Kjeldahl Determination of Nitrogen Elimination of the Distillation KALMAN MARCALI AND WILLIAM RIEMAN 111, School of Chemistry, Rutgers University, N e w Brunswick, N. J. Store in an alkali-resistant glass container protected from the atmosphere by an Ascarite tube. Concentrated Sulfuric Acid, c.P.,specific gravity 1.84. Potassium Sulfate Crystals, Baker's C.P. Methyl Red, 0.004 M*(O.l%). Dissolve 0.25 gram of indicator in 250 ml. of alcohol. Phenolphthalein, 0.03 M (1%). Dissolve 2.5 grams of indicator in 250 ml. of alcohol. Seutral Formaldehyde Solution, approximately 18%. Dilute 45 ml. of Merck's reagent-grade formaldehyde (36 to 3870) with an equal volume of water. Add 2 drops of 0.03 M phenolphthalein and neutralize with 0.1 N sodium hydroxide to the first detectable pink color. This solution should be prepared just before it is used. Sodium Bromide Solution, 60%. Dissolve 150 grams of Merck's reagent chemical in mater and dilute to 250 ml. Mercuric Oxide, reagent grade. Eleven organic compounds were purified by repeated crystallization till a constant sharp melting point was obtained. The compounds were dried to constant weight in vacuo in an Abderhalden dryer containing phosphorus pentoxide in the bulb.
A n acidimetric macroprocedure is described for determining nitrogen without a distillation. The ammonium ion i s titrated with standard sodium hydroxide in the presence of formaldehyde.
A
RTMANN and Skrabal (1) and Rupp and Rossler (7) have shown that ammonia can be accurately determined by oxidation to nitrogen in alkaline solution by means of standard hypobromite solution, the excess of the latter being determined by adding potassium iodide and acid and titrating the liberated iodine with thiosulfate. Willard and Cake (8) and more recently Haanappl (3) have used this method to eliminate the distillation in the Kjeldahl determination. This procedure has one serious disadvantage: The alkaline hypobromite solution employed to oxidize the ammonia must be kept between 0" and j o tsince it is unstable a t room temperature, Ammonia reacts rapidly with formaldehyde to form hexamethylenetetramine, which has very weakly basic properties (6) (K6 = 8.0 X 10-lo). Therefore ammonium salts may be sharply titrated in the presence of formaldehyde with sodium hydroxide and phenolphthalein indicator. Kolthoff (6) has found that the reaction between ammonia and an excess of formaldehyde goes so rapidly that the titration may be made directlyLe., without excess of alkali. This reaction has been employed in the present modification of the Kjeldahl procedure. The proposed method consists essentially of three. steps: (1) The usual destruction of the organic matter by oxidation with hot concentrated sulfuric acid in the presence of potassium sulfate and a mercury catalyst. (2) Neutralization of the excess sulfuric acid with approximately 10 N alkali to the methyl red end point. Sodium bromide is present during the neutralization to form a complex and thus prevent the precipitation of mercury compounds. Hg++
+ 4Br-
+HgBr4--
(3) Titration of the ammonium salt with standard 0.1 N sodium hydroxide to the phenolphthalein end point in the presence of formaldehyde. (XH&SO, 4s"
+ 2NaOH --+Na2SOI + 2H20 + 2KH3 + 6CH20 * (CH2)sNd + 6H20 REAGENTS
Sodium Hydroxide Solution, 50%. Dissolve 600 grams of sodium hydroxide (Baker's C.P. pellets, 0.004% silica) in 600 ml. of water. Let the solution stand several days in a paraffin-lined rubber-stoppered bottle and decant the clear supernatant liquid. Standard Sodium Hydroxide Solution, 0.1 N . Dilute approximately 8 grams of concentrated (500/0) carbonatefree sodium hydroxide solution to 1 liter with equilibrium water. Standardize a g a i n s t potassium biphthalate. Store Compound in an alkali-resistant glass container protected from the acid atmosphere with an Ascarite BenBamide tube. p-Chloroaniline Sodium Hydroxide Solution, approximately 10 N . Dilute p-Dimethylaminobenzaldehyde about 800 grams of concen~sum-Diphenylurea ,"~~$~~Pae,ine trated (5070) carbonate-free Sulfanilic acid sodium hvdroxide solution to 1 liter with equilibrium water.
izz$$
PROCEDURE
1r7eigh accurately a sample that will yield approximately 4 milliequivalents of nitrogen and transfer directly into a 500-ml. 29/42 standard-taper round-bottomed Pyrex flask. This flask is fitted with a 20-cm. (8-inch) removable neck and serves for both digestion and titration. Add 10 grams of anhydrous potassium sulfate, 0.6 to 0.7 gram of mercuric oxide, and 15 ml. of concentrated sulfuric acid. Put the neck of the flask in place, shake the mix-ture, and heat it in a fume chamber below the boiling point until frothing ceases. Then increase the heat so that the solution boils gently. If necessary, replace the sulfuric acid that has boiled off, but take care that the quantity of acid in the flask a t the end of the digestion does not exceed 15 ml. Continue the heating with sulfuric acid until the solution turns colorless and then 20 minutes longer. Cool the flask to a t least 50°, carefully add 50 ml. of water, and swirl the flask until the solid material has dissolved completely. Add 10 ml. of 60% sodium bromide, then 2 drops of methyl red. Neutralize the excess sulfuric acid with 10 N sodium hydroxide till the methyl red begins to change color. Now boil the solution gently for 3 minutes to expel carbon dioxide, cool to room temperature, and add 10 N alkali dropwise till the solution is just yellow. Next add N sulfuric acid dropwise till the pink color is restored. Then add the standard 0.1 Ai sodium hydroxide from a buret to the methyl red end point. Read the buret. Add 30 ml. of 18% formaldehyde solution. A t this point, the solution may become slightly pink. Disregard this and continue the titration till the solution is yellow again. Add 8 drops of phenolphthalein and complete the titration to the first distinct pink color. The alkali used between the methyl red and phenolphthalein end points is equivalent to the nitrogen present. When more than 17 ml. of concentrated sulfuric acid must be . neutralized before titration of the ammonium salt, sufficient silica is introduced as an impurity in the sodium hydroxide to buffer the solution a t the phenolphthalein end point and thus prevent a sharp color change. Thcrcfore it is suggested that no more
Table
1.
Determination of Nitrogen Recommended Procedure
No. of
l%e& 10.36 10.22 11.57 10.97 11.57 66.64 9.40 8.28 6.04 13.21
determinations 2 2 2 3 4 6 2 3 2 3
8.08
4
2-09
%N
mean 10.30 10.24 11.56 10.92 11.53 66.51 9.34 8.23 6.02 13.17 8.07
Mean deviation 0.03 0.02 0.04 0.00 0.01 0.01 0.01 0.00 0.01 0.06 0.02
Standard Procedure No. of deter%N Mean minations mean deviation 2 10.33 0.01 *. .. .. ..6 ..4
..
... ... ...
G6:54
...
8.23 ... ... ...
... ... ... ... 0.02 ... 0.04 ... ... ...
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
710
than approximately 15 ml. of sulfuric acid be present a t the end of the digestion. Determine and apply a blank correction, which is usually between 0.20 and 0.30 ml. The results of analyses of eleven organic compounds are indicated in Table I. DISCUSSION
As shown in Table I, the described method yields results with m average error of -0.04% nitrogen. Even this small difference may be due in part to failure to obtain perfectly pure organic compounds. The results compare very favorably with those of the standard Kjeldahl method with compounds of both high and Low nitrogen content. The disadvantage of the method is the fact that elements such as calcium, barium, copper, and iron interfere by forming precipitates which obscure the end point. Phosphorus also interferes because primary phosphate is titrated to secondary phosphate between the two end points. This renders the procedure inapplicable t o fertilizers and many biological samples. The method is applicable to samples containing organic and nitrate nitrogen when the usual sulfuric-salicylic acid modification is a p plied previous to digestion ( 2 , d ) .
Determination
OF
Vol. 18, No. 11
With the e k w t i o n of the distillation, approximately 20 in each determination* Work is in progress t o adapt the described procedure to the micro scale.
minutea
ACKNOWLEDGMENT
The authors are grateful to the Research Council of Rutgers UILiversity for financial aid in the investigation and to D. DeLapp of the American Cyanamid Company, Stamford, Conn., for a sample used in the analysis. LITERATURE CITED (1) Artmann, R., and Skrabal, A., 2. anal C h m . , 45,s (1907). (2) Aaaoc. Official Agr. Chem., “Official and Tentative Methods of Analysis”, 5th ed., p. 27,1940.
(3) Haanappl, T. A. G., Pharm. Weekblad, 75, 510 (1938). (4) Hillebrand, W. F., and Lundell, G . E. F., “Applied Inorganic Analysis”, p. 639,New York, John Wiley & Sons, 1929. ( 5 ) Kolthoff, I. M., Pharm. Weekblad, 58, 1463 (1921). (6) Kolthoff, I. M., Menrel, I. H., and Furman, N. H., “Volumetric Analysis”. Vol. 11, p. 163,New York, John Wiley & SOM,1929. (7) Rupp, E.,and Rossler, E., Arch. Pharm., 243, 104 (1905). (8) Willard, H. H., and Cake, W. E., J. Am. Chem. Soc., 42, 2646 (1920).
PaaeiNTEn before the Division of Analytical and Micro Chemiatry at the
CHEMICAL N SOCIETY,Atlantic City, N. J. 109th Meeting of the A M ~ R I C A
Carbon and Hydrogen by Combustion
Unitized Dual Apparatus and Improved Procedure D. D. TUNNICLIFF, EDWARD D. PETERS, LOUIS LYKKEN, AND
F. D. TUEMMLER, Shell Development Company,
Emeryville, Calif.
A dual unitized combustion apparatus and improved procedure are described for the determination of carbon and hydrogen in macrosize samples. The dual feature, absence of rubber connections, provisions for adequate combustion control, and well-defined procedure all combine to give accurate results in the minimum of time. Other noteworthy features are control and indication of all gas flow rates, adequate and convenient pyrometer temperature indication, addition of extra oxygen between the sample and catalyst, use of carefully determined combustion and operating condi-
tions, and a method for accurate analyses of volatile samples. Using the long, precision procedure, a skilled operator can make two determinations in an 8-hour day with a precision of *0.008~0hydrogen and *0.009% carbon and a probable accuracy of 0.011’%0 hydrogen and 0.015% carbon. Using the short, routine procedure, one experienced operator can make as many as eight determinations daily with a precision of *0.02% hydrogen and *0.05% carbon and with a probable accuracy of 0.05% for each element. Both procedures are applicable in presence of sulfur, halogens, nitrogen.
M
taneous combustion of two samples, by providing convenient and adequate control of the cambustion, and by carefully determining the minimum time required for each phase in the analysis. The conventional combustion method and apparatus for determination of carbon and hydrogen are not generally suitable for industrial use where speed is a consideration, and where analysts may be frequently replaced, and for applications where the greatest precision and accuracy are important. Using the simple apparatus and procedure given in the numerous references on the subject, one usually encounters the following difficulties: (1) loss of much time and effort in establishing the essential conditions, (2) nonsuitability of the apparatus for continuous use, (3) inadequate indications and control of flow rates, (4) inadequate control of furnace temperature, (5) lack of assurance of excess oxygen in combustion tube, (6) errors caused by use of rubber connections, (7) inadequacy of the combustiotl tube filling for materials Containing elements other than carbon, hydrogen, and oxygen, (8)requirement of 4 or more hours per analysis, (9) lack of qualitative evidence of complete oxidation of the carbon and hydrogen, and (10) lack of suitable technique for handling volatile samples. The method described here is a result of a study made to overcome or minimize these difficulties. The combustion apparatus was unitized in a compact assembly to increase the convenience of use and to allow standardization of technique and procedure; and since it waa found that two determinations could easily be made simultaneously, a dual unit
OST of the progress made in recent years in the determination of carbon and hydrogen has been in the field of microchemistry. Excellent methods and apparatus (1, 2, 6-9) have been developed which possess the desired attributes of speed, convenience, and accuracy when properly applied, but which too frequently produce results of only moderate accuracy when used on a routine basis (8). In this, as in many other laboratories, there has been a defhite need for a method which is accurate, precise, and rapid under normal industrial operating conditions. Because macromethods are less susceptible to variations in technique and are thus more suitable for industrial service analysis, it was decided that further development of the macromethod, along lines followed by the microchemists, offered the best possibilities of success. Wagman and Rossini (IO) have obtained very precise and accurate results on benzoic acid using large samples (1.8 grams) and improved techniques and apparatus developed a t the National Bureau of Standards. I n their work on the atomic weight of carbon, Baxter and Hale ( I ) also obtained excellent results using various hydrocarbons and even larger amounts (3 grams) of sample. These investigators had for their prime objective the greatest possible accuracy without regard for time required for analysis. In the present method, accuracy and precision have been achieved by adopting features of the Baxter and Hale a p paratus particularly suitable for routine work; speed has been secured by providing a dual apparatua which permita the rimul-
