T H E BEHAVIOR O F IRON AND NICKEL ANOS DEIN VARIOUS ELECTROLYTES Review of Theory and General Results BY E. P. SCHOCH AND C. P. RANDOLPH
In a previous communication by one of us ( I ) * it was shown that the nickel anode in sulphate electrolytes exhibits a behavior which practically proves the suggestion previously made by LeBlanc that the process of metal dissolution is a velocity phenomenon, It was also shown that when the current density is large enough t o produce anodic polarization exceeding a few millivolts, then another process may take place simultaneously-namely, the “oxidizing” discharge of anions. About the same time as the time of appearance of this article, LeBlanc published his studv of polarization with the aid of the oscillograph (2) as a result of which he sets forth essentially the same view of the details of anodic actions. He produces experimental evidence that polarization due t o concentration effects alone does not exceed a few rnjllivolts, and he considers that greater anodic polarization is due to the (simultaneous) discharge of oxygen producing anions. The latter he distinguishes as chemical polarization. This view, thus arrived at independently from different observations, may hence be considered to be fairly well established. LeBlanc did not study the effect of the discharge of oxygen producing anions upon the surface of the electrode, nor the peculiar relation of conditions that brings about the marked change in the rate of dissolution of the metal, which characterizes more particularly the phenomenon of passivity ; but he is of the opinion that passivity phenomena involve no actions other than those set forth above. One of us ( I ) had definitely established this view through the study of
*
The numbers in parenthesis given throughout the article refer to the list of references given a t the end.
720
E. P. Schoch and C. P. Randolph
experiments directed to this end. The theoretical view then proposed has been further confirmed and extended through the experimental work in this paper and is here given in its present form. The relative tendency to produce either one of the two possible modes of anodic action-i. e . , metal dissolution or oxidation-depends on (a)the specific metal; ( b ) the electrolyte, or more particularly, the specific anions; and (c) their manner of contact as influenced by diffusion, etc. With any particular metal and anion-e. g., nickel with sulphationthe diffusion influence is naturally largely dependent on the current density, or the potential gradient in the layer of electrolyte adjacent to the anode surface. With current densities that produce a polarization of only a few millivolts, no noticeable change is produced on the anode, but with larger current densities portions of the surface are oxidized, and thus rendered inactive as far as metal dissolution is concerned. The oxidized and the unchanged surface spots exert different opposing electromotive forces, hence the current density is no longer uniform all over the surface, but greater over the active spots. This produces here a steeper potential gradient and sets up an unstable, special or “accidental” diffusion relation which favors the tendency toward metal dissolution, so that this action may even take place exclusively. Though this stationary condition may maintain itself fairly indefinitely yet any uncontrollable or unknown influence may easily disturb it so that the oxidizing action may set in again. The interruption of the current, even for a moment, destroys any such particular diffusion relation. If a current of the same density is then reapplied as soon as possible, the oxidizing discharge will be found to be relatively prominent-i. e., the extent of the active surface is reduced-and this continues until another special diffusion relation such as the one above establishes itself. With extensive anodic polarization these special diffusion relations cannot, in general, maintain themselves, and hence the oxidizing discharge finally continues until practically
Behavior of Iron and Nickel Anodes
721
the whole surface has been rendered inactive, and the evolution of oxygen sets in. The two different kinds of surface spots-the active and the oxidized or inactive-may serve as the poles of a shortcircuited cell: by the action of this cell, the oxidized spots are discharged whenever the current is cut off or whenever its density is diminished so that the relation of the electromotive forces permits this local current to pass. In (I) the conclusiqn was reached that the particular effect wrought upon the surface of the anode by the oxidizing discharge could be either a film of free oxygen gas or a cover of metal oxide. The remarkable progress recently made in our knowledge of the potential of the oxygen electrode has thrown much light on this point, but as the whole subject will be found reviewed elsewhere in THISJOURNA-L (3) it is sufficient here t o present a summary of the general results as follows: (a) During the discharge of oxygen yielding anions all metal electrodes are oxidized. ( b ) The potential of the electrode is that of the oxide irrespective of any (adsorbed) oxygen gas also present. (c) The oxides specifically determine the potentials with which oxygen is evolved. ( d ) The amount of an oxide that must be actually present t o give all characteristic effects may be less than is optically perceptible. (e) Oxygen gas does not appear t o be directly electromotively active. ’ The formation of an oxide layer on a passivized metal electrode may be considered to be pretty well establishedat least all the arguments based upon optical observations, and the argument that passivity with acid or alkaline electrolytes respectively should be due t o different causesall these drop t o the bottom because there are positive indications of the formation of oxides when optical means fail, and the anodic formation of oxides in the presence of acids is also known t o be a fact. However, there is one point
722
E. P. Schoch and C. P. Randolph
that possibly needs some further consideration; i. e . , the faculty of some anodes to become soluble again at nobler potentials after they have become passivized. The example that is before us in this connection is that of chromium, which Hittorf has demonstrated so carefully (4). This example is cited particularly by the adherents of the “special metal surface’’ theory, or the “electron density” theory. W. J. Mueller (5) has recently published a study of the anodic behavior of thallium in which the author considers that he has found an example similar t o chromium. When we examine carefully into the details of these examples and eliminate what appears to us as doubtful cases-namely, the direct formation from the metal of trivalent chromium ions or of bivalent thallium ions (see below)-then we reach the general conclusion that after an anode has become passivized with reference to metal dissolution, the second stage of solubility entails the removal, by reaction, of oxygen together with the metal, usually in the form of a complex ion. Thus a chromium anode that has become passive with reference t o the formation of bivalent ions, does not in general become active again to form trivalent ions, but becomes active in forming chromations, in the formation of which oxygen also enters. Hence it is likely that these actions consume a part of the oxidized surface, somewhat as illustrated by E. Mueller (6) in his study of the anodic behavior of copper in sodium hydroxide solutions. The “special metal surface” theory or the “electron density”) theory lead us to expect that the direct formation of ferric ion would take place on an iron pole after it has become passive with reference t o the formation of ferro ion, yet this direct formation has never been observed and in our experiments we were particularly careful to watch this point. Upon examination we also find that the dissolution of chromium as a trivalent ion has never appeared as an intermediate stage of activity between that of the formation of the bivalent ion and of the chromation. Whenever the formation of trivalent chromion takes place it is under conditions which
Behavior of Iron and Nickel Anodes
723
make it possible for the formation to be indirect-either through the reduction of the chromation by alcohol, or the oxidation of the bivalent ion by a layer of oxide which had been previously formed by heating. The observations of W. J. Mueller on thallium which lead him to consider that a state of activity exists corresponding to the dissolution of the thallium anode as bivalent ion-intermediate between the state of formation of monovalent thallium and of thallic oxide respectively-this observation should not be interpreted as he has because bivalent thallium is otherwise not known to exist, and the voltage and amperage relations observed may be due just as well to an entirely different cause than that which the author considers. Thus there is no example left of the second stage of activity of an anode which does not involve the consumptiQn of some of the oxygen of the protective oxide cover. A phenomenon that is fundamentally identical with the one just described is presented by the electrolysis of alkaline solutions of acetates with iron or nickel anodes and this example supports our contention admirably. In its specific influence upon anode actions, the acet-ion appears t o be similar to the sulphation. The iron or nickel anodes passivize in this electrolyte with relatively small current densities, but at lower potentials the anodes become active again: the increase in current density at these low potentials is not surprising, because oxidation of the acet-ion takes place, but the extensive metal dissolution which takes place here and which has also been observed and commented upon by other investigators (7) requires special consideration. In alkaline solutions the acet-ion is extensively oxidized to methyl alcohol, particularly on iron and nickel anodes (8) and this action uses more oxygen than that corresponding to the discharge of the acet-ion. Hence portions of the oxide surface are probably reduced, the metal surface is temporarily laid bare and a part is dissolved before the oxide cover is reformed over it. Many essential elements of our theoretical view have
724
E. P. Schoch and C. P. Randolfih
been previously advanced by other investigators, notably by Haber (9) in his theory of “mobile pores” and again by Sackur and Alvarez who suggest that the formation of the oxide may be a velocity phenomenon of the reaction between the anions and the anode. Since these publications together with those previously referr+edto represent the latest views of the subject of passivity (except one-see below) we believe that our view extends the theory in the right direction. Foerster (11) 1ooks.upon the function of oxygen in this connection as a negative catalyzer of the process of metal dissolution and thus was led to consider hydrogen as a positive catalyzer of this process. Inasmuch as the quantities (concentrations) of hydrogen and oxygen theoretically present on the electrode are reciprocally related, the facts will fit his theory as well as the oxide theory; but we believe that the relation of the facts as presented by Foerster’s view is less direct than that presented according to our view. The experimental work in this paper was primarily directed to test further the theoretical views which so far had been established with nickel in sulphate and chloride electrolytes only. Experiments were made ( a ) with neutral solutions of potassium sulphate, nitrate, perchlorate, chlorate, bromate, iodate, chromate, and hydroxide, and with sodium acetate; ( b ) with acidified and alkaline solutions of the sulphates, nitrates, and acetates respectively; and finally (c) with mixtures of sulphate with chromate and sulphate with fluoride. Many of these solutions were tried on both nickel and iron anodes. The results of these experiments not only confirmed the theoretical view held by us but led us t o extend our view in so far as we realized that both modes of anodic a c t i o n i . e., metal dissolution and oxidizing action-are specifically influenced by the particular anions present. According t o the prevailing theory of metal dissolution, which assumes that the metal changes direct to cations, this particular process does not appear t o be necessarily influenced by the anions present; hence we held a t first that the specific influence of the anion was limited t o the oxidizing
~
Behavior of Iron and Nickel Anodes
725
action, and the fact that halides prevent the attainment of passivity was ascribed t o their reaction with oxygen. However, fluorides have the same effect as anode actions in chlorides, etc., and their effect cannot possibly be due t o any reaction with oxygen. This and other considerations forced us to the conclusions that the nature of the anion also asserts a specific effect upon metal dissolution. When arranged in the decreasing order of their tendency t o effect metal dissolution (or the increasing order of tendency to effect oxidation-which is naturally the reciprocal of the former) the anions present the following list: ( a ) Halogens, ( b ) sulphates, (c) acetates, (d) perchlorates, ( e ) nitrates, and ( f ) chromates, chlorates, bromates, iodates, hydroxidesthe latter all practically in one class. Of all anions, iodation seems to have the greatest tendency to anodic oxidizing action. The actions of the mixtures of two anions indicate that the specific influence of an anion shows itself irrespective of the presence or absence of other anions. The mixture will show a relative tendency toward metal dissolution as contrasted with oxidizing action which is just what we would expect if every anion would react individually with the electrode. The total effect is somewhat proportional to the concentrations of the different anions, although it is modified by the action of the cell formed between the active and inactive spots." Sackur and Alvarez ( I O ) have also found in a similar experiment that mixtures of chlorion with sulphation give results that vary as the relative concentrations of these ingredients vary. Although no attempt is t o be made here to formulate any details of the mechanism of the two modes of anodic change, yet we feel that we should point out that the marked specific influence shown by every anion in a mixture indicates that some individual interaction takes place between the anions
*
I n this connection it should be remembered t h a t the electromotive force of this cell-and hence its rate of action-depends directly o n ' t h e concentration of the hydroxylion (respectively, hydrion) in the electrolyte.
726
E. P. Schoch and C . P. Randolph
and the anode (or with the metal ions when they are first formed). Of this we are particularly impressed when we stop to realize that the free energy difference of either mode of anodic change is the same irrespective of the intermediate steps or of the anions that take part in the change; and furthermore, that there is no difference in the ease of electrical discharge of one anion as compared with another, because strictly spe-aking there is no such force as intensity of fixation of the electron upon the ion. In concluding this discussion of general results we wish t o point out that the results here attained are of importance in the problem of the corrosion of iron, because fundamentally considered the latter is an electrolytic phenomenon and hence involves the same facts as those here considered. With the conclusions reached in this paper it is readily seen that the corrosive character of an aqueous solution could not be extensively lessened by additions of certain electrolytes such as chromates, hydroxides, etc., as was once hoped for; again it is seen that any previous passivizing or “chromating,” etc., of an iron surface would not protect it appreciably against the action of a natural water containing sulphates or chlorides, etc. Experimental Details The immediate object of the following experiments is t o ascertain the potentials exhibited by an iron or a nickel anode as the latter is subjected to various current densities in different electrolytes. In designing the apparatus the following points were considered-the reasons for which are self-evident from the theoretical view advanced for the phenomena t o be studied: ( a ) To remove and exclude oxygen from both the electrolyte and the surface of the anode. ( b ) To prepare the surface of the anode in a definitely reproducible manner. (c) To stir the liquid next to the anode effectively. ( d ) To keep the composition of the electrolyte around the anode constant-above all to prevent the accumulation
Behavior of Iron. and Nickel Anodes
727
of ferrous ions, since their oxidation would complicate the phenomena. ( e ) To prevent access of hydrogen to the anode. The apparatus designed t o meet the above requirements and which was used in the experiments with iron anodes is shown in Fig. I . h is a three-liter flask, g a glass tube which acts as a siphon and conducts the solution t o the U-tube n ; c is a “return” condenser, b a cooling-jacket, and i a vessel containing ice. a is a three-way stop-cock, whereby any air that enters the tube g may be allowed to escape; and s is a glass tube which connects the capillary f with the calomel electrode, not shown, and which is filled through
the thistle tube d. In Fig. 2 , n is the U-tube 1 2 mm. inner diameter, and of thin glass, in which the electrolysis takes place, q is the anode, with an exposed area of 0.57 sq. cm., the vertical surface being covered by the rubber tubing i; $J is a very small rubber stopper; c, a copper wire soldered to the iron which makes the electrical connection for the electrolyzing current (through a brass sleeve, d , and a brush); e is a pulley; m , a brass tube; and d , a piece of rubber tubing. In the left arm of the U-tube, the thermometer I and the cathode (a platinum wire) lz are held in position by the stopper 0. The operation for making a series of measurements was the following: The U-tube was filled with dilute hydrochloric acid, and this was allowed t o remain long enough to
728
E. P. Schoch and C . P. Randolph
clean the surface of the anode thoroughly. The acid was then displaced with water which had been boiled in the flask a to free it from air; and the latter in turn was displaced by the boiled electrolyte. The electrolyte then in the U-tube the was boiled for several minutes and cooled to 23'-27', temperature a t which all the measurements were made. During electrolysis the electrolyte was passed through the U-tube at a rate of I O to 2 0 cc per minute, depending upon the current, and the anode was rotated a t the rate of 200 revolutions per minute. The potential was read as soon as it had become approximately constant after increasing the current, which in most cases was within two to three minutes after changingithe electromotive force.
Fig.
2
The apparatus and the method of measurement used in the experiments with nickel is essentially the same as the above except that a larger anode, 3.47 sq. cm. in area, was used. The concentrations of all solutions are given in gramequivalents ( I equivalent = I N). The table of polarization values for potassium permanganate, for ammonium persulphate and for ferric sulphate were not determined because these solutions react extensively on contact with these metals.
Behavior of Iron and Nickel Anodes
729
Ferrous sulphate was not used because it is oxidized a t the anode. All current densities in the following tables are given in milli-amperes per I sq. cm., and all.potentials are measured against the normal calomel electrode in international volts, and no correction is applied for diffusion potentials. The algebraic signs prefixed t o the potentials are those shown by the anode in the experimental arrangements-in accordance with the suggestion of Luther (13). The potentials are said t o decrease or fall when they become more noble and vice versa. For nickel, all preliminary experiments concerning the potentials shown without current in various electrolytes and their relation to the reversible potential in nickel salt solutions, as well as for the reproducibility of the surface condition, For iron, we made the following experiare found in ( I ) . ments: The potential of ordinary sheet iron in N/I FeSO, solution was found to agree with that given by Richards and Behr (14). The precautions taken t o prevent oxidation in making up the solution were as suggested in the abovementioned article, i. e., the flask was thoroughly washed out with hydrogen, then freshly boiled water was run in upon the salt which had previously been put into the flask, and hydrogen was passed through the solution. The potential of sheet iron in this solution rose in 4 hours to -0.714 volt which was the maximum value attained. The potentials of different samples of ordinary wrought iron in N/I K,SO, were found to be practically the same, -0.795 volt and these different samples also gave coinciding “current-voltage’” curves. Cast iron and steel, however, showed potentials. slightly higher than wrought iron, -0.828 and -0.830 volt respectively, yet they gave current-voltage curves t h a t coincide practically with those of wrought iron. Commercia1 wrought iron was used in all the following experiments. The numerical results obtained are presented in Table A (experiments with iron anodes) and Table B (experiments with nickel anodes). The “current-voltage” curves plotted
730
E . P . Schoch and C. P . Randolph
~
I
3
1
""I
I P I '9 I 1'9'9 " ? " ? * " p ' I 0
0 0
1 "
-
w
0
w
o o c o o , o o
'""" .'I
c? 0
Behavior of Iron and Nickel Anodes
731
from this data appear in the two accompanying figures. The list of the electrolytes employed, together with special observations, is given below; and the Roman numerals in the tables and in the plots of the curves refer to this list of electrolytes. Experiments I to XVI inclusive were carried out with iron anodes; the rest, XVII to X X I I , with nickel anodes. TABLEB
___-_-
____
_-
~
~
~~
Potentials of nlckel anodes, referred to the normal calomel electrode. In each colunin, the potentials above the heavy liorizoiital l i n e are negatlve; below, positt\e Ai1 asterisk Current I marks the beginnilig of a rapid passivizing drop. T h e elecdensity trolytes with whlcll these potentials were obtained are i n nlil-anlPs dicated by t h e roman numerals a t t h e head of these coluniiis (see list iii experimental p a r t ) Per Sq. cm XVII
I
-
xx
xx I
XXII
0.263
0.163
I
0.0
0.0029 0.0058
0.0115 0.023 0.0346 0.058 0.086 0.144 0.202
1 0.428
1 ~
-
0.326 0.280
0.300
0.224
0.265
0.224
0.244
*
1
1
0.250
0.347
*
0.75
1.18
I 1.18
0.288
I .20
0.432 0.576 0.864 I .44 2.88 5.76
-.
._ I .30
-
0.489 0.408 0.290 0.240 0.170
I
-
I - -
-_
0.240
0.230 0.184 0.126
1 I
0.075 0.040 0.030
0.0.5"
0.021
I
0 0.20
0.34 0.42 0.54
0.104
-
0.205
0.147 0.363 0.58 0.662
-
-
11
-_ -
-
0.402 0.538 0.66 0.787 0.91 I I . 14 1-35
1.55 1.57
~
-
0.745 0.859 0.948 I
.so -
I. N / I K,SO,. When the current is interrupted after the anode has been passivized, the potential of the anode rises slowly, but boiling brings it promptly to the initial potential, -0,795 volt, and the surface is then in the original condition again. If the solution around the anode is not renewed, i. e . , if it is not passed through the apparatus, the potential of the anode rises much more slowly than when
732
E . P. Schoch and C. P . Randolph
the electrolyte passes through the apparatus. After oxygen has once been evolved, the current can be decreased to a density of 0 . 1 1 mil-amps. without stopping the evolution of oxygen, provided the circuit is not broken. 1000 SM)
A00
300
zoo IO0
90
BO
10 -1.0
-0.5
0
*a6
t1.S
+2.6
Fig. 3
500
I00
0.10
-am
10 .
0
Fig. 4
11. N/I K,SO,, N/IOO KOH. As soon as the circuit is opened the potential of the anode rises slowly. On boiling, the original condition of the surface is restored. With a current density of I 7.5 mil-amps. a white precipitate (probably ferrous hydroxide) forms in the electrolyte. As soon as oxygen is evolved this precipitate is oxidized as shown by the dark green color it assumes.
Behavior of Iron and Nickel Anodes
733
111. N/I K,SO,, N/IO KOH. On opening the circuit the potential of the anode rises slowly and even on boiling volt. Furthermore it does not rise higher than -0.845 when the current is turned on again the behavior of the anode shows that the surface has not recovered its original condition. IV. N/I K,SO,, N/IOO H-SO,. On opening the circuit the potential rises immediately to -0.691 volt and on boiling it rises to -0.704, which is slightly higher than the initial potential. Of course the surface recovers its original condition immediately after opening the circuit. The behavior of iron with these four electrolytes is strictly similar t o that of nickel with the exception of the specific tendency of iron toward metal dissolution, which is much greater than that of nickel. Hence in general the return from the passive t o the active condition is much more rapid with iron than with nickel. V. N/I K,SO,, with 1/60 gram mol K,Cr,O, per liter. The anode does not regain its initial potential after polarization as quickly as it does with potassium sulphate alone; yet its potential begins to rise as soon as the current is cut off, and in five t o ten minutes it reaches 0.67 to -0.68 volt, which shows that in spite of the presence of the bichromate the surface rapidly attains its unimpaired, active condition. A comparison of Curves, I, V and XV, shows that the specific oxidizing ten-, dency of the chromate is certainly interfered with by the sulphate, which is best understood by considering that both actions-metal dissolution and oxidation-are specifically influenced by both kinds of ions present, but that the sulphation has a greater tendency t o effect metal dissolution than it has a tendency to oxidize, while for chromation the reverse is the case. VI. N/I K,SO,, N/IO KF. On opening the circuit volt, and the the potential arises immediately t o -0.749 surface soon becomes as effective as before. Evidently this mixture of electrolytes has a greater tendency toward metal dissolution than the pure sulphate solution. VII. N/IO KNO,. When the circuit is open the potential
734
E. P. Schoch and C. P . Randolph
of the anode begins to rise slowly. Boiling hastens the rise, but does not restore the original condition of the surface. VIII. N / I O KNO,, N/IOO KOH. As usual, on opening the circuit, the potential rises, but it is evident that the tendency for the surface to regain the active state is much less than with No. VII. This experiment together with IX were designed t o ascertain whether or no the influence of an addition of hydroxylion or of hydrion to nitrate electrolytes would have the same relative effect as when added to sulphate electrolytes. On comparing the relative positions of the curves it is seen that the influence is in strict accord with the theory advanced. Since it is a matter of common experience that the nitrate ion is a better oxidizing agent in the presence of hydrion than in its absence, one might have expected the addition of nitric acid to favor the attainment of passivity.. However, a careful consideration shows that ordinary oxidizing actions are accompanied by the formation of water, the free energy of which formation is increased by an increase in the concentration of hydrion; but these anodic actions are not accompanied by the formation of water, and hence it is not surprising to find that the addition of nitric acid lessens the tendency toward attainment of passivity. IX. N/IO KNO,, N/IOO " 0 , . In this electrolyte the anode does not exhibit as high a potential without current as in the neutral nitrate, which is probably due to a small amount of direct oxidation. The relative position of the curves shows that the surface is not extensively impaired by such oxidation. When after polarization the circuit is open the potential of the anode rises rapidly and the original Condition of the surface is soon restored. X. N / I O KOH. The values between -0.725 and $0.58 volts are unsteady. After opening the circuit the potential of the anode rises immediately; on boiling it rose to -0.845 immediately, and afterwards continued to rise slowly. It is remarkable that even with hydroxides, chromates, chlorates, bromates, and iodates the anode potentials return-
Behavior of Iron and Nickel Anodes
735
when the circuit is open-fairly readily t o the initial potential. This indicates that the tendency, to metal dissolution, of these anions is still appreciable. X I . N/3o K,Cr,O, (1/60 gram mol per liter). The potentials of the anode down to 0.197 volt are not constant. When the circuit is open the potential begins to rise slowly. On boiling it reaches 0.039 volt immediately and then continues to rise slowly. The results obtained with N/15 K,CrO, (1/30 gram mol per liter) are approximately identical with the results in X I . XII. 2N/3 K,Cr,O, (1/3 gram mol per liter). On breaking the circuit the potential rose to 0.087 volt immediately, and a t the end of forty hours it was at -0.416 volt, which is practically the initial potential. X I I I . N/3 KIO,. On opening the circuit the potential rose immediately to 0.29 volt, and in 2 hours more, t o -0.331 volt. On boiling it rose to -0.455 volt which is not quite as high as the initial potential. With this electrolyte the oxidation to periodate, corresponding to 'the oxidation of chlorate to perchlorate does not take place, for the wellknown reason that this oxidation requires a higher potential than the evolution of oxygen on iron or nickel anodes. The curve of these values has not been plotted. It would run close t o the horizontal axis up to 1.5 volts where it would turn so as to run almost parallel to the vertical axis. X I V . N/3 KC10,. When the potential of the anode reached 0.023 volt, the formation of perchlorate began, and hence the increase of current density and the unsteady potential observed. XV. N/8 KC10,. After breaking the circuit the potential of the anode rose in five minutes to --0.725 volt. The anode soon regained the original unimpaired state of the surface. This electrolyte gives a behavior very much similar to that of sulphates. XVI. N/3 KBrO,. The most notable thing about this electrolyte is the irregularity introduced through the formation of perbromate; this begins at 0.4 volt. Since the formation
736
E. P. Schoch and C. P . Randolph
of perchlorate takes place at 0.02 volt and the evolution of oxygen on an iron anode takes place at 1.5 volts, it is readily seen why the oxidation of iodates does not take place with iron anodes. The behavior of nickel in acetate solutions was studied very carefully because the complications arising from the variable oxidation of the acet-ion and its dependence upon the concentration of hydroxylion had made this case a very interesting one : this is so, particularly because extensive metal dissolution can take place even a t anodic potentials which otherwise characterize passivity-a point that has already been commented upon in the first part of this paper. Realizing that the current-voltage curve might change extensively with the concentration of hydroxylion, we made a number of trials with solutions which were just slightly on one or the other side of neutrality. All of these experiments were carried out with nickel anodes. XVII. N/I Na,CH,O, slightly acidified (0.047 N beyond the “phenolphthalein neutral point”). XVIII. N/I NaC,H,O, neutral toward litmus, which is still 0.015 N acid toward phenolphthalein. XIX. N/I NaC,H,O, neutral towards phenolphthalein, which is slightly alkaline toward litmus. XX. N/I NaC,H,O,, N / I O KOH. The general run and the relative positions of the Curves XVII t o X X is just what is theoretically to be expected on the assumption that in the main the acet-ion has the same relative tendency toward the two kinds of anodic action as the sulphation. In this connection it must be noted that on nickel anodes-just as on iron anodes-the evolution of oxygen is the predominating action with neutral solutions (see also (8)). The oxidation of the acet-ion, to methyl alcohol becomes extensive only as the solution becomes alkaline. Hence in XX currents of much greater density than may be obtained with sulphate electrolytes under the same conditions may be maintained here at potentials less noble than
Behavior of Iron and Nickel Anodes
737
those with which oxygen is evolved, which would be impossible if the oxidation of the acetion did not take place. XXI. N/3 KIO,, nickel anode. On breaking the circuit the potential of the anode rose immediately to 0.455 volt; in two hours more it rose t o ---o.165 volt. Beyond this point the rise in potential was inappreciable even on boiling. On account of the extremely small current densities obtained with this electrolyte it was impossible t o ascertain with certainty whether the surface had attained its original state. X X I I . 2N/3 K,Cr,O, (or 1/3 gram mol per liter) nickel anode. The potential rises very slowly after breaking a circuit. LIST O F REFERENCES. Schoch: Am. Chem. Jour., 41, 232 (1909). 2. LeBlanc: Abh. Bunsen Ges., No. 3. 3. “Potential of the Oxygen Electrode: a Review,” Jour, Phys. Chem., 149 665 (1910). 4. Hittorf: Zeit. phys. Chem., 25, 729. j . W. J. Mueller: Zeit. phys. Chem., 69, 460. 6. E. Mueller: Zeit. Elektrochemie, 13, 133 (1907). 7. Byers: Jour. Am. Chem. SOC.,30, 1737-38 (1908). 8. Foerster: Elektrochemie wass. Losungen, 477. 9. Haber and Goldschmidt: Zeit. Elektrochemie, 12, 49 (1906); Krassa: Ibid., 15, 490 (1909). IO. Sackur and Alvarez: Zeit. Elektrochemie, 14, 607 (1908). 11. Foerster: Abh. Bunsen Ges., No. 2. 12. LeBlanc and Levi: Boltzmann Festschrift, 187. 13. Luther: Zeit. Elektrochemie, 11, 777 (1901); and LeBlanc: “Textbook of Electrochemistry,” 4th E d . , 245. 14. Richards and Behr: Zeit. phys. Chem , 58, 301 (1907). The University of Texas Chemical Laboratory June 4, 1910 I.
