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 b y 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.
I
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, evaporated just to dryness oxidize the molybdenum to the 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
acid and 25 ml. of 25 per cent ammonium acetate, the solution was heated to boiling and the lead molybdate precipitated by the dropwise addition of 5 ml. of 4 per cent lead acetate solution. A large excess of lead acetate should be avoided, owin to the possible formation of lead sulfate to give accordingly higa results ( 2 , 5 , 9 ) . The solution was allowed to stand upon a hot plate just below boiling for 30 minutes to permit the precipitate to coagulate, then set aside overnight for the precipitate to settle. After filtering through a Gooch crucible, the lead molybdate was washed with hot water, or with a 2 to 3 per cent ammonium acetate solution if the amount of the precipitate was large, ignited at a dull red heat, then cooled and weighed. The weight of ' molybdenum present in the aliquot was equivalent to the weight of the lead molybdate found multiplied by the chemical factor 0.2614. Table I gives the results obtained in the analysis of two samples for molybdenum.
known solutions of molybdenum were tested and gave the results shown in Table 11. TABLE11. COLORIMETRIC DETERMINATION OF MOLYBDENUM Mo Extracts
Plant
Weight of PbMoOi Dram
A295 A237
Barley hay Barley hay, check Alfalfa Alfalfa, check
0,0086 0.0084 0.0014 0.0013
Av. Reading
7 10 10 15 25
0.001 0.01 0.07 1.00 10.00
15 30 20 40 70
Mo Used
M Q.
0.001 0.01 0.05 0.90 10.00
Ay. Reading
Mo Caldated
MQ. 15.05 30.16 27.60 44.99 69.90
0.00096 0.0095 0.051 0.89 10.02
Error
Me.
5%
0,00004
4 5 2 1 0.2
0,0005
0.001 0.01 0.02
The average error in the analysis of thirty-three known solutions containing from 0.001 to 10.00 mg. of molybdenunn was 4.6 per cent. Hence, the accuracy with which the molybdenum in the molybdenum sulfide precipitate was determined was approximately 95 per cent.
Determination of Molybdenum in Soils
Molybdenum
P. p .
Used
MQ.
iM1.
TABLEI. DETERMINATION OF MOLYBDENUM Samale
VOL. 7 , NO. 4
INDUSTRIAL AND ENGINEERING CHEMISTRY
274
m.
90 88 7 7
To determine the molybdenum content of most plants, which generally contained less than 5 mg. of molybdenum per 100 grams of sample, it was more convenient and accurate to use a colorimetric method. An aliquot of the unknown solution containing 0.5 ml. of 6 N hydrochloric acid was treated with 1 ml. of 1 N potassium thiocyanate and 2 ml. of l N stannous chloride solution, and immediately extracted with butyl acetate (8). A sufficient amount of this solvent should have been previously shaken with the stannous chloride and potassium thiocyanate solutions and separated from the aqueous layer before being used in the extraction. As the color is increasingly unstable in an aqueous solution in proportion to the amount of acid present, equal amounts (or 0.5 ml. of 6 N hydrochloric acid) were used in both the standard and unknown solutions. Approximately one-half of the total butyl acetate was used in the first extraction, followed by three or more extractions with smaller amounts until these were entirely colorless. The total volume of extract may vary from approximately 5 ml. for 0,001 mg. of molybdenum to about 25 ml. for 10 mg. of molybdenum. Any turbidity due to water may be removed by the addition of a few drops of alcohol.
A standard color was prepared by treating a suitable volume of a known solution of molybdenum as ammonium molybdate in the same manner as the unknown. T o obtain the most accurate result, the concentration of the molybdenum in the standard and the unknown should be nearly equal. The ammonium molybdate solution was prepared by dissolving 2.0425 grams of the salt in water and making it t o one liter. One milliliter of this solution contained 1mg. of molybdenum. Other standard solutions containing 0.1, 0.01, or 0.001 mg. of molybdenum per milliliter were prepared b y the dilution of this solution. The colored extracts of the unknown and standard solutions were made t o the same volume with butyl acetate and their relative concentrations determined by the comparison of their color intensities in a Bausch and Lomb colorimeter. By the substitution of these average values in the following equation the number of milligrams of molybdenum in the aliquot were calculated:
Mg. of Mo in unknown = mg. of Mo in standard X av. reading of standard av. reading of unknown T o determine the accuracy of the colorimetric method,
Molybdenum was determined in soils in much the same manner as described above for its determination in plants, except that a larger amount of material was used. Approximately 200 grams of the soil were covered with dilute hydrochloric acid, warmed upon a steam bath for 24 hours, filtered, and washed. The residue was repeatedly treated in the same manner with a mixture of hydrochloric acid and nitric acid until only a colorless residue remained. After the removal of the nitric acid by evaporation, the extracts were combined and the molybdenum sulfide was precipitated as described. Because of the large amount of material dissolved by the acid solutions, it was generally necessary to reprecipitate the molybdenum sulfide from the hydrogen sulfide precipitate before attempting its estimation. The minimum amount of molybdenum present as molybdenum sulfide that it was possible to determine by this colorimetric method was 0.001 mg. The results shown in Table 111were obtained in the analysis of several soil samples for molybdenum. TABLE111. DETERMINATION OF MOLYBDENUM IN SOILS Sample
Type of Soil
A779 4785 A777 A103 A150 A104 A137 A169
Iron concretions from Niobrara Iron concretions from Niobrara Shell layer from Niobrara Alkali seep Niobrara shale Alkali laver layer from Niobrara seep Niobrara shale Niobrari Niobrara wash
Molybdenum P. p . m. 20 15 5 5
1.0 0.8 0.5 0.08
Summary A gravimetric method is described for the determination of molybdenum as lead molybdate in plants and soils in amounts exceeding 5 mg. A colorimetric method based upon the extraction of the red colored molybdenum thiocyanate by butyl acetate is described for the determination of molybdenum in plants and soils.
Literature Cited (1) Bohm, J. S.,and Vostrebal, J., Z . anorg. allgem. Chem., 110, 81 (1920). (2) Bonardi, J. P.,Bur. Mines Bull. 212, 102 (1923). (3) Braun, A. D., Z . anal. Chem., 6, 86 (1867). (4) Brinton, P. H.M.-P., and Stoppel, A. E., J. Am. Chem. SOC.,46, 2454 (1924). (6) C h a t a r d , T. M., Am. J . Sci., (3) 1, 416 (1871). (6) Doerner. H.A.. Bur. Mines Circ. 6079 (1928). (7) Hall, D.; J. Am. Chem. SOC.,44, 1462 (1922). (8) James. L.H.. IND. ENQ.CHEM.,Anal. Ed., 4,89 (1932). (95 Murray, W.F.,Chemist Analyst, 18, 10 (1929). (10) Singleton, W., Ind. Chemist, 2, 454 (1926). (11) Tananaev, N.A,, and Panohenko, G. E., Ukrain. Khem. Zhur., 4, Sci. Pt., 121 (1929). RECEIVED April 29, 1935.
Aav
