OBSERVATIONS ON T H E EFFECT OF MECHANICAL AGITATION ON ELECTRODE POTENTIAL1 F. 0. KOENIG Department of Chemistry, Stanford University, California Received June 1.4, 1934 I. INTRODUCTION
The effect of mechanical agitation on the potential of an electrode immersed in an electrolyte solution is of interest because of its bearing on electrokinetics. I n the experiments hitherto carried out on this effect the mechanical agitation has been produced in three different ways: (i) by moving the electrode (15, 6); (ii) by moving the electrolyte (3); (iii) by scraping the surface of contact between the electrode and the electrolyte with some suitable foreign body (2). In this note only the first two methods are considered. The effect is conveniently described in terms of the quantity Acpgiven by
& = Prn
-c~r
(1)
where pr is the electric potential of the electrode with respect to the solution when the system is at rest, and pmthe potential when the system is subjected to agitation. The quantity Ap can evidently be measured by means of a suitable reference electrode. In recent years the quantities Acp have been extensively investigated for various metal electrodes in various electrolyte solutions by Stephan Procopiu (15, 16). This author does not describe his procedure in detail, but seems to have used method i exclusively, the electrode being attached to the armature of an electromagnetic vibrator. Procopiu’s conclusions may be summarized by the equation -Acp
f
(2)
where f is the electrokinetic potential of the electrode in the solution in question, defined in the usual manner. Equat,ion 2 may be called the “electrokinetic theory” of the effect. It is possible to account for equation 2 by making a number of assumptions as to the mechanism of the effect. Let that part of the double layer which, in the usual electrokinetic phenomena of electrosmosis etc., is 1 Presented before the Eleventh Colloid Symposium, held at Madison, Wisconsin, June 14-16, 1934.
455
456
F. 0. KOENIG
displaced relative to the solid phase be called the mobile part; it extends from the “rigidity boundary” (12)out into the interior of the solution and contains a certain net electric charge, say - u coulombs X cm-2. It is assumed (i) that the mobile part of the double layer is completely removed by the mechanical agitation, thereby removing - u coulombs X cm-2. of charge from the liquid side of the rigidity boundary, (ii) that the electroneutrality (10) of what is left of the double layer is restored by the simulfrom the solid side of the taneous disappearance of + u coulombs X rigidity boundary. As a result the potential of the electrode with respect to the solution will be changed by an amount ACP=
which is such that
LT
- pr
~ r n
< >
>
>
(iii) that -Av and { have also the same magnitudes2 It is to be noted that the picture contained in assumptions i and ii implies that the effect is qualitatively of an entirely different kind from the usual electrokinetic effects, in which the mobile part of the double layer is not removed, but is displaced parallel to itself without disturbance of its average structure. The validity of equation 2 is a question of some importance on account of the uncertainty (11) of the f values calculated from the usual electrokinetic measurements and the special difficulty of obtaining such measurements for metals. The uncertainty of the ordinary values also makes it impossible to test equation 2 rigorously. Nevertheless, the best that can be done at the present time is to compare A p and f for the same system, the f being obtained in the usual way. To this end Procopiu (16)has compared his Ap values for the metals platinum, silver, copper, bismuth, lead, and iron in water, with the f values obtained by Burton (5) from the electrophoretic velocities of colloidal particles of the same metals in water. For the metals silver and lead good agreement was found between ;-Ap and {, the metals platinum and iron agreed in sign but not in magnitude, and finally copper showed a discrepancy in both sign and magnitude. The deviations from equation 2 found here may be due to the fact that the colloidal metal particles, owing to their method of preparation, are in many cases covered with a layer of oxide or hydrated oxide (14). The comparison is therefore not decisive, and there are to the author’s knowl-
r
2 This picture of the electrokinetic theory of the effect differs in several features from that of Procopiu (16).
’
EFFECT OF MECHANICAL AGITATION ON ELECTRICAL POTENTIAL
457
edge no other data available which could lead to a similar comparison. What evidence does exist for equation 2 is therefore even more incomplete than that obtainable from a comparison of the usual calculated .( values with Acp. This evidence may be summarized under three heads: (i) The observed Acp values are always of the same order of magnitude as the values calculated from the usual electrokinetic data. (ii) For the metals silver, platinum, nickel, lead, iron, and zinc in water the sign of -Acp is the same as that of .( as found by Burton ( 5 ) or by Coehn and Schafmeister (7). (iii) The curves obtained by Procopiu (16) for Ap as a function of the electrolyte concentration, c, have the same general shape as the familiar - c curves (extrema, isoelectric points). The chief purpose of this note is to suggest that the electrokinetic theory is not the only possible explanation of the effect, a reasonable alternative being the theory that irreversible chemical reactions at the electrode are partly or wholly responsible. The following observations lend support to this idea.
r
11. EXPERIMENTS
The electrodes used in the experiments were of pure soft silver wire about 1 mm. in diameter, of the sort used as anode in a silver coulometer. The mechanical agitation was produced both by causing the electrode to vibrate (method i) and by stirring the solution (method ii). The vibrating electrode was made by soldering a copper lead onto one end of a 4-cm. piece of the silver wire and then sealing the silver wire with DeKhotinsky cement into one end of a Pyrex tube 3 mm. in diameter and 8 cm. long, in such a way that about 2 cm. of the silver wire projected beyond the seal. The copper lead was passed out through a small hole blown through the wall of the glass tube near its other end, which was attached, also by means of demotinsky cement, to the arm of a vibrator made by sawing off the gong and the clapper from a small electric bell. Three such electrodes were prepared: they all gave the same results. Before use, the silver electrodes were always thoroughly polished first with dry rouge and filter paper and finally with dry filter paper alone to remove traces of the rouge. The solution into which the electrode dipped was contained in a shortnecked wide-mouthed 200-cc. flask. Dipping into the latter were, besides the vibrating electrode, a glass stirrer run by a small AX. motor of 2000 R.P.M. and finally a glass siphon leading to another flask of the same solution, into which the snout of the reference electrode, a calomel electrode with 3.5 N potassium chloride, was immersed. With this arrangement the effects of vibrating and stirring could be studied side by side on the same system. The potentials were measured with a Type K potentiometer in conjunction with a Leeds and Northrup HS galvanometer.
458
F. 0. KOENIG
Preliminary experiments showed that only such solutions as enter into a fairly definite and reproducible electrochemical equilibrium with the silver give even approximately reproducible Acp values. Thus in an attempt to check Procopiu’s (16) value, Acp = 15 millivolts for silver in 0.2 N sulfuric acid, values were obtained lying anywhere between 6 and 30 millivolts depending on the length of time the electrode was allowed to remain immersed in the solution. Except where otherwise stated, the following experiments were carried out with potassium chloride solutions of various concentrations, saturated with silver chloride. 1. Nature of the vibration efect in time A few minutes after the silver electrode is plunged into a potassium chloride solution saturated with silver chloride, it reaches a state in which
CELLAgl3.0 M KC! SATURATED WITH AgC113.5 N KCl SATURATED WITH HgClIHg, AS A FUNCTION OF THE TIME A, vibration started; B, vibration stopped. The silver electrode is the negative pole, so that Ap>O. FIQ. 1.
E.M.F. OF THE
its potential changes less than 0.0001 volt per minute and may be regarded as constant. If then the electrode is caused to vibrate, its potential immediately increases (Acp > 0) to a new practically constant value which is maintained as long as the vibration lasts. On stopping the vibration the potential graduaEEy, during the course of five to eight minutes, returns to the value before vibration. Figure 1, showing an experiment with 3.0 N potassium chloride, illustrates this behavior. Acp is always taken 8s the difference (with proper sign) between the steady values. 2. Dependence of Ap o n the frequency of the vibrator
From the sound of the vibrator it was evident that its frequency increased with the voltage used to run it. Figure 2, in which the observed Acp for 1.0 N potassium chloride is plotted against the voltage applied to
EFFECT OF MECHANICAL AGITATION ON ELECTRICAL POTENTIAL
459
the vibrator, is typical of all the experiments with potassium chloride solutions and shows that Acp increases somewhat with the frequency, at least up to 6 volts. The application of 10 volts to the vibrator caused a sudden jump in frequency of two octaves. This increase in frequency in no case changed the Acp values appreciably from those obtained at 6 volts. This fact makes it probable that the’Acp for 6 volts is practically a “saturation value.” It is this “saturation value” which was always taken as the 2o
O
FIQ.
2. Arp
FOR
t
i
i
j
i
i
6’
7 VOLTS
1 N KCl SATURATED WITH SILVER CHLORIDE, AS VOLTAGEAPPLIEDTO THE VIBRATOR
A
FUNCTION OF THE
TABLE 1 A cp values as a function of the potassium chloride normality c, MOLES OF
KC1 PER LITER OF SOLUTION
AV
miZEinoZts
10-3
27 25 32 17 6
10-* 10-1 1.0 3.0
=k 2
f2 =I= 2 f2 1 2
Acp of the system. For a potassium chloride solution of a given concentration the Acp so obtained is reproducible t o rt 2 millivolts.
3. The Acp - c curve Table 1 gives the Acp values obtained by vibration, as a function of the potassium chloride normality, c. Each of the Acp values is the average of nine experiments (three electrodes, three sets of solutions). Figure 3 shows Acp plotted against log c. Each of the five points is represented by a vertical line indicating the limits of reproducibility of Acp (& 2 millivolts). The curve has the general characteristics of the familiar - c curves.
4. The efect of stirring If the solution is stirred, as described above, an effect of the same sign as that produced by vibration of the electrode results. The Acp increases
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F. 0. KOENIG
slightly with the rate of stirring, but the values obtained for the maximum rate of 2000 R.P.M. were 2 to 5 millivolts less, in the different solutions, than the maximum vibration values given in the above table, 6. Eflect of the removal of air
The stirring experiment was carriedout in an atmosphere of nitrogen as follows. Nitrogen from a tank was purified by passing it first through a strongly alkaline solution of sodium anthraquinone-B-sulfonate and sodium hyposulfite, then over dry calcium chloride, and finally over copper gauze at a dull red heat. An apparatus was set up in which the potassium chloride-silver chloride solutions to be investigated could be boiled and cooled while the pure nitrogen was bubbled through them, and then forced into a stoppered flask, previously filled with nitrogen and containing a glass stirrer introduced through an air-tight mercury seal, a silver electrode, and
4 3 I 0 I LOGC FIG.3. A p FOR KCI SOLUTIONS SATURATED WITH SILVER CHLORIDE, AS A FUNCTION OF THE LOQARITHM OF THE POTASSIUM CHLORIDE NORMALITY, c 0'
a siphon to the reference electrode. It was found that stirring gave Acp values of the same magnitude as in the presence of air, but that when, after conclusion of the experiment in nitrogen, air was bubbled through the solutions, the potential q,.of the silver electrode at rest increased by 3 to 5 millivolts. That is, the Ap effect persists when the air is replaced by nitrogen, but the silver electrode at rest is more positive in the presence of air than of pure nitrogen.
6 . Efect of sodium bisulfite The addition of 1 g. of sodium bisulfite t o 100 cc. of potassium chloride solution caused the complete disappearance of the effect in solutions of all concentrations up t o 3 N . 7 . Silver nitrate solutions
Finally, the effect of vibration and stirring was investigated with the silver electrodes immersed in silver nitrate solutions. It was found that N and 1N . Ap = 0 for silver nitrate concentrations between
EFFECT OF MECHANICAL AGITATION ON ELECTRICAL POTENTIAL
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111. DISCUSSION
The above observations can all be qualitatively explained either by the electrokinetic theory described above or by the following “chemical theory.” It is assumed that the silver immersed in potassium chloride solutions saturated with silver chloride tends towards electrochemical equilibrium not only with respect to the Ag+ ions: Ag+ (metal) G Ag+ (solution) but also with respect to the dissolved oxygen: 40H- (solution)
e 2Hz0 (solution) + O2 (solution) + 4 0
(metal)
Le., the silver metal functions also as an oxygen electrode, and is therefore the more positive the greater the concentrations of dissolved oxygen in its immediate neighborhood. It is furthermore assumed that the following irreversible chemical reaction : 4Ag (metal)
+ 4H+ (solution) + 4C1- (solution) + = 4AgC1 (solid) + 2Hz0 (solution)
0 2
(solution)
is always proceeding slowly a t the metal surface. This reaction tends to decrease the concentration.of dissolved oxygen in the immediate neighborhood of the electrode. Vibrating and stirring aid diffusion and therefore keep the oxygen concentration closer to its value in the bulk of the solution, thus producing a positive Ap. That an oxygen electrode effect is present a t the silver surface in potassium chloride solutions saturated with silver chloride is rendered fairly certain by observation 5 above, that the electrode is more positive in the presence of air than of pure nitrogen. That Acp nevertheless persists after boiling out the solutions in a current of pure nitrogen may be due to the fact that it is impossible to remove dissolved oxygen completely by this method. Sodium bisulfite, on the other hand, is known to remove the last traces of oxygen from a solution, so that the chemical theory is confirmed by observation 6. When the silver-ion concentration is appreciable compared with the concentration of dissolved oxygen, the effect of the latter on the electrode potential is negligible compared with that of the former. The chemical theory therefore predicts Acp = 0 for silver nitrate solutions of moderate concentrations, in accordance with observation 7. In this connection it is to be noted that Henry (9), using the method of the deflection of thin wires in an electric field, found practically zero deflection for the system silver-silver nitrate, but whether this means that { is really zero or merely that the specific conductivity of the wire is large compared with that of the solution (1) is not certain,
462
F. 0. KOENIG
Regarding the effect of stirring (observation 4), it is to be notedthatthis effect implies a conflict between the electrokinetic theory of A(a and the generally accepted ideas as to the kinetics of heterogeneous reactions. According to the latter, there is generally at the surface of a solid in contact with a solution an unstirrable liquid film (4,8,13). If this is true, then the mobile part of the double layer cannot, as demanded by the electrokinetic theory, be removed by stirring, and Ap should be zero. From the point of view of the chemical theory there is evidently no conflict of this sort. Regarding observation ‘1, it may be noted that practically nothing is known regarding the time necessary for the formation of the mobile part of the double layer. It is no doubt greater than the time required for the formation of an ionic atmosphere (“relaxation time”), which is of the order of ‘O-” -seconds. But that it should be a matter of minutes seems improbC
able. The time effect shown in figure 1 would therefore seem to favor the chemical rather than the electrokinetic theory. It is of course possible that the observed effects result from an overlapping of electrokinetic and chemical processes rather than from either kind of process alone. Finally, it is worth mentioning that the idea that chemical reactions involving dissolved gases are at least partly responsible for A(a is strongly supported by the observations of Charmandarjan and Peruschwin (6), who studied the electric current which flows from a platinum electrode agitated in a solution of sulfuric acid, hydrochloric acid, nitric acid, oxalic acid, or phosphoric acid to another platinum electrode at rest in the same solutions. It was found that the magnitude and, for hydrochloric acid, even the sign of the current is different depending on whether or not the solution has been subjected to electrolysis before the measurement. IV. SUMMARY
The electrokinetic theory of the effect of mechanical agitation upon electrode potential is critically discussed. It is pointed out that this theory is not the only possible explanation, an alternative being the theory that irreversible chemical processes are partly or wholly responsible for the effect. Experiments with silver electrodes in potassium chloride solutions saturated with silver chloride and in silver nitrate solutions are described which lend support to the chemical theory of the effect. REFERENCES
(1) ABRAMSON,H. A.: Electrokinetic Phenomena and Their Application to Biology and Medicine, p. 213. American Chemieal Society Monograph No. 66. The Chemical Catalog Co., New York (1934). (2) BENNEWITZ, K., AND SCHULZ, J.: Z. physik. Chem. 124, 115 (1926). BENNEWITZ, K., AND BIQALBE,I.: Z. physik. Chem. 164A, 113 (1931).
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BILLITZER,J.: Z. physik. Chem. 48, 542 (1904). BRUNNER,E.: Z. physik. Chem. 47, 56 (1904). BURTON,E. F.: Phil. Mag. 11, 440 (1906); 12, 472 (1906); 17, 583 (1909). CRARMANDARJAN, M. O., AND PERUSCHWIN, B. J.: Z. Elektrochem. 36, 248 (1930). (7) COEHN,A., AND SCHAFMEISTER, 0.: Z. physik. Chem. 126, 401 (1927). (8) DAVIS,H. S., AND CRANDALL, G. S.: J. Am. Chem. SOC.62, 3757, 3769 (1930). (9) HENRY,D. C.: Proc. Roy. SOC.London 133A, 106 (1931). (10) KOENIG,F. 0.:J. Phys. Chem. 36, 118 (1934). (11) MCBAIN,J. W.: J. Phys. Chem. 28, 706 (1924). (12) MULLER, H.: Cold Spring Harbor Symposia on Quantitative Biology 1, 2 (1933). (13) NOYES,A. A., AND WHITNEY,W. R.: Z. physik. Chem. 23, 689 (1897). (14) PENNYCUICK, S. W.: J. Chem. SOC. 1930, 1447. (15) PROCOPIU, S.: J. chim. phys. 19, 121 (1921). S.: Z. physik. Chem. 164A, 322 (1931). (16) PROCOPIU, (3) (4) (5) (6)
