ANALYTICAL EDITION
September 15, 1942
(4) Bernard, E., Z . angew. Chem., 27, 664 (1914) ; Landw. Vers. Sta., 86, 331 (1915). (5) Conway, E. J., Biochem. J . , 27, 430 (1933). (6) Conway, E. J., "Micro-Diffusion Analysis and Volumetric Error'', New York, D . Van Nostrand Co., 1940. (7) Davis, W. B., IND.ENG.CHEM.,NEWSED., 17, 762 (1939). (8) Eisner, A., and E. C. Wagner, IND.ENG.CHEM..AKAL.ED., 6, 473 (1934). (9) Hahnel, S., Suensk. Kem. Tids.,47, 4 (1935). (10) Markley, K. S., and Hann, R. M., J . Assoc. Oficial Agr. Chem., 8, 455 (1925). (11) Meeker, E. W., and Wagner, E. C., IND.EKQ. CHEM.,ANAL. ED. 5, 396 (1933).
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(12) Menefee, S. G., and Overman, 0. R., J . Dairy Sci., 23, 1177 (1940). (13) Scales, F. M.,and Harrison, A. P., J. IND. EXQ.CHEM.,12, 350 (1920). (14) Spears, H. D., J . Assoc. Oficial Agr. Chem. 5, 105 (1921). (16) Stover, N. M., and Sandin, R. B., ISD.ENQ.CHEM.,ANAL.ED., 3, 240 (1931). (16) Wanner. E. C.. Ibid.. 12. 771 (1940). (17j Winkler', L. W.', Z. angeb. C h e i . , 26, 231 (1913); 27, 630 (1914); 28, 48 (1915). OUTSIDEPublication No. 3639, Bureau of Agricultural Chemistry and Engineering, U. S. Department of Agriculture.
Kjeldahl Distillation without Absorbing Acid JAMES A. BRADLEY Newark College of Engineering, Newark, N. J.
I
7S T H E Kjeldahl method for the determination of nitrogen two procedures for running the distillation are in common use. I n the older method the ammonia is distilled over into a known quantity of absorbing acid and is measured by titrating the excess acid with a standard base. I n 1913 Winkler ('7) suggested that the ammonia be absorbed in a solution of boric acid and titrated directly with standard hydrochloric or sulfuric acid. This modification of the method has been studied extensively in recent years (1, 2, 6) and widely used. It eliminates the need for a standard solution of a base and simplifies the calculation of the analysis, but requires an additional solution in the form of a standard indicator solution by which the end point is matched and adjusted. Furthermore, the attainment of the end point is slow. If this additional solution can be eliminated and the time of the titration decreased without affecting the accuracy of the procedure, the Kjeldahl distillation will be further improved. I n the method described in this paper all absorbing acids are dispensed with. The ammonia is distilled directly into pure water and titrated with a standard acid, using methyl red or methyl orange as an indicator, and taking for the end point the first tinge of pink color. I n order to prevent the loss of ammonia from the distillate during the analysis, the operation is run in a closed still equipped with a safety balloon to relieve the pressure. The method has been compared with the usual Kjeldahl procedure in which an excess of absorbing acid is used and seems to give on the average a slightly higher degree of accuracy. This would be expected, since, as has been pointed out by Miller (4), ammonia is lost during the first few minutes of distillation. This loss would be prerented by running the distillation in a closed system. The proposed method decreases the time required for an analysis and calls for only one standard solution, a n acid. The end point is attained easily and quickly, and the calculation is simplified. From the point of view of time and motion study the efficiency of the three procedures may be compared as follows :
Boric acid method
Solutions Keeded Including Indicators
Sohtions Measured
EndPoint Adjustment
4
2
Slow
1.401 S
Rapid
1.401 S a
Calculation, yo N
2
1
W
Validity of the Procedure It may be asked whether, from a theoretical point of view, one might expect the proposed method to give reliable results. It is reasonable to assume that in a closed system all the ammonia can be distilled over into the receiver. But what, if any, of the distilled ammonia is lost from the absorbing water by passing into the gas phase in the receiver? For more concentrated solutions there are data on vapor pressure which would give this information. For the dilute solutions handled in the Kjeldahl analysis, however, i t was not possible to find reliable data concerning the partial pressure of the ammonia in the gas phase. It was possible, nevertheless, by the analysis of the work of Meldrum, Melampy, and Myers (3) on the aeration of ammonium hydroxide to get a fair idea as to what error, if any, might be expected from the loss of ammonia from a dilute solution to an inert gas in contact with it. The authors showed in this paper that the removal of ammonia from a dilute solution depended on the aeration rate, the temperature, and the concentration of the solution. They found that the rate was not affected by the size of the air bubble, a conclusion confirmed by the work of Shabilin, Krylov, and Obarin ( 5 ) . Such an aeration would result in the partition of ammonia between the liquid and gas phases comparable to the equilibrium attained in a closed system containing gas in contact with a dilute ammonia solution. It can be shown mathematically that in such an aeration the fraction of ammonia remaining in solution is given by the equation 2.303 log
'G
=
- KC
where c = the original ammonia concentration in moles per liter c - z = the final concentration u = the volume of the air used in aeration K = aconstant
a
W
Proposed methou
where iv, and Nb are normalities of acid and base, V , and Vb are volumes of acid and base in cc., and w is the weight of the sample. This comparison shows that the proposed method is more efficient than either of the two older methods in time, materials, and operation.
Ta '
Using the data of Meldrum, Melampy, and Myers for 25" C., the values for K were found to be 0.0057, 0.0043, 0.0034,
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INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
and 0.0040. This shows a fair agreement and gives an average value of 0.0043. In the work described in the present paper the average distillation resulted in about 300 cc. of solution in a closed receiver in contact with about 400 cc. of air. If this system were in equilibrium, it could be shown, using the constant obtained above, that only 0.5 per cent of the ammonia would be found in the gas phase. This would be, of course, an extreme, limiting case, impossible under the conditions of the distillation. The actual results show that the deviation from 100 per cent ammonia recovery is considerably less than this (as would be expected, since equilibrium is not attained) and that the process is essentially equal in reliability, and perhaps slightly better than the usual Kjeldahl method in which an absorbing acid is used. From the experimental point of view, furthermore, the va-’ lidity of the process is confirmed not only by the analytical results, but also by the observation, made repeatedly, that the distillate gave no odor of ammonia.
Procedure I n this work the interest was centered entirely in the distillation, not in the Kjeldahl method as a whole. The problem was to determine how much of the ammonia in the distilling flask was recovered in the receiver and measured in the titration. I n order to simplify the work and to avoid possible errors of weighing and calibration, the same buret was used both for introducing into the distilling flask a known quantity of ammonia and for making the final titration. Throughout the work one standard solution only was used, a sulfuric acid solution of normality 0.1996. The distillation was carried out in a closed system. This was made by modifying the usual Kjeldahl still by attaching the receiving flask by a one-hole rubber stopper to the lower end of the condenser. The receiving flask was a side-arm suction flask. To the side arm of this flask was fitted a small rubber balloon to relieve the pressure set up in the receiver as the distillation proceeds. The receiver was arranged so that it could be lowered during the last few minutes of distillation to permit drainage. A representative run might be described as follows: A definite volume of standard acid was delivered by buret to a beaker, 100 cc. of distilled water and 2 drops of methyl orange solution were added, and the resulting solution was titrated carefully to the end point with freshly distilled ammonium hydroxide solution having a concentration of about 0.2 N . This prepared solution which contained an accurately known quantity of ammonium sulfate was quantitatively transferred to the distillation flask. One hundred cubic centimeters of distilled water were then added t o the receiving flask, enough to cover the lower end of the condenser’s inner tube when the still was assembled. Enough water was added to the distilling flask to make a total volume of about 400 cc., 25 cc. of 10 per cent potassium hydroxide were also introduced, the assembly was closed, and distillation was begun. To ensure smooth boiling, fine capillary tubes, sealed at one end, were used in the distilling flask with their open ends down. After about 250 cc. of distillate were collected, the receiving flask was disconnected and lowered to the desk and the distillation was continued for a few minutes longer. This was to allow for drainage, but it is doubtful whether this part of the procedure is necessary. The receiving flask was then taken to the titration table, a few drops of methyl orange were added, and the contents were titrated with the original standard acid, using the same buret that was used initially in making up the ammonium sulfate solution for the distillation. The experimental results are given below. Four series of distillations were run. In Series A the usual Kjeldah1 procedure was followed, the receiver being open to the air, and the ammonia being absorbed in an excess of standard acid. In Series B absorbing acid was also used, but the distillation wa5 carried out in the closed still with the safety balloon. In Series C no absorbing acid was used. The ammonia was absorbed in pure water. For the sake of completeness, another series was run, Series D, in which an open still was used, but the ammonia was absorbed in pure water. The amounts of ammonia used in these distillations ran from about 70 t o about 200 mg.
Series Still used Absorbent used N o . of runs Ammonia recovered, % Maximum Minimum Average
B Closed Acid
A
Open Acid 21
C
Closed Water
4
100.0 99.4 99.7
12
100.0 99.4 99.8
100.0 99.9 99.9
D Open Water 7
99.4 98.3 98.7
The proposed method (Series C) gives slightly better results than the usual Kjeldahl procedure (Series A). Series D, as might be expected gave no useful results. When the closed still is used with the regular Kjeldahl procedure (Series B), the results are definitely improved. This can be explained by reference to the work of Miller (4) who found that a loss of ammonia occurred at the beginning of the distillation when it comes over so rapidly that some of it escapes absorption by the acid. An incidental study was made of the effect of the excess of the absorbing acid on the accuracy of the results. I n Series A some runs were made using 100 per cent excess acid; others using 4 per cent excess; others using no excess. As will be seen from the following table, the excess of acid seemed to have no effect on the results of the distillation: COMPOSITE SERIES
Excess acid used, % No. of runs Ammonia recovered, Maximum Minimum Average
A,
OPEN STILL, .4BsORBENT . h I D USED
4 3
0 11
99.9 99.9 99.9
100.0 99.2 99.7
100
7
1oo.n 99.4 99.7
Certain objections to the method naturally present themselves. First, will the pressure developed in the receiver blow out the balloon or a stopper? I n none of the distillations did this happen. The pressure seemed never to be greater than 20 to 25 em. (8 or 10 inches) of water. Second, will water condense in the balloon and absorb ammonia? The inside of the balloon was never observed t o be wet. If such a condition were found, however, it would be a simple matter to discard the balloon and fit the apparatus with a new one.
Summary The Kjeldahl distillation has been modified in such a way
as to dispense with the absorbing acid, thus decreasing the time and materials required for the process and simplifying the manipulations and calculations. The distillation is carried out in a closed still, the pressure being relieved by a small rubber balloon attached to the receiver. The ammonia is determined by titration directly with a standard acid. The results of this modified method have been found to be slightly better than those obtained in the usual Kjeldahl distillation in which an absorbing acid was used. In the regular Kjeldahl distillation, using absorbing acid, different excesses of acid were found to have no effect on the accuracy of the process.
(1)
Literature Cited Eisner, Abner, and Wagner, E. C., IND. ENG.CHEX.,AXAL.ED.,
(2) (3)
Meeker, E. W., and Wagner, E. C., Ibid., 5, 396 (1933). Meldrum, W. B., Melampy, R., and Myers, W. D., Ibid., 6 , 63
(4) (.5.)
Miller, Ibid., 8, 50 (1936). Shabilin. Krulov, and Obarin. J . Chem. I d .
6, 473 (1934). (1934).
( r ,S. S . R , ) . 16. No. 1, 10-14 (1939). (6) Stover, N. M., and Sandin, R. B., ISD. E N O .CHEM.,AXAL.ED., 3, 240 (1931). (7) Winkler, 2. angeu. Chem.. 26, 231 (1913).
