20
INDUSTRIAL AND ENGINEERING CHEMISTRY
Test solution A has given a positive test with chromium metal and chromium plating and a negative test with the stainless steels, stainless irons, and the chromium-nickel alloys. A chromium-cobalt-tungsten alloy with a chromium content of 25 to 31 per cent is the only alloy which has given a positive result with both test solutions, the reason being that this alloy remains passive with either test solution and the chromium dissolves as Cr04--. Numerous other metals and alloys have been tested with both solutions, and negative results have been obtained in each case: nickel-silver, solders, brasses, white metals, 15 per cent silicon steel, bronzes, copper-nickel alloys, carbon steels, nickel, copper, manganese, molybdenum, tantalum, tungsten, mercury, cadmium, aluminum, tin, zinc, vanadium, silver,
Vol. 13, No. 1
and lead. Vanadium gives a red spot with both solutions and silver yields a black spot with test solution B.
Literature Cited (1) Arnold, E., Chem. Listy, 27, 73 (1933). ( 2 ) Chazeneuve, P., Compt. Tend., 131, 346 (1900). (3) Feigl, F., 2.angew. Chem., 44, 739 (1931). (4) Feigl,
F., and Krumholz, P., Mikrochem., Pregl-Festschr., 77
(1929)
( 5 ) Glazunov, A,, and Kiholahv$, J., Chem. Obzor, 8, 176 (1933). Glarunov, A., and KEivolahv$, J., paper presented at International Founding Congress, Prague, 1933. (7) Heller, K., and Krumholr, P., Mikrochemie, 7 , 213 (1929). (8) Hittorf, W., 2.physik. Chem., 30,481 (1899). (9) Ibid., 34, 395 (1900). (10) Jirkovsky, R., Mikrochemie, 15, 341 (1934). (6)
Determination of Cobalt as Trioxalatocobaltiate G. H. CARTLEDGE AND PARKS RI. NICHOLS', University of Buffalo, Buffalo, N. Y.
I
N CONNECTIOX with the work on complex compounds in this laboratory t'he authors have developed a new spectrophotometric method of determining cobalt which is much more rapid than the gravimetric or electrolytic procedures. Numerous colorimetric methods are available (S), but certain of these require time-consuming separations, others are applicable only to minute amounts of cobalt, and others have only fair accuracy. The basis of this procedure is the measurement of the absorption a t 605 m p due to the trioxalatocobaltiate ion, Co(CzO&---, which is produced by oxidation of a cobaltous solution by lead dioxide in a weakly acid solution of potassium oxalate (4, according to the reaction
+
++
++
2COSOa 4HCzHaOz 7KzCzOr PbOp 2KsCo(Cz04)s PbCz04 4KCzHaOz
+ 2KzSO4 + 2H20
A small amount of lead remains in solution, presumably as acetate. The trioxalatocobaltiate anion produced has a dark emerald-green color with a maximum absorption at 605 m p ( 5 ) . The intensity of the absorption is used as a measure of the cobalt content. A Bausch & Lomb visual spectrophotometer was used for estimating the absorption, and with a maximum cell length of 5 cm. the solutions actually measured may contain from 2 to 50 mg. of cobalt per 100 ml. Since the volume of the final solution need not exceed 50 ml., the method may be applied to any sample or aliquot containing as much as 1 nig. of cobalt. Xickel, iron, and chromium may be present in considerable amounts; copper and manganese must be previously removed, if present. Blthough the authors have used a spectrophotometric procedure requiring no standard solutions for comparison, the same method could be adapted for use with a photoelectric colorimeter with a suitable red filter. The analysis can be completed in about 15 minutes from the time the aliquot of cobaltous salt solution is measured out. The accuracy is better than 2 per cent of the quantity determined. It is necessary to have a standard solution of the pure salt to determine the extinction coefficient a t the wave length selected for measurement of the absorpt'ion. The preparation of a standard solution of potassium trioxalatocobaltiate is difficult, however, since the n-ater content of the crystalline salt is very variable. Furthermore, the complex is not entirely stable, so that' its purity needs to be established. K h e n the salt is prepared by a modification of the procedure of Jaeger and Thomas ( 2 ) its formula approximates closely t'o 1
Prfsect addrees, Lewiston, N. T.
K3Co(CL0&. 3.5H20, but the water content varies with the atmospheric humidity from day to day. T'ranek (5) measured the absorption of trioxalatocobaltiate solutions. H e standardized his solutions by electrolytic determination of total cobalt, however, which of course would give an erroneous extinction coefficient if the salt had decomposed to a cobaltous compound to an appreciable extent. Since he gave no statement as to the history of his material, the authors redetermined the absorption a t 605 m p with solutions standardized by a procedure which they developed for estimating the complex cobaltic ion in the presence of cobaltous ions. Volumetric Determination of Trivalent-Cobalt Complex I n acid solution, potassium iodide does not reduce the trioxalatocobaltiate ion rapidly; ferrous sulfate reacts much more readily, but the formation of the greenish-yellow trioxalatoferriate ion in the solution obscures the end point. The authors found that ferrous sulfate may be used in an electrometric titration of the cobaltic ion, though the electrode is very sluggish until the end point is passed. The potential changes by about 150 millivolts a t the end point. I n the direct tit'ration with Mohr's salt the end point may be seen much more clearly if potassium fluoride is added near the end, but this is objectionable. Even so, the end point is reached slowly a t room temperature, and the authors therefore abandoned the attempt to make a direct titration of t rivalelit cobalt. The procedure finally developed consists in titrating the cobaltic solution with approximately 0.05 N Nohr's salt (20 grams per liter in 2 per cent, sulfuric acid) a t room temperature until the change from green to yellow shows that t'he end point has been slightly overstepped. The excess of ferrous ion is then titrated with standard potassium dichromate, using diphenylamine as the indicator. The back-titration of the excess ferrous salt is complicated by the induced reaction between dichromate and oxalic acid, however, so that i t is necessary to determine an empirical factor for the dichromate solution in each analysis. The solution of the trioxalato salt to be analyzed is prepared
so as to be nominally about 0.05 -11. To an aliquot (10 to 25 ml.),
add from a buret standard ferrous sulfate until about 0.5 ml. in excess is present. Let the mixture stand for 5 minutes, when the color change should be complete. Then add 1 ml. of a solution containing 10 ml. of phosphoric acid and 25 ml. of sulfuric acid per 100 ml., and dilute to about 75 ml. Add 3 drops of diphenylamine solution (1 gram per 100 ml. of sulfuric acid) and titrate with standard 0.05 N potassium dichromate. Not over 1.0 ml.
ANALYTICAL EDITION
January 15, 1941
should be used. Read the two burets, and add to the mixture as much ferrous sulfate as is estimated to be about equivalent to the dichromate consumed in the back-titration. Again titrate with the dichromate. The ratio between the ferrous sulfate and dichromate solutions under the conditions prevailing near the end point is calculated from the supplementary titration; this factor is used t o calculate the excess of ferrous salt found in the back-titration. The normality of the ferrous solution in its reduction of the trivalent cobalt may be determined by any of the usual methods. The following data show the concordance obtained in the titration of equal samples of potassium trioxalatocobaltiate:
Sample 1
10 26 0 16
Sample 2
10 68 0 38
Sample 3
10.15 Co Present
0 61
10 20
MQ.
10 16
1.56 3.89
hv. 10 17 FeSO4 0 04950 K:lO. 17 ml.
=
0.0297 gram of
Co+++
I n a comparison of the electrometric titration with the procedure just described the concordance was equally good. Unfortunately, there is no completely stable cobaltic complex which is readily reducible b y ferrous sulfate, so t h a t the method cannot be tested upon a pure compound of known composition. The data above correspond to 11.81 per cent of trivalent cobalt in the sample; the theoretical content in the pure salt with 3.3 moles of n a t e r is 11.71 per cent, and with 3 moles, 11.93 per cent.
Maximum Extinction Coefficient For the measurement of the extinction coefficient a t 605 mp, solutions of a recrystallized sample were made u p and titrated b y the procedure given above. Cartledge and Djang (1) have previously found that solutions of potassium trioxalatocobaltiate obey Beer’s law in the concentration range suitable for measurement. Small amounts of the purplish-red oxalatocobaltoate resulting from decomposition have no appreciable effect on the abaorption, since the extinction coefficient of this ion at 605 Inp is only 1.71, according to Vranek. The authors’ extinction curve for the trioxalatocobaltiate was in substantia1 agreement u ith \-ranek’s between 570 and 605 mp, b u t the authors found a slightly higher extinction coefficient at the maximum-namely, 162, instead of 161.1-the difference is probably not significant. To calculate the molar concentration, c, of the trioxalatocobaltiate ion we therefore have c=-
gram of manganese-free lead dioxide. Set the mixture aside at room temperature for 5 to 10 minutes away from bright light, and shake at intervals. After not over 10 minutes dilute the unfiltered mixture to either 50 or 100 ml., according to the amount of cobalt present. This can be readily judged by the depth of the co1.or of the oxalatocobaltoate before addition of the lead dioxide. Promptly after dilution filter the solution without suction through a good filter paper, such as Whatman No. 42. Discard the first portion of the filtrate and catch a sufficient volume directly in the absorption cell. It is essential that there be no turbidity. If it is necessary to filter a second time, an additional portion of lead dioxide is previously added. This is particularly necessary when the cobalt content is above 25 mg. For the measurement, the authors have used cell lengths of 1 to 5 cm. In the comparison cell they use a blank carried through the regular procedure.
TABLE I. DETERMINATIOX OF COBALT
0 15 0 21 0.56 0.44 0 61
10 72 0 56
21
7.78
11.75 11.75 11.94
15.56 29.5
31.1 41.46
Volume Ml. 50 50 100 100 100 100 100 100 100
log 9 and 1 is the cell ( If the oxidized solution is diluted to 100 ml.
in which d is the measured density
length in cm. before measurement of the absorption, the weight of cobalt in the sample is given by the relation Mg. of Co = 36.4 d / l
Spectrophotometric Determination of Cobalt For the determination of cobalt the authors have ascertained the conditions for the complete conversion of cobaltous salts into the trioxalato cobaltic complex. The solution may contain sulfates, nitrates, or chlorides. Keutralize an aliquot containing 1 to 50 mg. of cobalt with sodium hydroxide to the appearance of a slight permanent precipitate. The solution at this stage should not exceed 25 ml. Next add 2 ml. of glacial acetic acid, 5 ml. of 20 per cent ammonium acetate, 10 ml. of 1 -1f potassium oxalate, and about 1
Density
4
0.380
4
0.867 0.975 0.960 0.987
4 3 3 3
2 1 1
100
1
Co Found
0.848
0.852 0.815 0.845 1.13
Mg. 1.59 3.86
7.80
11.82 11.64 11.97 15.5 29.65 30.8
41.1
Err01
Mo. +0.03 -0.03
+0.02 f0.07 -0.11
+o.m
-0.1 + O . 15 -0.3 -0.35
Table I shows the results on a series of samples prepared from standardized solutions of cobaltous sulfate or cobaltous chloride.
Interfering Elements An advantage of the method lies in the fact t h a t substances having no appreciable absorption in the red do not interfere. Trivalent chromium interferes, but may be oxidized to the weakly absorbing dichromate by preliminary treatment with lead dioxide and nitric acid. I n the oxalate mixture ferric and nickel ions have a weak absorption a t 605 mp. To determine the degree of interference the authors have measured their absorption approximately; the results in Table I1 are expressed in terms of the number of milligrams of cobalt represented by the observed absorption of varying amounts of these elements. The volume of the measured solution was 100 ml. in each case, and the absorption was too weak to be measured accurately in a 5-cm. cell. EQUIVALENT OF INTERFEHING ELEMENTS TABLE 11. COBALT
Ni as Ni(N0s)z
Fe as FeSOl
d
162 1
Cell Length Cm.
Metal
Co Represented
Xg.
MQ.
20 40 100
0.4
20 40
0.2 0.2
20 40 100
0.1 0.2
100 Cr as KzCrz07
0.7 1.7
0.4
0.3
In the oxalate mixture copper has a strong absorption in the red and must be removed before oxidation of the cobalt. Manganese, if not previously removed, mould be oxidized to the trioxalatomanganiate ion. This complex is unstable at room temperature, and its decomposition induces the reduction of the cobaltiate complex, making a determination impossible.
Literature Cited (1) Cartledge, G. H., and Djang, T. G., J. Am. Cheni. Soc., 55, 3214 (1933). (2) Jaeger, F. M., and Thomas, IT,, Pwc. Acad. Sci. Amsterdam, 21, 693 (1919). (3) Snell, F. D., and Snell, C . T., “Colorimetric Methodsof Analysis”, Val. I, p. 321, Xew York, D. Van Nostrand Co., 1936. (I) Sorensen, S. P. L., Z. anorg. Chem., 11, 1 (1896). (5) Vranek, J., 2. Elektrochem., 23, 336 (1917).
