--
- -
0.84 0.95 0.95 0.88 1.39
0.93 1.05 1.05 0.98 1.55 1154.3= 1028.3
positions
1.12
1
1.11
1
and the others
peripheral.
The formation of the momentary current intensities on the electrode surface was then examined. The experimental conditions were the same as before, except that, instead of the polarograph, a memory oscilloscope was used to record the signal; also, the diameter of the jet was 0.3 mm, so that the applied flow rate was increased to 107.9 cm s-l. The results obtained are shown in Fig. 3. The current on the central electrode follows, with only a small distortion, the flow rate of the liquid controlled by the pump. At the peripheral electrodes, the signals are distorted a.nd of reduced amplitude. By tracing the momentary current intensities, the differences between electrodes in different positions are even more striking. When the distance between the electrode and the jet was increased, even the central electrode produced a strongly distorted signal, which was similar to those for the peripheral electrodes; the differences between the amplitudes were then less significant than in Fig. 3. The signals obtained are shown in Fig. 4. The momentary current intensities obtained in the course of the oscilloscopic examination are shown in Table 3; the data are the arithmetical mean values calculated on the basis 0115 curves. Since it was established in the previous experiment that electrodes situated at the periphery were nearly equivalent, the measurement was performed only with one of these. The data in
36
Fig. 3. Oscilloscope results obtained by means of differently placed electrodcs situated near the jet. (A) Peripheral electrode, 3. Amplification, 36; sensitivity, 0.2 V/div; time axis, 1 s/div. (B) Peripheral electrode, 4. Amplification, 35; sensitivity, 0.1 V/div; time axis, 1 s/div. (C) Peripheral electrode, t. Amplification, 36; sensitivity, 0.2 V/div; time axis, 1 s/div. (D) Peripheral electrode, 6. Amplification. 35; sensitivity, 0.1 V/div; time axis, 1 s/div. (E) Central electrode, 7. Amplification, 10; sensitivity, 0.5 V/div; time axis, 1 s/div. Fig. 4. Oscilloscope results obtained by means of differently placed electrodes situated 4-6 mm from the jet, (A) Peripheral electrode, 4. Amplification, 35; sensitivity, 0.1 V/div; time axis, 0.5 s/div. (B) Central electrode, 7. Amplification, 36; sensitivity, 0.1 v/div; time axis, 0.6 sfdiv.
Table 3 show that the electrode placed opposite the jet not only closely follows the flow rate but also produces a large signal in cases when its size is smaller than the diameter of the liquid jet and the flow of the jet is not retarded by the medium in front of the electrode. In conclusion, when turbulent flows are used, efficiency can be increased by reducing the surface area of the electrode. Thus, in contrast to earlier literary data, it was established that for the measurement of very low concentrations, the surface area of the electrode must be decreased to an optimal extent. The analysis of the distortion of the electrode signals will be discussed in a separate paper.
37 TABLE
3
Momentary Position of electrode” 3 4 6 6 7
signal levels on electrodes u, b (mV) --_---9.7 11.6 13.5 7.8 157.5
in different
-.-
positions
----___-..----____
10.8 15.1
aSee footnote, Table 2. bU, is the measured signal when the electrode is placed directly by the jet. ‘U, is the measured signal when the distance between jet and electrode is 4-5
mm.
SUMMARY In voltammetric analysis, when turbulent flows are used, i.e. the electrolyte passes through a jet on to the surface of an indicator electrode placed perpendicularly to the direction of flow, the current can be increased by decreasing the electrode surface area. When a system of this type was examined with a multichannel electrode, the momentary signal on the electrode placed opposite the jet was about 15 times larger than the signal measured on the electrode placed on the periphery about 3.5 mm from the central electrode.
REFERENCES 109. H. Matsuda, J. Electroanal. Chem., 15 (1967) A.J. Dikussr and MB. Bardin, Zh. Anal. Khim., 26 (1971) 1059. MB. Bardin and A.J. Dikusar, Zh. Anal. Khim., 26 (1971) 1068. H. Matsuda and J. Yamada, J. Electroanal. Chem. Interfacial Electrochem., 271; 44 (1973) 189. 5 M. Vliradi, Zs. Feht?r and E. Pungor, J. Chromatogr., 90 (1974) 259. 289. 6 E. Pungor and 8. Szepesvhry, Anal. Chim. Acta, 43 (1968)
1 2 3 4
30 (1971)
261,
Chimica Acta, 80(1976)31-37 Scientific Publishing Company,
TURBULENT TRIBUTION
M. VARADI* Institute (Received
HYDRODYNAMIC OF VOLTAMMETRIC
Amsterdam
-
Printed in The Netherlands
VOLTAMMETRY. PART I. THE DISCURRENT ON ELECTRODE SURFACES
and E. PUNGOR
for General
and Analytical
Chemistry,
Technical
University
Budapest
(Hungary)
14th May 1975)
The effects of hydrodynamic parameters in flowing systems on voltammetric currents have been thoroughly studied by several workers. For the case of an electrode surface perpendicular to the direction of laminar flow, Matsuda [l] gave the following correlation between the diffusion current and the various hydrodynamic parameters v-116 (U/L)112 (1) Id = const. n FACD2R where la is the diffusion limiting current, n the number of electrons taking part in the’electrochemical reaction, F the Faraday constant, A the electrode surface area, C the concentration of the electroactive component, D the diffusion constant, Y the kinematic viscosity, U the linear flow rate, and L the characteristic size of the electrode (e.g. in the case of disc-shaped electrodes L = nr/2, where r is the radius of the disc).. The validity of the above equation was also confirmed by the experimental data of Dikusar and Bardin [2,3] who studied on cone and disc-shaped electrodes, the effects of shape and size on the formation of mass transport. In further investigations, Matsuda and Yarnada [4] determined under well defined geometrical conditions the value of the constants shown in eqn. (1) and proved experimentally the correctness of their equations. They found that when the electrode is arranged so that the electrolyte flows through a jet onto the surface of the disc electrode, the minimum electrode radius which can still be used for the measurements, is 1.5 mm. In the work described here the optimal size of the electrode surface area and the distribution of the mass transport on the electrode surface were investigated empirically for a system in which the electrolyte flows directly from a nozzle onto the electrode surface. In front of the electrode is placed a jet which causes an increase in the linear flow rate of the liquid; because the flow is perpendicular to the surface, a turbulent liquid flow is ensured in the vicinity of the electrode. The construction of the cell and its application to liquid chromatography, have been described in an earlier paper [5]. Since in the case of a single electrode,
* Permnnent Budapest,
address: Hungary.
LABOR
MIM Laboratory
Instruments
and Equipment
Works,
32
the current densities originating from electrochemical reactions taking place on different parts of the electrode surface could not be established, the single electrode was replaced by a multichannel electrode consisting of several symmetrically placed electrodes. In this way it was possible to study the mass transport defined at the various points of the electrode by the flow conditions. EXPERIMENTAL
Reagents The solution examined was a 10-j M potassium hexacyanoferrate(I1) solution in 10” M KCl. The chemicals used were of analytical grade (Reanal, Budapest). Equipment The experimental arrangement is shown in Fig. 1. To transport the liquid, an LMIM LS-204 type micropump with piston was used, thus ensuring a precision of + 0.5% in the liquid flow rate in the range of 20-250 ml h-l to a pressure of 50 atm.
Fig. 1. Schematic diagram of the experimental device: 1, electrolyte reservoir; 2, pump; 3, jet; 4, indicator electrode; 5, reference electrode; 6, electric unit; 7, calibrated receiver.
The electric unit contained the equipment necessary to apply a voltage to the cell and to measure the current. Initially, a Radelkis type OH 102 polsrograph was used for this purpose; the sensitivity of the polarograph conld be adjusted between 8 lo- ’ 1 and 6.4 lo-’ A mm- ‘. Later, the polarizing voltage was obtained from a 3-V battery, and the signal was recorded by means of a Philips oscilloscope type PM 3251. The sensitivity of the oscilloscope and its resolution in time could be adjusted between 2 mV/div-20 V/div and 0.05 ys/div-1 s/div. For the fitting of the oscilloscope, a Keithley differential electrometer type 604 was connected into the circuit. The construction of the measuring cell was identical to that described earlier l
l
33
[5] ; the diameter of the jet opposite the indicator electrode was 0.5 mm or 0.3 mm. Silicone rubber-based graphite electrodes of various sizes served as indicator electrodes 163 for practical purposes, but for theoretical purposes a multichannel electrode was used. A Ag/AgCl reference electrode was employed. The construction of the multichannel electrode is shown in Fig. 2. The body of the electrode is a teflon rod of g-mm diameter which is supplied with five bores, each 1.2 mm in diameter. One of these bores is placed centrally while the other four are made at identical distances approximately 3.5 mm from the central electrode. The sensing surface of the electrode consists of silicone rubber-based graphite polymerized into the bores.
K ti -i-
--o
---CD
--a 42
I
!
J-
!
II
-___
5
I
b_ I
k I
Fig. 2. Conetruction of the multichannel electrode: 1, jet; 2, direction of the fluid streaming; 3-7, indicator electrode; 8, toflon body; 9, silicone rubbor-based graphite; 10, graphite powder; 11, electric connexions; 12, glass tube.
34
..
RESULTS
Effect
AND DISCUSSION
of the surface
area of the electrode
on the voltammetric
current
The effect of the sensing area on the voltammetric signal was studied. Graphite electrodes of different sizes were used with the arrangement shown, in Fig. 1, and the current intensities were measured in both stationary and flowing solutions by means of a polarograph. The linear flow rate of the electrolyte was 6.6 cm s-’ . In the flowing system the measurements were made at 0.76 V vs. the Ag/AgCl electrode. The results obtained are given in Table 1. The maximum values of current intensities measured on the electrode in the stationary solution served as a basis of comparison. As the data in Table 1 show, the maximum increase in current intensity in the flowing system was observed for the electrode with the smallest surface area. In the case of larger surface areas, only a small part of the electrode comes in contact with the fresh solution streaming at high speed from the 0.5-mm jet, and most of the other parts of the electrode receive the liquid transport through a Prandtl layer, the thickness of which depends on the distance from the point where the flow stream hits the electrode. Consequently, increasing the surface of the electrode decreases the current intensity using the same jet system. TABLE
1
Relationship Electrode
between
diameter
the geometrical
(mm)
i fl0wlng
size of the electrode
and the voltammetric -
2
6
7
9
2.31
1.79
0.68
0.50
current
istat.
Investigation
of current
distribution
with a multichannel
electrode
For the examination of the current distribution on the electrode surface, the multichannel electrode shown in Fig. 2 was used. The electrode was connected to the system shown in Fig. 1. First, the values of the current intensities were examined by means of a polarograph. The current intensity on each of the microelectrodes was measured separately, and after the five electrodes had been connected, the total current intensity was established. The applied flow rate was 13.2 cm s-l. The results obtained are summarized in Table 2. As in the earlier experiments, the tabulated data prove that the current intensities of the electrodes placed on the periphery are in fact lower than that on the electrode placed opposite the jet. The current intensities for the peripheral electrodes are nearly equivalent because of their almost symmetrical positions; the small differences
35
occurred because the positioning was not exactly symmetrical. When the flow rate was reduced to 6.6 cm s-l, the results obtained became less reproducible, and the differences found between the current intensities measured on the electrodes at different positions became significantly smaller. TABLE
2
The values of the current Position of electrode*
istat. (10-O
A)
intensities
Total Connected electrodes *Positions
_
204.0 189.3 238.0 181.0 216.0
190.8 198.8 249.9 180.0 334.8
1028.3
1154.3
935.6
1043.1
as indicated
i =
;:?‘“i)
.----
3 4 5 6 7
on electrodes
in Fig, 2. Position
in different i
iflowhp,
’ mcan ___--
istnt. _-.------.
1043.1 p= 935.6 7 is central,
--
- -
0.84 0.95 0.95 0.88 1.39
0.93 1.05 1.05 0.98 1.55 1154.3= 1028.3
positions
1.12
1
1.11
1
and the others
peripheral.
The formation of the momentary current intensities on the electrode surface was then examined. The experimental conditions were the same as before, except that, instead of the polarograph, a memory oscilloscope was used to record the signal; also, the diameter of the jet was 0.3 mm, so that the applied flow rate was increased to 107.9 cm s-l. The results obtained are shown in Fig. 3. The current on the central electrode follows, with only a small distortion, the flow rate of the liquid controlled by the pump. At the peripheral electrodes, the signals are distorted a.nd of reduced amplitude. By tracing the momentary current intensities, the differences between electrodes in different positions are even more striking. When the distance between the electrode and the jet was increased, even the central electrode produced a strongly distorted signal, which was similar to those for the peripheral electrodes; the differences between the amplitudes were then less significant than in Fig. 3. The signals obtained are shown in Fig. 4. The momentary current intensities obtained in the course of the oscilloscopic examination are shown in Table 3; the data are the arithmetical mean values calculated on the basis 0115 curves. Since it was established in the previous experiment that electrodes situated at the periphery were nearly equivalent, the measurement was performed only with one of these. The data in
36
Fig. 3. Oscilloscope results obtained by means of differently placed electrodcs situated near the jet. (A) Peripheral electrode, 3. Amplification, 36; sensitivity, 0.2 V/div; time axis, 1 s/div. (B) Peripheral electrode, 4. Amplification, 35; sensitivity, 0.1 V/div; time axis, 1 s/div. (C) Peripheral electrode, t. Amplification, 36; sensitivity, 0.2 V/div; time axis, 1 s/div. (D) Peripheral electrode, 6. Amplification. 35; sensitivity, 0.1 V/div; time axis, 1 s/div. (E) Central electrode, 7. Amplification, 10; sensitivity, 0.5 V/div; time axis, 1 s/div. Fig. 4. Oscilloscope results obtained by means of differently placed electrodes situated 4-6 mm from the jet, (A) Peripheral electrode, 4. Amplification, 35; sensitivity, 0.1 V/div; time axis, 0.5 s/div. (B) Central electrode, 7. Amplification, 36; sensitivity, 0.1 v/div; time axis, 0.6 sfdiv.
Table 3 show that the electrode placed opposite the jet not only closely follows the flow rate but also produces a large signal in cases when its size is smaller than the diameter of the liquid jet and the flow of the jet is not retarded by the medium in front of the electrode. In conclusion, when turbulent flows are used, efficiency can be increased by reducing the surface area of the electrode. Thus, in contrast to earlier literary data, it was established that for the measurement of very low concentrations, the surface area of the electrode must be decreased to an optimal extent. The analysis of the distortion of the electrode signals will be discussed in a separate paper.
37 TABLE
3
Momentary Position of electrode” 3 4 6 6 7
signal levels on electrodes u, b (mV) --_---9.7 11.6 13.5 7.8 157.5
in different
-.-
positions
----___-..----____
10.8 15.1
aSee footnote, Table 2. bU, is the measured signal when the electrode is placed directly by the jet. ‘U, is the measured signal when the distance between jet and electrode is 4-5
mm.
SUMMARY In voltammetric analysis, when turbulent flows are used, i.e. the electrolyte passes through a jet on to the surface of an indicator electrode placed perpendicularly to the direction of flow, the current can be increased by decreasing the electrode surface area. When a system of this type was examined with a multichannel electrode, the momentary signal on the electrode placed opposite the jet was about 15 times larger than the signal measured on the electrode placed on the periphery about 3.5 mm from the central electrode.
REFERENCES 109. H. Matsuda, J. Electroanal. Chem., 15 (1967) A.J. Dikussr and MB. Bardin, Zh. Anal. Khim., 26 (1971) 1059. MB. Bardin and A.J. Dikusar, Zh. Anal. Khim., 26 (1971) 1068. H. Matsuda and J. Yamada, J. Electroanal. Chem. Interfacial Electrochem., 271; 44 (1973) 189. 5 M. Vliradi, Zs. Feht?r and E. Pungor, J. Chromatogr., 90 (1974) 259. 289. 6 E. Pungor and 8. Szepesvhry, Anal. Chim. Acta, 43 (1968)
1 2 3 4
30 (1971)
261,