Determination of Small Amounts of Potassium by Means of Sodium Cobaltinitrite H. W. LOHSE, University of Toronto and the Ontario Agricultural College, Toronto, Canada
I
N T H E COURSE of a study
Experimental investigation of the pre,,ipitation of potassium as sodium-potassium cobaltinitrite indicates that the preCiPitate obtained varies in composition with respect to potassium content, depending upon the conditions of precipitation. The composition of the precipitate varies but
according t o Biilmann (2). Earlier investigators using Biilmann’s sodium cobaltinitrite Of amounts Of for quantitative determination sium by precipitation as potasof potassium include Gram (5)1 sium s 0 d i u m cobaltinitrite it was necessary to establish the and Tischer (16). Christensen a n d F e i l b e r g (41, J a n d e r relationship between the conand Faber (7), and B o n n e a u centration of potassium and the (3) h a v e s t u d i e d t h e p r e composition of the precipitate lies between KNa2 [co(No2)6] and IGNacipitation of potassium in the obtained with sodium cobaltini[Co(N02)61. trite under various conditions. presence of s o d i u m c h l o r i d e also. A c c o r d i n g l y solutions c o n I n the course of these experiments the author found that taining various amounts of potassium sulfate were analyzed the cost of preparing the sodium cobaltinitrite could be reby Taylor’s (11) procedure and by precipitation with sodium duced by using dry acetone, instead of the absolute alcohol cobaltinitrite prepared according to Biilmann (a), the potasand absolute ether of the original method, for drying the sium being precipitated in a water-alcohol solution saturated precipitate of sodium cobaltinitrite. He also found it adviswith sodium chloride. I n a third series of experiments poable to keep the dried sodium cobaltinitrite tightly stoppered tassium was precipitated from water-alcohol potassium chloand in a cool dark place, which precautions permitted him to ride solutions with Biilmann’s preparation, but the precipitate keep the material for more than 6 months without decompowas settled with the aid of a centrifuge and a different method sition. A further precaution recommended in the use of this of washing was used. preparation for precipitation of potassium is to dissolve it Precipitation by Taylor’s Procedure freshly, to be used immediately for the precipitation. The procedure used for precipitation in this second series The procedure described by Taylor (11) was used to prewas essentially the same as that (11) used in the first series, cipitate potassium from a series of standard potassium sulfate except for a different cobaltinitrite reagent and a slightly solutions of known concentration ranging from 0.20 to 1-00 different method of washing the precipitate. mg. of potassium per cc. The results obtained in this series of determinations are compared with the results obtained by Precipitation in Water-Alcohol Solution with alternate procedures in Table I. of the colorimetric estima-
Centrifuging
The third series presented in Table I involved a precipitation and washing procedure developed by the author on the basis of extensive experience in estimating small amounts of potassium in extracts from potash-bearing minerals and from soils.
TABLE I.
COMPOSITION OF COBALTINITRITE PRECIPITATE Potassium Potassium in Cobalt in y t . . of Precipitate in Sample Precipltate (Calcd.) Precipitate
Mo
.
0.20a 0.49 0.70 1.00
MQ.
%
Taylor’s Procedure 1.601 12.5b 3.991 12.3 5.276 13.3O 7.689 13.0
%
... ... .*.
Five cubic centimeters of a potassium chloride solution were mixed thoroughly with 1.25 cc. of 96 per cent alcohol in a graduated conical centrifuge tube, 5 cc. of a 20 per cent solution of Biilmann’s sodium cobaltinitrite were added, and the solutions were thoroughly mixed by shaking. The tubes were stoppered, held in an ice box for 24 hours at 2’ to 3” C., and then centrifuged and the supernatant liquor was carefully removed. The precipitate was washed three times with 10 cc. of 48 per cent alcohol, suspendin the precipitate after the second washing and, finally, was washef with absolute alcohol and dry acetone. The last traces of acetone were removed in a vacuum desiccator. After weighing the precipitate, the cobalt was converted into the sulfate and the cobalt content estimated electrolytically according to Pregl’s (9) procedure.
a , .
Precipitation in Water-blcohol Solution Saturated with NaCl 0.612 16.4 ,.. 0: 100 3.672 13.4 0.50 7.937 12.6d 1.00
... ...
Precipitation in Water-Alcohol Solution with Centrifuging 13.40d 12.85 3.65 0.489e 13.77 13.09 14.2 1.953 14.32 13.51 34.15 4.899 a As Kz904. b I n calculating the percentage of potassium in the precipitates it wa8 assumed that all the potasaium taken was preci itated. 0 The cobalt reagent in this case was 10 daya o?d. For all other precipitationa in these experiments freshly prepared cobalt, reagent was used. The cobalt reagent, undergoes contlnuous decomposltion even when tightly sto pered and in an ice,box. fl.00 mg. of, potassium precipitated. accordin { o the same procedure, with the exception of time held in the ice box w%ich wae 11 instead of 24 hours, gave a precipitate in which the calculated potassium content was 12.3 per ceni. e As KC1.
Discussion The experiments performed indicate that reasonably consistent results may be obtained by using recrystallized sodium cobaltinitrite as the reagent for precipitating potassium. It is evident, however, that the potassium content of the precipitate depends upon the concentration of potassium in the solution analyzed and upon the procedure followed in making the precipitation. The results of the present experiments are in agreement with observations made by Tischer ( l a ) , Gram (5), and Benedetti-Pichler (1). The chemical composition of the precipitate, on the basis
Precipitation in Water-Alcohol Solution Saturated with Sodium Chloride Table I also presents the results obtained when a series of potassium sulfate solutions of known concentration were precipitated in a water-alcohol solution saturated with sodium chloride, using a purified sodium cobaltinitrite prepared
1
The author’s study had been completed when he first learned of Gram’s
(6) investigation.
272
JULY 15, 1935
ANALYTICAL EDITION
of the calculated potassium content, is between KNa2[Co(NO&], which contains 9.3 per cent of potassium, and KzNalCo(NOs)s],which contains 17.9 per cent of potassium. Studies by Jander and Faber (7), Lewis and Marmoy ( 8 ) , and Sobel and Kramer (IO) also .indicate this. Since the potassium and sodium content of the precipitate varies with the conditions of precipitation, the formula (K/Na)s [Co(NOp)6],with K/Ka denoting the variable composition, has been suggested to designate this precipitate. Hamburger (6) has proposed a similar formula.
273
Acknowledgment The author is indebted to Helen Stantial and F. A. Beamish, Department of Chemistry, University of Toronto, for valuable suggestions and criticisms.
Literature Cited (1) Benedetti-Pichler, A,, 2. anal. Chem., 64, 429 (1924). (2) Biilmann, E., Ibid., 39, 284 (1900). (3) Bonneau, L., Bull. soc. chim., 45, 799 (1929). (4)
Christensen, H. R., and Feilberg, N., Landw. Vers. St., 97,
(5) (6) (7) (8)
Gram, Chr. N. J., Thesis, University of Copenhagen, 1932. Hamburger, Biochem. Z., 71, 428 (1915). Jander, G., and Faber, H., 2. anorg. Chem., 173, 225 (1928). Lewis, A. H., and Marmoy, F. B., J. Soc. Chem. Ind., 52, 177T
27-56 (1920).
Conclusions I n the quantitative determination of potassium by precipitation with sodium cobaltinitrite i t is necessary to take into account the fact that the potassium content of the precipitate is a function of the method of precipitation and the potassium concentration in the solution being analyzed. There seems to be a need for a very thorough analytical investigation in order to develop a satisfactory standard method for estimating small amounts of potassium by precipitating the potassium as sodium-potassium cobaltinitrite.
(1933). (9) Pregl, F., "Quantitative Organic Microanalyses," Philadelphia, P. Blakiston's Son & Co., 1930. (10) Sobel, A. E., and Kramer, B., J. Biol. Chem., 100, 561 (1933). (11) Taylor, F. H. L., Ibid., 87, 27 (1930). (12) Tischer, J., Biochem. Z., 238, 148 (1931)
RECEIVED December 8, 1934.
Determination of Molvbdenum in J
Plants and Soils KENNETH E. STANFIELD, University of W y o m i n g , Laramie, Wyo.
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N STUDIES made by the Department of Research Chemistry, University of Wyoming, on toxic minerals in native vegetation occurring on Cretaceous and post-Cretaceous shales, it has been observed that molybdenum is strongly indicated in some instances as being a contributing factor in the poisoning of range livestock. By growing barley on experimental plots that were treated with sodium molybdate, i t has been possible to determine, by the exclusion of selenium, the pathological conditions resulting from feeding livestock upon such grain. Chronic poisoning has been produced experimentally and the symptoms exhibited are like those produced by selenium in cereals. A survey of the methods in the literature for the determination of molybdenum showed the necessity of adapting them to plant and soil analysis. Most of these methods were concerned with the analysis of less complex mixtures containing relatively large amounts of molybdenum. In the analysis of plants and soils there is seldom more than a fraction of a milligram of the element present in 100 grams of sample. Owing to the presence of large amounts of the common elements, as well as smaller amounts of the rarer elements having similar properties to those of molybdenum it was necessary that the method developed be specific for this element.
]Determination of Molybdenum in Plants Approximately 100 grams of the air-dried sample were moistened with concentrated sulfuric acid, warmed to remove the excess acid, and slowly ignited, with occasional stirring, in an electric muffle furnace at a temperature preferably below 500" to 550" 0. (4). The ash was then extracted with dilute hydrochloric acid, liltered, and the remaining residue, which contained some carbon, again ignited. The remaining ash was extracted with dilute hydrochloric acid, filtered, and the residue warmed for 1 hour upon a steam bath with a mixture of hydrochloric and nitric acids. After diluting with water, the mixture was filtered, washed thoroughly with water, and the colorless siliceous residue discarded. The filtrate and washings from the latter treatment were then eva orated to dryness to remove the nitric acid, the residue was taien up in dilute hydrochloric acid, and the acid
solution united with the hydrochloric acid extracts previously obtained. In case tungsten was thought to be present (61, 5 grams of tartaric acid were added, and the solution was made strongly alkaline with ammonia and saturated while cold with hydrogen sulfide. The brown molybdenum trisulfide, MoSa, was precipitated by cautiously pouring the alkaline solution into an excess of 6 N sulfuric acid (or 6 N hydrochloric acid if considerable calcium was present) and heated to boiling to coagulate the precipitate. In case the presence of vanadium was suspected, the molybdenum was precipitated free of vanadium by saturating the slightly acid solution with hydrogen sulfide and heating 30 minutes in a pressure bottle placed in boiling water. After filtering off the molybdenum sulfide, it was washed with a cold saturated solution of h drogen sulfide and redissolved by pouring repeatedly through tge filter a warm solution of one part of 6 N hydrochloric acid and one part of concentrated nitric acid. The filtrate and washings were evaporated to dryness, and the residue was taken up in a dro or two of concentrated nitric acid, oxidize the molybdenum to the evaporated just to dryness hexavalent state), and finally taken up in water and 0.5 ml. of 6 N hydrochloric acid, or a sufficient amount to make the aliquot used contain 0.5 ml. of the acid.
80
To confirm the presence of molybdenum, a n aliquot of the solution was treated with either of the following reagents: 1. A few crystals of potassium ethyl xanthate stirred with the slightly acid solution containing 0.001 mg., or more, of molybdenum gave a distinctive rose coloration (7, IO). 2. A drop of a suitable reducing agent, such as stannous chloride or sodium thiosulfate solution, was stirred with the unknown to reduce any ferric iron present (which would interfere with the test) and to reduce the hexavalent molybdenum. The addition of a drop of sodium or potassium thiocyanate solution gave a red coloration that was easily detected in the presence of 0.001 mg. of molybdenum (1, 3, 11).
I n case the molybdenum sulfide precipitate indicated the presence of 5 mg., or more, of molybdenum it was determined gravimetrically as lead molybdate. The unknown solution, or aliquot, was made alkaline with ammonia, heated to boiling to precipitate any impurities of iron and aluminum, filtered, and the filtrate made just acid with hydrochloric acid. After the addition of 5 ml. of glacial acetio
