Volumetric Determination of Cobalt by Means of Ferrous Sulfate and Potassium Dichromate LANDON A. SARVER, School of Chemistry, University of Minnesota, Minneapolis, Minn.
E
NGLE and Gustavson ( 2 ) , Willard and Hall ( 5 ) , and others (9)have shown that cobalt can be determined easily and accurately by precipitating it as cobaltic hydroxide with sodium hydroxide in the presence of sodium perborate, decomposing the excess of the latter by boiling, dissolving the suspended precipitate by the addition of an acidified solution of potassium iodide, stannous chloride, or titanous sulfate, and finally titrating the liberated iodine, or excess of reducing agent by standard methods. However, the iodometric method is unsatisfactory in the presence of even a small amount of ferric iron, and requires from 0.5 to 2 hours for dissolving the precipitate, while stannous and titanous solutions require special handling because of their great sensitivity to atmospheric oxidation. Willard and Hall (6) attempted to use a strongly acid solution of ferrous sulfate as reducing agent, but obtained results which were consistently more than 4 per cent too low. On the other hand, the natural advantages of ferrous sulfate and potassium dichromate as titrating solutions have been recently increased by the discovery of the sensitive and reliable internal indicator diphenylamine sulfonic acid (4), and it has seemed desirable to develop a satisfactory procedure for their use in the determination of cobalt. It is probable that the low results of Willard and Hall were due to the great instability of cobaltic salts in acid solution, and that some free oxygen was lost while mixing with the strongly acid ferrous sulfate, since the latter is comparatively resistant to the action of oxygen in the cold. Such a possibility gave rise to the hope that an accurate procedure could be developed for the use of ferrous sulfate in the titration of cobalt, and has led to this research. SOLUTIONS The cobalt solution was made up from pure anhydrous cobalt sulfate, and proved to be free from nickel by testing with dimethylglyoxime; the mean of six electrolytic depositions ( 1 ) showed a cobalt content of 0.0495 gram (0.0495, 0.0494, 0.0495, 0.0495, 0.0496, and 0.0495 gram) for each 25cc. portion a t 25' C. The titrating solutions were 0.1 N , but much weaker ones could be employed if desired. The potassium dichromate was made up to a definite volume a i t h water from an accurately weighed portion of the pure recrystallized salt. The solution of pure ferrous ammonium sulfate, slightly acid with sulfuric acid, was standardized frequently against the dichromate in the presence of phosphoric acid, using 5 drops of a 0.2 per cent water solution of barium diphenylamine sulfonate as indicator. MANNEROF MIXINGREAGENTS EFFECT UPON FERROUS IRON.It is necessary in this analysis to mix an acidified ferrous sulfate solution with a considerable quantity of sodium hydroxide, without change of titer; otherwise, apparently correct results for cobalt would be due to a compensation d errors. Many experiments were made with the greatest care, allowing each solution to flow down
the walls of the flask underneath the other reagents, but some iron was always oxidized when air was present. A layer of ether prevented this fairly well when the ferrous solution was added to the base, but not when the order was reversed. The titer was unchanged, however, when the mixing was done in an oxygen-free atmosphere, regardless of the temperatures or the order of addition of the reagents. EFFECTUPON TITRATION OF COBALT.Lyw results were always obtained when the strongly alkaline solution containing precipitated cobaltic hydroxide was mixed with ferrous iron in the presence of enough sulfuric acid to neutralize all the base, regardless of the temperature or manner of mixing; naturally, the titration value of the iron was determined under parallel conditions in each case. The values were still lower when the mixing was performed under a layer of ether, and not even working in a completely closed air-free vessel sufficed to correct the error. However, when ordinary weakly acid ferrous sulfate solution was added to the alkaline mixture in an air-free flask, with subsequent acidification by sulfuric acid, the results were quantitative, even a t the boiling temperature. A 500-cc. Erlenmeyer flask fitted with a dropping funnel of 35 to 40 cc. capacity by means of a well-paraffined rubber stopper can be used for this purpose, but it is difficult to prevent the leakage of a little air, t o which ferrous hydroxide is very sensitive; an all-Pyrex apparatus with ground-in dropping funnel is preferable. RECOMMENDED PROCEDURE The cobalt solution, free from interfering ions and containing at least 5 cc. of 6 N sulfuric acid and 1 to 2 grams of dissolved sodium perborate, is treated with enough 6 N sodium hydroxide to leave about 10 cc. in excess, whereupon brownish black cobaltic hydroxide precipitates, accompanied by active effervescence; the mixture is boiled for 10 minutes to decompose excess of perborate and displace last traces of oxygen by water vapor, the dropping funnel being placed in position (with the stopcock o en) and paraffined near the end of this period. After removal Bom the hot late the apparatus is promptly closed and an excess of standarcfferrous sulfate measured into the funnel; upon opening the stopcock cautiously, the solution is drawn in by the reduced pressure inside the flask, taking care that no air is allowed to enter. The funnel is rinsed with two or three portions of water, always avoiding the entrance of air, the vessel is shaken a few seconds, and 25 to 30 cc. of 6 N sulfuric acid are admitted, whereupon the precipitate dissolves almost instantly. After cooling to room temperature, the stopcock is opened, and the fupnel removed and rinsed. About 10 cc. of 25 per cent phosphoric acid and 5 drops of a 0.2 per cent water solution of barium diphenylamine sulfonate are then added, and the excess of ferrous iron titrated to the appearance of a violet with standard dichromate. The method thus becomes extremely rapid.
IKTERFERING IONS.Kitrates and other oxidizing substances which give colorations with diphenylamine sulfonic acid must be absent. Nickel does not interfere. Cobalt can be easily separated in one operation from manganese, chromium, vanadium, etc., by means of phenylthiohydantoic acid ( 5 ) ; small amounts of iron are carried down with this precipitate, but not enough to cause incomplete oxidation of the cobalt by perborate.
275
ANALYTICAL EDITION
276
TABLEI. DETERMINATION OF COBALT EXPERI-0.1N KgCraOi.USED MINT Analysis Blank cc. cc . 24.28 1 15.89 24.28 2 15.87 24.28 15.88 3 24.28 4 15.91 24.28 5 15.91 24.70 16.28 6a 24.70 16.32 7a 24.77 16.34 8 16.37 24.77 9 24.77 16.41 10 24>77 16.40 11 24.77 16.40 12 24.77 16.35 13 24.77 16.37 14 24.77 16 33 15 24.77 16.34 16 24.77 17 16.32 24.77 16.37 18 24.77 19 8.02 24.77 20 8.00 a Experiments 6 and 7 contained I
COBALT Present Found Gram
ERROR %
Gram
0,0495 0.0 0.0495 0.0496 0.2 0,0495 0.0 0.0495 0.0495 -0.3 0.04940.0495 -0.3 0.04940.0495 0.3 0.0496 0.0495 -0.2 0.0494 0.0495 0.4 0.0497 0.0495 0.0495 0.0 0.0495 -0.4 0.0493 0.0495 0.04946 -0.3 0.0495 -0.2 0.04940.0495 0.3 0.0496 0.0495 0,0495 0.0 0.0495 0.049740.5 0.0495 0.5 0.0497 0.0495 0.0498 0.6 0,0495 0.0 0.0495 0,0495 -0.2 0.0988 0.0990 -0.1 0.0989 0.0990 20 and 40 mg. of nickel, respectively.
+
+
ACCURACY.A few typical results are given in Table I Lo illustrhte the accuracy which is possible by this method.
Experiments 1 to 5 were performed in an apparatus con-
Vol. 5, No. 4
structed entirely of Pyrex, while 6 to 20 were carried out in an ordinary apparatus with a paraffined rubber stopper.
SUMMARY The method described above has been developed for the use of sodium perborate, ferrous sulfate, and potassium dichromate in the volumetric determination of cobalt, and is believed to be the most rapid, convenient, and accurate so far proposed. Diphenylamine sulfonic acid is recommended as indicator. LITERATURE CITED (1) Classen. A.. and Cloeren. H.. tr. bx Hall., “Quantitative Analysis by Electrolysis,” p. 192, Wiley; 1919. (2) Engle, W. D., and Gustavson, R. G., J. IND.ENG.CHEM.,8, 901 (1916). (3) Gillis, J., and Cuvelier, V., Natuumu. Tijdschr., 1 1 , 2 0 , 1 2 3 (1929). (4) Sarver, L. A., and Kolthoff, I. M., J. Am. Chem. Soc., 53, 2902 (1931). ( 5 ) Willard, H. H., and
Hall, D., Ibid., 44,
2219, 2237 (1922).
RECBIVBD April 26, 1933.
Nomograph for Rapid Calculation of Sulfate-Carbonate Ratios ROBERTT. SHEEN,W. H. & L. D. Betz, Philadelphia, Pa. LARGE number of plants and laboratories calculate ratios of sodium sulfate to total alkalinity when exPressed 8s sodium carbonate, to ascertain whether Or not the result is within the limits as prescribed by the American Society of Mechanical Engineers for the inhibition of caustic metal embrittlement. Most laboratories report sulfates in terms of the radical SO4 and alkalinities in terms of c a l c i u m c a r b o n a t e . To convert ‘sulfates to sodium sulfate, it is necessary to multiply by 1.479-that is, SO4 x 1.479 = Na2S04. To calculate total a l k a l i n i t y as calcium carbonate in terms of sodium carbonate, it is necessary to multiply by the factor 1.06-that is, CaC03 X 1.06 = Na2C03. To calculate the sulfate-carbonate ratio, the first result is divided by the $ second. The accompanying nomograph accomplishes the same result in one operation. Given sulfates in terms of SO1 and total alkalinity in terms of CaC03, the ratio
A
For Pressures over 250 pounds, 3 parts sodium sulfate to 1 part The following examples illustrate the use of the chart: EXAMPLE 1. Given an analysis in parts per million, part of which reads as follows: Sulfates @or) Alkalinity as CaCOa: Bicarbonates Carbonates Hydroxides
s
2
NazCOa straight line from the total alkalinity axis at the given figure t h r o u g h the given figure on the sulfate axis and reading the result directly on the ratio axis. A. S. M. E. REQUIREMENTS. The ratio in each case is the minimum ratio required. For pressures up to 150 pounds, 1 part sodium sulfate t o 1 part total alkalinity. For pressures from 150 to 250 pounds, 2 parts sodium sulfate t o 1 part total
alkalinity.
A
0 154 196
The total alkalinity, the sum of carbonates and hydroxides as given in this anal sis, is 350 parts per million. Start a straigzi line from this point on the total alkalinity axis intersecting the sulfate axis at 300 parts per million and the corres onding ratio will be found to be 1.19 on tKe ratio axis. This ratio is satisfactory for boilers operating below 150 pounds pressure but is unsatisfactory for boilers over 150 pounds. Mathematically, the following calculations would have been required: S 0 r X 1.479 CaCOa X 1.06
Na2S04 may be read b y e x t e n d i n g a
300
300 X 1.479 360 X 1.08
1.19
EXAMPLE 2. Given an analysis in parts per million, part of which reads as follows: Sulfates (804) Alktlinity a0 CaCOa: Bicarbonates Carbon[ttes Hydroxides
300 0 188 312
The total alkalinity, the sum of carbonates and hydroxides as given in this FIGURE 1. NOMOGRAPH FOR CALanalysis, is 500 parts per million. Start a CULATING SULFATE-CARBONATEline from 500 parts per million on the total RATIOS FOR INHIBITION OF CAUSTIC alkalinity axis, intersecting the sulfate axis METALEMBRITTLEMENT at 300 parts per million, and it will strike .I
