A Rapid Kjeldahl Digestion Method Using Perchloric Acid LEONARD P. PEPKOWITZ, ARTHUR L. PRINCE, AND FIRMAN E. BEAR Agricultural Experiment Station, New Brunswick, N. J.
paper (4) suggested a rapid perchloric acid method for microdeterminations of nitrogen. The present paper describes a variation of this method which can be used in the ordinary Kjeldahl digestion. By the proposed procedure a considerable amount of time is saved in digesting, the necessity for using large amounts of salts is eliminated, and as a consequence, the cost of making nitrogen determinations is considerably reduced. An added advantage is that the resulting solution is usually water-clear. If iron is present a yellow tint may persist, but this will disappear upon cooling and dilution. Previous attempts t o use perchloric acid in Kjeldahl determinations have generally resulted in the loss of nitrogen either as the oxide or in the elemental form. Mears and Hussey (3) found that such loss could be prevented by decreasing the quantity of perchloric acid used, the increase in rate of digestion obtained was no greater, however, than if they had used mercury or selenium as catalysts. Stubblefield and DeTurk (6) refer t o a perchloric acid method proposed by Gauduchon-Truchot (1) and to a similar procedure of Le Tourneur-Hugon and Chambionnat (2) which involved the dropwise addition of perchloric acid, but they say that neither is applicable to general laboratory use. One reason for the failure of analytical chemists t o give more attention to the possibilities in the use of perchloric acid in nitrogen determinations is the tendency of the acid to react violently with easily oxidizable materials. I n the method proposed, there has never been any evidence of this explosive property. The cold dilute acid is not dangerous, and the small amount employed is further diluted by 50 times its volume of sulfuric acid before it is used. Furthermore, the easily decomposable materials have already been oxidized in advance of the introduction of the perchloric acid into the system.
preceded by the evolution of a small quantity of white fumes and a slight effervescence in the di est. The final solution is usually colorless, but certain materijs having insoluble components may yield a grayish turbid solution. This has no effect, however, on the accuracy of the nitrogen determination. For moist samples, such as fresh com ost, 1 ml. of the 35 per cent perchloric acid (rather than 0.5 ml.Pshould be used to compensate for the water contained in the sample. SOILS. Except for eats and other highly organic soils, the sodium sulfate shoulf be omitted. The sample is digested directly with 25 ml. of sulfuric acid and 1 ml. of the selenium oxychloride solution for 10 minutes over a vieorous flame. The subsequent perchloric acid phase of the digestion is as previously described. Since soils usually contain comparatively little organic matter, the solution is normally light colored when the perchloric acid is added. Accordingly, there is no definite color change to indicate the completion of the oxidation, and a standard period of 15 minutes is used for the perchloric acid phase of the digestion.
APREVIOUS
Experimental The perchloric acid procedure was tested by comparing the per cent nitrogen obtained on a variety of materials by this method with that found by the official A. 0. A. C. method (Table I). The values by the official method were determined by the members of the Soils Department in connection with other investigations. The fact that the results were
TABLE I. NITROQB~N OBTAINED BY RAPIDPERCHLORIC ACID AND A. 0. A. C. METHODS Material Sludge Hynite tankage Bovung Bmirow Garbage tankage Animal tankage Cocoa meal Peanut hulls Dried blood Tobacco stems Alfalfa Horse manure Dry fish scrap Acid fish scrap Chicken manure Processed tankage Cocoa tankage Beetle scrap dust hfilorganite Tankage 1 Tankage 2 Tankage 3 Tankage 9b-2200 Tankage 813-220 Mixed fertilizers 1 Mixed fertilizers 2 hIixed fertilizers 3 Mixed fertilizers 4 Compost 6A Compost 7A Compost 7 B Compost 8B Compost 8C Dutchess loam soil Ap Horizon A1 Horizon B Horizon Sassafras loam soil Plot 7A-Ap Horizon Plot ~ A - B IHorizon Plot 9A-Ap Horizon Plot 1lA-Ap Horizon Plot 18A-Ap Horizon Plot 18A-BI Horizon
Procedure PLANTMATERIALS AND FERTILIZERS. To 1 ram of dried material add 25 ml. of concentrated sulfuric acif, 10 grams of sodium sulfate, and 1 ml. of a selenium oxychloride-sulfuric acid solution (12 grams of selenium oxychloride er liter of concentrated sulfuric acid). Heat over a vigorous lame for 10 minutes. (During this interval the sample will char,.go into eolution, and pass to a dark red, light brown, or yellowish solution, depending on its composition: plant materials usually take on a reddish tint; fertilizers become brown.. For most materials, a digestion time of 10 minutes is sufficient.) If nitrates are present, as in mixed fertilizer, and zinc is used to reduce the nitro groups following the addition of salicylic acid, start the digestion with a low flame and continue for about 5 minutes in order to expel the hydrogen, after which heat it to vigorous boiling for an additional 10 minutes. Rotate the flasks occasionally to aid the refluxing acid to wash the walls of the flask clean. Turn off the flame and allow the flasks to cool for 10 minutes on the digestion rack and for a few minutes more under the cold water tap until they are cool enough to be grasped with the bare hands. Add 10 dro s (0.5 ml.) of a 35 per cent aqueous solution of perchloric acif(di1uted from 70 per cent acid) directly into the digest. Reheat the flasks over a flame so small that the digest does not boil. Continue the digestion at thls low temperature until the solution clears, normally between 10 and 15 minutes. After it has become clear, allow the digest to cool, dilute with 200 ml. of water, and distill by the usual procedure. The digest must be kept from boiling, otherwise a loss in nitrogen will occur, and if the erchloric acid is volatilized, the period of digestion may be proyonged. The end of the digestion is 856
Perchloric Acid Method
A. 0. A. C. Method
%
%
1.79 9.45 2.03 7.23 2.87 8.77 2.13 1.22 13.77 1.06 2.73 1.43 9.48 8.75 2.26 9.59 2.36 19.05 5.67 8.78 8.42 8.67 6.55 9.42 2.12 4.19 8.13 6.00 0.37 0.53 0.36 0.29 0.26
1.78 9.57 2.01 7.01 2.66 8.33 2.98 1.24 13.83 1 .oo 2.82 1.45 9.2s 8.54 2.25 9.76 2.52 19.02 5.66 8.85 8.50 8.65 6.60 9.42 2.14 4.20 8.33 5.93 0.31 0.48 0.35 0.26 0.29
Deviation
% +O.Ol -0.12 +0.02 +0.22 +o.21 4-0.44 +0.15 -0.02 -0.06 $0.06 -0.09 -0.02 +0.20 +0.21 +O.Ol -0.17 -0.16 4-0.03 +0.01 -0.07 -0.0s +0.02 -0.05 0.00 -0.02 -0.01 -0.20 +0.07 10.06 +0.05 +0.01 f0.03 -0.03
0.231 0.190 0,082
0.245 0.197 0,092
-0.014 -0.007 -0.010
0.057 0,020 0.084 0,100 0.132 0.047
0.064 0,020 0,089 0.104 0.143 0.053
-0.007 0.00
-0.005 -0.004 -0.011 -0.006
ANALYTICAL EDITION
November 15, 1942
obtained by independent analysts, one of whom used the A. 0. A. C. and the other the suggested method, indicates the reliability and accuracy of the proposed procedure. The average deviation from the official method for the 33 materials, other than the samples of soil, was 0.09 per cent. The average digestion time for materials other than soils by the perchloric acid method was 30 to 35 minutes, whereas that for the official method was between 1 and 3 hours. For soils, the digestion time by the rapid method was 35 minutes, in comparison with 2 hours by the official procedure.
Discussion The volume of perchloric acid that can be used without losing nitrogen is a direct function of the organic matter which remains after the preliminary sulfuric acid digestion. For this reason, an alternate method can be employed in which the use of sodium sulfate is not involved. Heat the sample with 25 ml. of cdncentrated sulfuric acid and I ml. of the selenium oxychloride solution for 25 minutes. Allow the digest t o cool, add 1 ml. of 35 per cent perchloric acid, and heat the mixture over a small flame until it is clear, as described above. This variation of the procedure is as accurate as the previously described method. The elapsed time for the complete digestion, however, is between 40 and 45 minutes. I n the absence of an organic substrate with which to react perchloric acid will oxidize the ammonia, with resulting loss of nitrogen. This explains why sodium sulfate cannot be used in soil digestions. The amount of organic matter in soils is comparatively low and most of it is oxidized by the sulfuric acid when sodium sulfate is added to raise the boiling point of the digest. During the perchloric acid phase of the procedure the temperature of the digest must be kept below the boiling point. If the solution is made to boil the perchloric acid will react a t too rapid a rate, with a resulting loss of nitrogen. The addition of salicylic acid did not materially lengthen the period of digestion. An average period of 32 minutes was necessary for samples containing approximately 2 grams of salicylic acid, Evidently the carbon contained in the salicylic acid is largely oxidized during the preliminary sulfuric acid treatment and therefore does not lengthen the total digestion period. This also explains why 0.5 cc. of 35 per cent perchloric acid is sufficient in spite of the large amount of carbon added in the salicylic acid. The perchloric acid method is also suitable as a wet-ashing procedure for the determination of plant ash elements, such as phosphorus, potassium, and calcium. The alternate rather than the first mentioned proceduie is to be preferred for this purpose.
857
(3) Mears, B., and Hussey, R. E., J. IND. ENQ.CHEM.,13, 1054-6 (1921).
(4) Pepkowitz, L. P., and Shive, J. W., IND.ENQ.CHEM.,ANAL.ED., 14, 915 ( 1 9 4 2 ) . ( 5 ) Stubblefield, F. M., and DeTurk. E. E., Ibid., 12, 396-9 ( 1 9 4 0 ) . JOURNALSeries paper of the Kew Jersey Agricultural Experiment Station. Department of Soils. Rutgers University.
Pressure Wash Bottle for Volatile Solvents LOWELL W. CHARKEY AND D. V. ZANDER Colorado Experiment Station, Fort Collins, Colo.
M
UCH time and energy can be saved in many laboratory operations by attaching to the mouthpiece of the con-
ventional wash bottle (of the type with the third tube for the thumb) a long piece of light rubber tubing connected to a compressed air line or air pump. I n using this device, a slow current of air is turned on and allowed to flow through the wash bottle continuously during the period of use, the air escaping through the thumb-hole tube. Delivery of liquid from the wash bottle is obtained instantly and for any desired period of time down to a small fraction of a second by placing the thumb over the thumb hole, thus creating pressure inside the bottle.
‘ompressed \_
’
clir
Thumb hole
Tip
hole i n r u b b e r t u b e (when t h u m b h d e , i s Stopped, Qir posses i n t o flusk h e r e , crratinq preb5ure,
-Small
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Summary A rapid Kjeldahl digestion procedure involving the use of 35 per cent perchloric acid is proposed. An alternate method eliminates the use of sodium sulfate, with only a small increase in the length of period of digestion. The average digestion time for 1-gram samples is approximately 30 minutes. KO explosive hazard, because of the perchloric acid used, is believed to be involved. The results check closely with the values determined by the official A 0. A. C. method. The cost of the determination is reduced by eliminating certain reagents employed in the A. 0. A. C. method, such as mercury and potassium sulfide.
Literature Cited Gauduchon-Truchot, H., “Contribution A l’fitude de la MBthode de Kjeldahl”, 1936. Le Tourneur-Hugon and Chambionnat. Ann. fals., 29, 227-9 (1936).
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I n case the device is used for volatile solvents such as ether, petroleum ether, alcohols, etc., evaporation and consequent loss of solvent which would take place by allowing a current of air to pass promiscuously through the bottle are easily prevented by incorporating into the design a small bypass, best described in the accompanying diagram. This device, the liquid flow from which is entirely under control of the thumb, is the most effective the authors have seen for delivering a small quantity of a liquid in a forcible stream exactly when and where it is wanted.
