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Enhancement of polarographic reduction currents by a static magnetic field
T&in@,
1972,Vd.I9,p~.497
to 503. PaeamonprrsS.prio~
in NonbMlIrolaad
EN~NCEMENT OF POLAR~G~P~IC REDUCTION CXJRRENTS BY A STATIC MAGNETIC FIELD SHIZUO FWIWARA and Y~SHIO UMFZAWA Ekpartment of Chemistry, Faculty of Science, University of Tokyo, Hong0 lf3, Tokyo, Japan (Received 19 June 1971. Accepted 9 September 1971)
Summary-The effect of a magnetic field on d.c. poIsrographic reduction currents was studied with a static magnetic field applied ~~~~~1~ to the dropping mercury electrode. In the presence of the magnetic field, diffusron or migration currents show a shght but distinct increase. The factors which can influence this effect have been examined experimentally, The effect is interpreted in terms of suppression of transfer of concentration polariration from one drop to the next. It is shown that certain types of maxima are enhanced by application of a magnetic field. IT WASFOUNDin this laboratory that the maximum wave current in d.c. polarography
decreases on appli~tion of a static magnetic Seld.l*a The decrease in current is always observable for the second kind of maxima, whereas for the first kind of maxima it occurs only with certain types of chemical systems. The magnetic field does not influence the drop-time and the flow-rate of mercury. The phenomenon has been interpreted in terms of retardation, by the magnetic field, of tangential motion at the surface of a mercury drop. The effect has also been examined in a.c. polaro~aphy3 and os~illopolaro~phy.7 In all these investigations 1--pthe magnetic field effect was to decrease the maximum wave current, and this can be interpreted in terms of ma~etohydrodyn~ics. However, it has also been founda that a magnetic field causes a small increase in the limiting current. This paper gives the expe~mental details and inte~re~tion of this effect, and shows that certain types of the maxima of the first kind are enhanced by the magnetic field. The effect of the magnetic field may be useful in elucidation of reaction processes at the surface of the dropping mercury electrode (DME). EXPERIMENTAL The d.c. polarograms were obtained with a Yanagimoto Polarograph Model PA-102 with a scan-rate of O-2V/ruin. Current-time curves during a single drop-lie at fixed potential were obtained by using a potentiostat constructed in this laboratory and a spectrum computer (Model JRA-5, Japan Electron Optics Co.). A magnetic field of 5.6 x 10’ A/m was applied perpendicularly to the DME. The magnetic field is homogeneous within about 10 ppm over the whole volume of the glass cell. Some sample solutions were passed through a charcoal filter bed before measurement in order to eliminate surface-active substances. Potent&is were measured o.r. the mercury pool anode. RESULTS
AND DISCUSSION
The magnetic field often increases the limiting current by only a smal1 amount (typically2 1%) and can be observed only if the current is magnified to some extent. This enhancement implies that mass transport from the bulk solution to the electrode surface is increased by application of the magnetic field, presumably because of a stirring motion of electrolytes at the mercury/solution interface, brought about by the magnetic field. This statement is consistent with the results obtained by others. Leont’ev and Smirnov found that application of a magnetic field resulted in mixing of the electrolyte during electrolysis, which in turn caused a re~st~butio~ 497
Sn~~ucr FUIIWARAand
498
Yosmo UMEZAWA
of cathodic current density .6 Center et al. studied the effect of a magnetic field on electrolysis at a mercury pool cathode and found that the mercury/solution interface is hydrodynami~iIy stirred by the magnetic field.@ In the present system, it is assumed that the surface of a mercury drop is stirred hydrodynami~lly by a force F, defined as F=JB CO (where B is the magnetic field, and J the electrolytic current flowing at the mercury~so~utio~ interface) and that this results in an increase, in some cases, in mass transport to the electrode surface. The possibility that the force is due to direct interaction of the magnetic field with a diama~et~c mercury drop is considered very slight, Eflect on dijiiusion current With the system 5mM lysine in 1.44 lithium chloride at pII 2-5, we observed a current minimum, i.e., lowering of the diffusion current just after the fall of the maximum of the first kind; in the region of this current minimum, a rna~e~~ field affects the current intensity appreciably as shown in Fig. 1. Similar results are
I%.
I.---Effect of
magneticfieldon the minimumfollowingthe first l&d of maximum. 5mh4@Sineand 1M LXX.pH 2-5 (a): Without magneticfield (b): App~~~o~ of noetic field
The observed for ImM nickel or cobalt chloride with no supporting electrolyte. current minimum following a maximum of the first kind has been studied by several workers,7*8 and is attributed to the transfer of concentration polarization (TCP) from one drop to the next, which causes depletion of the depolarizer in the neighbourhood of the electrode. After the fall of the negative maximum (of the first kind, i.e., a maximum which appears at a potential negative with respect to the el~tro~apillary zero potential), the depletion of the solution is especially enhanced,
Enhancement of poiarographic reduction currents by a static magnetic field
499
since at the maximum the depleted solution is transferred upwards and accumulates under the capillary in the vicinity of the drop, resulting in the current intensity being decreased after the m~mum to less than the limiting diffu~on current. Hence it may be concluded that the magnetic field can partially suppress the TCP, through the stirring action produced by I; at the surface of the DME. This is consistent with the result obtained by Smoler,7*s*10who found that the current minimum does not appear when the capillary orifice is bent at an angle of 90” or 45”. He interpreted this in terms of removal of depleted solution from the capillary orifice, brought about by a falling drop. He observed a feeble downward streaming of the solution along the capillary orifice when the drop falls, which is a characteristic phenomenon of the inclined capillary and not observed in the case of the ordinary capillary. As shown in Fig. 2, the magnetic field effect varies for different parts of the polarogram. It is most pronounced at the region of limiting diffusion current,
h
I
008pA
Fro, 2.-Comparison of magnetic field effect at various portions of the poiarogram. 2mM NiCI, and no supporting electrolyte drop-time 3.49 set (at - 140 V, h = 0.70 m). + : magnetic field switched on - : magnetic field off Sample solution was passed through a charcoal filter bed before measurement.
and almost imperceptible on the wave itself (as might be expected since depletion increases with wave-height). At the beaning of the wave the current is slightly decreased by the applied field (3 in Fig. 2)s The enhancement of the limiting diffusion current (1 in Fig. 2) is most marked at the beginning of growth of a mercury drop. This is consistent with the results of Hans et al. IJ who found the contribution from TCP is somewhat greater during the early growth of the drop. Because of difficulty in reproducing the hydrodynamic conditions, 7 the results are poorly reproducible, however (c$ Fig. 3). Eflect of surface-active substances. Addition of O*OOl% of gelatin eliminates the increase in the limiting current caused by the magnetic field. It has been established7*10 that the addition of surface-active substances lowers the TCP, and
500
SHIZUO FUIIWARAand Yosmo
UMEAZWA
FIN. 3.-Effect of the drop-time on the magnetic field effect. 2mM NiCl, and no supporting electrolyte at -1.60 V (indicated by arrow on the d.c. polarogram) (b): t = 7.87 set (a): t = 4.28 set, + : magnetic field switched on - : magnetic field off sample solution was passed through a charcoal filter bed before measurement.
this has been explained in terms of formation of a film of surface-active substance round the drop, this film being retained by the falling drop and carrying with it the depleted layer of the solution. As the surfactant has already suppressed the TCP, the magnetic field of necessity has no effect. Influence of the drop-time. As shown in Fig. 3, the magnetic field enhancement of the limiting diffusion current is more pronounced when the drop-time is reduced. Smoler’ found that the minimum after a negative maximum becomes more marked if the drop-time is shortened. When the drop grows slowly, the TCP effect is nullified because there is enough time for diffusion to occur. Rapid growth enhances TCP, which in turn is reduced by the magnetic field. Concentration of supporting electrolyte. Increasing the concentration of supporting electrolyte diminishes the enhancement and may even cause a decrease in the limiting current when a magnetic field is applied. This may be explained as follows. TCP is expected to appear almost irrespective of the concentration of supporting electrolyte. However, addition of too much supporting electrolyte results in appearance of the second kind of maxima (which are due to the’ tangential motion brought about at the surface of a mercury drop by high flow-rateg). It has been shown that the second kind of maxima are reduced appreciably by application of a magnetic field.2 According to Hans et al. l1 the second kind of maxima result in an increase in TCP (for the reasons given for the first kind). An increase in concentration of supporting electrolyte therefore results in formation of the second kind of maximum and then two opposing effects occur: (a) the effect of the magnetic field is enhanced because of the increase in TCP due to the maximum of the second kind, but (b) the magnetic field appreciably decreases the current of the second kind of maximum. With the SmoIer-type capillary, i.e., a capillary inclined at 45”, the magnetic field decreased the limiting current when the concentration of supporting electrolyte
Enhancement of polarographic reduction currents by a static magneticfield
501
was decreased, which can be explained in terms of the second kind of maxima, caused by the high flow-rate of mercury which is characteristic of this capillary. The dependence of the magnetic field effect on the magnitude of current intensity, i.e., a test of equation (l), was also studied. However, the results were poorly reproducible, because of differences in adsorbability of reaction products, which may determine the hydrodynamic conditions at the interface (cJ effect of surface-active substances, above). Enhancement of thejrst
kind of maxima
As shown in Fig. 4, (2mM uranyl sulphate, no supporting electrolyte), a maximum of the first kind may be enhanced appreciably by a magnetic field. Increasing the
.
-I.-*.*
m-m,vL
(III-.-IC
FIO. 4.-Relation between magnetic field effect on the lirst kind of maximumand mercury reservoir height h. 2mM UOISOIand no supportingelectrolyte (a), (a’): no magneticfield (b), (V): magneticfield on. mercury head, however, increases the current at the maximum and the magnetic field effect becomes unobservable. A similar result is obtained for oxygen in 10-3M potassium chloride with a trace of sodium polyacrylate, where the potential at which the maximum falls abruptly becomes more negative when the magnetic field is applied or the mercury head increased. This effect has been examined for a variety of chemical systems, but only a few show enhancement. Indeed, many show a decrease instead,lea and their current-time curves during growth of a mercury drop fluctuate violently, that will be discussed elsewhere. The enhancement can be correlated with the anomalous shape of the current-time curve (i-t curve) during growth of a mercury drop at the maximum. For example, for 5mM copper chloride in 0.1M ammonia-ammonium chloride, the current at the maximum increases with time, according to i = KF3, but falls abruptly at the later stages of drop growth until the diffusion current (i = kP) is attained (Fig. 5). A similar trend is observed for the uranyl sulphate and oxygen systems. The abrupt fall is probably due to formation of a weakly absorbed layer of surface-active substance,
Smzuo FUIIWARAand Yosmo UMEZAWA
502
Fm. S--Relation
between magnetic geld effect on the first kind of maxima and the shape of the c~ent-me curve at the maxima. 5mM CuClp and O+lMNH,-NH&I Cmrent-time curves are recorded at -0.42 V. (A), (a): with rna~et~~ field (B), (lj): no magnetic field
or an electrolysis product at the surface of the DME, resulting in disappearance of the maximum. Coverage of the surface of a DME by adsorbents is especially pronounced in the later stages of drop growth, the rate of expansion of surface area then being small.* The magnetic field is clearly demonstrated in Fig. 5, the abrupt fall of the i-t curve being eliminated by the magnetic field. A possible explanation is that the force F disturbs the surface layer and removes any surface-active substances from the neighbourhood of the capillary orifice, resulting in a continuation of the current at a value i = liW until the drop falls. When the mercury head is increased, the abrupt faI1 of current is reduced by the increased rate of expansion of the surface area of the DME. * The surface area A of the growing me~y
drop can be ~pre~nt~ as a fiction of time,lt and dA/dt = f kt-“t8. From this it can be Concluded that dA/dt be-mm small with increase in t. A = kP,
Enhancement
of poiarographic
reduction currents by a static magnetic field
503
It appears that the present model for the effect of the magnetic field is reasonable; further study of this effect may be usefui in exploration of the structure of the electrode double layer and the mechanism of transport phenomena at the DME. Acknowledgement-The authors thank Teruhiko Kugo, Yanagimoto discussions and help in the experimentai work.
Co., Kyoto, for his valuable
Zusanunenfassung-Der EinfluD eines Magnetfelds auf gleichstrompoiarographische Reduktionsstrome wurde untersucht. Ein stat&ches -M&etfeid wurde senkrecht zur Qu~ksiI~~ropfelekt~de aneeleet. In Gegenwart des Magnetfeides nehmen Diffusions- und W&rd&mgsstr&e urn einen kl&ren, aber deutlichen Retrag zu. Die Faktoren, die diesen Pro& beeini?ussen k&men, wurden experimentell untersucht. Der Effekt wird dadurch erkhtrt, daf3 die Ubertragung der Konzentrationspolarisation von einem Tropfen zum nachsten unterdriickt wird. Es wird gezeigt, daO beim Anlegen eines Magnetfeldes bestimmte Typen von Maxima verst&rkt werden. Resume-On a CtudiC i’effet dun champ magnetique sur les courants de reduction polarographique en courant continu avec un champ ma~~iique statique appliqu6 ~~ndiculairement & I’electrode a goutte de mercure. En la presence du champ magnetique, les courants de diffusion ou de migration montrent un accroissement leger mais net. On a examine experimentalement les facteurs qui peuvent influer sur cet effet. L’effet a Cti: interpret6 en fonction de la suppression du transfert de la polarisation de concentration dune goutte a la suivante. On montre que certains types de maximums sont renfor& par I’application dun champ magnetique.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
REFERENCES S. Fujiwara, H. Haraguchi and Y. Umezawa, Anal. Chem., 1968,40,249. S. Fujiwara, Y. Umezawa and T. Kugo, ibid., 1968,40,2186. S. Fujiwara, H. Kojima, Y. Umezawa and T. Kugo, J. Eleetrortal, Chem., 1970,26,53. S. Fujiwara, Y. Umezawa, T. Kugo and M. Hikasa, Anal. Ckem., 1970,42, 1005. A. V. Leont’ev and A. G. Smimov, Tr. Kazansk. Khim-Tekrwl. Inst., 1964, No. 33,36, Chem. Abstr, 1966,65, 1763C. E. J. Center, R. C. Overbeck and D. L. Chase, Anal. Chem., 1951,23,1134. I. Smoler, J. Electroanal. Chem., 1963,6,465. L. Holleck, and A. M. Shams El Din, J. Electroanal. Chem., 1968,17,365. J. Heyrovskj and J. Kuta, Princ@les of Paiarograpky, Academic Press, New York, 1966. J. Kdta and I. Smoler, Progress in Polurography, Vol. I, p. 43. Interscience, New York, 1962. W. Hans, W. Henne and E. Menrer, 2. Elektrochem.. 1954.58.836. G. S. Smith, Nature, 1949,164,664.
1972,Vd.I9,p~.497
to 503. PaeamonprrsS.prio~
in NonbMlIrolaad
EN~NCEMENT OF POLAR~G~P~IC REDUCTION CXJRRENTS BY A STATIC MAGNETIC FIELD SHIZUO FWIWARA and Y~SHIO UMFZAWA Ekpartment of Chemistry, Faculty of Science, University of Tokyo, Hong0 lf3, Tokyo, Japan (Received 19 June 1971. Accepted 9 September 1971)
Summary-The effect of a magnetic field on d.c. poIsrographic reduction currents was studied with a static magnetic field applied ~~~~~1~ to the dropping mercury electrode. In the presence of the magnetic field, diffusron or migration currents show a shght but distinct increase. The factors which can influence this effect have been examined experimentally, The effect is interpreted in terms of suppression of transfer of concentration polariration from one drop to the next. It is shown that certain types of maxima are enhanced by application of a magnetic field. IT WASFOUNDin this laboratory that the maximum wave current in d.c. polarography
decreases on appli~tion of a static magnetic Seld.l*a The decrease in current is always observable for the second kind of maxima, whereas for the first kind of maxima it occurs only with certain types of chemical systems. The magnetic field does not influence the drop-time and the flow-rate of mercury. The phenomenon has been interpreted in terms of retardation, by the magnetic field, of tangential motion at the surface of a mercury drop. The effect has also been examined in a.c. polaro~aphy3 and os~illopolaro~phy.7 In all these investigations 1--pthe magnetic field effect was to decrease the maximum wave current, and this can be interpreted in terms of ma~etohydrodyn~ics. However, it has also been founda that a magnetic field causes a small increase in the limiting current. This paper gives the expe~mental details and inte~re~tion of this effect, and shows that certain types of the maxima of the first kind are enhanced by the magnetic field. The effect of the magnetic field may be useful in elucidation of reaction processes at the surface of the dropping mercury electrode (DME). EXPERIMENTAL The d.c. polarograms were obtained with a Yanagimoto Polarograph Model PA-102 with a scan-rate of O-2V/ruin. Current-time curves during a single drop-lie at fixed potential were obtained by using a potentiostat constructed in this laboratory and a spectrum computer (Model JRA-5, Japan Electron Optics Co.). A magnetic field of 5.6 x 10’ A/m was applied perpendicularly to the DME. The magnetic field is homogeneous within about 10 ppm over the whole volume of the glass cell. Some sample solutions were passed through a charcoal filter bed before measurement in order to eliminate surface-active substances. Potent&is were measured o.r. the mercury pool anode. RESULTS
AND DISCUSSION
The magnetic field often increases the limiting current by only a smal1 amount (typically2 1%) and can be observed only if the current is magnified to some extent. This enhancement implies that mass transport from the bulk solution to the electrode surface is increased by application of the magnetic field, presumably because of a stirring motion of electrolytes at the mercury/solution interface, brought about by the magnetic field. This statement is consistent with the results obtained by others. Leont’ev and Smirnov found that application of a magnetic field resulted in mixing of the electrolyte during electrolysis, which in turn caused a re~st~butio~ 497
Sn~~ucr FUIIWARAand
498
Yosmo UMEZAWA
of cathodic current density .6 Center et al. studied the effect of a magnetic field on electrolysis at a mercury pool cathode and found that the mercury/solution interface is hydrodynami~iIy stirred by the magnetic field.@ In the present system, it is assumed that the surface of a mercury drop is stirred hydrodynami~lly by a force F, defined as F=JB CO (where B is the magnetic field, and J the electrolytic current flowing at the mercury~so~utio~ interface) and that this results in an increase, in some cases, in mass transport to the electrode surface. The possibility that the force is due to direct interaction of the magnetic field with a diama~et~c mercury drop is considered very slight, Eflect on dijiiusion current With the system 5mM lysine in 1.44 lithium chloride at pII 2-5, we observed a current minimum, i.e., lowering of the diffusion current just after the fall of the maximum of the first kind; in the region of this current minimum, a rna~e~~ field affects the current intensity appreciably as shown in Fig. 1. Similar results are
I%.
I.---Effect of
magneticfieldon the minimumfollowingthe first l&d of maximum. 5mh4@Sineand 1M LXX.pH 2-5 (a): Without magneticfield (b): App~~~o~ of noetic field
The observed for ImM nickel or cobalt chloride with no supporting electrolyte. current minimum following a maximum of the first kind has been studied by several workers,7*8 and is attributed to the transfer of concentration polarization (TCP) from one drop to the next, which causes depletion of the depolarizer in the neighbourhood of the electrode. After the fall of the negative maximum (of the first kind, i.e., a maximum which appears at a potential negative with respect to the el~tro~apillary zero potential), the depletion of the solution is especially enhanced,
Enhancement of poiarographic reduction currents by a static magnetic field
499
since at the maximum the depleted solution is transferred upwards and accumulates under the capillary in the vicinity of the drop, resulting in the current intensity being decreased after the m~mum to less than the limiting diffu~on current. Hence it may be concluded that the magnetic field can partially suppress the TCP, through the stirring action produced by I; at the surface of the DME. This is consistent with the result obtained by Smoler,7*s*10who found that the current minimum does not appear when the capillary orifice is bent at an angle of 90” or 45”. He interpreted this in terms of removal of depleted solution from the capillary orifice, brought about by a falling drop. He observed a feeble downward streaming of the solution along the capillary orifice when the drop falls, which is a characteristic phenomenon of the inclined capillary and not observed in the case of the ordinary capillary. As shown in Fig. 2, the magnetic field effect varies for different parts of the polarogram. It is most pronounced at the region of limiting diffusion current,
h
I
008pA
Fro, 2.-Comparison of magnetic field effect at various portions of the poiarogram. 2mM NiCI, and no supporting electrolyte drop-time 3.49 set (at - 140 V, h = 0.70 m). + : magnetic field switched on - : magnetic field off Sample solution was passed through a charcoal filter bed before measurement.
and almost imperceptible on the wave itself (as might be expected since depletion increases with wave-height). At the beaning of the wave the current is slightly decreased by the applied field (3 in Fig. 2)s The enhancement of the limiting diffusion current (1 in Fig. 2) is most marked at the beginning of growth of a mercury drop. This is consistent with the results of Hans et al. IJ who found the contribution from TCP is somewhat greater during the early growth of the drop. Because of difficulty in reproducing the hydrodynamic conditions, 7 the results are poorly reproducible, however (c$ Fig. 3). Eflect of surface-active substances. Addition of O*OOl% of gelatin eliminates the increase in the limiting current caused by the magnetic field. It has been established7*10 that the addition of surface-active substances lowers the TCP, and
500
SHIZUO FUIIWARAand Yosmo
UMEAZWA
FIN. 3.-Effect of the drop-time on the magnetic field effect. 2mM NiCl, and no supporting electrolyte at -1.60 V (indicated by arrow on the d.c. polarogram) (b): t = 7.87 set (a): t = 4.28 set, + : magnetic field switched on - : magnetic field off sample solution was passed through a charcoal filter bed before measurement.
this has been explained in terms of formation of a film of surface-active substance round the drop, this film being retained by the falling drop and carrying with it the depleted layer of the solution. As the surfactant has already suppressed the TCP, the magnetic field of necessity has no effect. Influence of the drop-time. As shown in Fig. 3, the magnetic field enhancement of the limiting diffusion current is more pronounced when the drop-time is reduced. Smoler’ found that the minimum after a negative maximum becomes more marked if the drop-time is shortened. When the drop grows slowly, the TCP effect is nullified because there is enough time for diffusion to occur. Rapid growth enhances TCP, which in turn is reduced by the magnetic field. Concentration of supporting electrolyte. Increasing the concentration of supporting electrolyte diminishes the enhancement and may even cause a decrease in the limiting current when a magnetic field is applied. This may be explained as follows. TCP is expected to appear almost irrespective of the concentration of supporting electrolyte. However, addition of too much supporting electrolyte results in appearance of the second kind of maxima (which are due to the’ tangential motion brought about at the surface of a mercury drop by high flow-rateg). It has been shown that the second kind of maxima are reduced appreciably by application of a magnetic field.2 According to Hans et al. l1 the second kind of maxima result in an increase in TCP (for the reasons given for the first kind). An increase in concentration of supporting electrolyte therefore results in formation of the second kind of maximum and then two opposing effects occur: (a) the effect of the magnetic field is enhanced because of the increase in TCP due to the maximum of the second kind, but (b) the magnetic field appreciably decreases the current of the second kind of maximum. With the SmoIer-type capillary, i.e., a capillary inclined at 45”, the magnetic field decreased the limiting current when the concentration of supporting electrolyte
Enhancement of polarographic reduction currents by a static magneticfield
501
was decreased, which can be explained in terms of the second kind of maxima, caused by the high flow-rate of mercury which is characteristic of this capillary. The dependence of the magnetic field effect on the magnitude of current intensity, i.e., a test of equation (l), was also studied. However, the results were poorly reproducible, because of differences in adsorbability of reaction products, which may determine the hydrodynamic conditions at the interface (cJ effect of surface-active substances, above). Enhancement of thejrst
kind of maxima
As shown in Fig. 4, (2mM uranyl sulphate, no supporting electrolyte), a maximum of the first kind may be enhanced appreciably by a magnetic field. Increasing the
.
-I.-*.*
m-m,vL
(III-.-IC
FIO. 4.-Relation between magnetic field effect on the lirst kind of maximumand mercury reservoir height h. 2mM UOISOIand no supportingelectrolyte (a), (a’): no magneticfield (b), (V): magneticfield on. mercury head, however, increases the current at the maximum and the magnetic field effect becomes unobservable. A similar result is obtained for oxygen in 10-3M potassium chloride with a trace of sodium polyacrylate, where the potential at which the maximum falls abruptly becomes more negative when the magnetic field is applied or the mercury head increased. This effect has been examined for a variety of chemical systems, but only a few show enhancement. Indeed, many show a decrease instead,lea and their current-time curves during growth of a mercury drop fluctuate violently, that will be discussed elsewhere. The enhancement can be correlated with the anomalous shape of the current-time curve (i-t curve) during growth of a mercury drop at the maximum. For example, for 5mM copper chloride in 0.1M ammonia-ammonium chloride, the current at the maximum increases with time, according to i = KF3, but falls abruptly at the later stages of drop growth until the diffusion current (i = kP) is attained (Fig. 5). A similar trend is observed for the uranyl sulphate and oxygen systems. The abrupt fall is probably due to formation of a weakly absorbed layer of surface-active substance,
Smzuo FUIIWARAand Yosmo UMEZAWA
502
Fm. S--Relation
between magnetic geld effect on the first kind of maxima and the shape of the c~ent-me curve at the maxima. 5mM CuClp and O+lMNH,-NH&I Cmrent-time curves are recorded at -0.42 V. (A), (a): with rna~et~~ field (B), (lj): no magnetic field
or an electrolysis product at the surface of the DME, resulting in disappearance of the maximum. Coverage of the surface of a DME by adsorbents is especially pronounced in the later stages of drop growth, the rate of expansion of surface area then being small.* The magnetic field is clearly demonstrated in Fig. 5, the abrupt fall of the i-t curve being eliminated by the magnetic field. A possible explanation is that the force F disturbs the surface layer and removes any surface-active substances from the neighbourhood of the capillary orifice, resulting in a continuation of the current at a value i = liW until the drop falls. When the mercury head is increased, the abrupt faI1 of current is reduced by the increased rate of expansion of the surface area of the DME. * The surface area A of the growing me~y
drop can be ~pre~nt~ as a fiction of time,lt and dA/dt = f kt-“t8. From this it can be Concluded that dA/dt be-mm small with increase in t. A = kP,
Enhancement
of poiarographic
reduction currents by a static magnetic field
503
It appears that the present model for the effect of the magnetic field is reasonable; further study of this effect may be usefui in exploration of the structure of the electrode double layer and the mechanism of transport phenomena at the DME. Acknowledgement-The authors thank Teruhiko Kugo, Yanagimoto discussions and help in the experimentai work.
Co., Kyoto, for his valuable
Zusanunenfassung-Der EinfluD eines Magnetfelds auf gleichstrompoiarographische Reduktionsstrome wurde untersucht. Ein stat&ches -M&etfeid wurde senkrecht zur Qu~ksiI~~ropfelekt~de aneeleet. In Gegenwart des Magnetfeides nehmen Diffusions- und W&rd&mgsstr&e urn einen kl&ren, aber deutlichen Retrag zu. Die Faktoren, die diesen Pro& beeini?ussen k&men, wurden experimentell untersucht. Der Effekt wird dadurch erkhtrt, daf3 die Ubertragung der Konzentrationspolarisation von einem Tropfen zum nachsten unterdriickt wird. Es wird gezeigt, daO beim Anlegen eines Magnetfeldes bestimmte Typen von Maxima verst&rkt werden. Resume-On a CtudiC i’effet dun champ magnetique sur les courants de reduction polarographique en courant continu avec un champ ma~~iique statique appliqu6 ~~ndiculairement & I’electrode a goutte de mercure. En la presence du champ magnetique, les courants de diffusion ou de migration montrent un accroissement leger mais net. On a examine experimentalement les facteurs qui peuvent influer sur cet effet. L’effet a Cti: interpret6 en fonction de la suppression du transfert de la polarisation de concentration dune goutte a la suivante. On montre que certains types de maximums sont renfor& par I’application dun champ magnetique.
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
REFERENCES S. Fujiwara, H. Haraguchi and Y. Umezawa, Anal. Chem., 1968,40,249. S. Fujiwara, Y. Umezawa and T. Kugo, ibid., 1968,40,2186. S. Fujiwara, H. Kojima, Y. Umezawa and T. Kugo, J. Eleetrortal, Chem., 1970,26,53. S. Fujiwara, Y. Umezawa, T. Kugo and M. Hikasa, Anal. Ckem., 1970,42, 1005. A. V. Leont’ev and A. G. Smimov, Tr. Kazansk. Khim-Tekrwl. Inst., 1964, No. 33,36, Chem. Abstr, 1966,65, 1763C. E. J. Center, R. C. Overbeck and D. L. Chase, Anal. Chem., 1951,23,1134. I. Smoler, J. Electroanal. Chem., 1963,6,465. L. Holleck, and A. M. Shams El Din, J. Electroanal. Chem., 1968,17,365. J. Heyrovskj and J. Kuta, Princ@les of Paiarograpky, Academic Press, New York, 1966. J. Kdta and I. Smoler, Progress in Polurography, Vol. I, p. 43. Interscience, New York, 1962. W. Hans, W. Henne and E. Menrer, 2. Elektrochem.. 1954.58.836. G. S. Smith, Nature, 1949,164,664.