Use of Aeration in Kjeldahl Distillations W. B. MELDRUM,R. MELAMPY,AND W. D. MYERS, Haverford College, Haverford, Pa.
T
H E extremely useful method for the determination of nitrogen in organic substances, brought forward by Kjeldahl in 1883 (6) as an extension of a method first applied to urea in 1875 by Heintz and Ragsby (C), has undergone a number of modifications to widen the scope of its application and to facilitate the analysis. One has been the aeration of the alkalized solution to expedite the sweeping over of the ammonia into the standard acid solution. Suggested by Folin in 1903 (5) in connection with urine analysis,
ing wet with the colored acid solution, served to show whether or not the ammonia was being completely absorbed and to absorb any such that came through; change to the alkaline color at the base of the tube usually did occur, more markedly in the case of high aeration rates. The sodium sulfide solution to precipitate the mercury and the excess sodium hydroxide solution were admitted through a funnel temporarily attached to the inlet tube, B. The temperature of distillation was read from a thermometer attached to the inlet tube inside the flask. To this temperature the solution was raised as rapidly as practicable before aeration was begun. The effects of varying the conditions upon which the rate of distillation might depend were in turn examined. The more important conclusions so far as this investigation was concerned are brought out by the data given in Tables I and I1 and plotted in Figures 2 and 3. TABLE I. NITROGEN RECOVERY FOR VARIOUS A ~ R A T I OR.4TES N AT VARIOUS TEMPERATURES A~RATION TBMPERATURE RATE a
c.
L./min. 0.2 0.4 0.6 0.8 0.2 0.4
25
NITROQEN RECOVERED
PER CENT RECOVERY
%
0.36 2.24 0 54 3.36 0 64 3.9s 1.00 6.22 50 0.62 3.85 1.28 7.96 0.6 1.63 10.13 0.8 1.87 11.62 76 0.2 1.91 11.88 0.4 3.74 23.24 0.6 5.40 33.56 0.8 7.01 43.57 90 0.2 6.05 37.60 0 4 9.21 57.21 11.21 69.67 0 6 0.8 12 33 76.63 100 0.2 13 56 84 27 04 15.26 94.84 0.6 16 00 99 44 0.8 16 06 99 81 Data obtained using a sample of 0.2 gram, a total volume for distillation of 250 ml., and with 20-minute distillation. Complete recovery equals O9 per cent nitrogen.
S
it was proposed as a general procedure in Kjeldahl distillations by Kober in 1908 (6) and by Sebelien in 1909 (7). Dillingham in 1914 (1) and Falk and Sugiura in 1916 (8) demonstrated that complete recovery of the nitrogen by aeration alone was not practicable but that heat was also necessary; aeration alone, even when carried on for several hours# in the recovery Of less than 95 per cent Of the nitrogen. Although aeration with simultaneous heating has been used by many analysts with satisfactory results, no critical examination of the method seems to have been made. The authors of this paper failed to find any data in the literature on nitrogen recovery under various experimental conditions or any information as to the most desirable procedure. Accordingly a critical examination was carried through, some of the results of which are summarized in this communication.
It was found that increasing the dilution of the solution greatly slowed up the rate of ammonia distillation, With a solution temperature of 90" C. and an aeration rate of 0.8 liter per minute, the recovery for 15 minutes' distillation 100
PROCEDURE 80
The substance selected for analysis was N-butyrainide, procured from the Eastman Kodak Co., and found to give satisfactorily reproducible analyses from a large batch in close agreement with the theoretical nitrogen content of 16.09 per cent. A sample of about 0.2 gram was oxidized with 40 grams of concentrated sulfuric acid, to which were added 10 grams potassium sulfate and 0.5 gram of mercury. The digestion was continued for 30 minutes after the contents of the Kjeldahl flask became colorless. The ammonia distillation was carried out using the apparatus shown in Figure 1. By the application of suction a t D,air from outside the laboratory was drawn in through the flowmeter A and the inlet tube B, passed over together with steam and ammonia, bubbled through the standard acid solution, and drawn out through the guard tube C. The guard tube C contained glass beads without perforations, and through it the acid solution and the methyl orange indicator were introduced into the receiving flask. The beads, remain-
2 $60
8
&! E
A0
g ! I Z
M 20
0
0
0.2
0.4
AERATION RATE
0.6
0.8
1.0
L I T E R S P E R MIN.
OF NITROGEN RECOVERY TO FIGURE 2. RELATION AERATION RATE
63
ANALYTICAL EDITION
64
TABLE 11. NITROGENRECOVERYFOR VARIOUS TIMES OF DISTILLATION
100
ASRATION TIMBOF N I T R O G ~ N P E R CENT RATE DIHTILLATION RBCOVERED RECOVBRY L./min. Min. % 0.2 5 0.95 6.90 15 3.93 24.42 30 7.20 44.74 60 11.08 68.89 90 12.78 79.43 0.8 5 3.41 21.19 16 12.32 76.57 30 14.89 92.64 46 16.69 96.88 60 16.11 100 1.6 5 10.51 65.32 10 13.72 85.27 15 16.27 94.94 20 16.75 97.89 25 15.95 99.12 30 16.10 100 Data obtained using a sample of 0.2 gram a total volume of solution of 260 ml., and a distillation temperature of 90°'C. Complete recovery equals 16.09 Der cent nitrogen.
80
n LO
a" z g 40 C 0
k
z x LO
a
Vol. 6, No. 1
0
20
A0
TIMEDISTILLED
FIGURE 3.
ao
60
WIN.
VARIATION OF NITROGEN RECOVERY WITH TIME OF DISTILLATION
was 95.93, 79.86, and 58.67 per cent, from total volumes of 100, 300, and 500 ml., respectively. It was also found that it makes little' difference whether the inlet tube delivers the air in large bubbles or in small bubbles distributed more widely through the solution.
SUMMARY 1. The conditions favorable for carrying out Kjeldahl distillation using aeration are indicated. With fairly rapid aeration at the boiling point of the solution distillation is complete in less than 15 minutes. k, 2. I n addition to the saving of time involved, the use of
aeration is recommended because of the entire absence of bumping and the obviation of the danger of acid solution backing up into the condenser. The apparent disadvantage of more complex apparatus is actually very slight, the only additional requisites to that needed for an ordinary distillation being the guard tube and inlet tube, neither of which requires more than rinsing between distillations. LITERATURE CITED Dillingham, J. Am. Chem. Soe., 36, 1310 (1914). Falk and Sugiura, Ibid., 38, 916 (1916). Folin, 2.phusiol. Chem., 37, 161 (1903). Heinta and Ragsby, in Hoppe-Seyler's "Handbuch der physiol. und pathol. chem. Anal.," 1875. Kjeldahl, 2.anal. Chem., 22, 366 (1883). Kober, J. Am. Chem. Soc., 31, 1131 (1908). Sebelien, Chem.-Ztg., 33, 795 (1909). RECEIYED July 10, 1933.
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Determination of Lead as Dilead Hydrogen Arsenate C. L, DUNNAND H. V. TARTAR, Chemistry Department, University of Washington, Seattle, Wash.
A"
data afforded evidence that a still greater control of the I VESTIGATION in this laboratory (2) on the system IY Pb0-Asz06-Hz0 showed that dilead hydrogen hydrogen-ion concentration would be necessary to obtain arsenate, PbHAsOe, is very insoluble; the results indicated precipitates corresponding in composition to dilead hydrogen that its solubility is comparable to that of the chromate and arsenate. Further experiments showed that lead can be precipitated sulfate. Since this compound exists over such a wide range of acidity, the suggestion was made that lead might be deter- quantitatively from a solution of lead nitrate as dilead hydromined quantitatively by precipitation as the dilead hydrogen gen arsenate by the use of a disodium hydrogen arsenatearsenic acid buffer solution. The proper acidity necessary arsenate. The aim of this paper is to present in very brief form the for precipitation is pH 4.6-that is, just alkaline to methyl results of an attempt to devise a quantitative method for the orange. Any marked change in the acidity due to the determination of lead in commercial lead arsenate. The liberation of nitric acid will materially change the composimethod is not so adaptable as the chromate and sulfate tion of the precipitate obtained. The acidity can be kept methods for easy control and consequently it is not neces- sufficiently constant during the precipitation by the gradual addition of a dilute solution of sodium hydroxide using sary to give in detail the experimental results ( I ) . Preliminary experiments made on the precipitation of methyl orange indicator. The arsenate solution should lead as dilead hydrogen arsenate, by adding disodium ar- be subjected to mechanical stirring during the precipitation. Attempts were also made to precipitate as dilead hydrogen senate solution to a standard solution of lead nitrate, showed that the acidity of the solution had a marked influence on arsenate the lead in samples of commercial lead arsenate. The samples were dissolved in 0.5 N nitric acid. The results the composition of the precipitate. A series of determinations was next carried out using indicated that fairly accurate results can be obtained. The disodium hydrogen arsenate-arsenic acid buffer solutions of procedure is laborious and necessitates too close control to varying pH. Precipitation was accomplished by adding a rival the present accurate sulfate and chromate methods. definite quantity of the lead nitrate solution to varying LITERATURE CITED amounts of the arsenate buffers. All precipitates were (1) Dunn, C. L., M. A. thesis, University of Washington, 1932. collected and washed on asbestos pads in Gooch crucibles and dried a t 120" C. The pH of the solutions changed consider- (2) Tartar, Rice, and Sweo, J. Am. Chem. Soc., 53, 3949 (1931). ably during the precipitations, becoming more acid. The R E C ~ I V EOotober D 12, 1932. Relrubmitted September 26, 1933.
