THE ELECTROLYTIC PRGCIPITATION OF’ SILVER BY RALPH C. SITOWDON
The precipitation of metals by electricity has, until very recent years, been regarded as something more or less in the hands of an invisible and uncontrollable power, and if good or bad results were obtained in specific cases, the greater portion of the conditions governing them were attributed to luck. It has been the object of about a year’s work in this laboratory to show that the precipitation of metals from aqueous solutions particularly for plating purposes is as easily controlled and as purely chemical as any of our processes, the electric current being simply a means to an end and having the nature of any of our power factors. Considerable work has been done on the precipitation of the metals, zinc, copper and tin, to furnish a part of the experimental data in a report by Professor Bancroft before the International Electrical Congress at St. Louis1, on Chemistry of Electroplating,’’ and we have followed out the same lines in the subsequent work on silver. It is a well known and accepted fact in general chemical practice that substances which are precipitated from solutions separate out larger crystals if they are precipitated slowly, and that an approximate relation exists between the size of the crystals and the speed of precipitation. The ordinary formation of barium sulphate from soluble barium salt solutions and of crystals of copper sulphate of extraordinary size from copper sulphate solutions may be cited as examples of this fact. Now the current controls the speed of the reaction in the case of electrolytic precipitation, so we are justified in saying that the size of the crystals of the metal d‘ecreasewith the increase in current density at the cathode unless secondary or extraordinary reactions take place. Also the decrease in the size of the crystals due to the addition of various organic and inorganic compounds to the electrolyte is to be expected from our present con((
Jour. Phys. Cliem., g, 277 (1905).
The EZectvobtz’c Pvec$itatio?z of SiZver
393
ception; zinc, copper and tin all fall in line with the theory and they also show a marked increase in size of crystal between zoo and 70°, those at 70°, all other conditions the same, being the largest. Rotating the cathode or stirring vigorously by other means tends to flatten out the crystals probably by simple skin friction, and it is possible to obtain a bright polished deposit in many cases by running up the speed of the cathode and using a correspondingly high current density. This apparent limiting speed and current density in the case of decreasing crystal size is of some interest in the precipitation of silver from solutions which usually do not give a satisfactory plating deposit. I t is true that even our voltameter deposits are sometimes unsatisfactory because of their tendency to form large crystals and fall off. Therefore the object in this investigation was to proceed with the study of the effect of current density and speed of rotation of the cathode upon the electrodeposition of silver, and to ascertain the possibility of obtaining a plating deposit of silver from the nitrate solution. The first experiments were tried with a normal silver nitrate solution, silver anode and small rotating disc of copper plated with silver for cathode. This method of operation was of no use whatever because, the reactions at the’ anode and cathode acted together in a manner detrimental to the purpose of the investigation, giving results seemingly independent of current density and speed of rotation. After several unsatisfactory runs this method of procedure was abandoned in the light of some recent work on the silver voltameter by Richards’ and others where they use a porous diaphragm to separate the anode and cathode solutions. By means of the diaphragm the peroxynitrate of silver, 3Ag,0.50.AgS0,,2which is formed at the anode is kept from the cathode and its influence is thus eliminated. Our cell was then changed to suit the new conditions by the Richards, Collins and Heirnrod : Proc. Am. Acad., 35, 123 (1899) ; Richards aud Heimrod : Ibid., 37, 415 (1902) ; Guthe : Phys. Rev., 19, 138 (1904). * Mulder and Heringa : Recueil Trav. Pays-Bas., 15, I (1896).
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addition of a IOO cc. porous cup which contained the silver nitrate solution under examination, and in which the cathode rotated. Outside the diaphragm and near its bottom was the platinum anode in its 2 0 percent nitric acid electrolyte. All was contained in a 500 cc. beaker. The cathodes were small copper discs 7 cn?. in area soldered to a vertical shaft and speeded to about 2 2 0 0 R. P. M. by a small electric motor. The lower face of the disc was plated with silver from a cyanide solution to be described later and then burnished. This operation was repeated two or three times in order to ensue a non-porous deposit. The remainder of the disc stem and the drop of solder were coated with paraffin and we had by this means an inexpensive and efficient cathode. These cathodes were then rotated within the porous cup, a fresh one being used for each change in conditions. After each of the longer runs and each two of the shorter ones, we changed the silver nitrate solution which by continued use became more or less dilute and affected by diffusion of nitric acid from the anode compartment. Endosmose was also prevented by keeping the level of the cathode solution higher than that in the anode compartment and by previously saturating the porous cup with the silver nitrate solution. The diaphragm worked very successfully in keeping the anode formation from the cathode chamber and very excellent results were thereby obtained, which followed the line of prediction very closely. The solutions of silver nitrate used were made up as follows : I . Normal AgNO,. 2. Normal AgNO,, normal "0,. 3. 1 / 1 0 normal AgNO,, 1/10 normal " 0 , . Five runs on each of the solutions were made, using the current densities, actual currents, voltages and times of run as indicated in the following table : We made photomicrographs (enlargement, g I diameters) of each cathode as soon as it was dry, and then lacquered it to check tarnishing as much as possible. These photomicrographs show that the size of crystal decreases with the increase in current density, although the real difference between the size of the
.
The EZectvolytic Pvec$itation of Silvev SOLUTION No.
i 3
I
1
Volts
3
4.0 10.0 20.0
Time Minutes
30 IO IO
8 5
10.0
0.5
5
Actual current amperes
20.0
I 2
4
T = 19’
0.5 2 .o 4.0
I 2
4 5
Current density Amp/dm2
I.
395
2.0
1 2
0.5
3 4 5
4.0
2.0
10.0 20.0
0.280
0.70 1.40
-
-
7.4
cvystal faces in the first two runs of each set is very small and at times seemingly negative but the cvystal aggwgaiions change regularly in size. The decrease in size of crystals between the third and fifth runs is genuine, although the change is not great and it seems as though the maximum speed attainable with our motors is not suitable for the higher current densities. I t is also noticed that the crystals in the cases of high current densities are laid on in mound-like masses following the direction of rotation, and are not separate individuals. From these experiments we are justified in saying that in the case of silver nitrate solutions, the size of the crystals of electrolytically precipitated silver decreases with the increase in current density and that the chemistry ” idea is based on firm foundations. Some people have observed that rapid stirring of the electrolyte and a high current density at the cathode tend to make a metal deposit smoother and more homogeneous. The Elmore process for making seamless copper tubes is an instance of this. All other experimenters along this line have varied ‘(
both conditions simultaneously and since this explained but little, the present work was carried out, varying only one condition at a time. I n the previous work on the silver nitrate solutions, we kept the speed of the cathode very nearly the same in all cases, so the actual effect of stirring could not be observed excepting in a general way. If this betterment of deposit in the case of rapidly rotated electrodes or unusually agitated solutions is due to simple friction of the electrode against the solution or the rapid renewal of discharging ions, then we should expect that a decrease in the rate of stirring would allow the deposit to become less homogeneous, more crystalline or “tree-like” and ultimately bad. In order to observe the effect of such a decrease in the rate of stirring, the silver cyanide solution again came into play because it gave the nearest to a bright deposit of any of the plating solutions. T h e solution is : I 5 g AgNO,. Enough KCN to redissolve the precipitate formed. 1000 cc. water. Under ordinary conditions in a simple electrolytic cell with silver anode, this solution precipitates the silver as a beautifully white coating having a matte appearance, but if the cathode be speeded to about 2 2 0 0 R. P. M. the deposit comes out almost bright. The conditions and results of experiments follow : POTASSIUM SILVERCYANIDESOLUTION. ‘ I ’ = zoo I. 2 . 0 amp/dni2r Speed 2 2 0 0 R. P. M. zl/, minutes Deposit nearly bright 2. 2.0 amp/dm2 Speed 500 R. P. M. z ’ / minutes ~ Deposit decidedly matte 3. 1.0amp/din2 Speed 2 2 0 0 R. P. M. 5 minutes Deposit decidedly matte 4. 10.0amp/dm2 Speed 2 2 0 0 R. P. M. 5 minutes Deposit decidedly matte The copper cathodes previously described were used but they were not plated with silver before the runs. A silver anode was used. Several trials were necessary before the conditions were obtained for a bright deposit, working from a low current
The Electrolytic Prec$itation of Silver
397
density and a high current density toward a happy medium. T h e data for these have been omitted because they indicate but little. T h e reason why the current density of IO amp/dnizdid not give a bright deposit is because the speed and current density are factors depending upon each other, and in this case the silver was precipitated out of the solution too rapidly to be burnished by the swiftly moving cathode. However, the difference between Run I and Run 2 shows that the ideas advanced are correct as far as we have gone and that the speed is a very important factor. T h e difference between I and 3 confirms still further the conceptions in regard to increase in crystal-size with decrease in current density. There is but little doubt that if means were at hand by which the cathode could be rotated at higher speeds a place would appear where a burnished deposit might be obtained from even a silver nitrate solution, one of the poorest solutions from which to precipitate a good deposit of silver. During the investigation, the effect of glue on the deposition of silver was studied since it is usually conceded that the addition of organic “colloids” causes wonderful effects when precipitating metals chemically or by electricity. Three experiments, which were made on silver nitrate solution No. I, gave rather interesting results. These were done in the diaphragm cell previously described. The lowest current density was used because it usually forms large crystals of the metal. Our conditions were as follows: MOLECULARSILVER NITRATESOLUTION No. I , ‘I= ’ 19’. Speed rotation 2 2 0 0 R. P. M. I . 0.5 g glue per liter. C. D. 0.5 amp/dm2. Smooth purple deposit. 2. 3.0 g glue per liter. C. D. 0.5 amp/dmz, Smooth yellow deposit going purple. 3. I O o g glue per liter. C. D. 0.5 anip/dm2. Smooth purple deposit . A precipitate of colloidal silver was obtained in each of the cases, Nos. I and 3 being purple in color and almost perfectly
398
The Electrolytzc Pvecipitation of SiZver
amorphous while No. 2 had a yellowish color which darkened slowly on standing to nearly purple. The three colloidal deposits which we did obtain had no visible crystal structure and were nearly bright. They resembled those precipitated by M. Carey Lea from silver salts of organic acids by purely chemical means. The phenomenon is rather unusual and we have not tried to work out the conditions under which the various forms of collodial silver may be obtained, but leave this for a further research. The general results of this paper are: A very finely crystalline deposit of silvei can be obtained from a silver nitrate solution by rotating the cathode rapidly and keeping the anode and cathode solutions separated. The size of the crystals decreases with the increase in current density and the increase in the rate of stirring at the cathode. The addition of small amounts of organic colloids ” to the solution makes the deposit very amorphous and causes the precipitated metal to assume a colloidal state. The presence of free nitric acid decreases the size of crystals but slightly. This work was suggested by Prof. Bancroft and carried out under his direction. ((
Cornell Universify.
