TRANSFERENCE NUMBER OF COBALT SULFATE* BY R.
c.
CANTELO A N D E.
c.
PAYNE~
This paper records the results of an investigation which was undertaken to determine how the transference number of cobalt sulfate varies with the concentration of the solution. A review of the literature indicates that no data exist for the transference number of cobalt sulfate. I n fact, the only such investigations on cobalt salts are those of Bein2 for cobalt chloride, and of Denham3 for cobalt bromide. Bein gives only one value, T& = 0.596 for a concentration corresponding to 0.199 per cent chlorine. Denham’s results are given in Table I.
TABLE I Cobalt Bromide Solutions Conc’n. moles per liter 0.090 0.459 1.345 2.448 3.106 4,731 5.554 T &-: +0.409 0.413 0.340 0 . 3 2 2 0 . 2 1 5 0.005 -0.444 Denham explained these decreasing and negative values by assuming the formation of “auto-complexes” such as C o B r s and coB1-4 which transfer cobalt towards the anode. With respect to these kinds of phenomena McBain and Van Rysselberghe* have shown that a common ion, when added in large excess, to the solution of a bivalent cation (and to less extent for univalent cations), will suppress the movement of that ion towards the cathode, and may even reverse its direction, causing it to show a negative transference. Further, they have shown that these phenomena likewise occur with very concentrated solutions of a single salt. They say: “If complexes occur in mixtures of two salts, they must also occur in solutions of one salt of the same (anion) concentration. From the mass action principle, even greater amounts of the complexes will be formed, but less in proportion to the total salt present.’’ McBain and Van Rysselberghe have attempted to show that these results are “incompatible with any complete dissociation theory,” since they believe it to be necessary for complex formation: that the simple anion combine with the undissociated molecule to form the complex ion. *Contribution from the Department of Chemistry, College of Llberal Arts, University of Cincinnati. 1 An abstract of a art of the dissertation presented by E. C. Payne to the Graduate School, University of 8incinnati; in partial fulfillment of the requirements for the degree of Doctor of Philosophy. Bein: Z. physik. Chem., 27, I (1898). 3 Denham: Z. physik. Chem., 65,641 (1909). 4 McBain and Van Ryaselberghe: J. Am. Chem. SOC.,50, 3009 (1928);52, 2336 (1930).
1046
R. C. CANTELO AND E. C. PAYEiE
Materials Cobalt Sulfate.-The cobalt sulfate was the so-called C.P. grade which originally contained one per cent nickel as nickel sulfate. It was purified in two ways, each of which gave an apparently pure product. The first method was that used by Cantelo and Bergerl in their work on the electrical conductance of cobalt sulfate. The second method of purification was based on that described in “U. S. Bureau of Mines Information Circular 6331.” Cobalt Sulfate Solutions.-Conductivity water was used in the preparation of all solutions; the solutions were made up fresh for each determination. Analytical Method All analyses were made by electrolysis. The method was essentially that described by Lundell and Hoffmann? but certain modifications were used. A portion of solution was taken for analysis which contained from 0.15 to 0 . 2 5 grams of cobalt. To this was added a mixed solution containing the equivalent of I O cc. concentrated sulfuric acid and 5 5 cc. of strong ammonia. The solution was made up to 1 5 0 cc. and was electrolyzed for I-% hours, with a current of 0.6 to I ampere and a voltage of 2 . 5 to 3.5. Acylindricalcathodeof platinum gauze and a rotating platinum spiral anode were used. At the end of one hour, or when the solution became colorless, the cover glass and the sides of the beaker were washed down and about 0.5 gram of sodium bisulfite was added. The sodium bilsulfite aids apparently in the deposition of the last portion of cobalt, but tends to form small amounts of sulfide on the cathode. It was found inadvisable to remove all the cobalt by electrolysis as platinum may be plated onto the cathode. Accordingly, after I-% hours of electrolysis, the residue was precipitated by ammonium sulfide, after the excess ammonia had been removed by boiling. The cobalt sulfide was filtered, washed with water containing ammonium chloride and ammonium sulfide, ignited and weighed, The weight of oxide so obtained, seldom more than I milligram, was and the result, (as cobalt) was added to multiplied by an empirical factor, the weight of cobalt deposited on the cathode.
x,
Apparatus In this investigation, we have tried to follow the suggestions given by Noyes and Falk,3 namely: “Both cathode and anode portions should be analyzed.” This was I. done, but the anode portion did not give reliable results. “The change in concentration should be as large as possible.’’ 2. 3 . “It is advisable to analyze three middle portions.’’ 4. “The character of the electrodes should be such as to form no migrating substance likely to cause error in analysis.” 5. “Apparatus should be so designed as not to cause convection currents and stirring, during electrolysis and removal of the solution.” Cantelo and Berger: J. Am. Chem. SOC.,52, 2648 (1930). Lundell and Hoffman: Ind. Eng. Chem., 13, 540 (1921). 8 Noyes and Falk: J. Am. Chem. Soc., 33, 1436 (1911). 1
TRANSFERENCE NUMBER OF COBALT SULFATE
1047
The apparatus used was essentially that of A. A. Noyes.' The inside diameter was 23 mm. and was uniform throughout. The total distance between the electrodes was about 7 0 cm. The apparatus was made in two parts, and joined at the middle by rubber tubing, wired tight, and coated with ceresine wax. Several electrodes were tried but the only satisfactory ones were: a cathode of very fine platinum gauze, and an anode of similar material plated with cobalt. These electrodes were flat circular pieces of gauze about I-K cm. in diameter. They were welded to pieces of platinum wire, which were sealed into the end of a 6 cm. length of 4 mm. glass tubing. On the inside of the tube contact was made with a copper wire by means of Wood's metal. This tube, joining the electrode, was joined to a longer one by means of rubber tubing; and the longer one was held in place at the top of the transference apparatus by passing through a rubber stopper. Electrical connection was made by means of a copper wire which passed through the long tube and was connected to the wire in the short one by means of a loop and hook. The electrode end, thus, could be detached and weighed. The gauze cathode left nothing to be desired. The number of equivalents of cobalt plated on it in an experiment checked very well with the amount calculated from the silver coulometer. The anode, however, did not dissolve, on electrolysis, with any degree of regularity, and consequently, few of the anode portions could be checked against the cathode portions. After electrolysis the anode was coated with a material which was, apparently, an oxide. This oxide varied in color from pink, blue or green to brown. Some of this material fell to the bottom of the anode compartment as a sludge. Current was measured by a silver coulometer which was constructed as follows: The cathode was a deep platinum dish of about 200 cc. capacity; the anode was a bar of pure silver, coated with electrolytic silver and wrapped in ashless filter paper. The silver nitrate was purified by the method of Rosa and Vinal.z Current was indicated roughly by a milliammeter, and thestrength of current was adjusted by a slide-wire resistance. Experimental Procedure The two halves of the apparatus were cleaned and dried; and were joined as described previously, and the solution was poured into the completed cell. The levels were adjusted to 3-4 cm. above the bends in the outer arms, and the small withdrawal tubes were stoppered. The electrodes were then inserted and adjusted so as to be just below the surface of the solution. The anode had been weighed previously. The apparatus was immersed almost to the top of the withdrawal tubes in a thermostat kept a t the temperature z 5' i0 . 0 2 . After at least thirty minutes the current was applied at a sufficiently high voltage to pass the desired current, 5 to 80 milliamperes. Since storage cells were used as the source of electricity, the current could be kept very constant. A. A. Noyes: J. Am. Chem. SOC.,23, 42 (1901). *Rosa and Vinal: Bur. Standards, Bull., 13, 479 (1916-17).
1048
R . C. CANTELO AND E. C. PAYNE
Current was passed for as long a time as, as found by experience, would give the maximum change in concentration about the electrodes, without changing the concentration of the middle portions. This time varied from 3 to 1 2 hours with 4 to 6 hours as the most usual. After the current was stopped, the solution was separated into five parts. The middle portion (M), and the two adjacent portions, (AM and CM), in the vertical arms of the tube, were carefully pipetted directly into dried and weighed flasks by means of suction. The tip of the pipette was kept just below the surface of the solution to avoid stirring. The two electrodes were removed and carefully rinsed with a very small amount of the original solution into their respective compartments. They were then rinsed with water, dried and weighed. The cathode deposit was dissolved in nitric acid, again dried and weighed, and the weight of the cobalt deposit was determined. The coulometer was emptied, carefully rinsed, dried and weighed and the weight of the silver deposited was taken as a measure of the current passed. As has been mentioned previously, there was good agreement between the number of equivalents of silver deposited, and the number of equivalents of cobalt deposited on the cathode. The two halves of the apparatus were separated and stoppered. The exterior of the glass was wiped off and the apparatus and contents weighed. The solutions were well mixed, poured out into dry flasks, and the tubes were rinsed, dried and weighed. The five portions were analyzed electrolytically and the transfer of the cobalt sulfate was calculated. As has been mentioned above, the anode results were unreliable. Results Table I1 contains the transference numbers of cobalt calculated from the change in concentration of the cathode portion.
TABLE I1 Transference Numbers of Cobalt Sulfate Solutions Concn. N
Faradays passed
3.994 3.I04 2.833 2.331 I . 860 1.454 1.444 0.996 0.753 0.473
0.006627 0.005624 0.010504 0,003335 0.008620 0.003I 76 0.002632 0.007536
0,233
E uivalents lost by eathode middle
Cathode
0.002550
0.00
0.002929 0.002610
0.00
0.005640 0.004320 0.007776 0.002408 0.006154 0.002 195 0.001801 0.004850 0.001854 0,001 787
0.00
0.001577
0.00 0.ooooa 0.00 0.00012 I
0.000025
0.00 0.00
0.000103
Transference No. of Go++ 3 . I49 0.232 0.260
0.278
0.286 0,309 0.316 0.356 0.373 0,390 0.396
TRAXSFERENCE XUMBER O F COBALT SULFATE
I049
Discussion of Results Cantelo and Berger' found for the equivalent conductance of cobalt sulfate The ion-conduca t zero concentration the value A, = 134.6 mhos for 25'. tance of SO=c at 25' is 78.4. Hence the transference mumber of Co++ in cobalt sulfate is 56.2/134,6= 0,419for solutions of zero concentration. An examination of Table I1 shows a t once how greatly the determined transference numbers deviate from the value calculated for zero concentration. The measured values vary from 0.396 for a 0.233 N solution to 0,149for a 3.994 normal solution. These results indicate that with increasing concentration, either there is a fiery great chanoe zn the ratio of the mobelities of the cobalt and sulfate ions or that there is the appearance of complex ions in increasing concentration as the solutions increase in stoichiometrical concentration. If we consider that the rapid increase in the transference numbers with increasing concentration is due to a change in the ratio of the ion mobilities, we can find a possible explanation in excessive hydration of cobalt ion.2 It is necessary to assume that the cobalt ion is more highly hydrated than the sulfate ion. Suppose that each Co++ ion carries ten molecules of bound water more than the anion. During transference this water is carried to the cathode, thereby displacing the whole solution towards the anode. Then for one Faraday carried by CO++,there would be an apparent movement towards the anode of j moles or 90 grams of water. For 0.233 N cobalt sulfate solution, this would give 0.233 X 0.09 = 0.021 equivalents change in the apparent migration. But this is further reduced, if we use the value for TZ6 for zero concentration to 0.021 X 0.42 = 0.009 equivalents for the total effect on the migration. The actual change in migration is 0.419 - 0.396 = 0.025. This method of explanation is, therefore, inadequate to explain our results. A correction two and one-half times greater would be required to explain them in this way. For the 3.994N solution, the change in the apparent migration due to I O molecules of water of hydration would be 3.994 X 0.09 X 42 = 0.1j~ equivalents. Actually the change in migration is 0.419 - 0.151 = 0.268 equivalents, a value nearly twice as great. Such great hydration seems very improbable. The second explanation lies in the admission that we are dealing here with complex ions, such as CoTS04)~. Pfanhause9 found that in a saturated solution of nickel ammonium sulfate, the nickel moves entirely to the anode in the form of the complex ion, NiTS04)2. Thus, it seems probable that such complex ions exist in solutions of cobalt sulfate. The decreasing values of the tranference numbers of Co++ are due to the increasing amounts of cobalt carried to the anode as complex ions.
L,
Cantelo and Berger: loc. a t . See McBain and Van Rysselberghe: J. Am. Chem. Soc., 50, 3016 (1928). Pfanhauser: 2. Elehtrochemie, 7, 698 (1901).
IO50
R . C. CANTELO AND E. C. PAYNE
Summary The transference number of cobalt ion in cobalt sulfate solutions at concentrations 0.233 to 3.994 N has been measured at 25'. I n this range the transference number varies from 0.396 to 0.149. The value calculated from the ion conductance is 0.419. It has been shown that the assumption of excessive hydration of the cobalt cation is inadequate to explain the deviations of the T, values from 0.419. It has been suggested that the admission of complex anions of the type C O ( ~ Oetc., ~)~ affords a much more logical explanation. Cincinnati, Ohio.
