ELECTRICAL ENDOSMOSE BY WILDER D. BANCROFT
Perrinl has made the generalization that all porous diaphragms become charged positively in a solution which is sufficiently strongly acid and become charged negatively if the solution is sufficiently strongly alkaline. This is the same as saying that electrical endosmose will carry the acid solution through the diaphragm from cathode to anode and will carry the alkaline solution through the diaphragm from anode to cathode. I do not intend now to discuss whether this generalization is or is not absolutely accurate for all diaphragms; but I do wish to show how certain apparent exceptions may be accounted for. If we take an acidified copper sulphate solution and-a porous cup we find that electrical endosmose carries the solution through the walls of the cup from anode to cathode which is not the direction that Perrin’s law would lead us to expect. On the other hand the solution passes to the anode if we substitute copper nitrate and nitric acid for the corresponding sulphates. The sign of the charge on a diaphragm depends on the relative adsorption of cation and anion, being positive if the cation is adsorbed to a greater extent than the anion and negative if the reverse is the case. Let us assume that, for equal concentrations, hydrogen as ion is adsorbed somewhat more readily than sulphate as ion, while copper as ion and nitrate as ion are only adsorbed to a relatively slight extent. In that case the diaphragm will be charged positively in sulphuric acid and in nitric acid; but will be charged negatively in copper sulphate. Electrical endosmose will then carry sulphuric acid and nitric acid solutions to the anode and a copper sulphate solution to the cathode, which is what happens. An acidified copper nitrate solution will also move to the anode because hydrogen as ion will be the de-
Jour. Chim. Phys., 2, 601 (1904).
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cidiiig factor. The situation is different with an acidified copper sulphate solution, especially if the concentration of copper sulphate is high and that of sulphuric acid is low. Since the adsorption of sulphate as ion increases with increasing concentration, there will come a point at which sulphate as ion is adsorbed more strongly than hydrogen as ion, The charge on the diaphragm will then reverse and electrical endosmose will carry the solution through the diaphragm from the anode t o the cathode, which is what happens experimentally. This enables us to explain some facts observed by Reed ten years ago. A containing vessel was divided into three compartments by means of two porous diaphragms. In the anode compartment there was a copper sulphate solution (sp. gr. 1 . 1 7 ) slightly acidulated with sulphuric acid. The other two compartments contained H$O, solution (sp. gr. 1 . 1 7 ) . “Immediately on closing the circuit and causing a current to flow through the apparatus both the volume and the weight of the solution in the middle compartment begin t o rapidly increase and continue to increase a t an apparently uniform rate as long as the current flows. Even with a current of about ’/, ampere per square inch of cross-section of electrolyte, the increase in volume amounts to 1 . 7 cc in I hour.” Electrical endosmose would carry the sulphuric acid solution from the cathode compartment to the middle compartment. At the anode compartment we should have the copper sulphate solution tending to be carried into the middle compartment while the sulphuric acid solution would tend to be carried back into the anode compartment. The total transfer through this diaphragm will therefore be the difference of the two and will depend on the actual conditions in the diaphragm a t any moment. I am inclined to believe that there will be a flow of solution from the anode compartment to the middle compartment; but the details are not sufficiently accurate t o enable one to be certain of this. I t is clear however that there must be an increase in the amount of solution Trans. Am. Electrochem. SOC.,2 , 238 (1902).
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in the middle compartment and that electrical endosmose is the important factor in Reed’s work. Perrin’ has found that a saturated solution of barium carbonate moved to the anode through a chromic chloride diaphragm. Since such a solution is slightly alkaline by hydrolysis, the movement is apparently in the wrong direction. This might be due to barium as ion being adsorbed to a greater extent than hydroxyl as ion. An experiment made for me by Mr. Briggs disproved this assumption, because a barium hydroxide solution moved to the cathode through a porous cup. The only possible explanation then was that this was analogous to the case of copper sulphate and sulphuric acid. Let us assume that hydroxyl as ion is adsorbed a little more readily than barium as ion while carbonate as ion is only slightly adsorbed. The total concentration in a sakurated barium carbonate solution is low; but the amount of hydrolysis is not very large and so the ratio of barium to hydroxyl is rather large and we therefore have conditions favorable to barium being adsorbed more than hydroxyl. To test this Mr. Briggs took a barium chloride solution and added a small amount of barium hydroxide. This solution moved slowly to the cathode through a porous cup, which was not what it should have done; but we found that dilute acid behaved similarly with this diaphragm so that the alkaline solution does behave like a dilute acid. We hope to repeat this experiment some day, using a chromic chloride diaphragm. It was also found that a barium hydroxide solution passes much less rapidly than a sodium hydroxide solution having the same hydroxyl concentration. Since we know that barium is adsorbed much more readily than sodium, this is exactly as it should be. Muller and Bahntje’ found that, in acidified copper sulphate solutions, starch and gum arabic did not move to the cathode and did not cut down the size of the copper crystals when the solution was slightly acid; but did both these things Jour. Chim. Phys., 2, 643 (1904). Zeit. Elektrochemie, 12, 320 (1906).
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when the solution was made more acid, The movement of the colloid t o the anode in a slightly acid solution is just what happened with the porous cup. Of course the reversal will come a t different degrees of acidity with different colloids. I n fact, Muller and Bahntje found that gelatine moved to the cathode in solutions in which starch moved to the anode. The effectiveness of gelatine in cutting down the size of lead crystals in the Betts process is due largely to the fact that the solution is markedly acid. It has been claimed that gelatine has no effect on the quality of the deposit from lead acetate solution; but this of course can only be due to faulty experimental considerations. To prove this Mr. Bennett, a t my suggestion, made up two solutions containing I O percent lead acetate and 5 percent acetic acid. To one solution there was added I percent of glue. The two solutions were then run in series, using lead electrodes, the current density being 0.3 amp/dm2. The beneficial effect of glue was very marked. Aluminum sulphate is often added to zinc baths to improve the quality of the zinc deposit. The explanation for this has been given by Schlotter.’ “Since aluminum sulphate is dissociated hydrolytically, in aqueous solution, into sulphuric acid and colloidal aluminum hydroxide, the action of aluminum sulphate is clearly that of a colloid. We get a similar result when copper is deposited electrolytically in presence of aluminum sulphate.’’ Though Schlotter does not mention it, it seems certain that the action of tin salts2 in a copper bath comes under the same head. It seemed as though a good way of confirming this would be to predict in regard to an entirely new case. If we boil a ferric acetate solution we shall get a certain amount of colloidal ferric oxide. If we add this to an acidified lead acetate solution, it ought to cut down the size of the lead crystals obtained by electrolysis. Mr. Bennett was good enough to try this experiment for me. He made up the following solutions: A. Ten percent lead acetate and 5 percent acetic acid. Galvanostegie, I, 38, 51 (1910). “Elektrometallurgie,” Third Edition, 193 (1903).
* Borchers:
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B. Three grams ferric acetate and a few drops of acetic acid made up to 7 0 cc boiled and filtered. In one beaker was placed 300 cc A and in a second 300 cc A 50 cc B. The two solutions were run in series a t 43’ with a current density of 0.3 amp/dm2, using lead electrodes, The deposit was distinctly better from the solution containing colloidal ferric oxide, though the ferric oxide is not as efficient as gelatine. Another point that we can now clear up is the electrolytic precipitation of black, pulverulent copper from a dilute copper sulphate solution. I n my paper’ on “The Chemistry of Electroplating ” I was forced to leave this point unsettled. We know that it is not due to the simultaneous precipitation of hydrogen because we get all the copper out of certain copper sulphate solutions as a good deposit when we are doing electrolytic analysis. It cannot be due to the low concentration because the concentration of copper as ion in a complex salt is much lower. If we have a dilute, nearly neutral copper sulphate solution, we shall have certain amounts of hydrolysis and a certain formation of colloidal cupric oxide. Within certain limits this will be beneficial to the deposit, just as a certain amount of gelatine is beneficial. If we get in too much gelatine, we get a black deposit instead of a brilliant one. If we get too much colloidal cupric oxide relatively to copper as ion, we shall also get a black deposit. The favorable conditions are a dilute and nearly neutral solution. When we are determining copper electrolytically from a sulphate solution, the excess of acid cuts down the hydrolysis to a negligible amount. Hydrolysis is also prevented if we have a complex salt of copper; and if the solution is neutral or alkaline, there is normally no tendency for the colloid to pass to the cathode. The same reasoning applies also, to a greater or lesser extent, to other metals. Of course one is not justified in concluding that all black deposits are necessarily the result of hydrolysis. Since the iso-electric concentration is the one a t which
+
Bancroft: Jour. Phys. Chem., 9,
282
(1905).
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the anions and cations are adsorbed in equivalent amounts, it follows that we cannot draw any conclusions in regard to single potentials from measurements of the iso-electric point except for the special and somewhat improbable case that we have a zero adsorption of anions and cations.' This is also the explanation for the flowing electrode mercury not giving true single pstentials.2 Another interesting consequence is that we can no longer maintain that the potential difference between a metal and an aqueous solution is a function of the concentration of the ions of the metal only. It will vary to some extent with the nature of the anion and with the nature of the other cations present in so far as these are adsorbed by the metal in question. Cornell University
lCf. Billitzer: Zeit. phys. Chem., 45, 327 (1903); 48, 513, 452 (1904); Drude's Ann., 11, goz (1903). Goodwin and Sosman: Trans. Am. Electrochem. S0c.s 71 83 (1905). Smith and Moss: Phil. Mag., [ 6 ] 15, 478 (1908).
