ANALYTICAL EDITION
March, 1944
Moore, L. A., IND.ENG.CHEM.,ANAL.ED., 12,726 (1940). Moore, L.A.,and Ely, Ray, Ibid., 13,600 (1941). Peterson, W. J.,Ibid., 13,212 (1941). . . Strain, H. H., “Chromatographic -4dsorption Analysis”, New York, Interscience Publishers, 1942. (11) Wall, M. E.,and Kelley, E. G., IND.ENG.CHEM.,ANAL.ED., 15, 18 (1943).
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(12) Wiseman, H.G.,Kane, E. A., Shinn, L. A,, and Cary, G. .I. J. Agr. Research, 51,635 (1938). (13) Zimmerman, W.I., Tressler, D. K., and Maynard, L. A., Food Research, 6,No. 1, 57 (1941). APPROVED by the Director as paper No. 161, Scientific Journal Series. Colorado Agricultural Experiment Station.
Determination of Manganese after Oxidation to Tri-Dihydrogen Pyrophosphatomanganiate Use of Pyridine to Separate Iron, Chromium, Vanadium, and Cerium from Manganese J. 1. WATTERS
AND
I. M. KOLTHOFF
School of Chemistry, University of Minnesota, Minneapolis, Minn.
Vanadyl, vanadate, ceric, cerous, and chromic ions are quantitatively coprecipitated with an excess of ferric iron when hydrous ferric oxide is precipitated by pyridine. Chromate divides between the precipitate and Filtrate. Manganous and ferrous ions remain quantitatively in the filtrate. Use of pyridine is advantageous in separating the interfering metals, vanadium, chromium, cerium, and a large excess of iron from manganese prior to the polarographic determination of manganese as tri-dihydrogen pyrophosphatomanganiate.
IN
APPLYING the authors’ (1) polarographic procedure for the determination of manganese, after its oxidation to tridihydrogen pyrophosphatomanganiate, to the analysis of ores and steels it was found that large amounts of iron and even small amounts of chromium, vanadium, and cerium interfered with the determination. These elements must be separated from manganese before the polarographic procedure can be applied. It will be shown in a subsequent paper that as little as 1 or 2 mg. of manganese can be determined in the presence of 0.2 gram of iron without a separation; if more iron is present, a separation is necessary. The interference of chromate can be eliminated by reduction with arsenious acid. Otherwise the pyridine separation was found to be the most effective and simple method for eliminating the interference. I n this procedure chromium, vanadium, and cerium are removed from solution by coprecipitation with hydrous ferric oxide. Pyridine, a very weak base having an ionization constant of 1.4 X 10-9,has been recommended fpr the quantitative precipitation of certain trivalent metals as the hydrous oxide without appreciable coprecipitation of cobalt, nickel, manganese, and copper ( I , 3). A mixture of approximately equal concentrations of the pyridinium salt and pyridine is well buffered and is slightly acidic, having a pH of approximately 5.2 a t 25’ C. The solid hydrous oxide thus acquires a positive charge due to its primary adsorption of hydrogen ions. Consequently anions such as the sulfate ion, and not cations such as cobalt and nickel, are adsorbed as counter ions and are carried down with the precipitate. A second property of pyridine undoubtedly contributes to the excellent separations of the hydrous oxides from cobalt, copper, nickel, and manganese. The pyridine molecule, owing to the lone pair of electrons on the nitrogen atom, is able to occupy a place in the coordination sphere in complex-forming metal ions. A great many complexes of the above metals with pyridine are known. The authors found that the formation of a complex of chromic chromium with pyridine actually is a factor in prevent1 Present address. Metallurgical Laboratory, University of Chicago, Chicago, Ill.
ing the precipitation of chromic ion in the absence of ferric iron or aluminum. I n order that the pH shall not exceed 5.2 when 5 ml. of pyridine are added to approximately 25 ml. of aqueous solution, it is necessary to have 31 milliequivalents of strong acid or its equivalent of metallic ion such as ferric iron present. Each millimole of ferric iron liberates 3 milliequivalents of acid upon precipitating as the hydrous oxide. If 0.6 gram of ferric iron were present, no excess of mineral acid would be necessary. For each 0.1 gram oi ferric iron present, the amount of strong mineral acid present should be decreased by 5 milliequivalents. However, 5 ml. of pyridine will precipitate quantitatively as much as 1.0 gram of ferric iron from 100 ml. of solution containing no appreciahle excess of acid. APPLICATIONS
The use of pyridine to separate quantitatively trivalent chromium, iron, or aluminum from manganese, cobalt, and nickel; or uranium from calcium, barium, and strontium, has been recommended by Ostroumov ( 3 ) . According to him, a sum of 0.1264 gram of the oxides of trivalent iron, chromium, and aluminum may be quantitatively separated from a solution containing about 0.02 gram of divalent manganese, cobalt, or nickel with lese than 0.2% coprecipitation of these ions with the mixed hydrous oxides. Zinc, if present, divides between the precipitate and solution. Lingane and Kerlinger ( 2 ) more recently used a similar procedure to separate ferric iron and chromic chromium from copper, cobalt, and nickel before determining the last three elements polarographically. One interesting difference was observed in the results of Ostroumov and those of Lingane and Kerlinger. According to Lingane and Kerlinger, trivalent chromium is separated only through coprecipitation with the hydrous ferric oxide, while Ostroumov found that trivalent chromium can be quantitatively precipitated in the absence of iron. The following factors undoubtedly account for this difference. I n Ostroumov’s experiments the precipitation was carried out in a hot solution containing no appreciable amount of mineral acid, while Lingane and Kerlinger added 24 milliequivalents of hydrochloric acid before adding the pyridine to the cool solution. In Ostroumov’s procedure the higher temperature is favorable for the decomposition of the complex of chromic ion with pyridine and the higher pH decreases the solubility of hydrous chromic oxide. EXPERIMENTAL
REAGENTS. Pyridine solution (1 to 2 ) . Dissolve 50
ml. of pyridine in 100 ml. of water. Sodium bisulfite, 20 per cent. Dissolve 2 grams of sodium bisulfite in 8 grams of water. Prepare a fresh solution daily.
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Ferric vanadate is insoluble in neutral solutions but dissolves readily if a small amount of mineral acid is added to the solution. The above separation of vanadate is not, then, necessarily one of Erperiment Chromium i n FPrrir Irun IU Chromium Remaining coprecipitation. However, if a simple preciphtion of ferric NO. Sample %mplr in Filtrate vanadate occurred, one would expect a larger percentage of vanMu. Mu % adate to be precipitated. The fact that this is not the case indi1 260 C r I I I 3V0 0 390 C r I I I 2 ,500 1.76 cates that the vanadate is actually coprecipitated with the hy1.00 CrVI 279 1 3
