the kinetics of electrode processes. ─part iii. the...

89

The Kinetics of Electrode Processes.—Part III. The Behaviour of Platinum and Gold Electrodes in Sulphuric Acid and Alkaline Solutions containing Oxygen.

By G. Armstrong, Ph.D., F. R. H imsworth, B.Sc., and J. A. V. B utler, D.Sc.,University of Edinburgh.

(Communicated by J. Kendall, F.R.S.—Received May 12, 1933.)

The first paper of this series* contained a general survey of the reactions of hydrogen and oxygen at platinum electrodes in dilute sulphuric acid solutions. We have now investigated in greater detail some of the phenomena which were observed, using mainly small polarizing currents, by means of which they can be more conveniently studied, and extending the scope of the experiments to other metals and solutions. In this paper an account is given of further experiments on platinum electrodes in the presence of oxygen, and of an investigation of the behaviour of gold electrodes in sulphuric acid and alkaline solutions. The experimental arrangements were similar to those previously described.

Platinum Electrodes in Oxygen Saturated Solutions.Cathodic Depolarization hy Oxygen.—If a platinum electrode in sulphuric

acid is anodically polarized until oxygen is liberated and the current is then reversed, a marked depolarization process occurs at about eH = + 0 - 0 5 . Bowden5 sf suggestion that this process is the reduction of a platinum oxide was shown in Part I to be untenable, because the length of the arrest of the potential is much reduced even by gentle stirring. The depolarization must be caused by some substance which is formed in the solution during the anodic polarization. Since it appeared that ordinary oxygen in the solution is not very active as a cathodic depolarizer in this region, it was suggested that the discharged oxygen remained for a time in a particularly active condition, but tests of the solution failed to reveal any enhanced oxidizing power.

When a platinum electrode is cathodically polarized in sulphuric acid containing oxygen, the cathodic depolarization which can be ascribed to the reduction of oxygen is variable in amount and in the potential at which it occurs. I t has now been found that if the electrode is previously polarized

* ‘ Proc. Roy. Soc.,’ A, vol. 137, p. 604 (1932) ; cited later as I. t Ibid., A, vol. 125, p. 446 (1929).

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for a short time to the point at which oxygen is formed, well defined and reproducible cathodic depolarization curves are obtained. Some of the curves obtained in this way are shown in fig. 1(a). The electrode had an apparent area of 4 cm.1 2. Before each curve the electrode was polarized anodically with

eH

90 Gr. Armstrong, F. R. Himsworth and J. A. Y. Butler.

1-118 0-951 0-792

0-795 0-73

1-155 0-979 0-738 0-661

+ 0*6

SecondsF ig. 1.—Cathodic depolarization by oxygen at platinum electrodes which have previously

been polarized anodically for a short time. Currents in milliamperes. (a) M/10 H aS 04 ; (6) pH 8 solution ; (c) N /5 NaOH.

1 *5 X 10~3 amps, for 15 seconds, and the solution was then stirred by a streamof oxygen for 5 minutes. The oxygen produced in the anodic polarizationhas no appreciable influence on the length of the cathodic curve, for apart



The Kinetics of Electrode Processes. 91

from the stirring, the amount liberated is insignificant compared with the amount of cathodic polarization. The cathodic current was stopped when the potential reached sH = + 0 • 05, to avoid the formation of hydrogen in the solution.

Beans and Hammett* have shown that while in acid solutions traces of oxygen have no appreciable effect on the reversible hydrogen potential a t platinized electrodes, its electromotive activity is much greater in alkaline solutions. On examining the depolarization produced by oxygen in N/5 sodium hydroxide, we found that reproducible cathodic curves were obtained without the preliminary anodic treatment, if the solution was well stirred between successive experiments. Almost identical curves were obtained when the preliminary anodic polarization was carried out, fig. 1 (c). In a buffer solution of pH = 8 (0 • 05 M K H 2P 0 4 ; 0*047 N NaOH) a considerable amount of cathodic depolarization occurred at electrodes which had not been anodically polarized, but the curves were somewhat unreproducible. Very reproducible curves were obtained after a short anodic polarization, fig. 1 (6).

These curves have characteristics similar to those obtained for cathodic depolarization by solutions of methylene blue and quinhydrone.f In these cases it has been shown that as a result of the gradual depletion of the depolarizer in the vicinity of the electrode, the length of the depolarization curve to the final rapid rise of potential is, for times greater than about 100 seconds, in accordance with the equation

i t co = a + f& ao , (1)

where is the transition time, i.e., the time taken to reach an arbitrarily chosen potential on the rapidly rising part of the curve and i the current.

Table I gives the transition times with various currents for electrodes which had the previous anodic treatment.

The equation (1) holds for transition times above about 80 seconds. The slopewhich has been shown to be approximately proportional to the concentration

of the depolarizer is practically the same in the sodium hydroxide and sulphuric acid solutions, but is appreciably greater in the p L 8 solution. This is probably due to the somewhat greater solubility of oxygen in the buffer solution. I t is evident that the cathodic depolarization in this region which was previously observed at electrodes which have been the anode both in oxygen and hydrogen saturated solutions must be ascribed to a high local concentration of oxygen.

* ‘ J. Amer. Chem. Soc.,’ vol. 47, p. 1215 (1925). t ‘ Proc. Roy. S op . , ’ A, vol. 139, p. 406 (1933).



Anodic Polarization.—After the cathodic depolarization process representedin fig. 1, a considerable amount of anodic polarization is necessary before the potential reaches a steady value. This was previously investigated with large currents. Fig. 2 (a) shows the anodic curves of a 1 cm.2 electrode in sulphuric

92 G. Armstrong, F. R. Himsworth and J. A. V. Butler.

Table I.—Transition Times for Oxygen Depolarization at Platinum Electrodes.i, amps. X 10-3 ; tx, seconds ; Coulombs X 10 3

Sodium hydroxide. P = 3-7 X 10"4.

pn 8 solution. = 6*0 X 10-4.

69140171284494

1035

1-1180-9540-9720-6300-466

7586

105143260

1-1140-7950-7300-6200-5860-569

77111125176289589

6790

133227557

7089

125141170206354

6191

169213288400802

1-1550-9790-7380-6440-5890-5140-442

SecondsFig. 2. (a) Anodic curves of platinum in sulphuric acid (currents X 10~7 amps.). (b)

Anodic curves of platinum with 31-4 X 10~7 amps, after cathodic polarization to various potentials. (The last two curves, initial voltages +0*71 and +0-56 , are displaced horizontally relative to the others. The curve for the initial voltage + 0 -36 is shown in (a).)



93

acid which has been polarized cathodically to +0*36 before each curve. The fall of potential is nearly linear with the time and the quantity of electricity required to cause the potential to change by 0-1 volt increases only slowly as the current is decreased. In Table II is given the number of coulombs required for 0*1 volt between +1*96 and +1-26 volts.

The Kinetics of Electrode Processes.

Table II.

Current .......................... 31-4 58 120 254 372 X 10“7 amps.Coulombs for 0 • 1 volt.... 1-63 1-48 1-56 1-27 1-12 X 10"4

These quantities are much greater than corresponds with the capacity of the double layer, for when the voltage was allowed to decay and a small anodic current was re-started only 1-0 X 10~5 coulombs were required for a change of 0 • 1 volt, and the potential rose initially on cathodic polarization at the same rate. Similar curves at more negative potentials were obtained in N/10 barium hydroxide. The total passage of current during the linear parts of the curves, about 0-6 volts, is 9 x 10 ~4 coulombs. If the true area of the electrode is taken as three times its apparent area (see I) it may be estimated that there are 4*8 X 1015 atoms of platinum at the surface, and the quantity of electricity required to form a layer of oxygen atoms spaced one to each platinum is 16 X 10“4 coulombs, which corresponds fairly closely with the amount observed. I t may therefore be supposed that in the anodic polarization an approximately complete layer of adsorbed oxygen is formed.

It has been suggested (I) that on account of the energy of adsorption the formation of adsorbed oxygen atoms would require a smaller expenditure of energy and would therefore occur at a more negative potential than is required for their continuous formation in the solution. This hypothesis would require that the ions should already be in or near the adsorption positions of the subsequently formed atoms. If the discharge of the ions occurs by the transfer of electrons to the electrode by a process similar to that investigated by Gurney, an equation of the type i = &NeaV, where N is the number of ions in such adsorption positions, would be expected. If N is maintained constant by ion migration during the process the potential should remain constant. The linear fall of the potential during the formation of the adsorbed layer would be explained if the adsorbed oxygen atoms, on account of their electron affinity, behave as dipoles. There would then be a potential difference proportional to the number of adsorbed atoms superimposed on the normal potential difference between the electrode and the charged ionic layer in the solution.



In order to find at what stage of the cathodic polarization this adsorbed layer is destroyed the electrode was polarized cathodically successively to more negative potentials and after each such treatment an anodic curve was taken with a small current. The curves obtained are shown in fig. 2 (b) and in Table III the slopes of the linear parts are given.


Table III.

Initial voltage ...................Slope of anodic curve .......

+ 1*26 + 1-06 + 0-96 +0*8610“3 coulombs/volt.0*10 0 1 0 0*15 0-19 X

Initial voltage ................... + 0-76 +0-71 + 0-56 + 0-36Slope of anodic curve ....... 0-38 0-86 1-41 1-63 X 10~3 coulombs/volt.

So long as the potential does not become more negative than +1-0 volts the slope of the anodic curve corresponds with the capacity of the double layer (1*0 X 10~4 coulombs per volt), but at more negative potentials and as the electrode passes along the oxygen depolarization stage the adsorbed layer is gradually reduced as is shown by the increasing quantity of electricity which is required to replace it. The reduction of the adsorbed layer and depolarization by oxygen thus occur simultaneously. In the cathodic depolarization by methylene blue and quinone it was found that the variation of the electrode potential during the process was given by V = Y0 — RT/^F log q, where q = qo — it + a + represents the amount of the depolarizer near the electrode after the time t. For oxygen the potential rises more rapidly than according to this equation. This can be explained if it be supposed that the depolarization occurs on those parts of the surface which are covered by oxygen. As this adsorbed oxygen is destroyed the available surface diminishes and the depolarization potential will rise more rapidly than if it were dependent only on the concentration of the depolarizer.

Gold Electrodes in Oxygen Saturated Solutions.Behaviour in M/10 Sulphuric Acid.—The gold electrodes were made of

thin gold foil having an apparent area of 1 cm.2, fused to a thin gold wire which was melted into glass tubes. Typical curves showing their behaviour in dilute (M/10) sulphuric acid are given in fig. 3. The first time-potential curve, of which curve I is an example, was somewhat variable, but afterwards they gave very reproducible curves. As with platinum, in the first anodic treatment a considerable amount of depolarization occurs before the potential




reaches a constant value, but if the oxygen overvoltage is allowed to decay for a time and the electrode again made the anode, curve II, the potential falls much more quickly and initially linearly with the time, about 1-8 X 10“5 coulombs being required in this case for the change of 0*1 volt. As with platinum, this is interpreted as being due to the charging of the double layer without depolarization. When the current is reversed the potential begins to rise linearly at the same rate as in curve III and at sH = -f-1 • 1 a depolarization process begins. When this has proceeded for a time an anodic curve of the first type, curve IV, is obtained on subsequent anodic polarization.

After anodic polarization two definite depolarization stages, a, (3, curve V are observed on continued cathodic treatment. The upper stage [3, which is in the same region as the single stage observed with platinum is like the latter much reduced by even gentle stirring,* as in curve VI. When similar experiments were made in an atmosphere of nitrogen, the anodic polarization being stopped at +1 • 8 to avoid the liberation of oxygen, the stage (3 did not appear although a was wTell developed. The former is therefore due to cathodic depolarization by molecular oxygen in the solution. Some cathodic depolarization curves in an oxygen saturated solution at a gold electrode which has not been anodically polarized are given in the inset diagram, fig. 3.

The lower stage a, which is quite unaffected by stirring the solution, and remains unaltered even when the electrode has been removed from the solution and washed, must be due to a depolarizer which adheres to the electrode. After long continued anodic polarization the electrode becomes covered with a black film, which is probably an oxide of gold and since this disappears again at this stage during cathodic polarization the process here must be the reduction of this substance. This black substance has been the subject of a number of investigations.f Jirsa and Buryanek found J that after drying over sulphuric acid its composition was close to Au(OH)3, and after drying at 142° and over phosphorus pentoxide, the proportion of gold was somewhat less than that required for Au20 3.

Curve VII, fig. 3, was obtained on anodic polarization after the potential had been taken cathodically to + 0 • 2. The potential falls linearly from + 0 • 8 to +1 * 3 (A, A) and then falls slowly along the curve CC, which is similar to

* This applies to large currents. With small currents the length of the process is increased by stirring, because the rate at which oxygen diffuses to the surface is then an appreciable factor.

t Jirsa and Bury&nek, e Chem. Listy,’ vol. 16, pp. 189, 299, 328 (1922) ; Jirsa and Jelinek, ‘ Z. Electrochem.,’ vol. 30, p. 286 (1924).

% 6 Z. Electrochem.,’ vol. 29, p. 126 (1923).




I but more extended. The quantity of electricity required in the linear change AA is from two to three times that required for changing the potential of the double layer (curve II) at more positive potentials. This is considerably less than would be required to form an adsorbed oxygen layer, as with platinum. It seems likely that the change is merely a double layer effect, but the capacity of the double layer is considerably greater than that at the oxide covered surface.

When the electrode has been taken along the stage (3 during the cathodic polarization a small break BB appears in the anodic curve at about +1-1,

400Seconds

anC ^creases as the amount of cathodic polarization inage is increased. It appears to be due to small quantities of a reduction

produet; formed at the electrode during the cathodic polarization.The Anodic Process.-The process occurring along CC, fig. 3, had been

s u led in great detail. The curves obtained with comparatively small currents with an electrode having a double layer capacity after anodic polarization of about 1 x 1 0 - Coulombs/0-1 volt, are shown in fig. 4 (a). In order



97

to obtain reproducible conditions the electrode was polarized cathodically to +0-66 before each experiment, after which the solution was stirred with a stream of oxygen for 100 seconds. The total quantity of electricity required to bring the potential to +1-7 volts is nearly constant and is considerably greater than the corresponding amount for platinum electrodes of the sam e

e„

The Kinetics of Electrode Processes.

xlO coulom bs

SecondsFig. 4.— (a) Anodic polarization of gold electrodes in dilute sulphuric acid.

— 0 — 63, — X — 137, — A — 268,X \ /

— □ — 442, — o — 684, — O — 2920 X 10-7 amps.

(b) Cathodic curves after anodic polarization to various stages, 45 X 10-7 amps.

capacity. That the formation of oxide occurs along these curves is shown by fig. 4 (6), in which the electrode was taken by anodic polarization successively to more positive potentials and the current then reversed. The break a appears in the cathodic curve when the electrode has passed +1*27 and its length

vol. c xliii.— a , H



increases as the potential falls to lower values down the curve CC. The efficiency of oxide formation, as given by the ratio of the length of the cathodic break a to the amount of anodic polarization initially approaches 100%, but decreases as the process continues, as can be seen from the following table.


Table IV.—Anodic and Cathodic Polarization of Gold Electrodes in M/10Sulphuric Acid.

Current = 45 X 10~7 amps.

Time of anodic polarization.

Time of anodic polarization

below -f-1 * 27.Length of

cathodic break.Efficiency of

oxide formation (%).

sec. sec. sec.50 0 — —

100 50 50 100200 150 125 84300 250 170 68600 550 340 62

1800 1750 480 27

Jirsa and Buryanek (loc. cit.) found by an analytical method that the current yield of oxide when gold electrodes were anodically polarized in 1 • 1 N . H2S04 at 18° C. was 1*3%. The amount of oxide formed after various periods of anodic polarization can be accurately and simply determined by finding the quantity of electricity required to reduce the oxide by cathodic treatment along the stage a.* The following table shows that this quantity is practically independent of the cathodic current employed.

Table V.—Anodic Polarization, l - 9 x 10 3 Coulombs with 670 X 10 7 amps.

Cathodic current ....................... 670 106 63 13 X 10-7 amps.Cathodic polarization (to -j-0 -67

volts) ....................................... 1-54 1-57 1*56 1*42 X 10~3 Coulombs

Fig. 5 shows the number of Coulombs passed in the stage a, plotted against the total amount of previous anodic polarization. The inset diagram, curve I, gives a small part of the curve for small anodic polarizations on a much greater scale. The efficiency is high during the early stages of the electrolysis, but it rapidly diminishes, and when the current is continued for a long period the

* A few measurements of this kind have been made by Shutt and Walton (c Trans. Faraday Soc.,’ vol. 28, p. 740 (1932)).




amount of oxide increases linearly with the quantity of anodic polarization the efficiency being about 0*9% (15°). The points shown in the diagram were obtained with currents of 4*5 X 10~3 amps, and 0*069 X 10~3 amps. They lie on the same curve and it appears that between these limits at least, the amount of oxide depends on the total amount of anodic polarization and is practically independent of the current density. Some similar measurements at 50° showed that this increase of temperature had little effect on the efficiency.

It has been shown that the formation of oxide can be traced without any discontinuity from the appearance of visible amounts to the first stages of the

10000 20000Anodic polarisation, coulombs x 10~3

30 000

F iq. 5.—Efficiency of oxide formation at gold electrodes. Anodic currents,

— O — 4-5 X 10~3 amps., — X — 0-069 X 10~3 amps.,. — 0- 37 X 10~3 amps.

depolarization process at +1*27 volts. The primary process might be either (a) the passage into solution of gold ions which are immediately hydrolysed and precipitated as auric oxide or hydroxide, the solubility of which has been shown to be very small in dilute sulphuric acid (Jirsa and Jelinek, loc. tit. ) ; or (b) the deposition of oxygen atoms in adsorption places at the surface of the electrode, followed by their re-arrangement leading to the formation of definite molecules of gold oxide Au20 3, or a hydrated form. The forces between the gold atoms at the surface and those in the underlying layers might be so



reduced by the presence of adsorbed oxygen that a re-arrangement, as in the following scheme, could easily occur :—

100 G. Armstrong, F. R. Himsworth. and J. A. V. Butler.

Au An.. 0 Au(Au 20 3)

Au Au. .0 Au

Au Au. .0 Au Au

Au Au. .0 Au Au. .0

In order to distinguish between these possibilities it would be necessary to know whether gold ions are capable of dissolving ionically at the potential at which the process begins. Gold dissolves anodically in dilute hydrochloric acid at about the same potential, but this process is probably affected by complex ion formation, in the absence of which a more positive potential would be required. The mechanism (6) is thus more probable. In either case as the electrode becomes covered with oxide the effective area of the electrode is reduced and the potential at constant current must fall. I t ultimately reaches a value at which the liberation of free oxygen can occur. The oxide is evidently in a porous condition, for it continues to be formed at a slow constant rate even after long electrolysis.

SecondsF ig. 6.—Anodic and cathodic curves of gold electrodes in N/10 NaOH. Currents, amps. X 10“7. Cathodic curves with 104 X 10~7 amps.

Behaviour in A/10 Sodium Hydroxide.—The anodic polarization of gold electrodes in N/10 sodium hydroxide is shown in fig. 6. The curves are linear for a considerable part of their course and are similar to those of platinum in sulphuric acid. The following table gives the quantity of electricity required for a change of 0 • 1 volt in the linear region.




Table VI.—Double Layer Capacity of Electrode, 1*3 X 10 5 Coulombs/0-1volt.

Current ........................... 24-3 39-2 77*1 104 139 206 X 10~7 amp.Coulombs for 0-1 volt .... 2-55 2-51 2*39 2-39 2-23 2 16 X 10-*

The ratio of these quantities to the double layer capacity of the electrode is not much greater than that observed for platinum. After anodic polarization a distinct depolarization stage appears in the cathodic curves, of which examples are shown in fig. 6, at about +0-3. This stage is not so flat as in the corresponding curves in the acid solution, but the process is quite distinct from the oxygen depolarization which begins at +0-16. The quantity of electrolysis required for this process after varying periods of anodic polarization is given in Table VII.

Table VII.—Anodic and Cathodic Current, 104 X 10 7 amps.

Time of anodic polarization ........... 26200

50300

75600

100 seconds 1560 seconds

Anodic polarization ........................... 0-272-08

0-523 1 2

0-786*24

1-05 X 10-3 Coulombs 16-2 X 10~3 Coulombs

Cathodic polarization to +0*2 ....... 0 1 50-64

0-430*72

0-510*79

0-58 X 10-3 Coulombs 0-83 X 10~3 Coulombs

With this current the potential fell linearly on anodic polarization for 90 seconds. As the electrode passes down this linear stage the subsequent cathodic process increases steadily in length, but when the linear fall of potential is completed the rate of increase becomes very small. The following figures obtained with another electrode of rather greater capacity show that the length of this stage continues to increase at a very slow rate even after long continued anodic polarization.

Table VIII.

Anodic polarization ....... 24 307 768 4620 27200 x 10"3 CoulombsCathodic break ............... 1 19 1-36 1-43 1-64 1-72 x 10-3 Coulombs

It is evident that the greatest amount of electrolysis required here in the cathodic process is of the same order as that required to remove a single layer of oxygen atoms at the surface of the electrode. The fact that the anodic process begins at a more negative potential than is required for the ionic



solution of gold, and the similarity between the anodic curves and those of platinum, suggest that the primary process is the same as that postulated for the latter, viz., the formation of an adsorbed layer of oxygen atoms.

At an electrode which had previously been used for the determination of the efficiencies of oxidation in sulphuric acid, however, a considerable amount of oxidation occurred, although the amount, fig. 5, curve II, was less than under similar conditions in sulphuric acid. For example, after 10 Coulombs had been passed with a current of 5-6 X 10“3 amps., 66 X 10~3 Coulombs were required to reduce the oxide formed as compared with 120 X 10-3 Coulombs in sulphuric acid. I t therefore appears that the electrode which has previously been oxidized and reduced in sulphuric acid is much more open to attack in alkaline solution than one which has not been so treated.

Action of Reducing Agents on the Oxide Films.—When platinum electrodes have been anodically polarized in solutions containing hydrogen, the potential returns in a short time at open circuit to the neighbourhood of the reversible hydrogen potential. After anodic polarization under similar conditions the potential of gold electrodes did not become more negative than +1*2 even after a considerable time. If any reduction of the oxide by hydrogen occurs it is a very slow process. However, reducing agents such as potassium ferro- cyanide and hydroquinone are able to reduce the oxide. After anodic polarization in solutions containing these substances the potential becomes rapidly more negative to the point at which the reduction of oxide occurs, remains nearly constant for a time during its reduction and then changes further to the reversible potential characteristic of the solution. On account of the close resemblance between this process and the spontaneous recovery of the potential of anodically passivated gold in solutions of chlorides (Shutt and Walton, loc. cit.) some observations were made of the variation of the time of recovery with the concentration of the reducing agent, which are given in Table IX.

102 Gr. Armstrong, F. R. Himsworth and J. A. V. Butler.

Table IX.—Reduction of Gold Oxide by Hydroquinone.N/10 Sulphuric acid. Anodic polarization, 1*0 X 10~3 amps, for 30 seconds.

Concentration of hydroquinone, gm./ litre ......... 0-33

59A. 7 1 AA O AA

Time of recovery (seconds) .0*0/

271 -uu

172 -00 6-5

o-UU1 _

N/10 Sodium hydroxide. Anodic polarization, 0*5 X 10 3 amps, for 5 seconds.Concentration of hydroquinone, gm./

litre ..........................Time of recovery (seconds)

0 0 198

0-01559

0-0238

0-0322

0-059-5

0-103-5




The concentration of hydroquinone for a given time of recovery is much less in alkaline than in acid solution. Except for the shortest times the results are in accordance with the equation tc — K — ac, where t is the time of recovery and c the concentration of hydroquinone and K and a are constants having the values 21 and 4 in M/10 H 2S04, and 1-1 and 14 in N/10 NaOH. The comparatively small term ac apparently represents the amount of reduction of the oxide layer by hydroquinone during the anodic process. The rate of reduction, as represented by the reciprocal of the time, is therefore proportional to the concentration of hydroquinone. I t is significant that Shutt and Walton (loc. cit.) found a similar relation between the time of the spontaneous recovery of passivated gold in solutions of chlorides and the hydrogen ion concentration. The nature of the reduction process in this system has not, however, been definitely established.

We wish to express our gratitude to the Carnegie Trustees for a Scholarship held by G. A., and a Teaching Fellowship held by J. A. V. B., and to the Committee of the Moray Fund and Imperial Chemical Industries, for grants for photographic paper and apparatus.

Summary.(1) Experiments are described which support the view that on the anodic

polarization of platinum electrodes in sulphuric acid or alkaline solutions a single layer of adsorbed oxygen atoms is formed. On cathodic polarization, the reduction of the adsorbed layer occurs simultaneously with depolarization by dissolved oxygen in the solution.

(2) When gold electrodes are polarized in dilute sulphuric acid, the formation of a definite oxide begins when the potential reaches +1 • 27 volts. The efficiency of oxide formation, which is about 100% in the earliest stages, steadily decreases as the electrolysis proceeds and finally reaches a constant value of about 0*9%. In alkaline solutions the behaviour of gold is very similar to that of platinum, and even after long continued electrolysis the amount of oxidation is not more than corresponds with a single layer of oxygen atoms at the surface. The reduction of the oxide or oxygen films by hydroquinone has been studied.



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