Polarographic Determination of Cobalt in Presence of Nickel LOUIS MEITES' Sterling Chemistry Laboratory, Yale University, N e w Haven, Conn.
A new modification of the Kolthoff-Watters procedure for the determination of traces of cobalt in the presence of nickel involves oxidation of cobalt(I1) in an ammoniacal ammonium chloride solution of the sample to the $3 state with excess permanganate, followed by the destruction of the excess permanganate with excess hydroxplammonium sulfate. The results are accurate to within &5%, and a determination is easily completed in 15 minutes or less.
A
LTHOUGH much effort has been devoted to the development of procedures for the analysis of mixtures of nickel and cobalt, very few satisfactory methods have been proposed for the determination of traces of cobalt in the presence of a large excess of nickel or zinc. The best of these are doubtless the ones described by Kolthoff and Watters (1, 2, 7 , 8),both of which involve the oxidation of cobalt t o the +3 state and the measurement of the polarographic diffusion of the cobaltic complex. Such a procedure is necessary in any polarographic method for two reasons. First, the nickel wave precedes that of cobalt(I1) in nearly every known supporting electrolyte, so that it would be virtually impossible to measure the very small diffusion current of the cobaltous wave. Secondly, no medium is known in which cobalt(I1) gives a wave at a potential sufficiently different from that of zinc t o permit these elements t o be distinguished, and zinc is of course a ubiquitous contaminant of nickel salts. Probably the more elegant and useful of the methods proposed by Kolthoff and Watters, because of its greater freedom from interferences, is that in which the cobalt is oxidized by sodium perborate in an ammoniacal solution (?, 8). Unfortunately, the excess perborate has to be removed by boiling the solution (under reflux t o prevent excessive loss of ammonia), which must then be cooled before its polarogram can be recorded. The time consumed in these operations makes the procedure rather too lengthy for the routine laboratory. The method described in this paper is very similar to that of Kolthoff and Watters, but uses potassium permanganate instead of sodium perborate. The excess permanganate is instantly destroyed even a t room temperature on addition of an excess of hydroxylammonium sulfate. This eliminates the necessity of refluxing and cooling the solution. The excess hydroxylamine does not reduce the cobalt(II1) complex a t a measurable rate at room temperature.
copper and nickel, and also to reduce any cobalt(II1) formed by reaction with dissolved oxygen. A current integrator (Analytical Instruments, Inc.) was then connected into the electrolysis circuit and the potentiostat was readjusted to keep the working electrode potential constant a t -1.45 volts va. S.C.E. At this potential cobalt(I1) is completely reduced to the metal, and the difference between the initial and final coulometer readings gave the number of milliequivalents of cobalt directly. The results of seven such experiments are shown in Table I. For the reasons described above, this procedure actually gives the sum of cobalt and zinc, but the percentage of the latter element present could hardly have been significant in this work. When the amount of zinc present can be neglected, this procedure would appear to be one of the most accurate and convenient available for the standardization of a cobalt solution. RECOMMENDED PROCEDURE
Weigh 2.50 grams of the nickel salt to be analyzed into each of two 100-ml. volumetric flasks, and to each add about 50 ml. of water, 2.5 f 0.5 grams of ammonium chloride, and about 10 ml. of concentrated ammonia. T o one of the two flasks add 2 ml. of saturated potassium permanganate solution and let stand for a minute or two t o ensure complete oxidation of the cobalt. During this period a brown turbidity of manganese dioxide will form slowly: The appearance of a large precipitate immediately after the permanganate is added indicates the presence of a considerable amount of arsenic, antimony, chromium, or manganese. I n that event, add more permanganate until the solution is definitely purple. Finallv add 2 ml. of saturated hydroxylammonium sulfate and 1 ml, of 0.2% Triton X-100 (Rohm & Haas Co., Philadelphia) to each flask, and dilute each solution t o the mark. Allow a few seconds for the excess permanganate and manganese dioxide in the oxidized solution to react completely and for vigorous evolution of nitrogen to cease, then stopper and shake thoroughly. Transfer a portion of the oxidized solution to a polarographic cell, deaerate i t with hydrogen or nitrogen, and record its polarogram from -0.2 to -0.8 volt us. S.C.E.
Table I. cos04 Used, MI 9.891
9.998
14.900
Coulometric Standardization 51illifaraday Consumed 2,0543 2.0617
lMillifaraday/MI Normalit) 0.2076 0.2084: 0 20647 0.20715 0.20721 0.20721
2.0643
2.0741 2.0717 2.0717 3.0924 Mean
EXPERIMENTAL
All olarographic measurements were made with a pen-and-ink r e c o d n g polarograph (6) and an entirely conventional dropping mercury electrode assembly. The modified H-cell ( 6 ) , equipped with a sintered borosilicate glass gas dispersion cylinder t o permit rapid deaeration, was secured from E. H. Sargent and Co. (S29438). All polarograms were recorded at 25.0" f 0.5". A stock solution of cobaltous sulfate was prepared from the reagent grade salt and standardized by coulometry a t controlled potential (5, 4 ) . An accurately known volume of the cobalt solution was added to about 75 ml. of 1 M ammonia-1144 ammonium chloride in a double-diaphragm cell for controlled potential electrolysis ( 4 ) . Dissolved air was removed by a rapid stream of prepurified nitrogen; then 25 ml. of mercury was added and the solution was electrolyzed a t -1.10 volts us. S.C.E., using a potentiostat (Analytical Instruments, Inc., Bristol, Conn.) t o maintain a constant electrode potential. This served t o remove traces of
=
0.207jj 0.2074 I- 0.0004
The resulting curve Till resemble curve B of Figure 1, which was secured during the analysis of a sample of reagent grade nickel chloride. The half-wave potential of the cobalt(II1) wave is approximately -0.40 volt us. S.C.E. This is close to the mean of the values reported in the literature for the waves of the hexammino- and aquopentammino-cobalt(II1) ions. KO doubt it is a mixture of these which results from the oxidation with permanganate and is responsible for the abnormally small slope of the wave. Discard this solution and replace it with a portion of the solution which had not been treated with permanganate. Deaerate this and record its polarogram under exactly the same conditions and with the same polarograph settings that were used in recording the first polarogram. The resulting polarogram will resemble
I Present address, Department of Chemistry, Polytechnic Institute of Brooklyn, 99 Livingston St.. Brooklyn 1, N. Y.
404
V O L U M E 28, NO. 3, M A R C H 1 9 5 6
405 Table 111. Effects of Possible Impurities in Nickel Salts on Determination of Cobalt (Each sample contained 2.50 grams of nickel nitrate, 1.21 mg. of cobalt, and specified amount of one other element) Substance Added, Cobalt Found, 1Mg.
75 4s 200 Bi 1200 Cd 100 Cr (BaC12 added) 1cu 10 cu
P lA
w Y P
25 cu 10 F e 100 Fe 30 h l n 100 hIo 200 Pb (NazSOd added)
4Q 2
U
3
120 Sb 120 Sn
' 2
c
u
z 21 P
120 50 \' 180 w 1400 Zn
3
Mg. 1.20 1.11 1.22 1.24 1.21 1.26 1.2 1.22
0.95 1.21 0.97 1.24 1.16 1.18 1.15 0.0 0.0 1.24
U
C
- 0.9
-1.0
Figure 1. Polarograms of reagent grade nickel chloride treated by recommended procedure A.
Without permanganate
C.
After addition of known amount of cobalt and oxidation with permanganate
B. After oxidation with permanganate
Table 11. Proportionality between Cobalt Concentration and Diffusion Current bIg. Co/lOO h11.
Millimole Co/Liter
0.153 0.307 0.615 1.23 1.84 2.40 3.07 3.69 6.15 15.4 36.9 61.5 184
0.0260 0.0521 0.1042 0.208 0.312 0.416 0.521 0.625 1,042 3.12 6 . 25 10.42 31.2
id, pa.
Mesn
4 . 1 1 f 0.07
curve A in Figure 1. It always contains a small wave a t about -0.5 volt us. S.C.E. This is due to the reduction of the cuprous ammonia complex formed by the reaction between the copper contained in the sample and the excess hydroxylamine. It is evident that a single polarogram of the oxidized solution would both give an erroneously high value for the cobalt concentration and fail to reveal that an error was being made. Measure the vertical distance between the t r o curves a t a potential on the plateau of the cobalt wave, roughly -0.70 volt i s . S.C.E. Compare the diffusion current thus measured with the values obtained when solutions containing known amounts of cobalt are treated in the same way. DATA AYD DISCUSSIOY
Table I1 shows some of the results obtained when known cobalt solutions were treated by the recommended procedure. The diffusion current is closely proportional to the cobalt concentration over a very wide range. The final solutions are about 0.5M in ammonium chloride and 1J f in ammonia. Decreasing the ammonium chloride concen-
tration to 0.2M gave much less reproducible results. The diffusion current was always lower than the value predicted from Table 11, and the ratio of the diffusion current to the cobalt concentration decreased considerably with increasing cobalt concentration. These phenomena became even more pronounced if the ammonium chloride was omitted altogether, or if its addition was deferred until after the addition of the hydroxylammonium sulfate. In the absence of a sufficient concentration of ammonium ion-i.e., a t pH values much higher than those attained in the recommended procedure-the oxidation of cobalt( 11) by permanganate apparently proceeds with considerable difficulty and fails to reach completion even after a considerable length of time. Whereas a solution containing the recommended concentration of ammonium ion becomes cherry red almost immediately when permanganate is added, and slowly deposits manganese dioxide, a solution containing little or no ammonium ion remains purple long after the permanganate is added. On the other hand, the wave secured in the presence of 1M ammonium ion is appreciably less well defined-it begins earlier and ends later, and the plateau is considerably shorter. Though a variation of about *20% in the ammonium ion concentration has little or no effect on either the rate of the oxidation or the characteristics of the wave, a much wider deviation from the recommended value should be avoided. If a strongly acidic solution (such as that obtained on dissolving a sample of nickel metal in nitric acid) has to be analyzed, it should be nearly neutralized with sodium hydroxide rather than n-ith ammonia before beginning the analysis. The concentration of ammonia is much less important. No detectable error results from using even 3M ammonia. Care should, however, be taken to ensure that the ammonia concentration is high enough to prevent any precipitation of hydrous nickel oxide, for this might coprecipitate an appreciable amount of cobalt. Table I11 illustrates the results obtained when solutions containing 2.50 grams of nickel nitrate, enough added cobalt to give a total of 1.21 mg. of this element, and the specified amounts of other elements were analyzed by the recommended procedure. Arsenic, cadmium, antimony, tin, and zinc do not interfere. Bismuth, copper, iron, manganese, and molybdenum can be tolerated in moderate amounts. [The cobalt diffusion current becomes difficult to measure precisely in the presence of a large excess of copper. Bismuth and iron interfere by coprecipitating cobalt, and molybdenum by forming a moderately insoluble cobalt(II1) molybdate. The presence of a large amount of nickel, incidentally, greatly reduces the errors caused by bismuth and iron.] Relatively large amounts of chromium and lead can be handled bv adding an excess of either barium chloride or
ANALYTICAL CHEMISTRY
406
Table IV.
Determinations of Cobalt in Reagent Grade Nickel Salts Cobalt Present, % Polarographic Manufacturer analysis
Salt Ni(0Ac)z NiCh Ni(N0i)r NiSOk Ni(NHSz(S0Sa Ni(meta1)
0.016 0.10 0.05 0.03
0.05 0.01
0.149 f 0.003 0.096 f 0 . 0 0 3 0.0049 f 0.0006 0.0140 f 0.0002 0.0079 i 0.0005 0.0065 =k 0.0009
sodium sulfate, respectively. Tungsten and vanadium must be absent. The reproducibility attainable by the procedure described is illustrated by the fact that 13 analyses of a sample of reagent grade nickel chloride (stated by its manufacturer to contain no more than 0.10% cobalt) gave a mean cobalt content of 0.096%, with a standard deviation of i 2 . 6 % , the extreme values being 0.092 and 0.100%.
In Table IV are given the results obtained when samples of various nickel salts were analyzed by the recommended procedure. Each value in the last column is the mean of at least three results; that given for the chloride was taken from the analyses described in the preceding paragraph, and the interesting figure for the acetate was derived from six analyses of material taken from various parts of a bottle whose seal was intact when it reached the author. LITERATURE CITED
(1) Kolthoff, I. M.,Watters, J. I., ANAL.CHEM.15, 8 (1943). (2) Ibid., 22, 1422 (1950). (3) Lingane, J. J., J. Am. Chem. SOC.67, 1916 (1945). (4) Meites, L.,ANAL.CHEM.27, 1116 (1955). (6) Meites, L., “Polarographic Techniques,” p. 16, Interscience. New York, 1955. (6) Meites, L., hleites, T., Zbid., 23, 1194 (1951). (7) Watters, J. I., Ph.D. thesis, University of Minnesota, 1943. (8) Watters, J. I., Kolthoff. I. M., ANAL.CHEM.21, 1466 (1949). RECZIVEDfor review September 20, 1955. Accepted November 7, 1945. Division of Analytical Chemistry, 128th Meeting, ACS, Minneapolis, Minn., September 1955. Contribution 1322, Department of Chemistry, Yale University.
Rapid Gravimetric Determination of Mercury in Organic Compounds HAROLD F. WALTON and HOWARD A. SMITH’ University o f Colorado, Boulder, Colo.
In a rapid method for determining mercury in certain organic compounds the compound is decomposed by refluxing with hydriodic acid containing iodine. Mercury forms the HgI,-- ion, which is then precipitated and weighed as cupric propylenediamine mercuriiodide, CupnzHgId. Compounds such as methyl mercuric hydroxide, bromide, and iodide and diphenylmercury give results which are precise but about 1% low. Part of this error is caused by incomplete precipitation.
A
REASONABLY fast and accurate method is needed for
determining mercury in organic compounds. Most published methoda depend on oxidation of the compound, either by oxygen in a combustion tube or by such agents aa sulfuric and nitric acids, ammonium persulfate, or potassium permanganate. Combustion with oxygen is long and tedious, though reliable. Some of the wet oxidation methods cannot be used in presence of halogen. A few methods depend on reduction t o metallic mercury, but these generally give low results. Iodine has long been known to attack organic mercury compounds (4, 6). Hydrogen iodide, of course, is a very effective reducing agent and decomposes many substances. A combination of iodine and hydriodic acid was found to attack alkyl and aryl mercuric halides and hydroxides very rapidly, forming the Eltable complex ion HgII--. The next step was to find a way of quantitatively determining mercury in the form of this ion. Spacu and Spacu ( 2 ) precipitate the salt CupnrHgIc (pn = 1,2propanediamine) by adding cupric propylenediamine sulfate and weigh it. They claim an accuracy of 1 part in 1000 for 0.5-millimole quantities of mercury added as mercuric chloride. To explore the possibilities of this method for organic compounds, two things were necessary-a study of the Spacu method for inorganic mercury and a study of the effectiveness of the hydriodic acid-iodine digestion. 1
Present address, E. 1. du Pont de Nemours & Co., Houston, Tex
Spacu and Spacu Method. To 100 to 250 ml. of a solution of a mercuric salt add 2 g r a m of potassium iodide per 100 ml., and make weakly basic with ammonia. Heat to boiling, add an excess of a boiling concentrated solution of cupric propylenediamine sulfate, then cool to room temperature, filter, and wash the precipitate first with a solution containing 1 gram each of potassium iodide and cupric propylenediamine sulfate per liter, then with alcohol and ether. Dry in a vacuum desiccator at room temperature and weigh. Duval and Dat Xuong ( f ) showed that drying a t room temperature was unnecessary, and that the precipitate could be heated to 157” C. without decomposition. They consider this method one of the best gravimetric methods for mercury. The precipitate is eaaily filtered and dried, and its molecular weight, 920.2, gives a very favorable gravimetric factor. EXPERIMENTAL
Modification of Spacu Method. The cupric propylenediamine sulfate reagent was made by mixing 1 volume of 1,Z-propanediamine (Eastman practical grade, redistilled) with 5 volumes of 1Jf cupric sulfate. The question arose whether this grade of propylenediamine was sufficiently pure for the purpose, for ethylenediamine, a probable impurity, would not be removed by distillation. A quantity of propylenediamine was therefore made by converting the redistilled “practical” amine to the sulfate, recrystallizing this from aqueous methanol, and reconverting to free amine by distilling from sodium hydroxide. Reagent made from this purified amine gave the same analytical results as that made from the redistilled practical amine. Furthermore, the addition of 5% ethylenediamine to the propylenediamine had practically no effect on the analysis. Cupric propylenediamine sulfate reagent which was 6 months old gave the same results as fresh reagent. This is in contradiction to Spacu and Spacu, who say that the reagent must be freshly mixed and should be heated before being added to the ammoniacal mercury solution. This heating was found to be unnecessary. Not only is it unnecessary to wash the precipitate with ether,
