E L E C T R O L U S I S O F COBALT A N D NICKEI, T A R TRATES BY JAY E. ROOT
I n some experiments made on apparent decomposition voltages under the conditions used in electrochemical analysis, the value of 1.75 volts was found for cobalt in a n alkaline tartrate solution and 2.8 volts for nickel.' If this difference continued to exist when both cobalt and nickel were present in the solution, a quantitative separation by the constant voltage method would be very simple. Vortinann" states that a separation can be effected in such a solution, but he gives no data and practically no details. He says that the addition of a few drops of potassium iodide solution prevents the formation of cobaltic oxide at the anode. I n my work I found this not to be the case. Furthermore, the presence of iodide interferes with the precipitation of the metal at the cathode. T h e precipitation of any oxide at the anode can, however, be reduced to a nriniinum by making the anode current density high. If a few drops of concentrated nitric acid be added to the solution, there is still a deposit at the anode when the current density is low ; but the deposit is then chiefly nickelic oxide. It was soon fcund that the range of voltage is not as great as was hoped fro 11 the behavior of the metals when taken singly. If the voltage be kept below 2.1 volts the metallic cobalt contains no nickel. A t higher voltages the cathode deposit always tests for nickel. As no detailed study of cobalt and nickel alloys has yet been made, the theory of this phenomenon cannot be discussed here. While pure cobalt is obtained at voltages below 2.1 volts, it has proved impracticable to get all of the cobalt out of the solution in any reasonable time. One reason for this is theoxidation of cobaltous salt to a green cobaltic salt Jour. Phys. Chem. 7 , 428 (1903). Monatsheft, 14,536 (1893).
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Jay E. Root
at the anode. Addition of hydroxylamine sulphate, fortnaldehye, sodium bisulphite or even hydrogen peroxide helps somewhat, but does not make the precipitation satisfactory, With the bisulphite there is also precipitation of sulphur at the cathode. I n the experiments to be described, the salts used were crystallized CoS047H20from the laboratory stock and crystallized nickel sulphate or nitrate. No one of the three salts was free from the metal of the other. T h e cobalt sulphate was recrystallized and contained no iron. I t lost some water while being powdered and contained about 22.5-23.5 percent of cobalt instead of the more orthodox 21 percent. This introduces no error because it was only necessary to obtain metal, oxide, apd solution which would pass the test. T h e following tests were used in all the work : Test for nickel in presence o f cobalt. - T h e hydroxides of nickel and cobalt are filtered, washed and dissolved in the least possible amount of hot concentrated potassium cyanide solution. T o this solution add three times its volume of strong NaOH solution, then bromine water in excess and warm. T h e appearance of a blackish coloration is due to nickelic hydroxide. Test foy cobalt in presence of nickel. -- Solutions, if acid, are made alkaline with NaOH. Add slight excess of acetic acid, then large excess of saturated KNOz solution. T h e appearance of a yellow precipitate, after standing over night in a warm place, indicates traces of cobalt. This test will easily show 0.005 mg cobalt per cubic centimeter. I n the electrolytic runs, a Classen dish was always one electrode, while the other was either a platinum disk or a platinuni wire. T h e distance between the electrodes was always I cm. T h e first series of runs was made with the dish as cathode and the disk as anode. T h e solution was not stirred. T h e runs were made at 18". T h e stock solution cotisisted of I O g sodium potassium tartrate and 50 cc NaOH (sp. gr. 1.3), made up to 150 cc after the cobalt and nickel had been added.
EZecdrodysis of Cobalt and NickeZ Tarlrntes
3
RUN I . 0.3070 g crystallized cobalt sulphate. 0 . I g nickel as nickel sulphate, 1.9 volts ; 0.45 amp ; 6 hours. Cathode deposit of 0.0645 g Co (== 2 1 . 1 c/o), free from nickel. Slight anode deposit which is easily detached and is not weighable. Solution gives slight test for cobalt. RUN 2 .
0.7352 g crystallized cobalt sulphate. 0. I g nickel as nickel sulphate. 1.9 volts; 0.45 amp; 13 hours. Cathode deposit of 0.1505 g Co (= 20. j %), free from nickel. Considerable anode deposit. Solution gives slight test for cobalt. RUN 3. 0.2817 g crystallized cobalt sulphate. 0.1 g nickel as nickel sulphate. 1.9 volts ; 0.45 amp; 5 hours. Cathode deposit of 0.0556 g Co (= 19.7 %), free from nickel. Slight anode deposit. Solution tests for cobalt. In all three runs a whitish insoluble salt was noticed after lengthy electrolysis at both electrodes and through the solution. The second series of runs differed from the first in that the solution was stirred by rotating the anode. RUN 4. 0.4 575 g crystallized cobalt sulphate. 0 . I g nickel as nickel sulphate. 1.9 volts ; o 45 amp ; 20' ; 2 hours. Cathode deposit of 0.086 g Co, free from nickel. Anode deposit of 0.0065 g cobaltic oxide,' free from nickel. Total cobalt 0.0895 g (== 17.8 %). The anode deposit is approximately Co,O,.3H,O ; see Run 12. The anode was dried in air about one inch above the tlp of the Bunsen burner flame. I t was kept ill motion while being dried. T h e anode was allowed to cool f o r fifteen minutes aud was then weighed. The deposit was dissolved in boiling " 0 , and tested in the usual way.
Juy E. Root
4
Solution tests for cobalt. This run was purposely made short to see whether the bulk of the cobalt precipitates in the first two hours, RUN 5. 0.8440 g crystallized cobalt sulphate. 0.19g nickel as nickel sulphate. 1.9volts ; 0.45 amp ; 20' ; 8 hours. Cathode deposit of 0.1668 g Co, free from nickel. Anode deposit of 0.0153 g cobaltic oxide, free from nickel. Total cobalt 0 . I 7 5 g (1 20.7 % ) . Solution tests for cobalt. When using the rotating anode, the insoluble white salt is not formed. Rapid rotation tends, however, to detach the peroxide a t the anode. I n the third set of runs a needle anbde of platilium wire was used and the solution was not stirred. Owing to the increased resistance the current dropped almost to one-sixth its previous value. RUN 6. 0.9288 g crystallized cobalt sulphate. 0.I g nickel as nickel sulphate. 1.95volts ; 0.08 amp; 18' ; 2 0 hours. -Cathode deposit of 0.1948 g Co (= 19.9 %), free froin nickel. No anode deposit. Solution still contains cobalt. RUN 7. I .078 g crystallized cobalt sulphate. 0 . I g nickel as nickel sulphate. 1.95 volts; 0.08amp ; 18' ; 24 hours. Cathode deposit of 0.1958 g Co (= 18.2 %), free from nickel. Anode wire coated with whitish insoluble salt. Solution contains cobalt. RUN 8. 0.7443 g crystallized cobalt sulphate. 0 . I g nickel as nickel sulphate. 1.95volts ; 0.08 amp; 18' ; 24 hours. Cathode deposit of 0.1558 g Co (= 20.9 %), free from nickel. Solution contains cobalt. \
Electrolysis of Cobalt and Nickel Tavtvates
5
When using needle anode, care must be taken to prevent voltage rising during the run. T h e temperature s,hould be kept between 15' and 18'. Some runs were made at 5" and a t IO', but there is then a green iridescent precipitate left on the cathode after the metallic cobalt has been dissolved. T h i s precipitate disappears on ignition, sulphur dioxide being formed. In some cases as much as 0.001j g of sulphur were found. T h e results of these runs are that the cobalt precipitates pure, but that the last traces are very difficult to remove from the solution. Experiments were also made to see whether better results could be obtained by varying the concentrations of caustic soda and sodium potassium tartrate. Reducing the caustic soda from j o cc to 5 cc reduces the amount of anode deposit, but increases the precipitation of nickel at the cathode. Decrea3ing the amount of tartrate increases the anode deposit and also the amount of nickel at the cathode. Increasing the amount of tartrate cuts down the anode deposit and prevents the precipitation of nickel at the cathode, but it makes the precipitation of the cobalt more difficult. Raising the temperature makes the nickel precipitate more readily. T h e best conditions seem to be I O g sodium potassium tartrate, j o cc N a O H (sp. gr. 1.3), temperature of 15'-20' and a voltage not to exceed 2 . 0 volts. Since Vortmann recommends the use of potassium iodide a run was made to test the effect. RUN 9.
0.7 I 3 g crystallized cobalt sulphate. 0. I g nickel as nickel sulphate, z g potassium iodide. 2 . 0 5 volts ; 0.75 amp ; 12' ; 2 2 hours. There was a thick green deposit bn the cathode and the solution was murky. There was a black residue at the anode. Cathode deposit of 0.079 g
co (=
11.1
%).
As one reason for the difficulty in precipitating the last traces is the formation of the green cobaltic salt at the anode,
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Jay E. Root
some experiments were made with reducing agents. Addition of pyrogallol prevents the precipitation of oxide for a while, but more must be added every hour. T h e solution, of course, beconies black and dirty. With a potential difference of 2.2 volts the cobalt was not all precipitated a t the end of twenty-five hours, Sodium bisulphite prevents the anodic precipitation of oxide, but sulphur is precipitated with the cobalt and the last traces of cobalt come out hard. Addition of 2 g resorcine prevents the precipitation of cobalt. After the voltage had been kept four hours a t 2.5 volts, there was no cobalt on the cathode. Formaldehyde, I O cc, ,prevents the precipitation of oxide, but the cobalt was not all out a t the end of six hours. Hydroxylamine sulphate does not change the decomposition voltage of cobalt perceptibly; but the last traces of metal do not precipitate under low voltages. When there is no nickel present, there is no difficulty in getting all the cobalt out of the solution. T h e following experiments may be cited : RUN IO. 0.7361 g crystallized cobalt sulphate. 5 g sodium potassium tartrate. 5 cc NaOH (sp. gr. 1.3). Solution made up to 130 cc. Rotating disk as anode. A few crystals of hydroxylamine-sulphate, were, added to prevent formation of oxide. hours. 4.75 volts ; 3.25 amp ; i.5' Cathode deposit of Solution contains n .a7430 g, crystadize
Other conditions A@ ,Ca.fhodedaposjt, of, Solution contains no -4 In, the mext #run,the hpJrozxlatq@e pulphate was omitted fhe-dniountlof c i c Soda incyqas$d '? slightly t p dTsTea resistance. Y
EZectvoGysis of Cobalt and Nickel Tartrates RUN
7
12.
0.0870 g metallic cobalt dissolved in hot concentrated
sulphuric acid. The excess acid was neutralized with caustic soda. 5 g sodium potassium tartrate. 7 cc NaOH (sp. gr. 1.3). Solution made up to I jo cc. Rotating disk as anode. 4 volts; j amp; 1S0-4jo; 2 hours. Cathode deposit of o.oSoz g Co. Anode deposit of 0 . 0 1 2 2 g cobaltic oxide. Solution gives no test for cobalt.
If we assume that the cobaltic oxide has the formula Co203.3Hz0 the total cobalt is 0.0868 g instead of 0.0870 g, a difference which is inside the experimental error. On any assumption the composition of the oxide must correspond very closely to the formula Coz03.3HZ0,which confirms the results of Coehn'. Another run was made in which a needle anode was used, the solution being stirred by means of a rotating glass paddle. T h i s was to prevent anodic precipitation of oxide. RUN 13. 0.0802 g metallic cobalt dissolved in hot concentrated
sulphuric acid. The excess acid was neutralized with caustic soda. 5 g sodium potassium tartrate. 7 cc NaOHqsp. gr. 1.3). I
Jay E. Root
8
RUN 14. 0.0394 g metallic cobalt dissolved in least possible hot concentrated "0,. 7 cc water. 7 g crystallized sodium potassium tartrate dissolved in a very little water. 7 cc NaOH (sp. gr. 1.3) plus amount necessary to neutralize free acid, Solution made up to I jo cc. 4 volts ; 5 amp ; 18O-joO ; 2 hours. Cathode deposit of 0.0393 g Co. Solution gives no test for cobalt.
If there is any tendency for cobaltic oxide to precipitate at the anode, a drop or two of hot yoncentrated nitric acid should be added. RUN 15. 0.0863 g metallic cobalt dissolved in nitric acid. Time of electrolysis, 3 hours. All other conditions the same as in Run 14. Cathode deposit of 0.0862 g Co. T h e quantitative results of Runs 12-15 are given i n Table I. T h e methods are entirely satisfactory for the case when one has to make deterininations of cobalt only.
TABLEI. -~-
~~
__
Cobalt
Cobalt Taken
0.0870 0.0802
I
[
Pound
0.0868
0.0800
~1
0.0394 0.0863
~
Found
0.0393 0.0862
While the electrolytic determination of cobalt in itself is simple and easy, the separation from nickel in a n alkaline tartrate solution takes so long that it is useless for analytical purposes. Experiments were then made on electrolytic purification, to see whether the bulk of the cobalt could be obtained free from nickel and the bulk of the nickel obtained free froin cobalt.
EZecty0.olysi.s o f Cobali af2d Nickel Tavtraies
9
Nickelic oxide, Niz03, precipitates more readily, the more carbonate and the less tartrate there is in the solution. As cobaltic oxide tends to come down with the nickel, it was found best first to precipitate nearly all the cobalt, then to add sodium carbonate and electrolyze with low current density. There are three cases to be considered : cobalt solutions with nickel as irnpurity ; nickel solutions with cobalt as impurity ; solutions containing nickel and cobalt, neither being present i n large excess. I n the first run 150 cc of solution contained I O g sodium potassium tartrate 5.01 g cobalt sulphate 0.50 g nickel nitrate I O drops hot conc. HNO, 50 cc caustic soda (sp. gr. 1.3).
A Classen dish was used as cathode and a needle anode. T h e solution was electrolyzed thirty hours at 20' and 2 volts. T h e cathode deposit weighed 1.069 g and gave no test for nickel. T h e solution still contained cobalt. A clean cathode was substituted and the run continued for twenty-four hours more. T h e cathode deposit weighed 0.058 g and was free from nickel. T h e solution gave only a faint test for cobalt. A platinum dish was now made anode and a rotating platinum disk the cathode. After adding 5 g Na2C03the solution was electrolyzed for another twenty-four hours. T h e cathode deposit weighed 0.0016 g and contained no nickel. T h e anode deposit did not adhere well and the solution was therefore filtered through a Gooch crucible. I n this way there were obtained 0.1643 g nickelic oxide on the anode and 0.0356 g in the Gooch. T h i s oxide gave a faint trace for cobalt, probably due to occluded mother-liquor.' T h e solution was colorless and gave no test for cobalt or nickel. T h e yield is I. I 285 g of pure cobalt and 0.1998 g of nickelic oxide containing the merest traces of cobalt. By dissolving and reprecipitating, this trace of cobalt could be removed. I n a second run with 5.004 g cobalt sulphate and 0.1256 g nickel nitrate, 1.1233 g pure cobalt were obtained and 0.0638 g of nickelic oxide containing traces of cobalt. Coehn.
Zeit. anorg. Chem. 33,
22
(1902).
Jay E. Roof
IO
I n the next run the conditions were the same as before, except that the solution contained 2.0 g cobalt sulphate and 2.041 g nickel nitrate, and that the voltage was 1.9 volts. Electrolyzed 2 0 hours Deposit = 0.2672 g Co, pure Electrolyzed with clean electrodes, 7 hours Deposit = 0.0452 g Co, pure Electrolyzed with clean electrodes, I 7 hours Deposit = 0.0752 g Co, pure Electrolyzed with clean electrodes, 24 hours Deposit = 0.0478 g Co, pure Electrolyzed with clean electrodes, rg hours Deposit = 0.0067 g Co, pure Electrolyzed with clean electrodes, I 7 hours Deposit = 0.0017 g Co, pure .Electrobzed with clean electrodes, 48 hours Deposit = 0.0044 g Co, pure Added I O g sodium potassium tartrate and 25 cc NaOH Electrolyzed with clean electrodes, 48 hours Deposit = o 0042 g Co, pure Electrolyzed 5 hours, with a wire gauze anode, $ in. by 4 in. , wrapped in parchment paper. Solution stirred: . ,' 1 Deposit = 0.0662JgCo,pure.'L On1 c. posit on gauze. Solutiw gr.w-frotni !Added 5 g Na,CO, and eIectroly&d* as anode and stat Atlode deposit = o! bait. : N o .tabhode deposit. from nickel and cobalt., cr
b l
A,
1
