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 by 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)
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).
(IO) Fillard, H. H., and Diehl, H., “Advanced Quantitative Analysis,” p. 81, iXew York, D. Van Kostrand Co., 1943. (11) Willard, H. H., and Hall, D., J . Am. Chem. SOC., 4 4 , 2 2 1 9 (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. The 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. The 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,
1650
ANALYTICAL CHEMISTRY
potassium iodide is added, and the liberated iodine is titrated with thiosulfate. The most direct titration of cobalt was proposed by Flaschka (9). After the solution was made basic, the cobalt was titrated with ethylenediaminetetraacetic acid. Murexide was used as the indicator. However, this is a micromethod and the largest weight of cobalt for which analytical data were given was 0.3849 mg. In addition, murexide does not give a clear end point in the presence of appreciable salt concentrations. Therefore, this method would not be expected to give accurate results in the strongly acid cobalt solutions that were necessarily used in the present work, since in the process of making the solution basic prior to the titration, a large quantity of salt would be formed. The procedure described in the present paper involves the addition of an excess of a standard ethylenediarninetetraacetic acid (EDTA) solution to an acidic solution of the cobalt and the titration of this excess with a standard zinc solution in the presence of the indicator, Eriochrome Black T. After the standard ethylenediaminetetraacetic acid is added to the acidic cobalt solution, the pH must be adjusted to about 10 in order that the indicator function properly. The ethylenediaminetetraacetic acid-cobalt complex is sufficiently stable so that the pH adjustment may be made with ammonium hydroxide without the formation of the brown ammine complex. Back titrations in which ethylenediaminetetraacetic acid is used have been described for a number of metal ions by Biedermann and Schwarzenbach ( 3 ) and Flaschka (8).
Table 1. Analytical Results Cobalt Known to Be Present,
a fen- drops of ammonium hydroxide. Standard EDT.4 solution. Dissolve 5.5 grams of disodium dihydrogen ethylenediamine tetraacetate dihydrate in distilled water and dilute to 1liter. Standard cobalt solution. Dissolve 1 gram of high purity (99.99yo) cobalt sponge (Johnson, Matthey and Co., Ltd., Hatton Garden, London, E.C. 1) in 20 ml. of 1 to 5 nitric acid and dilute to 1liter. Standard zinc solution. After drying a sample of zinc oxide for 2 hours at 110" C., dissolve 1.4 grams in a minimum amount of 1 to 1 nitric acid and dilute to 1 liter with distilled water. Standard nickel solution. Dissolve 1 gram of nickel metal in 15 ml. of 1to 2 nitric acid on a hot plate and dilute to 1liter with distilled water.
Mg.
Mean value
Maximum Deviation from Mean Abeolute, Relative,
Accuracy Absolute Relative error, m g . error, yo
mg.
70
49.77
49.78 49.73 49,73
49.75
0.03
0.06
0.02
0.04
44.79
44.71 44.76 44.76
44.74
0.03
0.07
0.05
0.11
39.81
39.79 39.79 39.74 34.82 34.82 34.82
39.77
0.03
0.08
0.04
0.10
34.82
0.00
0.00
0.02
0.06
Mg.
34.84 29.86
29.86 29.81 29.81
29.83
0.03
0.10
0.03
0.10
24.88
24.79 24.84 24.84
24.82
0.03
0.12
0.06
0.24
19.91
19.87 19.87 19.92
19.89
0.03
0.15
0.02
0.10
14.93
14.90 14.90 14.95 9.971 9.971 9.971
14.92
0.03
0.20
0.01
0.07
9.971
0.00
0.00
-0.018
-0.18
9.953
4.977
4.988 4.988 4.988
4.988
0.00
0.00
-0.011
-0.22
3.981
3.989 3.989 3.994
3.991
0.003
0.08
-0.01
-0.25
2.986
2.991 2.986 2.986
2.988
0.003
0.10
-0,002
-0.07
1.991
1.987 1.987 1.987
1.987
0.00
0.00
0.004
0.20
0.995
0.994 0.994 0.994
0.994
0.00
0.00
0.001
0.10
REAGENTS
Indicator solution. Dissolve 0.2 gram of Eriochrome Black T in 100 ml. of distilled water which have been made basic with
Cobalt Found,
solutions containing from 15 to 50 mg. of cobalt. Use standard solutions of ethylenediaminetetraacetic acid and zinc having one tenth the concentrations described above for determining sample solutions containing from 1 to 10 mg. of cobalt. DISCUSSION
STANDARDIZATION OF SOLUTIONS
The nickel solution was standardized by precipitation with dimethylglyoxime in the usual manner. The zinc solution was standardized by precipitating the zinc as the ammonium phosphate and weighing as the pyrophosphate after ignition a t 550' C. (14). The ethylenediaminetetraacetic acid solution was standardized against the standard zinc and nickel solutions. Five milliliters of 1 to 1 ammonium hydroxide and 3 drops of Eriochrome Black T solution were added to 50 ml. of the standard zinc. This was titrated with the ethylenediarninetetraacetic acid solution until the color changed from red to blue. Five milliliters of 1 to 1 ammonium hydroxide and 0.2 gram of murexide indicator (0.1 gram of murexide ground with 100 grams of sodium chloride) were added to a 50-ml. portion of the standard nickel solution. This was titrated with the ethylenediaminetetraacetic acid solution until the color changed from yellow to purple. The results of these two methods of standardization agreed with each other to within 2 parts per thousand. PROCEDURE
Transfer the acidic cobalt solution to a 250-ml: Erlenmeyer flask. ildd an excess of the standard ethylenediaminetetraacetic acid and adjust the pH of the resulting solution to approximately 10 with 1 to 1 ammonium hydroxide. Add sufficient Eriochrome Black T indicator to produce a distinct blue color and titrate to a red end point with the standard zinc solution. The quantity of indicator needed varies with the amount of cobalt present. Use the standard ethylenediaminetetraacetic acid and standard zinc solutions for determining the cobalt content of
The proposed method is not recommended for analyzing a sample (or aliquot of a sample), that contains more than 50 mg. of cobalt, since the end point becomes increasingly difficult to determine as the cobalt content increases above this limit. The reason is that the color of the cobalt ethylenediaminetetraacetic acid complex interferes with the end point when too large a quantity is present. The analytical results are shown in Table I. The precision, expressed as the maximum relative deviation from the mean, was found to be 0.20% or less and the average value was 0.068%. The accuracy, expressed as the relative error of the mean, was found to be 0.24% or less and the average value was 0.13%. The method that is given in the present paper is not satisfactory if polyvalent cations other than cobalt are present in the acid solution. However, as the procedure given was designed to determine the cobalt in the solution that results from dissolving organocobalt precipitates in an acid solution, it suggests a method that could be applied generally for the determination of cobalt. This would involve: separation of the cobalt from other polgvalent cations by means of a suitable organic precipitating agent, TTashing of the precipitate, dissolution of the precipitate in acid, and accurate analysis of the resulting acid solution according to the procedure given in the present paper. This method will become more generally useful with, and perhaps provide a stimulus for, the development of better organic reagents for the separation
V O L U M E 2 6 , NO. 10, O C T O B E R 1 9 5 4 of cobalt from the elements rvith which it is frequently associated. There is no need to restrict the organic reagents to those that give good weighing forms. LITERATURE CITED
(1) Rarbieri, G. 4 . , Atti accad. nazl. Lincei, 8 , 405 (1928). (2) Biedermann, W., and Schwarzenbach, G., Chimia (Switz.), 2, 56 (1948). (3) Dickens, P., and Maassen, G., Arch. Eisenhutcenw., 9,487 (1936). (4) Dobbins, J. T.. and Sander, J. P., IXD.ESG. CHEM.,ANAL.ED., 6, 469 (1934). (5) Engle, K.D., and Gustavson, R. G., J . Ind. E p g . Chem., 8, 901 (1916). (6) Evans, B. S..Analysc, 62,363 (1937).
1651 (7) Faleev, F. V., Gorno-Obagatitel. D e b , 10, 53 (1932). (8) Flaschka, H., Chemist Snalyst, 42,56 (1953), (9) Flaschka, H., Mikrochim. Acta, 39,38 (1952). (10) Job, A,, Ann. chim. phys., 20, 214 (1900). (11) Laitinen, H. A., and Burdett, L. W., ANAL. CHEW. 23, 1268 (1951). (12) K'ikolow, C., Przemyd. Chem., 17, 46 (1933). (13) Prible, R., and Malicky, V., Collection Czechoslot. Chem. Commum.. 14,413 (1949). (14) Vance. J. E.. and Boruu. R. E.. ANAL.CHEM..25. 610 (1953). (15) Willard, H. H., and Diehl, H., "bdvanced Quantitative Analysis,'' p. 382, Xew York, D. Van Nostrand Co., 1943. (16) Willard, H. H., and Hall, D., J . Am. Chem. Soc., 44, 2237 (1922). ,
RECEIVED for review April 10, 1054. Accepted July
I
14, 1954
Determination of Pyridinium Nitrogen FRANCIS E. CRANE, JR., and RAYMOND M. FUOSS Chemical Laboratory, N e w Jersey College for W o m e n , N e w Brunswick, Y a l e University, N e w Haven, Conn.
Treatment of pyridinium salts with alkaline peroxide liberates about '75% of the nitrogen in titratable form and renders the balance accessible to standard Kjeldah1 determination. A n analytical procedure based on these facts is described : using methylpicolinium iodide 0.13% nitrogen (14 analyses) as a test compound, 5.84 =I= was found, compared to 5.969'0 theoretical. -4t least 2 grams of sodium peroxide per millimole of nitrogen must be used.
N. J.,
and Sterling Chemistry Laboratory,
1.2 grams of red phosphorus (0.038 mole), and 13 ml. of 57% hydriodic acid (0.10 mole) \vas heated for 10 hours at 95' to 100" C. The hot reaction mixture was filtered through an asbestos mat to remove phosphorus and was then washed with 35 ml. of water. Filtrate and washings were allowed to stand overnight; 3.0 grams of white crystals melting a t 172-6" C. separated. Two recrystallizations from water (20 ml. per gram) gave a product melting a t 177-8" C. Potentiometric iodide, 34.5, 34.9%, us. 35.16% Lheoretical; total iodine (after alkaline hydrolysis), 70.3% u8. (0.27% theoretical. PROCEDURE
H
ETEROCYCLIC nitrogen frequently is difficult to determine quantitatively, especially when it is present in the quaternized state. Even the Dumas method gives erratic results when the salt is a halide such as methylpicolinium iodide. A variety of modifications of the conventional Kjeldahl procedure have been suggested (1-15). This note describes another procedure for the quantitative determination of pyridinium nitrogen which is simple and reasonably rapid. illkaline peroxide (from sodium peroxide and water) converts pyridinium salts (via the pyridone) to aliphatic cleavage products, the nitrogen of which is then accessible to quantitative determination. The method is based on this sequence of reactions. MATERIALS
Methyl-4-picolinium iodide was prepared from methyl iodide and 4-picoline in ethyl alcohol, and was twice recrystallized from 50:50 ethvl alcohol-ether and once from 90 to 10 ethvl alcoholether; melting point, 157-8 '. Potentiometric iodide, 53.9, 53.9%, os. 54.0% theoretical. p-Iodoethylpyridinium Iodide. -4mixture of 21 grams (0.27 mole) of pyridine and 34 grams (0.27 mole) of ethylene bromohvdrin was daced in a flask under a reflux condenser and, after b&ng heated gently to start the reaction, the mixture was kept from overheating by immersion in cold water. Too vigorous reaction gives vinylpyridinium bromide and other by-products. The mixture set 'to a white solid which was remelted and heated for 15 minutes more t o ensure completion of the reaction. The crude product (8-hydroxyethylpyridinium bromide) was washed with three 60-ml. portions of dry ethyl ether to remove unreacted reagents and was stored under vacuum over phosphorus pentoxide; it is extremely hygroscopic. Yield, 39 grams = 71%; melting point 108-10' C.; potentiometric bromide, 39.0, 39.1, 39.2, 39.2y0 us. 39.16% theoretical. The iodide was then prepared by the following reaction:
A mixture of 5.0 grams of the above bromide (0.025 mole),
The sample (enough to give 2 to 6 mg. of nitrogen) is weighed from a long-handled weighing bottle into a 100-ml. Kjeldahl flask. A borosilicate glass tube (3 X 1 cm.) containing 2 to 3 grams of sodium peroxide (Mallinckrodt's reagent grade or Baker's C.P. analyzed) is then dropped into the flask, care being taken not to spill peroxide on the sample. The conventional Kjeldahl apparatus is then assembled, with the outlet condenser dipping under 0.025N sulfuric acid in a 250-ml. Erlenmeyer flask. The connection to the steam generator is closed a t this stage of the procedure. rlbout 10 ml. of water are run into the Kjeldahl flask through the addition funnel. By gentle swirling, the sample can be dissolved (or be thoroughly w d ) without peroxide coming in contact with sample or water. Then 20 ml. more of water are admitted (taking care to keep a water seal above the entrance stopcock), and the peroxide tube is tipped so that water and peroxide mix. The mixture is allowed to stand with occasional swirling for 20 minutes; then the contents of the Kjeldahl flask are slowly heated to about 80" C. Too rapid heating here produces serious foaming. Steam distillation is then started (taking care that a positive pressure of steam exists in the generator before the stopcock to the Kjeldahl flask is opened) and is continued until the volume in the Erlenmeyer flask reaches about 150 ml. The Kjeldahl flask is also heated to prevent undue condensation of steam. The Kjeldahl flask is allowed to cool and, after a drop of phenolphthalein solution is added, the contents are cautiouslv neutralized with concentrated sulfuric acid. An excess of 7" to 12 ml. of acid is added. &lost of the water is evaporated over a microburner and then the residual solution is heated in a fume chamber below the boiling point until frothing ceases. Next, the temperature is raised so that the solution boils gently. After the iodine is evaporated, 5 to 10 mg. of catalyst (1 part of red mercuric oxide and 3 parts of selenium) are added and the solution is digested a t 300" t o 350" C. for at least 1 hour (some samples required 2 to 3 hours for complete digestion). Evaporated acid is replaced to maintain approximately constant volume during the digestion. During this operation, the first distillate in the Erlenmeyer flask is evaporated to 40 to 50 ml. -4fter the digestion, the Kjeldahl flask is cooled to 50" C. or below, and 25 ml. of water are cautiously added to dissolve the sodium sulfate. One drop of phenolphthalein solution is added and the Kjeldahl apparatus is reassembled with the Erlenmeyer flask, containing the concentrated distillate, a t the condenser tip
