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Measuring streaming potentials on flat surfaces with rotating electrodes
Electroanalytical Chemistry and Interracial Electrochemistry, 56 (1974) 315 - 319
315
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
PRELIMINARY NOTE
Measuring s t r e a m i n g p o t e n t i a l s o n flat surfaces w i t h r o t a t i n g e l e c t r o d e s
R. KNODLER, A. K(JHLINGand G. WALTER Battelle Institute. V., am R6merhof 35, 6000 Frankfurt am Main 90 (G.ER.)
(Received 22nd August 1974)
The measurement of streaming potentials is a well-known method for the determination of the so-called ~'-potential. The application of the method, however, is usually confined to capillaries, diaphragms or porous plugs I . When measuring streaming potentials with this method difficulties may arise especially with regard to the reproduction of the packing characteristics of the plugs 2. Therefore the reproducibility is poor. We have overcome these disadvantages by using a rotating ring-disc electrode with an unusually wide gap between ring and disc. The rotation of the electrode in an electrolyte causes a fluid to move along the electrode surface and thus the development of a streaming potential between ring and disc. An equipment which is also based on a rotating disc system has been described earlier 3 , experimental results however are scarce.. Experimental
We used a commercially-built rotating system with exchangeable electrodes (A. Jaissle, Neustadt/Waibl., G.F.R.) and with ring and discbf silver. The gap between ring and disc consists of black polyethylene ("electrode B"), white polyethylene ("electrode W") and (red) polyvinyl chloride ("electrode R"). The diameter of the silver disc was 3 mm for electrode B, 12 mm for electrodes W and R. The distance between disc and ring was 6.5 mm for electrode B, 4.5 mm for electrodes W and R. In all cases the ring was 2 mm thick. The rotational speed V could be controlled within 0.1% and varied between 20 and 4000 r.p.m. Using a triangular voltage sweep generator, we developed an arrangement which allowed a continuous and automatic variation of the speed with respect to time (dV/dt = const.), This can be done in a wide range, about 20 to 20,000 r.p.m. ~ . Most experiments were carried out at 2500 r.p.m. 2 . At higher values hysteresis effects were observed in some cases,/.e, we obtained different streaming potentials when the speed increased and vice versa.
316
PRELIMINARY NOTE
The solution used was AgNO3 of various concentrations. At concentrations lower than 1 0 - s mol 1-1, difficulties arose in cleaning the solution, whereas at concentrations higher than 10 - 3 the effects which were to be studied, were strongly decreasing. The use of a silver electrode together with silver salt suppressed to a large extent the appearance of a rest potential at zero speed, which is often observed with platinum electrodes 4 . Since sometimes a small rest potential is still present, the values of the streaming potential in the figures were transformed to zero potential at zero speed. This procedure means only a parallel shifting of the curves. Results
Figure 1 shows typical curves in which the streaming potential E is recorded continuously vs. the rotational speed V. Here the concentration is varied using electrode W. The curves proved to be sufficiently reproducible.
erectrocle W
'7
Ag NO3 uJ 6!
~ 4
3
5.10"
2
I
0
'
I
t
500
1000
1500
2000
2500
3000
rotation speed V/r.p.m.
Fig. 1. Dependence o f the streaming potential on the rotational speed at various concentrations for
electrode W. Figure 2 shows the effect of the different electrode materials. Electrode R exhibits the highest values of the streaming potential, whereas at electrodes W and B the valt~es were significantly lower. The shape of the curves corresponds to expressions of the type E=a
Vm
The magnitudes a and m in Fig. 2 are represented in Table 1. If the simple theory of a linear
PRELIMINARY NOTE
317
tO0
electrode R
/
/
electrode W
~u
/
T: I
electrode B
0.1
10-s M
Ag NO:j
0.01
1000
10000 rotQtiono~ speed V / r.p,m,
Fig. 2. Dependence of the streaming potential on the rotational speed for various electrodes in 10-s M AgNO~. TABLE 1 CHARACTERISTIC MAGNITUDESOF THE CURVES IN FIG. 2 Electrode
a
m
R W B
6.4 x 10 -3 [Volt(min) °'27] 3.4 x 10 - s [Volt(min) °'7] 5.5 x 10 -11 [Volt(rain)2"11
0.27 0.7 2.1
fluid stream in a tube according to Helmholtz-Smoluchowski is applied to the system and if the well-known hydrodynamics of the rotating disc s are taken into account, the dependence E = f(F) is expected to be E = V l"s. Obviously, this is not in accordance with our experiments and we therefore intend to investigate the conditions in the electrolytic double layer more closely in order to obtain an expression for the potential in the shear
318
PRELIMINARY NOTE
plane (~-potential). However, a first estimation gave values for this potential which lie in the same order as cited in the literature for powder systems 6 . We observed that with increasing speed the ring becomes more positive. This means that in the electrolytic double layer opposite to the plastic material positive ions are in excess. In order to obtain information on the kinetic behaviour of the double layer, we looked for the response of the streaming potential after a sudden disconnection of the motor. Figure 3 shows a typical example of such an experiment. The decrease in speed was linear and about 1400 r.p.m, s -1 . The potential decreased simultaneously, but did not
uJ 5 c
! :=
4
electrode W 10"s M AO NO3 3
0
I
2
3
time/s
Fig. 3. Dependence of rotational speed and streaming potential on time after a sudden disconnection of the motor. immediately reach zero potential at zero speed. This effect may be attributed to the slow decrease o f the fluid stream at the electrode as well as to desorption processes. The behaviour of the three electrodes investigated showed significant differences in their responses to sudden changes in the rotational speed. The following characteristic magnitudes are listed in Table 2. Vo = rotational speed prior to disconnection (dE/dl0s,o = dE/dV after sudden change in spee d, taken at t = 0 (dE/dF)stat = d E / d F of the stationary curve (see Fig. 1), taken at V = Fo.
319
PRELIMINARY( NOTE TABLE 2 CHARACTERISTICQUANTITIES DESCRIBING THE CHANGE OF POTENTIAL WITH THE CHANGE IN ROTATIONAL SPEED IN 10-s M AgNO3
Electrode
Volt.p.m.
(dE/dV)s,o/mV (r.p.rtt) -~
(dE/dV)stat/mV (r.p.m.) -1
w
1960 250 1960 250 250
1.2 x 10-3 2.1 x 10 -3 3 x 10 - s 2.1 x 10 -2 7 x 10 - s
2.4 x 10-3 6.1 x 10 -3 3.5 × 10 -a 4.5 x 10 -2 7 x 10-s
R B
These values show that (dE/dV)stat is always higher than (dE/dV)s,o except for electrode B. This means that in the case of electrode B the new environment is rearranged rapidly after the change of speed, whereas electrodes W and R need some time to rearrange. The differences between the non-stationary and the stationary cases are even greater at lower velocities, as is also illustrated in Table 2. These facts demonstrate that there are basic differences between the materials investigated in electrolytic solutions. This may be due to the different structure of the electrolytic double layer adjacent to the solution. The current work, which yielded the above preliminary results, is aimed at elucidating some of these aspects. In a further paper we shall give a more quantitative explanation of the effects described here. REFERENCES 1 P. Ney, Zeta Potentiale und Flotierbarkeit yon Mineralen, Springer, Wien, New York, 1973. 2 J.M.W. Mackenzie, MineralsSci. Engng., 3, No. 3 (1971) 25. 3 L.G. Levashova and V.V. Krotov, Vestn. Leningr. Univ., 16 (1969) 137. 4 R.J. Hunter and A.E. Alexander, J. Colloid Sci., 17 (1962) 781. 5 V.G. Levich, PhysicochemicalHydrodynamics, Prentice Hall, Englewood Cliffs, 1962. 6 P. Bene~and M. Paulenova, KolloidZ. Z. Polym., 251 (1973) 766.
315
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
PRELIMINARY NOTE
Measuring s t r e a m i n g p o t e n t i a l s o n flat surfaces w i t h r o t a t i n g e l e c t r o d e s
R. KNODLER, A. K(JHLINGand G. WALTER Battelle Institute. V., am R6merhof 35, 6000 Frankfurt am Main 90 (G.ER.)
(Received 22nd August 1974)
The measurement of streaming potentials is a well-known method for the determination of the so-called ~'-potential. The application of the method, however, is usually confined to capillaries, diaphragms or porous plugs I . When measuring streaming potentials with this method difficulties may arise especially with regard to the reproduction of the packing characteristics of the plugs 2. Therefore the reproducibility is poor. We have overcome these disadvantages by using a rotating ring-disc electrode with an unusually wide gap between ring and disc. The rotation of the electrode in an electrolyte causes a fluid to move along the electrode surface and thus the development of a streaming potential between ring and disc. An equipment which is also based on a rotating disc system has been described earlier 3 , experimental results however are scarce.. Experimental
We used a commercially-built rotating system with exchangeable electrodes (A. Jaissle, Neustadt/Waibl., G.F.R.) and with ring and discbf silver. The gap between ring and disc consists of black polyethylene ("electrode B"), white polyethylene ("electrode W") and (red) polyvinyl chloride ("electrode R"). The diameter of the silver disc was 3 mm for electrode B, 12 mm for electrodes W and R. The distance between disc and ring was 6.5 mm for electrode B, 4.5 mm for electrodes W and R. In all cases the ring was 2 mm thick. The rotational speed V could be controlled within 0.1% and varied between 20 and 4000 r.p.m. Using a triangular voltage sweep generator, we developed an arrangement which allowed a continuous and automatic variation of the speed with respect to time (dV/dt = const.), This can be done in a wide range, about 20 to 20,000 r.p.m. ~ . Most experiments were carried out at 2500 r.p.m. 2 . At higher values hysteresis effects were observed in some cases,/.e, we obtained different streaming potentials when the speed increased and vice versa.
316
PRELIMINARY NOTE
The solution used was AgNO3 of various concentrations. At concentrations lower than 1 0 - s mol 1-1, difficulties arose in cleaning the solution, whereas at concentrations higher than 10 - 3 the effects which were to be studied, were strongly decreasing. The use of a silver electrode together with silver salt suppressed to a large extent the appearance of a rest potential at zero speed, which is often observed with platinum electrodes 4 . Since sometimes a small rest potential is still present, the values of the streaming potential in the figures were transformed to zero potential at zero speed. This procedure means only a parallel shifting of the curves. Results
Figure 1 shows typical curves in which the streaming potential E is recorded continuously vs. the rotational speed V. Here the concentration is varied using electrode W. The curves proved to be sufficiently reproducible.
erectrocle W
'7
Ag NO3 uJ 6!
~ 4
3
5.10"
2
I
0
'
I
t
500
1000
1500
2000
2500
3000
rotation speed V/r.p.m.
Fig. 1. Dependence o f the streaming potential on the rotational speed at various concentrations for
electrode W. Figure 2 shows the effect of the different electrode materials. Electrode R exhibits the highest values of the streaming potential, whereas at electrodes W and B the valt~es were significantly lower. The shape of the curves corresponds to expressions of the type E=a
Vm
The magnitudes a and m in Fig. 2 are represented in Table 1. If the simple theory of a linear
PRELIMINARY NOTE
317
tO0
electrode R
/
/
electrode W
~u
/
T: I
electrode B
0.1
10-s M
Ag NO:j
0.01
1000
10000 rotQtiono~ speed V / r.p,m,
Fig. 2. Dependence of the streaming potential on the rotational speed for various electrodes in 10-s M AgNO~. TABLE 1 CHARACTERISTIC MAGNITUDESOF THE CURVES IN FIG. 2 Electrode
a
m
R W B
6.4 x 10 -3 [Volt(min) °'27] 3.4 x 10 - s [Volt(min) °'7] 5.5 x 10 -11 [Volt(rain)2"11
0.27 0.7 2.1
fluid stream in a tube according to Helmholtz-Smoluchowski is applied to the system and if the well-known hydrodynamics of the rotating disc s are taken into account, the dependence E = f(F) is expected to be E = V l"s. Obviously, this is not in accordance with our experiments and we therefore intend to investigate the conditions in the electrolytic double layer more closely in order to obtain an expression for the potential in the shear
318
PRELIMINARY NOTE
plane (~-potential). However, a first estimation gave values for this potential which lie in the same order as cited in the literature for powder systems 6 . We observed that with increasing speed the ring becomes more positive. This means that in the electrolytic double layer opposite to the plastic material positive ions are in excess. In order to obtain information on the kinetic behaviour of the double layer, we looked for the response of the streaming potential after a sudden disconnection of the motor. Figure 3 shows a typical example of such an experiment. The decrease in speed was linear and about 1400 r.p.m, s -1 . The potential decreased simultaneously, but did not
uJ 5 c
! :=
4
electrode W 10"s M AO NO3 3
0
I
2
3
time/s
Fig. 3. Dependence of rotational speed and streaming potential on time after a sudden disconnection of the motor. immediately reach zero potential at zero speed. This effect may be attributed to the slow decrease o f the fluid stream at the electrode as well as to desorption processes. The behaviour of the three electrodes investigated showed significant differences in their responses to sudden changes in the rotational speed. The following characteristic magnitudes are listed in Table 2. Vo = rotational speed prior to disconnection (dE/dl0s,o = dE/dV after sudden change in spee d, taken at t = 0 (dE/dF)stat = d E / d F of the stationary curve (see Fig. 1), taken at V = Fo.
319
PRELIMINARY( NOTE TABLE 2 CHARACTERISTICQUANTITIES DESCRIBING THE CHANGE OF POTENTIAL WITH THE CHANGE IN ROTATIONAL SPEED IN 10-s M AgNO3
Electrode
Volt.p.m.
(dE/dV)s,o/mV (r.p.rtt) -~
(dE/dV)stat/mV (r.p.m.) -1
w
1960 250 1960 250 250
1.2 x 10-3 2.1 x 10 -3 3 x 10 - s 2.1 x 10 -2 7 x 10 - s
2.4 x 10-3 6.1 x 10 -3 3.5 × 10 -a 4.5 x 10 -2 7 x 10-s
R B
These values show that (dE/dV)stat is always higher than (dE/dV)s,o except for electrode B. This means that in the case of electrode B the new environment is rearranged rapidly after the change of speed, whereas electrodes W and R need some time to rearrange. The differences between the non-stationary and the stationary cases are even greater at lower velocities, as is also illustrated in Table 2. These facts demonstrate that there are basic differences between the materials investigated in electrolytic solutions. This may be due to the different structure of the electrolytic double layer adjacent to the solution. The current work, which yielded the above preliminary results, is aimed at elucidating some of these aspects. In a further paper we shall give a more quantitative explanation of the effects described here. REFERENCES 1 P. Ney, Zeta Potentiale und Flotierbarkeit yon Mineralen, Springer, Wien, New York, 1973. 2 J.M.W. Mackenzie, MineralsSci. Engng., 3, No. 3 (1971) 25. 3 L.G. Levashova and V.V. Krotov, Vestn. Leningr. Univ., 16 (1969) 137. 4 R.J. Hunter and A.E. Alexander, J. Colloid Sci., 17 (1962) 781. 5 V.G. Levich, PhysicochemicalHydrodynamics, Prentice Hall, Englewood Cliffs, 1962. 6 P. Bene~and M. Paulenova, KolloidZ. Z. Polym., 251 (1973) 766.