POTASSIUM BICHROMATE AS DEPOLARIZER BY G. P. VINCENT
Bengough and Stuart1 stated that "the action of bichromate solutions in passivifying metals is not readily understood on electrochemical lines, since bichromate is a powerful depolarizer, and would be expected greatly to enhance any electrochemical action that took place in distilled water. Miss Souders2 showed that copper, iron, and zinc become passive when made anodes in a dilute potassium bichromate solution with a low current density. She also showed qualitatively that hydrogen was given off a t a platinum cathode in a dilute bichromate solution which means that the solution is not acting as an effective depolarizer. The present investigation was undertaken under the direction of Professor Bancroft to determine under what conditions and to what extent a potassium bichromate solution functions as a depolarizer to nascent hydrogen. Two gas coulometers were set up in series, with molecular sulphuric acid in one and the potassium bichromate solution in the other; the reaction was followed gas analytically by comparing the relative amounts of hydrogen set' free in the two coulometers. The electrodes were of platinum and the cathodes were so arranged that an inverted burette could be placed over each. It was expected that there would be little or no reduction in the dilute bichromate solutions and that the amount of reduction would idcrease with increasing concentration of the bichromate solution. This expectation was not fulfilled. With M/40 potassium bichromate and an impressed voltage of 13.5 volts, 9.0 cc of hydrogen were obtained from the sulphuric a d d coulometer and 9.1 cc from the bichromate coulometer, readings which are identical within the experimental error. There is therefore no depolarizing action a t a platinum cathode under these conditions. The next runs were made with a saturated bichromate solution containing, approximately I 3 5 grams potassium bichromate per liter a t 25'. With an impressed voltage of 15 volts the hydrogen from the sulphuric acid was 37.8 cc and from the bichromate also 37.8 cc, showing that no depolarization occurred. When the voltage was dropped to 10.5 volts, the two values were 35.7 cc and 35.6 cc respectively. With an impressed voltage of j volts, gas was of course evolved very slowly; but the results were 5.1 cc and 5.1 cc. The bichromate solution was then heated to 63.5' and a run made at I O volts; but 37 cc of gas were obtained from each coulometer. There is therefore no depolarization with a saturated solution of potassium bichromate using a platinum cathode and it seems desirable to abandon the myth that such a solution is a powerful oxidizing agent. There was of course the possibility that the solution in contact with the electrode might be potassium chromate after the first few moments of electrolyJ. Inst. Metals, 28,
52,
(1922).
* Bancroft: J. Phys. Chem. 28, 831 (1924).
876
G. P. VINCENT 1
sis, so a special coulometer mas constructed with the anode and cathode in the same compartment and only two centimeters apart. The first run with saturated potassium bichromate and six volts gave 2 3 . 2 cc hydrogen from the sulphuric acid coulometer and 35.2 cc of oxyhydrogen gas or about 2 3 . j cc hydrogen from the bichromate coulometer. In another run the values were 24. I and 36.3 cc. ( $ 2 4 . 2 ccHz) respectively, which indicated no depolarization. When the voltage was made so low that 3.8 cc hydrogen were obtained in the sulphuric acid coulometer in one and a half hours, the amount of oxyhydrogen gas was 4.0 cc, corresponding theoretically to 2 . 7 cc hydrogen. This would indicate some depolarization; but it is more probable that the calculated result is wrong owing to some oxygen having dissolved in the solution. Some may also have diffused to the cathode, which would cut down the yield irrespective of any depolarizing action of the bichromate. Some experiments were next made with other cathodes to see whether a higher over-voltage would make a difference. With a lead cathode, not prepared in any special way, and six volts, 23.8 cc hydrogen were obtained in the coulometer in a three-hour run and 2 2 . 4 cc from the saturated bichromate solution. In a longer run the values were 54.1 cc and 51.0 cc respectively. If there is any depolarization, it does not exceed six percent which is not enough to be interesting. Quite different results were obtained with a mercury cathode. With an impressed voltage of 9 volts, the hydrogen in the sulphuric acid coulometer was 12.9 cc and in the bichromate coulometer about 1.8 cc, indicating about 86% depolarization. It was noticed, however, that 100% oxidation of the hydrogen seemed to occur a t the start, and that the efficiency decreased with the time, I t seemed probable that this was due to the formation of a protecting film over the surface of the mercury, consisting of some chromium compound, presumably hydrous chromic oxide. When the mercury cathode was so arranged as to make it possible to expose a fresh surface of mercury to the solution, one hundred percent efficiency was obtained, 4.2 cc hydrogen being evolved in the acid coulometer and none a t the mercury cathode. The reduction potential of a potassium bichromate solution is therefore at such a point that the depolarizing efficiency is zero at a smooth platinum cathode and one hundred percent at a clean mercury cathode, This is a very interesting demonstration of the effect of overvoltage. As was to be expected, there was no depolarization when an iron cathode was used. When M/40 potassium permanganate was electrolyzed with platinum electrodes, 29.6 cc hydrogen was obtained in the acid coulometer and 18.3 cc hydrogen in the permanganate coulometer, indicating about 3 87', depolarization. A neutral permanganate solution is therefore a much stronger hydrogen depolarizer than the corresponding potassium bichromate solution. Since it was not possible to construct a curve varying from zero to complete depolarization with platinum electrodes by varying the concentration of the potassium bichromate, it was decided to get a similar result by adding varying amounts of acid. The first run was made with 400 cc saturated potassium
877
POTASSIUM BICHROMATE AS DEPOLARIZER
bichromate solution plus 2 0 cc concentrated sulphuric acid and an impressed voltage of 6 volts. I n the sulphuric acid coulometer there were evolved 30.4 cc hydrogen and only 0.3 cc in the chromic acid coulometer. I n other words addition of five percent by volume of concentrated sulphuric acid to a saturated potassium bichromate solution increased the depolarization at a platinum cathode from zero to practically one hundred percent. I n the next set of runs the voltage was kept always a t 9 volts and 250 cc saturated bichromate solution were taken in each case. The data are given in Table I, the column entitled acid showing the amount of sulphuric acid added, the next two columns showing the hydrogen evolved from the acidified bichromate and from the sulphuric acid coulometer respectively, while the fourth column shows the percentage oxidation of hydrogen.
TABLE I cc saturated KzCrzOi+sulphuric acid Platinum electrodes; 9 volts 250
Acid added cc
Bichromate cc
Hz
0. I
21.1
0.3 0.5 I .o
18.9 18.3 21.3 8.3 8.5
1.5 .o
2
Coulometer
CCHP 20.6 18.3 18.3 26.8 23.9 28.2
Oxidation
Acid added
Bichromate
Coulometer
% ’
cc
cc Hz
cc Hz
%
0
2.5 3.0 4.0 5.0 6.0
10.4 2.8 3.9 2.0
29.5 16.5 33 .o 42.7 27.6
64.5 83 .o 88.0 95.5 98.5
o o 20.5 65.0 69.8
0.4
Oxidation
It will be noticed that there is no oxidation until 1.0cc of sulphuric acid has been added. This means that under these conditions, depolarization does not begin until some amount between 0.5 and 1.0cc sulphuric acid has been added to 250 cc saturated potassium bichromate solution. In order to determine more closely where this point occurred, another set of runs was made which differed from the preceding one only in the amounts of concentrated sulphuric acid added. The data are given in Table 11. TABLE I1 250 cc saturated K&rzO,+sulphuric Platinum electrodes; 9 volts Acid added cc
Bichromate cc
Hz
0.I
21.7
0.3
.o 28. I
0.5
22
Coulometer
CCHZ
21.7 21.7 28.2
Oxidation
Acid added
acid
Bichromate
Coulometer
Oxidation
cc HP 23. I 36.1 42.2
0.9 14.4 18.2
70
cc
CCHZ
0
0.6
22.9
0
0.7 I .o
30.9 344
0
7c
878
G. P. VINCENT
While there is apparently some depolarization when 0.6 cc sulphuric acid is added, the value is within the experimental error, so that the first significant oxidation occurs on the addition of 0.7 cc concentrated sulphuric acid. In Table I11 are given data obtained for a single sample of 250 cc saturated potassium bichromate solution to which varying amounts of sulphuric acid were added.
TABLE I11 250 cc saturated KzCrz07fsulphuric acid Platinum electrodes; 9 volts. Acid added cc
0.7 I .o 1.5 2 .o 3.5
Bichromate
Coulometer
CCH~ 16.4 13.8 8.9 8 5 3.9
Oxidation
Acid added
70
cc
CCHB 20.1 20.8 30.0 20.1
18.4 33.6 70.3
3.0 4.0
57,7
20.2
80.6
Bichromate cc
Hz
Coulometer cc
Hz
5.0
4.7 1.6 1.7
23.9
6.0
1.1
20.2
7.0
0.4
23.5
20.7 20.2
Oxidation
76 77.3 92 .o 92.9 94.5 98.2
These results are so wobbly that only a qualitative curve can be drawn from them, Since the presence of reduction products at the electrodes might affect the results, another set of data was obtained, using a fresh sample of saturated bichromate solution each time. In order to minimize the time necessary, the voltage was increased to 2 0 volts which meant a large increase in current density. The data are given in Table IV.
TABLE IV z j o cc saturated KzCrzO,+sulphuric acid Fresh sample of bichromate each run. Platinum electrodes; 2 0 volts
Acid added cc 0.0
0.2 0.4 0.5 0.7 0.8 I
.o
1.5 2.5
Bichromate
CCHB 20.4 31.9 26.6 48.0 28.8 25.9 17 .o 14.9 14.8
Coulometer
Oxidation
Acid added
Bichromate
ccHz
%
cc
20. I
0
31.7 26.4 48.2 29.4 27.4 18.7 17.3 20.7
0
0
3.5 5. 6. 7.
3.8
2.0
IO
2.5
15
3.9 3.6 7.3 8.8
o
5.5 9.2 13.9 28.5
20
25 40
CCH~
22.4 5.6 4.7
Coulometer cc H, 37.5 23. I 29.5 31.2 28.2 43 .o 33 .o 44.5 46. I
Oxidation
% 40.3
75.7 84.0 87.8 91.1 90.9 89.0 83.6 90. I
These results give a fairly smooth curve and are plotted in Fig. I , in which there is also given a smoothed curve for the data from Table 111. We get much higher efficiencies with IO volts than with 2 0 volts and the 20-volt
POTASSIUM BICHROMATE AS DEPOLARIZER
879
curve passes through a maximum about I O cc sulphuric acid. This is a question of current density. For a constant-voltage drop across the terminals of the two coulometers, the current increases with the amount of acid added. This may easily cause an apparent maximum in the curve. To show this another run was made with I O cc sulphuric acid added and 2 0 volts. The hydrogen values were 2.1 cc and 2 0 . 2 cc, indicating 89.676oxidation as against 91.1% in Table IV. The current was about 5.5 amperes. A run was made with 40 cc acid added and the voltage adjusted so that the current was j.; amperes. The hydrogen values were 2.1 cc and 20.6 cc, indicating 89.8% oxidation as against 80. I Ycin Table IT. In other words the current efficiency depends on the current density after a certain amount of sulphuric acid has been added and can undoubtedly be varied at least over the range 80-100%
FIG.I 2 5 0 C.C.
saturated K, Crz0 7 and sulphuric acid; platinum electrodes Left curve T O volts, right curve 2 0 volts.
for the case of 40 cc concentrated sulphuric acid added to 2 j o cc saturated potassium bichromate solution. Since no immediate use could be made of the figures, it did not seem worth while t o make a set of runs at a very low and constant current density. A few experiments were now made on the actual corrosion of zinc, iron, and copper in saturated potassium bichromate solutions under various conditions a t room temperature. A strip of zinc in a saturated bichromate solution showed no signs of corrosion in two months exposure. X strip of iron showed no corrosion in the solution; but there was corrosion in the portion exposed to the air. The strip of copper lost its original bright luster but did not waste away. This means that the protecting film is visible to the eye, In order to show that heterogeneity is not necessarily sufficient to cause corrosion, a piece of platinum wire was wound tightly round a strip of copper and the whole immersed in a saturated bichromate solution. Although this is a short-circuited cell, there was no action on the copper beyond what was noted in the previous experiment with copper alone. Zinc, iron, and copper strips were short-circuited with mercury cathodes in saturated bichromate solutions. With iron and copper the presence of the mercury cathode had no visible effect. With zinc there was a very slight corrosion, the surface ceasing to be highly lustrous and becoming slightly
880
G. P. VINCENT
whitish. The change was so slight that it might easily have been overlooked by anybody making a hasty examination. With zinc and iron strips in M/40 potassium permanganate solution there was distinct, though limited, corrosion, which is what one would expect because neutral permanganate is a better hydrogen depolarizer than potassium bichromate. Some experiments were also made to determine the amounts of concentrated sulphuric acid that one must add to a saturated potassium bichromate solution in order to cause appreciable corrosion of zinc, and copper. In each case I O O cc of potassium bichromate solution were taken. With zinc addition of 0 . 2 cc acid and exposure for 44 hours caused a slight brownish tinge on one face. There was no marked corrosion and the film may have been protecting the metal to some extent from further attack. Addition of 0.3 cc acid and exposure for 44 hours gave about the same results. With addition of 0.5 cc acid the effect was more marked and the exposure was therefore continued for six days. Corrosion was very apparent and the solution had turned dark brown. Addition of 0.5 cc acid is therefore sufficient to cause very appreciable corrosion of zinc. With iron addition of 0.3 cc acid caused no visible corrosion in 44 hours and addition of 2 . 0 cc of 3.0 cc acid caused no appreciable corrosion in six days. With addition of 5.0 cc acid and exposure for six days, the iron was found to have corroded completely and the solution was black. It evidently takes a good deal more acid to cause the corrosion of iron than of zinc. These experiments do not show whether the difference is due primarily to the difference of chemical potentials or to the difference in the protecting films. With copper there was perceptible corrosion on addition of 0.1 cc acid and the copper corroded completely on addition of 0.3 cc acid. This makes it probable that the important factor in these cases is the nature of the protecting film and not the electromotive force of the pure metal. Bengough and Stuart' have called attention to the fact that dilute acetic acid corrodes zinc more rapidly than does dilute sulphuric acid. To test this, strips of zinc were immersed in solutions of I:;OOO acetic acid and I : ~ O O O sulphuric acid. The zinc in the acetic acid showed quite appreciable corrosion in two months, the corrosion products precipitating all over the surface of the zinc. I n the sulphuric acid solution the zinc showed a slight amount of corrosion, the edges of the strip having turned black; but the amount of corroded metal was not at all equal to that in the acetic acid solution. It seemed possible that this was due to the fact that zinc acetate hydrolyzes much more completely than zinc sulphate and that the hydrogen ion concentration does not decrease as rapidly in the acetic solution as in the sulphuric acid solution. To test this hypothesis a weighed piece of extremely pure zinc was placed in a 1:5ooo acetic acid solution. Another weighed piece of the same zinc was placed in a I:IOOOO sulphuric acid because sulphuric acid is bivalent; but the sulphuric acid solution was renewed periodically. Under J. Inst. Metals, 28, 89 (1922).
POTASSIUM BICHROMATE AS DEPOLARIZER
88 I
these conditions there was no appreciable difference in the action of the two acids and it seems justifiable to conclude that zinc corrodes as badly in extremely dilute sulphuric acid as in extremely dilute acetic acid if the corrosion products in the sulphate solution are removed. The general conclusions of this paper are as follows:A saturated potassium bichromate solution has practically no deI. polarizing action on hydrogen set free a t a smooth platinum cathode. A saturated potassium bichromate solution shows IOO percent de2. polarization for hydrogen set free a t a clean mercury cathode. 3. Addition of about 0.7 cc concentrated sulphuric acid to 2 5 0 cc saturated potassium bichromate solution is necessary before there is appreciable depolarization of hydrogen a t a smooth platinum cathode. 4. Zinc, iron, and copper do not corrode appreciably in a saturated potassium bichromate solution. j. Copper does not corrode appreciably when short-circuited with platinum in a saturated bichromate solution. 6. Iron and copper do not corrode appreciably when short-circuited with mercury in a saturated potassium bichromate solution; but zinc corrodes slightly. 7. Different amounts of concentrated sulphuric acid must be added to a saturated potassium bichromate solution in order to cause marked corrosion of zinc, iron, and copper. It seems probable that the important factor is the nature of the protecting film rather than the electromotive force of the pure metal. 8. Neutrql M/40 potassium permanganate has a distinct depolarizing action on hydrogen at a platinum cathode and causes appreciable corrosion of zinc, iron, and copper. 9. Zinc corrodes more rapidly in an extremely dilute acetic acid solution than in an extremely dilute sulphuric acid solution; but this difference in rate disappears if the sulphate corrosion products are removed. Cornell University
