Determination of Cobalt in Cobalt-Nickel Solutions WILLIAM F. HARRIS and THOMAS R. SWEET McPherson Chemical Laboratory, The O h i o State University, Columbus 70, O h i o
A convenient method has been developed for separation and volumetric determination of cobalt in mixtures containing varying ratios of cobalt and nickel.
T
HE purpose of this work was to develop a convenient method for the separation and volumetric determination of
cobalt in mixtures containing widely varied ratios of cobalt and nickel. T h e precipitstion of cobalt as the robalticyanide ( 2 ) provides a good method for the separation of cobalt from nickel, but it is very slow. The digestion period that is usually recommended is 24 hours. Lowry (4)developed a separation that is based on the fact that nickel carbonyl is more volatile than cobalt carbonyl. This is somewhat inconvenient, since both the nickel carbonyl and the reagent, carbon monoxide, are poisonous and the nickel carbonyl is highly explosive. Tillu ( 7 ) utilized the difference in solubility of nickel and cobaltous chlorides in acetone as a means of separation. However, in order to effect this separation, it is necessary to convert the cobalt and nickel to the form of the monohydrated chloride salts. Nenadkevich and Saltykova ( 5 )have suggested a separation based on the ability of cobalt to form a stable cyanide complex in acid solution and on the instability of the nickel cyanide complex under the same conditions. They precipitated the cobalt cyanide complex with a heavy metal such as silver. Such an analysis would necessarily involve special precautions to avoid the health hazard. Several organic precipitating agents have been proposed for this separation. If cobalt and nickel are precipitated as the xanthates (9),the nickel xanthate can be dissolved in ammonium hydroxide and the cobalt xanthate collected by filtration. Phenylthiohydantoic acid ( 2 1 ) has been used for the separation of cobalt from nickel. However, a slight amount of nickel is carried down with the cobalt precipitate. Portnov (6) suggested p acetaminophenyl arsonic acid, although the separation does not appear to be complete with this reagent. One of the most successful reagents for the separation of cobalt and nickel is 1-nitroso-2-naphthol (3). It has been used chiefly in the separation of small amounts of cobalt from large amounts of nickel (IO). The bulky red cobalt precipitate has an indefinite composition; therefore, the precipitate must be converted to the oxide before it can be weighed. Since the formula Cos04 only approximates the composition of the oside that is formed by igniting the precipitate, it is generally further converted to the sulfate if the sample contains more than a few milligrams of cobalt. METHOD
T h e method proposed in the present paper involves a determination of the combined nickel and cobalt content of the sample by the addition of excess ethylenedianiinetetraa ce t i c acid (EDTL1)and the titration of the excess with zinc, using Eriochrome Black T as the indicator. This is done with one portion of the sample solution. Using a second portion, the 0.005 0.00 51.93 41.54 9.960 9.953 cobalt and nickel are separated 31.06 19.87 19.91 by means of a l-nitroso-225.96 24,94 24.94 29.88 29.86 20.77 naphthol precipitation, 10.39 39.94 39.81 0 . 0 0 49.78 49,77 followed b y a chloroform e x t r a c t i o n of t h e c o b a l t
complex ( 1 ) . The nickel content of the aqueous layer is determined by the addition of an excess of ethylenediaminetetraacetic acid and a back-titration with zinc, using Eriochrome Black T as the indicator. The difference between these two determinations represents the cobalt content. An attempt was made to determine the cobalt more directly by decomposing the cobalt 1-nitroso-2-naphthol in acid, adding a n excess of ethylenediaminetetraacetic acid, and back titrating with standard zinc. However, this does not represent a satisfactory approach to the present problem, because of the exceptional resistance of cobalt 1-nitroso-2-naphthol to treatment with acids. APPARATUS
The precipitation of cobalt with 1-nitroso-2-naphthol and the extraction of the precipitate with chloroform m r e done in a 250-ml. separatory funnel. A one-hole rubber stopper was substituted for the glass stopper. A &inch straight drying tube, cut off about 1 inch above the bulb, was inserted in the stopper and served as a combination funnel and trap. It was necessary to use a device of this type because the high vapor pressure of the chloroform frequently caused some loss of sample when the glass stopper was removed. REAGENTS
Indicator solution. Dissolve 0.2 gram of Eriochrome Black
T in 100 ml. of distilled water which has been made basic with a few drops of ammonium hydroxide. Standard zinc solution. Dissolve 1.4 grams of zinc oxide (dried a t 110" C. for 2 hours) in a minimum amount of 1 to 1 nitric acid and dilute to 1 liter with distilled water. Standardize the solution by precipitating the zinc as the ammonium phosphate and weighing it as the pyrophosphate after ignition a t 550" C. Standard E D T A solution. Dissolve 5.5 grams of disodium dihydrogen ethylenediaminetetraacetate dihydrate in distilled water and dilute to 1 liter. Standardize against the standard zinc solution. -4dd 5 ml. of 1 t o 1 ammonium hydroxide and 3 drops of Eriochrome Black T solution to 50 ml. of the standard zinc. Titrate with the EDTA solution until the color changes from red to blue. PROCEDURE
Transfer a 50-ml. portion of the sample solution to a 250-ml. Erlenmeyer flask and add an excess of the standard EDTA. Adjust this solution to a pH of about 10 n i t h 1 to 1 ammonium hydroxide and add enough Eriochrome Black T indicator t o produce a distinct blue color. Titrate to a red end point with t h e standard zinc solution. Transfer a second 50-ml. aliquot of the Eample to a 250-ml. separatory funnel containing 50 nil. of 10% hydrochloric acid. Add a twofold excess of freshly prepared reagent solution (1 gram of 1-nitroso-2-naphthol dissolved in 15 ml. of glacial acetic acid), One gram of 1-nitroso-2-naphthol will precipitate about 0.1 gram of cobalt, Mix well by vigorously agitating the separatory funnel several times over a period of about 15 minutes. Add a 5-ml. portion of chloroform and shake the funnel. Allow the,chloroform to settle for a few minutes and shake the mixture again. Allow the chloroform to settle for about 5 minutes and drat! off the chloroform layer, leaving a very small amount of
Table I. Analytical Results
51.84 41.59 31.09 25,92 20.79 10.40 0.003
1648
0,005 0.007 0.04 0.00 0.02 0.13 0.01
...
0.07 0.20 0.00 0.07 0.30 0.02
0.09 0.05 0.03 0.04 0.02 0.01 0.003
0.17 0.12 0.10 0.16 0.10 0.10
...
V O L U M E 2 6 , NO. 10, O C T O B E R 1 9 5 4 the chloroform in the funnel. Add two more 5-ml. portions of chloroform and repeat the above process after each addition. At this point the aqueous solution is yellow orange in color. Pass the solution in the separatory funnel through S and S Black Ribbon filter paper into a 250-ml. Erlenmeyer flask. K a s h the funnel and filter paper with distilled water. Add 0.5 ml. of 3070 Superoxol to the mixture and heat for 15 to 20 minutes. llaintain the solution a t near boiling for the last 5 minutes. After the solution has cooled, neutralize it n i t h concentrated ammonium hydroxide and add a 3 ml. excess. Add an excess of the standard E D T A i solution and enough Eriochrome Black T indicator to produce a distinct blue color. Titrate to a red end point with the standard zinc solution. DISCUS SIOY
This method is recommended for the srparation and determination of cobalt and nickel in samples (or aliquots of samples) containing up t o a total of 50 mg. of cobalt and nickel. The analytical results are shown in Table I. The precision, expressed as the maximum observed relative deviation from the mean vvas found t o be nithin 0.25% for cobalt and 0.12% for nickel. The accuracy, evpressed as the absolute error of the mean, was found to vary from 0.005 nig. of 0.13 nig. of cobalt and from 0.09 mg. to 0.003 mg. for nickel. This covers the range of mixtures from no cobalt and 50 mg. of nickel to 50 nig. of cobalt and no nickel. Both cobalt and nickel can be readily determined b y the proposed method provided that the unknoxn solution contains only those two metal ions. If, in addition to cobalt and nickel, other metal ions are present that react with ethylenediaminetetraacetic acid, then the nickel could not be determined by the proposed method. However, it is reasonable to believe that if the additional ion or ions do not react with I-nitroso-2-naphthol, the cobalt content of the unknown mixture could still be determined without any modification of the proposed procedure
1649 or of the calculations involved. I n this case, the first titration would give the total concentration of cobalt and all the other ions that combine with the ethylenediaminetetraacetic acid. The second titration would give the concentration of all these cations, with the exception of cobalt. The difference again represents the cobalt content. There are a number of distinct advantages inherent in the present method. The voluminous 1-nitroso-2-naphthol precipitate does not have to be filtered and washed and the time ordinarily required for digesting the precipitate and converting it to the proper weighing form is eliminated. Furthermore, any adsorption of nickel as a simple salt on the cobalt precipitate will not result in an error. The reason is that during the extraction process, the nickel ions will be released when the cobalt is dissolved in chloroform and will dissolve in the aqueous layer. There they will be titrated together with the rest of the nickel. LITERATURE CITED (1) (2) (3) (4) (5)
(6) (7) (8) (9)
(IO) (11)
Fischer, Hellmut, Mikrochemie, 30, 38-56 (1942). Fischer, iY.W.,Pogg. Ann., 71, 545 (1847). Knorre, G., 2. angew. chem., 264 (1893). Lpwry, T. AI., Chemistrv & Industry, 42, 462 (1923). henadkevich, K. A, and Saltykova, V, S., Zhur. Anal. Khrm., 1 , Xo. 2 , 123 (1946). Portnov, A. I., Zhur. Obshchei Khim., 18, 601 (1948). Tillu, SI. M., J . Indian Chem. Sac., 20, 139 (1943). Vance, J. E., and Borup, R. E., ANAL.CHEM.,25, 610 (1953). Whitby, A., and Beardwood, J. P., Chem. Met. Mining Soc., S. Africa, 21, 199 (1921). Fillard, H. H., and Diehl, H., “Advanced Quantitative Analysis,” p. 81, iXew York, D. Van Kostrand Co., 1943. Willard, H. H., and Hall, D., J . Am. Chem. SOC.,44,2219 (1922).
RECEIVED for review -4pril 29, 1954. Accepted July 1, 1954.
Volumetric Determination of Cobalt Complexometric Titration with Ethylenediaminetetraacetic Acid WILLIAM
F. HARRIS and THOMAS R. SWEET
The McPherson Chemical Laboratory, The Ohio State University, Columbus, O h i o
-4rapid and accurate method is described for determining 50 mg. or less of cobalt in acid solution. It involves the addition of excess ethylenediaminetetraacetic acid to the cobalt solution and titration of the excess reagent with a standard zinc solution, using Eriochrome Black T as the indicator.
I
N A study of some organic precipitating agents, it was found desirable to make determinations of the weight of cobalt in
a series of precipitates. This was done by dissolving the precipitates in an appropriate acid and determining the cobalt in the resulting solution by a volumetric method. hlthough a number of volumetric methods for cobalt have been described in the literature, none of these was found to be sufficiently rapid, accurate, and convenient. T h e formation of cobaltic h j droxide with alkaline peroxide or perborate has been used as the basis for many of the volumetric methods for cobalt that have been proposed in the literature ( 6 , 1 0 ) . ;Ifter removal of the excess oxidizing agent, which may be done by boiling, the trivalent cobalt is titrated with a reducing agent. However, the determinations are complicated by the great instability of the cobaltic ion in acid solution and by the slowness n-ith which the cobaltic hydroxide dissolves in reducing agents (15, 16). Barbieri ( 1 ) oxidized cobaltous ions to cobaltic with an excess
of standard permanganate. T h e excess permanganate was measured by the addition of an excess of ferrous sulfate, which was back-titrated with permanganate. Faleev ( 7 ) and Sikolow ( 1 2 ) follolved a very similar procedure. Faleev used oxalic acid instead of ferrous sulfate and Nikolow reduced the excess potassium permanganate with potassium iodide and titrated the liberated iodine with thiosulfate. Dobbins and Sander (4)precipitated cobalt as CO(C~H,N)~(CSS)z with an excess of ammonium thiocyanate. This was followed by the addition of an excess of silver nitrate and a backtitration R ith standard ammonium thiocyanate. Ferric alum was used as the indicator. Dickens and Maassen ( 3 )proposed the ferricyanide titration of cobalt, in which cobalt is oxidized in an ammoniacal citrate solution with an excess of potassium ferricyanide. This was follom ed by the potentiometric titratiou of the excess ferricyanide with cobaltous nitrate. Evans (6) has suggested a modification of the cyanide titration that is used for the determination of nickel. Prible ( 1 3 ) determined cobalt by titrating it with ceric ion in the presence of ethylenediaminetetraacetic acid. The end point is determined potentiometrically. Hoviever, the time between the addition of the reagent and t h e reading of the potential is very important and varies with the amount of cobalt present. Laitinen ( 11 ) suggested an amperometric titration, in which cobalt is oxidized with hydrogen peroxide in a bicarbonate solution,
