THE ELECTRODEPOSITION OF METALS FROM T H E I R LIQUID AMMONIA SOLUTIONS BY ROBERT TAFT AND HAROLD BARHAM*
It has been known for many years that the form of electrodeposited metals from aqueous solutions of their salts could be varied a t will by changing the conditions under which the metal was deposited. A systematic study of the factors influencing the form of the deposit has been made by Bancroft,‘ Aten? Hughes,3 Kohlschiitter,4 and especially by Blum and co-workers.6 Blum has shown the influence of several factors upon the form of the metallic deposit from a purely experimental standpoint: Some of these determining factors as listed by Blum include current density, concentration of salt and agitation, temperature, conductivity, metal ion concentration, hydrogen ion concentration, addition agents, and structure of base metal. The similarity between water and liquid ammonia in their chemical properties has long been known. Many salts are solvated by these two substances to form respectively hydrates and ammoniates. They are also similar in their solvent action, which is probably chemical, dissolving many substances, both organic and inorganic. Each of them possesses the property of causing dissociation when inorganic salts are dissolved in them and, consequently, show the phenomenon of electrolysis when an electric current is passed through their solutions. On account of this similarity between water and liquid ammonia it was thought that a fairly extensive study of the form of metals deposited electrolytically from liquid ammonia solutions would be of interest in that the influence of controllable factors, such as has been done for aqueous solutions, could be studied by the use of this different solvent, further the influence of the solvent on the form of the metal could be shown; and lastly, there would be the possibility of commercial utilization of any one of the electroprocesses which appeared to possess any advantages over its corresponding aqueous analog. For our purposes the electrodeposition of the following metals was studied : lead, nickel, cadmium, copper, silver, zinc, chromium, and aluminum. This list is representative both from a chemical and industrial standpoint; and further, Groening and Cady6 have shown that most of these metals possess a higher metal overvoltage in liquid ammonia than in water, a property which, as pointed out by Bancroft, tends toward the production of a “good” deposit. In describing a deposit, it was necessary to * University of Kansas, Lawrence, Kansas. ‘Trans. Am. Electrochem. SOC.,6 27 ( 1 9 0 4 ) ; 23, 266 (1913). * Rec. Trav. chim., 39, 720 (1920).’ Bull. 6, Dept. Sei. and Ind. Res., London, 1922; Beama, 12, 215 (1923). ‘2. Elektrochemie, 24, 300 ( 1 9 1 8 ) . Trans. Am. Electrochem. SOC., 36, 2 1 2 ( 1 9 1 9 ) . J. Phys. Chem., 30, 1597 (1926).
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ROBERT TAFT A N D HAROLD BARHAM
have a concept as to what a “good” deposit should be. The following is a statement of the characteristics which me have considered as belonging to a good deposit : A deposit may be said to be “good” when it is non-porous, compact, hard, fine-grained, and adherent.
Method and Apparatus Our experimental procedure consisted in varying several or all of the following factors. When the individual metals are considered, it will be indicated as to which of these factors were varied. I. 2.
3. 4.
5‘ 6.
8. 7.
Current density Metal ion concentration Hydrogen ion concentration, acidity Anion, nature of anion Temperature Metal base Addition of colloids Stirring
The method employed consisted in weighing out the desired quantity of salt and dissolving it in IOO C.C. of liquid ammonia which had been placed in a wide-mouthed test tube shown in Fig. I . The liquid ammonia employed came from a cylinder of the commercial product. It was drawn from the cylinder into a Dewar flask and used directly in the preparation of solutions, All of the electrolyses were made a t the boiling point of liquid ammonia unless stated to the contrary. Electrodes two cm. wide and about 18 cm. long, which had been thoroughly cleaned with acid dip, polished with fine pumice stone and weighed, were placed in rubber spacings two cm. apart and the whole inserted into the cell. It was difficult with thin electrodes to keep the electrodes from bending slightly in or out. The electrolyses were made out of contact with air, the electrodes being set in sealing-wax and fitted into the rubber stopper, A mercury trap was arranged and attached to the cell to exclude the air. The cell was placed in a wide-mouthed Dewar flask and around the cell was poured some liquid ammonia which served as a bath. To obtain lower temperatures, enough liquid air was added to the ammonia to give the required temperature. Stirring was done with an auger-shaped glass stirring rod, introduced between the electrodes, and driven by an electric motor. In proceeding with the electrolysis, the required current was shunted from a I I O volt D.C. line, and time and voltage data taken. The number of coulombs which had passed through a cell was calculated from current (read from an ammeter in series with the cell) and time data, the ammeter used having been previously compared with an iodine coulometer. The current efficiencies were then calculated from the metal deposited (or dissolved a t the anode) in
ELECTRODEPOSITION O F METALS FROM LIQUID AMMONIA
93 I
accordance with Faraday's laws on the basis of the valency of the metal as it existed in the salt used in making up the solution. Current densities were computed on the basis of the area of one side.' The resulting deposits were removed after the electrolysis, washed, dried, and weighed. Some of them were difficult to wash entirely free from adhering salts without the removal of some of the deposits. This introduced an error iuto the current efficiency values, especially when the deposits were poor. After weighing the deposits, they were studied as carefully as possible, described, and finally photographed. Microphotographs with a magnification of twenty times were made on lead deposits. The remainder were photographed a t approximately natural size. Several deposits of lead, cad- .. mium, copper, silver and zinc are reproduced in Figs. 3 to 7, inclusive.
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For the study of cadmium deposits, it being desirable to carry out the electrolyses at room temperature and at a corresponding pressure (about I;O lbs. per sq. in.), a pressure bomb was constructed, a cross-section of which is shown in Fig. 2 . The walls of the bomb were made of heavy-walled gas piping, and the top and bottom caps were ordinary gas-pipe fittings. To the top cap were welded two heavy steel lugs to be used in tightening and removing the top cap. Within the walls was placed a lead sheet which protected them from the splattering of the cadmium salts which are highly corrosive under these conditions. The rubber gasket served the double From the current aDd the potential drop across the cells as tabulated below, approximate values of electrlcal resistances of the solutions may be computed. From the current densities and potential drops, as given, and the distance between electrodes ( z cm.) a p proximate values of the specific resistances of the solutions may be computed.
93 2
ROBERT TAFT Ah-D HAROLD BARHAM
purpose of preventing leakage from the top and corrosion of the cap. Into the lower cap were placed two carefully-machined packing glands, through which passed the copper leads. The copper wire was set in litharge-glycerine cement within a heavy-walled, soft-glass tubing, one end of which was sealed with rubber cement to prevent leakage through the porous cement below. The tube and wire were packed into the gland with pure rubber packing. A cast-iron valve was also fitted to the lower cap t o permit the release of the pressure a t any desired rate. It was found that comparatively rapid release of the pressure was necessary as the long standing of the cadmium deposits in contact with the solution destroyed them. The cell and contents were placed in a metal cylinder inside the bomb (this permitted equilibrium to be established rapidly) as is shown in Fig. 2 . Solutions of Lead Salts in Liquid Ammonia Among the salts of lead listed by Franklin' and Kraus as being soluble in liquid ammonia, were lead acetate, lead nitrate, and lead iodide. Of these lead acetate and lead nitrate were used in our experiments. The anhydrous character of lead nitrate made it the more useful in carrying out electrolyses under conditions in which a minimum quantity of water was present. A number of electrolyses were made, the results of fifteen of these trials being recorded in Table I, and the form of several of the cathode deposits being photographically recorded in Fig. 3. The results of these trials may be summarized briefly in terms of the factors previously stated under method. I. Current density. Increasing the current density decreases the size of the nodules as shown in the data for Cells I to 3. At the same time the area covered by the deposit becomes greater. As the current density becomes greater, however, the tendency for treeing becomes more pronounced. 2. Metal ion concentration. A comparison of the results of cells q and 5 and #g and I O showed that as the concentration of salt increased the tendency to treeing decreased. 3 . Acidity. The ammonium salts in liquid ammonia behave as acids, Le., solvated hydrogen ions are formed when dissolved in ammonia. In aqueous solutions acids are added for several purposes, e.g., t o increase conductivity; to prevent hydrolysis of salts and deposition of oxides, etc. The solutions of lead nitrate were invariably opalescent and the deposits were striated. The opalescence and production of striae are evidence of the presence of colloidal material in the solution, undoubtedly a basic salt (rather its ammonia analog) of lead. The addition of the acid (XH4SO3)removed the opalescence and the deposit was no longer striated (Cf. Cells 7 and 8). The statements made in Paragraph 7 below should also be considered in this connection. 4. Nature of the anion. The substitution of lead acetate for nitrate produced non-striated clearly crystalline deposits. I t should be noted in Am. Chern. J., 20, 820 (1898).
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'i'l,at Ihr: prccipitatc react. wit.li sonic substarice formcd during clect rol i s a. cuprous compound i s s1101vn by i t s drcoloriaing potxsiimn permanganate solut,ion and upon i t s ready oxidation by air. 1!IC wsulls of twenty-five rlertroiyscs are recorded in 'Table I\'. The solute i n all CRSPS wits the tctraaniiriino nitrate as previously descrikd. hi the last sixteiw trials five grams'of arrirnoniurrr nitrate was added to every hisnrlred cubic cin. of liquid ftlnnloili5 in addition t u tlas copper salt. 'lhrse results imy bc brirfly snsuinariacd as follows. j. \ariatioil of ('nrrent. Ilcnsity. .Altbougli llrcrc was no case i n which i i good &pusit was obtained, due to the fact that all were non-adhrrent, iiicreasing t h e current dieiisit.y improrrs Ihc dqxmit up t o ii wrtnin poini l x yvood which il Ix~eoniwpoorpi'. I .
ELECTRODEPOSITIOS OF METALS FROM LIQCID AJIMONIA
945
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946
ROBERT TAFT AND HAROLD BARHAM
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Solutions of Zinc Salts in Liquid Ammonia observationa of zinc deposits from liquid :mimoiiin aoliitiona w x t ~ rnadc. T l ~ csalt uacd was zinc nitrate, licsahydratr, iron bcing riiiployed as the cathodr: and ziiie strips as tlw nnodr:. ' F h rrsolt,~of t i i t w &ctrolyscs nrr givcn i n 'i'atile \ I Sr:veral
ROBERT TAFT AND HAROLD BARHAM
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ELECTRODEPOSITIOS OF METALS FROM LIQUID AMMONIA
949
Conclusions similar to those already drawn in the case of other metals, with respect to the influence of changing current density, metal salt concentration, and acidity can be made here. Solutions of Chromium Salts in Liquid Ammonia Chromium chloride and chromic acid were the only salts given by Franklin and Kraus as being a t all soluble and they are only slightly soluble. Several other compounds were tried with the following results: C~Z(PO~)Z.~HZ~ Insoluble C~Z(C&~Z)~.ZHZ~ Slightly soluble Crz(SOd3.sHzO Insoluble KCr(S04)z.12H20 Insoluble Insoluble ZnCr04 Slightly soluble (NH4Wr207 Slightly soluble K2Cr207 Insoluble K2CrO4 C ~ ( N 0 ~ ) aHzO .x Very slightly soluble Effect of the Addition of N a S H 2 on the Solubility of Chromium Salts Crz(P04)2.6H20 Slight coloration CrdSOd z.sH20 ?io coloration Cr(K03)3 . ~ H 2 0 Very slight coloration C ~ Z ( C Z H ~ O Z ) ~ . ~ H ~ O ?io detectable change KO coloration K2Cr04 No detectable change Cr03 Rather a large number of experiments were carried out with various chromium salts, chromium acetate and chromic acid, in particular, all with negative results. The attempt was made to duplicate the conditions under which chromium deposits are produced in aqueous solutions-that is, by the use of chromic acid solution containing a small quantity of a salt whose anion is not decomposed in the process, such as the sulphate or chloride,-but the best that could be obtained was a black, non-adherent, amorphous coating. The great difficulty lay, no doubt, in the solubility of the salts available. The cell whose conductivity was the greatest was that in which a current of 0.3 amp. was passed under a potential of I I O volts. Solutions of Aluminum Salts in Liquid Ammonia Aluminum nitrate, which was found to be very soluble in liquid ammonia, was employed in neutral, basic, and acid solutions in the attempt to obtain an aluminum deposit. There was no case in which there was the slightest deposit but gassing always occurred a t both electrodes. Cryolite was found to be insoluble.
Summary and Conclusions To illustrate how changes in the operating conditions alter the form of deposits made in liquid ammonia solutions, specific examples for each variable previously outlined may be given.
950
ROBERT TAFT AND HAROLD BARHAM
I. Increase in Current Density. Anickel solution containing 2 grams of hexammino-nickel nitrate per 100 C.C. of liquid ammonia, gave the best deposit (Table 11, g9) a t a current density of 0.15 amps./sq. dm. The deposit a t a lower current density, 0.07 amps./sq. b.was somewhat darkened (Table 11, $3) and a t a higher current density, 0 . 2 3 amps./sq. dm., it was covered with green colored growths (Table 11, #IO). This factor was more marked in its effect upon silver deposits. A solution containing 4 grams of silver nitrate per IOO C.C. of liquid ammonia gave, a t a current density of 0.42 amps./sq. dm., the best deposit in this group (Table V, #z) but at 0.21 amps./sq.dm., it was partly burnt and torn away from the electrodes (Table T', #6), and a t 1.85 amps./sq.dm., it was badly burnt Table V, $8). Cells I, 2 and 3 (Table I) of the lead deposits, formed from a normal solution of lead nitrate, show that as the current density increases, the size of the nodules decreases and the area covered by the deposit increases. There was also some treeing along the electrode of Cell 3; Although the lead deposits given above were not good, it is evident that increased current density favors a good deposit up to the point where treeing occurs. Lowering of the Temperature. Lead deposits of Cells 5 and 6 were 2. prepared under the same conditions as were those of Cells I and 4 except that the former were obtained a t lower temperatures, namely: a t -55' and - 57°C respectively. If there is any difference in the form of the corresponding deposits, it is in the size of the nodules; the area covered by the deposit has not been noticeably changed. It seems, therefore, that lowering the temperature has only a slight effect upon the form of the deposit. 3. Addition of Colloidal Materials to Solution. When the colloidal material gliadin was added to a solution identical with that used to form the lead deposit of Cell 7 and electrolyzed at the same current density, no difference between the deposits could be detected (Cell 13.) The above result, together with the similarity between these deposits and those made from aqueous solutions containing colloids, had lead us to believe that neutral solutions of lead nitrate in liquid ammonia contain a colloidal basic salt which exerts a like effect upon deposits made in liquid ammonia solutions. The addition of casein in varying quantities to a solution of the same composition as was used t o form deposit of Cell #b, Table 111, electrolyses made at approximately the same current density, causes the deposits (Table 111, # I I , $12, $14) produced to be of poorer quality in that the film is less tenacious and the grainy character increased. Since very little is known about the behallor of colloids in liquid ammonia, it was not possible to choose intelligently among the colloidal materials which are soluble in liquid ammonia, for one which would be carried toward the cathode in the course of electrodeposition. For this reason any beneficial effect upon deposits caused by the addition of colloidal material to liquid ammonia solutions would be entirely accidental, since these substances were chosen at random.
ELECTRODEPOSITIO?; O F METALS FROM LIQUID AMMONIA
951
4. Increase in Metal Salt Concentration. A series of cadmium deposits made at a current density of approximately 1.15 amps./sq.dm. from a solution containing I O grams of ammonium nitrate and variable quantities of the hexammino salt can be selected from Table 111. A solution containing 8 grams gave the best deposit, (#b), another with z grams (#F) formed a “spongy” and “arboreal” deposit, and one with 1 2 grams gave a deposit possessing a distinct grainy character. Deposits $1 and $2 of Table I11 from solutions containing respectively 4 and approximately 8 (saturated) grams of zinc nitrate hexahydrate per I O O C.C.of liquid ammonia show a pronounced effect due to difference in metal concentration. #I was brittle, burnt, and non-adherent, while $2 was tough, bright, and adherent,. j. Increase in Conductivity of Solution. Lead deposits of Cells 7 and 8, Table I, show how the effect of increased conductivity, effected by the addition of I gram of ammonium nitrate to one of two identical solutions, alters the deposit. In Cell 7 large ridges or striations consisting of nodule-like growths of acicular crystals, have grown out into the solution. In Cell 8 the striations are not present, the tendency to grow out into the solution has decreased and, although they are larger, the nodules are much closer together. Cells $ 2 , $,; $8, $9 and # I O of Table VI represent deposits from solutions containing 4 grams of zinc nitrate hexahydrate per I O O C.C. of liquid ammonia and 0, 2 , 4, 6, and 8 grams respectively of ammonium nitrate. The deposit in Cell $2 was bright and adherent over most of the electrode but the film was less tenacious than Cell $ 7 . As the acidity increases from z to 8 grams, the deposits become more grainy. 6. Agitation. Deposits in Cells $17 and 18, Table I, were made under the same conditions as that of Cell 8, except that for the first two the solutions were stirred. The base metal for Cell 17 was lead and iron for Cell 18. The effect of stirring was to eliminate the growths, forming a smooth, adherent film. Conclusions
Our purpose in this investigation, as mentioned in the introduction, was threefold: ( I ) To determine if the same general factors influence the form of metals deposited from liquid ammonia as from aqueous baths, ( 2 ) the influence of the solvent upon deposit’s, and (3) the possibility of commercial utilization. As to the first point, ample confirmation has been given in the discussion hitherto given. I n the case of the second objective, direct comparisons are difficult to make in all cases. For this purpose we have prepared a table of deposits made in aqueous and in liquid ammonia baths. From the table of comparison of deposits made in aqueous and liquid ammonia solutions, it appears that the effect of the solvent upon deposits is as follows: I. In the majority of cases the C.D. for the best deposit is lower in liquid ammonia solutions than in aqueous solutions. They are, however, about the same for silver and zinc.
ROBERT TAFT AND HAROLD BARHAM
a,
c, L
ELECTRODEPOSITION O F METALS FROM LIQUID AMMONIA
953
2. For three metals, Pb, Cd, and Ag, the metal concentrations were approximately the same; for the other three, the concentrations were lower in liquid ammonia solutions. The difference between the two solvents are apparently due chiefly t o the much greater fluidities of solutions in liquid ammonia. Thus, Fitzgerald' gives approximately 340 as the fluidity of .gN CuN03.4NHa a t -33.5' in liquid ammonia, while Blanchardz found for the corresponding salt at 25' in water a fluidity of less than half this value; for the corresponding silver salt the fluidity is reduced nearly one third in aqueous solutions over that in liquid ammonia solutions. Furthermore, Franklin and Cady3 in their work on ionic velocities in liquid ammonia a t -33' found that the ionic velocities are two and one half to three times greater than in water at 18'. No data are available for the temperature coefficients of viscosity of liquid ammonia solutions, but if the differences (which are slight) in the form of the lead deposits obtained by us a t -33OC and a t -57OC are accountable solely to change in viscosity the temperature coefficient of viscosity of solutions in liquid ammonia must be very much less than that in water, which is generally given as 2 % per degree. The difficulties involved in preparing good lead and cadmium deposits in aqueous solutions, such as cost and adequate control of rather complex plating baths, make it seem at least possible that electrodeposition of these metals from their liquid ammonia solutions may find a practical application in industry.
* J. Phys. Chem.
16, 644 (1912). J. Am. Chem. hoc., 26, 1324(1904).
J. Am. Chem. Soc., 26,499 (1904).
