May, 1944
ANALYTICAL EDITION
aftcr an addit,ional 5 weeks. Air impurity blanks of 0.002 and 0.003 ml., respectively, were found. In all these tests the generator was vonnected to the azotometer by means of a short glass connection which was 9 cm. long, but otherwke identical to a regular micro combustion tube. Since it requires about 1 hour to pass 500 ml. of carbon dioxide intodhe microaxotometer, any air which diffum through the two rubber connections during this time is included in the observed air blank. Hence the actual
345
purity of the cwhon dioxidcs i* plohrrbly w e n higher than the tests indicnt,r. LITERATURE CITED
(1) Lowe, E. W.,arid Guthriiaiiii. W , S., IND. ENG.CHEM..A s ~ L . ED.,4,440(1932). (2) Poth, E. J.. Ibid., 3,202 (1981). (3) Ibid., 11, 518 (1939). Ihid., 12,684 (1940). (4) Rausclier, W.H.. (5) Shclherg, E.I:.. Ibid., 10,704 (1938).
Rapid Digestion Method for Determination of Phosphorus DONALD W. B O L l N AND O L O F E. STAMBERG, Department of Agricultural Chemistry, University of Idaho, Moscow, Idaho
A
ItAPIl> quantitative mrthotl f ( J r thr determination of phosphorus is always useful in thr analytical laboratory. Perchloric acid either alone or in mixtures with other acids has been used to oxidize organic matter previous to the determination of mineral constituents ( 8 , 4 ) . Work in this laboratory has shown that the presence of molybdenum in a perchloric-sulfuric acid mixture markedly increases the rate of oxidation of organic matter. Most colorimetric methods for the determination of phosphorus are modifications of the Misson (6) or Fiske and Subbarow (8) methods in which molybdenum is a reagent. The use of molybdenum in small amounts as catalyst proved not to interfrre with the quantitative colorimetric determinat,ion of phosphorus; hence a rapid method was developd for determining phosphorus by the combined techniques of perchloric-sulfuric acid digestion in the presence of molybdenum followed by colorimetric analyses. Results of phosphorus determinations on feeds digested with perchloric-sulfuric acid mixture in the presence of molybdenum aa a catalyst are presented here and compared with results using the official ashing method (I). ANALYTICAL PROCEDURE
DIGESTIOX MIXTURE.Dissolve 30 grams of sodium molybdate in 150 ml. of distilled water, then slowly add 150 ml. of concentrated sulfuric acid to the molybdate solution. Allow this solution to cool and then add 200 ml. of 70 to 72% perchloric acid. DIGESTION OF ORGANIC MATERIAL.Transfer not more than a 500-mg. sample to a dry 100-ml. Kjeldahl flask, and add 5 ml. of the digestion mixture and a few glass heads to prevent bumping. Heat the flask slowly over a microburner. Oxidation will begin in 1 or 2 minutes. At this time the burner may be turned off and the digestion allowed to proceed under its own generation of heat. Wash down any adhering particles on the side of the flask by swirling the flask gently, add 2 ml. of perchloric acid, place the flask back on the burner, and heat until the digestion is romplete. Digestion is usually complete within 3 or 4 minutes, and the solution is then clear and no charred material remains. Dilute the digestion mixture to a volume of 100 ml. with distilled water. Filter this solution or let it stand to permit any silica present to settle out. Take a suitable aliquot for the colorimetric determination, and adjust the acidity with perchloric acid to the approximate range of acidity stated in the method used. Since there is a relatively wide acid range in these methods, the approximate adjustment of perchloric acid concentration can easily be made. The phosphorus was determined by two different colorimetric methods-the development of a blue color by reducing the phosphomolybdate as described by Sherman ( 7 ) and the development of a yellow color by the formation of a phosphovanadiomolybdate compound by the method of Koenig and Johnson (5). The per cent transmission was determined with the Cenco photelometer, using a 420 mp filter for the yellow color, 600 mg for the blue color, and a reagent blank as a reference liquid. A standard reference curve was made by plotting values of known amounts of phosphorus against the photelometer readings on semilogarithmic graph paper. For routine analysis with samples in which the phosphorus
range is suitable, the Kjeldahl digestion flask may be calibrated t o 100 ml. and the reagents added directly to this flask. By the use of the phosphovanadiomolybdate method, samples containing 0.05 to 0.5% may be determined directly without further dilution. For materials containing less than 0.1% phosphorus and with a .500-mg. sample the reduced phosphomolybdate method (7) i5 more suitable, while for material containing higher amounts of phosphorus the vanadate method (5) appears preferable because of the greater stability of the color. DISCUSSION OF RESULTS
Table I shows good agreement of phosphorus in some typical fcLcdsby the perchloric-sulfuric acid digrstion method as compared with the longer ashing method. Values are also given as obtained by two previously published colorimetric procedures. Replicate results by t,he shorter acid digestion method werr always as good as by the ashing method. Several samples have been digested with perchloric-sulfuric acid in the presence of molybdenum, but no explosions have resulted. With a sample of 500 mg. or less, the reaction proceeds smoothly and a set of six samples can be digested in less than 10 miniitrs.
-Sample
Table I. Phosphorus Valuer
Bluea
%
AshingYelAverIowa age
%
-DigestionYelBlue0 lows
R
%
%
AverDifage ference'
%
%
Clover chaff 0.094 0.090 0.0940 0.095 0.096 0.0955 0.087 0 , 0 8 8 0.0875 0.087 0.092 0.0895 Beet pulp Corn silage 0.234 0.225 0.2295 0.220 0.227 0.2235 Range grass 0.060 0.060 0.0600 0,060 0.064 0.0620 Barley 0.362 0.357 0.3595 0.362 0.357 0.3595 0.306 0.312 0.3090 0,309 0.306 0.3075 Corn Alfalfa 0.164 0.168 0.1660 0.167 0.170 0.1685 0.191 0.193 0.1920 0,195 0.200 0.1975 Alfalfa Alfalfa 0.110 0.106 0.1080 0.104 0,102 0.1030 4.560 4.560 4.5600 4.800 4.800 4.8000 Meat meal Soybean 0.554 0.552 0.5530 0.567 0.555 0.5610 2 920 2.970 2.9450 3.070 3.100 3.0350 Fish meal Wheat 0.420 0.419 0.4195 0,417 0.417 0.4170 Peas 0.480 0.480 0.4800 0.460 0.468 0.4640 Poultry ration 0.815 0.800 0.8075 0.803 0.820 0.8116 a Phosphomolybdate blue, Sherman method (7). b Phosphovanadiomolybdate yellow, Koenig and Johns'on (6). C Ashing method, basis of 100.
fl.6 +2.3 -2.6 +3.3 0.0 -0.5 +1.5 +2.9 -4.6 +5.3 f1.4 f3.1
-0.5 -3.3
4-0.5
LITERATURE. CITED
(1) Assoc. Official Agr. Chem., Official and Tentative Methods of Analysis, 5th ed., p . 133 (1940). (2) Fiske, C. H . , and Subbarow, Y., J . B i d . Chem., 66,375 (1925). (3) Gerritz. H.W.. IND. ENQ.CHEX..ANAL. ED.. 7. 116 (1935). i4j Giesiking, J. E.: Snider, H. J , - a n d Getz, C. A . , - j b i d . , 7, 185 (1935). ( 5 ) Koenig, R. A., and Johnson, C. R., Ibid., 14, 155 (1942). (6) Misson, G., Chem-Ztg., 32, 633 (1908). (7) Sherman, M.S.,IND.END.CHEM.,ANAL.ED.,14,182 (1942). I
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PVBLISHED with approval of the Director of the Idaho Agrioultural Expennient Station as Research Paper No. 223.
