Determination of Ammonia by a Diffusion ARTHUR N. PRATER, EDWARD J. COWLES, AND ROBERT P. STRAKA of Agrieultnral Chemistry and Engineering, Western Regional Research Laboratory, Bu-u U. S. Department of Agriculture, Albany, Calif.
R
ECENTLY it became necessary to run a large number of ammonia determinations in this laboratory. Of the methods available, the diffusion cell technique originally d e
wash water already adjusted t o the correct pH and observing any change in pH. At a DHof 5.0 to 5.1 the indicator has iust
scribed by Conway (6, 6) seemed the most satisfactory. Since there were no standard Conway diffusion units available, an analytical technique was developed, using apparatus and supplies which were readily obtainable. With a minimum of effort and expense, a large number of units were prepared for multiple determinations. While several modifications of the diffusion cell technique have already appeared in the literature (8,S), i t is believed that the one described in this paper is the simplest and moat economical. To demonstrate its reliability representative data are presented on the analysis of ammonium salt solutions, a meat extract, and a meat extract plus ammonium chloride solution.
--~ pH that accompanies thedilution of the glyceml-boric acid solutions. This change in pH is enhanced bv the presence of dycerol, ~I ~~
~~
~~
~
~~
~~-~~~
The .. boric acid-glycero!.so!ution wfs, prepared in O.?litefc&@iLies ana was convemenuy aispensen irom B aroppmg oocue wicn a glass pipet stopper.
Outline of Methods The diffusion unit consists of either the top or bottom of a Petri dish with a circular glass plate cover. -The ammonia or other volatile base is absorbed in drops of a saturated solution of boric acid in elvcerol sumended from the cover. After absomtion is corn&< the suspended drops are washed into a flask br beaker n u d i n r a r d , e i t h rlecrron.ktncully or by the uae 61 x i r able indicators. The use of Iwrie acid RS 2 n aliwrhent fur ammmin w a s lirnt rerommenilrd bv \Vinhler f / ? ) :xnd hns Since been advocated by many authors (Ijb,4,8,10-16): It histhe ad& tage of requirin only one standard solution for the complete determination. %oreover, in the present method, the viscous boric acid-glycerol solution need not he weighed or accurately measured volumetrically. The electrometric titrations were R carried out with a Beckman glass electrode OH-meter U S ~ micro ~~~~~~~
~
~~~~~
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~.~ ~
~~~~~~~
"
~
~~
(9). The data reported here were oht&ed by the~ele&ro&tric
FIGURB 1. ARRANGEMENT OF A P P ~ A T UDURINO. S ANALYSIS (10O-Xa. PETRIDISHES)
method and checked carefully by the indicator method. The Latter was found satisfactory and has the advantage of being faster but may he affected by personal error.
The wasb water, which was made up in a large Pyrex bottle, was acidified t o pH 5.0 to 5.1 with hydrochloric acid. If the determination was to he carried out with indicators, the methyl red-bromocresol green indicators were added to the wash water in equal proportions. Enou h indicator was added to produce a distinct color chance under t%e conditions of the exueriment. Using thr plycr;oJ solution and flir wash war& prepared as de>rnbrd, rhr method Lad 3 ZQTO blink value. Orrirsionally it was nrcrssary i o readjust t l ~ epR 01 the solution5 afrer standing. SrnndnrJired 0.01 .V hydrochloric acid m'uj used fur the titrat1on. For alkalizing agents t o liberate the volatile bases, both a solution saturated with sodium metaborate and potassium chloride and a 4 per cent sodium hydroxide solution have been used. The choice of alkaliaiue aeents is eoverned hv the character of the material being anahrzcd. Stro& caustic should be avoided when
Apparatus and Reagents covers and.._ bottoms Petri dishes,of two sizes were used, .-The .. -~ ., . " of *~ ~
AN
~~
this operation requires only a few minutes per d&h. Resistanceglass Petri dishes with ground rims are available from some supply houses. The circular glass plates were cut from window glass, about 1 cm. in diameter larger than the dishes used. Similar circular glass plates may he iurchased. The saturated solution of boric acid in glycerol was made up in advance at room temperature and decanted from the excess boric acid. This solution contains about 28 grams of boric acid per 100 pams of glycerol. To eliminate the necessity of running blank determinations the boric acid-glycerol solution was carefully acidified, so that when mixed with the wash water in the approximate proportions prevailing at the end of the titration, the mixture remained at a pH of 5.0 to 5.1, the end point of the titration. The final volume may be varied in accordance with the amounts of wash water and titration acid used. In most of this work a final volume of 60 ml. was used. Because of the effect of dilution and the fact that the pH of the color change of the indicator in glycerol solution is not the same as in an aqueous solution, the indicator could not be added directly to the glycerol solution. Instead it was necessary to follow the course of the acidification by dropping the boric acid-glycerol solution into the
tory use bv Davis (7). A 25-gram muscle samole and 100 grams
duced to a creamy suspension. A portion of this was used for the analysis directly, while another portion was clarified by centrifuging before anrtlysis. If the sample wss used without clarification. care had to be exercised to prevent SBIIipling errors due t o air bubbles. 703
704
Vol. 14, No. 9
INDUSTRIAL AND ENGINEERING CHEMISTRY
of a solution saturated with sodium metaborate and potassium chloride as the alkalizing agent. After 3 hours the absorption was complete. Curve 1 represents a clear meat extract, while curve 2 represents the same extract containing suspended meat particles before clarification. These two curves show that all of the volatile base was quickly evolved and that no further change occurred on longer standing. Samples which were allowed to stand 24 hours showed no change in the titration value. Curve 3 represents 5 ml. of the same clear meat extract plus 5 ml. of 4 per cent sodium hydroxide solution, and indicates the effects of strong caustic in causing a continual slow evolution of volatile base due to reactions with the meat extract, Curve 4 represents 5 ml. of a standard ammonium chloride solution plus 5 ml. of the boratepotassium chloride solution. Actual recovery was 0.598 mg. of ammonia nitrogen, whereas the calculated requirement for the solution used was 0.602 mg. Curve 5 reuresents 5 ml. of the standard amFIGURE 2. ABSORPTION OF AMMONIANITROGEIV AT VARYING TIMEINTERVALS monium chlo'ide plus ml* Of the 'Iarified meat extract, plus 5 ml. of the borate mixture. 1. Clarified meat extract plus sodium metaborate plus otassium chloride Actual recovery was 0.728 mg. of ammonia y 2. Same as 1 except t h a t meat extract was not clarified f ~ centrifugation 3. Clarified meat extract lus sodium hydroxide aolution nitrogen, whereas addition of the titers of the 4. Standard ammonium cEloride solution plus sodium metaborate plus potassium chloride solutions gives 0.734 mg. In this case the ab5. Clarified meat extract plus standard ammonium chloride solution alus sodium metasorution urocess was slowed down because the borate plus potasaium chloride vohme in' the absorption cell waq 15 ml. Curves 4 and 5 demonstrate the reliability of the method, since both a pure ammonium salt solution and an Analytical Procedure ammonium salt added to a meat extract were recovered quantiThe rims of the Petri dishes were coated with vaseline. Aliquot tatively. For reference the calculated values of 0.602 mg. and samples of the solution to be analyzed were i etted into the 0.734 mg. have been indicated in Figure 2 along the scale on the dishes. The size of the sample and the size o f t i e dish were so left side. chosen that the final liquid volume in the dish just covered the bottom. Ordinarily in a 100-mm. dish, a total volume of 10 ml. was used. Larger volumes required longer absorption time. The glycerol-boric acid solution was then carefully dropped on to a TABLE I. ANALYSES OF A STANDARD AMMONIUMSULFATE cover plate, each drop spaced so that no two drops would coalesce. SOLUTION With the 100-mm. Petri dishes 13drops were used, while with the Ammonia Nitrogen 0 . 0 1 N Acid Ammonia Nitrogen 50-mm. dishes 5 drops were used. The cover plate was then Taken Found Found quickly inverted, the alkalizing agent added, and the cover MQ. MQ, MI. placed over the dish. If the solution under analysis and the 0.246 1.76 0.247 alkalizing agent were added carefully to the dish, it was possible 1.78 0.249 to avoid mixing the two solutions until after the cover was in 1.76 0.247 0.249 1.78 place; then the unit was rocked gently to ensure thorough mix1.77 0.248 ing. Figure 1 shows the arrangement of the lycerol-boric acid Av. 0 . 2 4 8 drops during an analysis in which a 100-mm. fish was used. KO difficulty was experienced in inverting the plate carryin the glycerol drops. On long standing the drops absorb water an! become more fluid, but even after 48 hours it was possible to invert the plate into a funnel without any loss. The units were allowed to stand for 3 hours a t room temperature. For the effect of temperature, rocking the absorption cell, 1.640 1.64 11.71 etc., the reader is referred to a recent summary (6) on the sub1.636 11.68 ject. 1.647 11.76 After the absorption was complete the cover plate was care1.639 11.70 1.643 11.73 fully removed and held vertically over a glass funnel in a 125-ml. 1.632 11.65 flask or beaker. The plate and funnel were thoroughly washed .4v. 1 . 6 4 with the re ared wash water and the solution was titrated with 23,47 3.288 3.29 the stanzrcfacid. It was not found necessary to boil the wash 23.42 3.281 water or the final titration mixture. 23.45 3.285 23.36 23.39
Experimental Results Representative data are given in Table I and are presented graphically in Figure 2 to demonstrate the precision and accuracy of the method. Table I contains the results of replicate analyses of varying amounts of a n ammonium sulfate solution. I n each case, 5 ml. of a saturated solution of sodium metaborate and potassium chloride were used as the alkalizing agent; the final volume in the cell was 10 ml.; and the dishes were allowed to stand for 3 hours at 25" C. I n Figure 2 each curve represents the results of an experiment in which seven dishes were allowed to stand at room temperature (25" C.) for varying time intervals before titration of the evolved volatile bases.
Av. 8.21
Av.
The method has been used successfully for amounts of ammonia nitrogen varying from 0.1 to 9 mg. By using 150mm. Petri dishes, still larger amounts of ammonia can be accommodated.
Literature Cited Adler, L., 2.ges. Brauw., 39, 162 (1916). (2) Allen, W. F., IND.ENG.CHEM.,ANAL.ED.,3, 239 (1931). (3) Bandemer, S. L., and Sohaible, P. J., Ibid., 8, 201 (1936). (1)
Curves 1 and 2 are practically identical and represent the volatile base evolved from 5 ml. of a meat extract by the use of 5 ml.
58.36 58.44 58.14 58.30 58.50
3,272 3.276 3.28 8.175 8.186 8.144 8.167 8.195 8.17
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).
705
(12) Menefee, S. G., and Overman, 0. R., J . Dairy Sci., 23, 1177 (1940). EXQ.CHEM.,12, 350 (13) Scales, F. M.,and Harrison, A. P., J. IND. (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. In 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. In 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, an 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
Adjust-
4
2
Slow
1.401 S
Rapid
1.401 S a Ta'
EndPoint ment
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.
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,
