MAY 15, 1935
ANALYTICAL EDITION
tain klsrosene gave a reddish color, readily distinguished from the benzene color.
Precautions If suitable concentration methods and adequate precautions are used, concentrations of benzene well below 1 in 10,000 should be detected. Exclusion of chlorides from the extraction is necessary, as the subsequent nitration seems sensitive to their presence. A lower salt concentration than that used gave erratic results and a high salt concentration precipitated solids into the bottom layer. Exclusion of water from the nitration mixture is imperative. The pres-
185
ence of oxidizable material, such as excessive amounts of alcohol from the extraction or stopcock lubricant, interfere probably by their effect in the nitration. Care in preventing this effect is fully as important as efficient extraction of the benzene in the carbon tetrachloride.
Acknowledgment The writer wishes to express appreciation for the interest taken by A. Izsakin this work, and to the Commercial Solvents Corporation for permission to publish the results. RECEIVED January 10,1936.
Destruction of Organic Matter in Plant Material by the Use of Nitric and Perchloric Acids J. E. GIESEKING, H. J. SNIDER, and C. A. GETZ Department of Agronomy, University of Illinois, Urbana, Ill,
T
HE destruction of organic matter by some form of ashing generally precedes the determination of the mineral constituents in plant material. Two undesirable features of the ashing process suggest the need of a wet oxidation method similar to the Kjeldahl digestion: (I) The method of ashing commonly used for calcium and magnesium determinations is not applicable for potassium and phosphorus determinations on account of the loss of the more volatile compomds of potassium and phosphorus while ashing; and (2) the residues after ashing may be in a slowly soluble form. Perchloric acid either alone or in mixtures with other acids has been widely used in the oxidation of organic materials of animal origin previous to the determination of the mineral constituents. Kahane and associates (4) have made the most comprehensive study of these methods and have developed a method for determining silica in plant materials ( 5 ) . Winter and Bird (11) have used perchloric acid similarly for determining aluminum in plants. The properties of perchloric acid, as well as those of the perchlorate ion, make it a very desirable oxidizing agent for the analysis of organic substances. No water-insoluble perchlorates of the metals have been reported. Furthermore, cold perchloric acid, either dilute or concentrated, is not affected by ordinary reducing agents. The dehydrating action of perchloric acid on silica, shown by Willard and Cake ( l o ) ,aids in the quantitative separation of silica. Since hot concentrated perchloric acid may react violently with organic substances, the reaction intensity must be controlled. It has been found advisable to pretreat samples of plant material with nitric acid before adding perchloric acid. With substances very high in fat it may be necessary to pretreat the sample several times with nitric acid before it can be oxidized with perchloric acid without a loss of a portion of the sample. The perchloric acid should be diluted with water and nitric acid. Experimental Procedure The following method of wet oxidation was applied to a wide variety of plant materials, including sweet clover (roots and tops), alfalfa hay, red clover hay, alsike clover hay,
timothy hay, redtop hay, wheat straw, cornstalks, corncobs, corn (grain), and soy beans: Place a 4-gram sample of the material to be oxidized in a 400ml. beaker and add 10 ml. of nitric acid (RP.gr. 1.42). Cover the beaker with a watch glass and heat gently until any rapid initial reactions have subsided. Then heat to boiling and boil until the contents of the beaker are almost dry. Remove the beaker from the hot plate and add 10 ml. of dilute nitric acid (1 to 1) and 10 ml. of perchloric acid (70 to 72 per cent). Replace the cover glass and heat very gently to a low boiling temperature (avoid superheating). Maintain this temperature until all organic material has been removed from the sides of the beaker and from the solution, which will be indicated by a colorless or slightly colored solution. Remove the cover glass, allow the beaker to cool a few minutes, and wash any adhering salts into the beaker, (If the cover glass is washed with perchloric acid, the contents of the beaker need not be cooled.) Evaporate t o dryness at a temperature just below the boiling point in a clean hood. If potassium is to be determined on the residue the ammonium salts should be removed at this oint. After the removal of ammonium salts, add 5 ml. of hydrociloric acid (I to 1) and 10 ml. of water. Heat until all salts are dissolved. Filter into a suitable volumetric flask. Wash the silica residue thoroughly with hot water and make the filtrate up to volume. Aliquot portions of the filtrate may be taken for subsequent analyses. The above method was applied to the plant materials studied and calcium, magnesium, potassium, and phosphorus were determined. Known amounts of calcium from a standard solution of calcium acetate, magnesium from a standard solution of magnesium sulfate, and potassium and phosphorus from a standard solution of potassium dihydrogen phosphate were then added to a duplicate sample of the material and the determinations repeated, using the same procedure. The acids and salts used were taken from the usual laboratory stock of c. P. reagents. Calcium was precipitated as the oxalate and titrated with permanganate as directed by Wiley (9). Magnesium was determined by the method of Handy (2) as modified by Truog and Chucka (8). The method of Schueler and Thomas (6) was used for potassium. Phosphorus was precipitated as the phosphomolybdate and titrated with sodium hydroxide according to the method given by Treadwell and Hall (7). Table I shows the amounts of calcium, magnesium, potassium, and phosphorus recovered.
INDUSTRIAL AND ENGINEERING CHEMISTRY
186
OF PHOSPHORUS, POTASSIUM, CALCIUM, AND TABLE I. RECOVERY MAGNESIUM ADDED TO PLANTMATERIAL AND OXIDIZEDWITH PERCHLORIC ACID
Wheat atraw Sweet clover
Wheat straw Sweet clover
Added, Total, Mg. Mg. Phosphorus 0 2.50 2.00 4.50 2.00 4.48 0 2.00 2.00 4.00 2.00 3.95 Calcium 0 8.2 14.5 22.6 22.6 14.5 0 31.4 45.9 14.5 14.5 45.5
R~COYered Added, Mg.’ Mg. 0 2.00 1.98
0
2.00 1.95
0 14.4 14.4 0 14.4 14.1
Total, Mg.
Potassium 3.29 0.60 3.91 0.60 3.94 0 1.92 0.60 2.46 0.60 2.49 Magnesium 0 3.01 9.90 7.10 7.10 9.99 0 10.76 7.10 17.91 7.10 17.82 0
RECOYered, Mg. 0 0.62 0.65 0 0.54 0.57 0 6.89 6.98 0 7.15 7.06
TABLE 11. TOTAL CALCIUM AND TOTAL MAGNESIUM FOUND IN PLANT MATERIAL (When oxidized with nitric-perchloric acid and when ashed without treatment according to A. 0.A. C. method) Magnesium Sample Calcium NumBy By By By ber HClOi ashing HClO4 ashing
VOL. 7, NO. 3
ment calcium and magnesium are always lower when the samples are ashed. With the nitric-perchloric acid method one obtains a white residue of dehydrated silica, but when the sample is ashed for calcium and magnesium determinations, the residue is usually gray, indicating the incomplete oxidation of carbon. Furthermore, the calcium and magnesium in the ash are not readily soIuble in the dilute acid used in extraction. Low results under such ponditions suggest the incomplete removal of calcium and magnesium from the residue obtained upon ashing. Likewise, the ashing method for potassium tends to give lower results than the nitric-perchloric acid method. Since potassium compounds are volatile at high temperatures, a loss of potassium might be expected in using the ashing procedure. The results of the three methods used for the phosphorus determinations agree very closely. In view of these results and the plausible explanations thereof, it would seem that the proposed method of wet oxidation of this class of materials is from the standpoint of both accuracy and convenience superior to the ashing methods.
Summary A method involving the wet oxidation of the organic matter 0.11 0.38 0.30 0.14 902 Wheat straw 0.08 0.22 0.18 0.10 907 in plant materials previous to the determinations of calcium, 0.50 1.53 0.55 W407 1.68 Sweet clover tops magnesium, potassium, and phosphorus has been devised. 0.23 0.29 0.38 0.36 N309 Sweet clover roots 0.61 0.67 0.76 C 0 ..74 Sweet clover tops This method has been found to be rapid in that only one 0.17 0.31 0.25 0.18 C Sweet clover roots 0.18 0.27 0.16 0.26 WS sample is required for the four determinations. The calcium, Redto hay 0.19 0.17 0.16 0.30 R40SE TimotRy hay magnesium, potassium, and phosphorus are left in a condition 0.51 2.29 0.51 2.24 Alfalfa hay R400E in which they are readily soluble in dilute acids and no loss through volatilization of phosphorus or potassium was enTABLEI 111. TOTAL PHOSPHORUS AND TOTAL POTASSIUM FOUND countered. IN PLANT MATERIAL The method used for preventing violent reactions between (When ashed with H&Od (method given by Wiley) and oxidized with nitriathe perchloric acid and the organic substances was found to be perchloric acid) effective in all cases. Phosphorus Potassium %
!E,9‘,”, Alfalfa hay Red clover hay Alsike clover hay
Wheat etraw
401 403 404 408 409 401 402 902 907
%
a2Zg
%
%
~204asBgLg
%
%
%
%
0.15 0.20 0.20 0.20 0.20 0.15 0.29 0.25 0.25
0.14 0.22 0.23 0.20 0.21 0.15 0.25 0.22 0.22
0.96 0.73 0.93 0.80 0.96 1.16 1.12 1.69 2.33
0.83 0.69 0.83 0.67 0.93 0.99 1.01 1.50 2.13
In order to test the accuracy of the nitric-perchloric acid method of destroying organic matter a corresponding set of samples was ashed, and calcium, magnesium, potassium, and phosphorus were determined as before. The method of the Association of Official Agricultural Chemists (1) was used for ashing the samples previous to the determination of calcium and magnesium, and the sulfuric acid method of Wdey (9)was used for potassium and phosphorus. As a further comparison, the method of Howk and DeTurk (3) was used for phosphorus. Samples of wheat straw, redtop hay, sweet clover tops, and sweet clover roots were found to contain 0.26, 0.15,0.18, and 0.32 per cent of phosphorus, respectively, by the Howk and DeTurk method, as compared to 0.27, 0.16, 0.19, and 0.32 per cent by the nit-ric-perchloric acid method. The results of the other comparisons are given in Tables I1 and 111.
Discussion of Results The amounts of calcium, magnesium, potassium, and phosphorus added were very satisfactorily recovered with the use of the nitric-perchloric acid procedure (Table I). Tables I1 and 111show that there is not always good agreement between the nitric-perchloric acid and the ashing methods. It will be noted from Table I1 that in cases of disagree
Literature Cited (1) Assoc. Official Agr. Chem., “Official and Tentative Methods,” 3rd ed., p. 102 (1930). (2) Handy, J. O.,J. Am. Chem. SOC.,22,31 (1900). (3) . . Howk, B. W.. and DeTurk, E. E., IND.ENQ.CHEM.,Anal. Ed.,
.-,
4,111 (1932).
(41 Kahane. E.. “L’Action de 1’Acide Perchloriaue sur lea Matihres
Organiques. I. Generalitbs. 11. Applications,” Paris, France, Hermann et Cie, 1934. (5) Lematter, L.,Boinot, G., Kahane, E., and Kahane, M., Compt. rend., 192, 1459 (1931). (6) . . Schueler. J. E., and Thomas, R. P.. IND.ENG.CHEM..Anal. Ed., 5, 163 (1933). (7) Treadwell, F. P., and Hall, W. T., “Analytical Chemistry,” 6th ed., Vol. 11, p. 503, New York, John Wiley & Sons, 1924. (8) Truog, E., and Chucka, J. A., unpublished data, University of Wisconsin, 1934. (9) W h y , H. W., “Principles and Practices of Agricultural Analysis,” p. 554,Easton, Pa., Chemical Publishing Co., 1914. (10) Willard, H. H., and Cake, W. E., J . Am. Chem. Soo., 42,2208 (1920). (11) Winter, 0.B.,and Bird, 0. D., Ibid., 51,2964(1929).
RECEIVED March 20, 1935.
“Cold Light” Filter Developed A filter for obtaining “cold light” has been developed by K. S. Gibson, chief, colorimetry section, National Bureau of Standards. This a t e r transmits a very narrow band in the green, and the luminous efficiency of this transmitted energy is extremely highabout 99 per cent of the maximum possible. The result is “cold light” or light practicallv without heat. The transmitted band of light has a wave length of 560 millimicrons, a spectral region which is of particular importance in the colorimetry of sugar solutions in optical pyrometry, in abridged spectrophotometry, and in Lotometry. Four components are used in the construction of t i e filter, two being made of Corning and two of Jena glass.
