Determination of Nitrogen in Pyridine Ring-Type Compounds by the Kjeldahl Method R A Y L. SHIRLEY
AND
W. W. BECKER,
Hercules Experiment Station, Hercules Powder Company, Wilmington, Del.
I
S THIS laboratory nitrogen must be determined by the
Kjeldahl method on many types of research samples. The necessary modifications of methods for determining nitro, nitrate, azo, and other nitrogen groups are well known and reasonably satisfactory. The voluminous literature on the Kjeldahl method, however, is conflicting with respect to the correct procedure for the analyris of such compounds as pyridine, nicotinic acid, nicotine, and quinoline, which contain a refractory ring-type nitrogen. Bradstrwt (d), in an excellent review of the Kjeldahl method, mentioned several examples of cyclic nitrogen compounds, such as pyridine, which are extraordinarily resistant to oxidation, but included no modifications of the method to obtain quantitative results. Acrec (f), in a collaborative report of the Association of Official Agricultural Chemists on the analyses of a large number of compounds, using mercuric oxide as the digestion catalyst in a seminiicromcthod, obtained only a 25% recovery of the nitrogen in nicotinic acid. However, some other workers apparently were able to analyze this type of compound successfully. Belcher and Godbert ( 3 ) used a mixture of mercuric sulfate and selenium as a catalyst and obtained good results on a micro scale for nitrogen in picolinic acid and nicotinic acid. Clark ( 5 ) used mercuric oxide and obtained on a semimicro scale the theoretical nitrogen yield on y, y-dipyridyl dihydrate. Phelps and Daudt (8) ohtained satisfactory results for nitrogen on nicotinic acid, pyridine-zinc chloride, and quinoline derivatives, using mercuric oxide as a catalyst. Need arose in this laboratory for an exact procedure to determine, on a macro scale, the nitrogen content of compounds containing refractory ring-type nitrogen. Therefore, a brief study was made of digestion catalysts and time of digestion. The catalysts tested were copper sulfate, mercury, selenium oxychloride, and a mixture of mercury and selenium oxychloride. The time of digestion was varied from 1 to 4 hours, Numerous workers, among them Taylor (fl), Prince (Q), Osborn and Krasnitz ( 7 ) , and Sreenivasan ( I O ) , used the mercury plus selenium oxychloride mixture in their work, but unfortunately did not report results on refractory ring-type nitrogen compounds,
digestion, the stopper was removed, and the neck of the fl:i.k was rinsed with an additional 10 ml. of sulfuric acid; t h w t i i t . catalyst and potassium suifate were added. DISCUSSION
The effectiveness of the various digestion catalysts was teAteil on caffeine, quinoline, and nicotine; a digestion time of approximately 2.5 hours was used. Caffeine was chosen as a control; its nitrogen, though present in the pyrimidine:iminazole form, i j easily converted to ammonium sulfate during the Kjeldahl dips. tion. The results obtained (Table I) show that both copper sulfate and selenium oxychloride, while effective on caffeine, gave nitrogen results ranging from approximately 45 to 85% of thcory on both quinoline and nicotine. Mercury alone and the mixture of mercury plus selenium oxychloride were much more eff ective digestion catalysts; the results obtained on all three compounds were approximately 99 to 100% of theory. Since preliminary results on nicotinic acid and pyridine werci erratic, these two compounds were selected for studying thc: effect of the time of digestion. T h e results obtained using t h e four catalysts and varying digestion time are shown in Table 11. Both copper sulfate and selenium oxychloride gave low recoveries of the nitrogen present, and, therefore, appeared unsatisfactory as digestion catalysts for this type of compound. I n t h e analysis of nicotinic acid, using mercury as the catalyst, a digestion time of 4 hours yielded practically theoretical results for. nitrogen; using mercury plus selenium oxychloride, a digestion time of 3 hours gave equally good results. K i t h pyridine, both mercury and mercury plus selenium oxychloride gave essentially the same results, approximately 98 to 99% of theory, after digesting for 3 hours; only a negligible increase was obtained after digesting for 4 hours. The time of digestion therefore proved to be of utmost importance. Usually, a Kjeldahl sample is digested for only 1.5 times the time required for the sulfuric acid solution to become clear.
PROCEDURE
The Kjeldahl method used was essentially the same as that of the ;Issociation of Official Agricultural Chemists ( 2 ) . The analyses were done on a modern commercial Kjeldahl nitrogen apparatus : 500-watt heaters were used for digesting the samples, and the necks of the digestion flasks were set at 30" angles. The amounts of catalysts used were: 1.0 gram of copper sulfate, 0.12 to 0.15 gram (5 drops) of selenium oxychloride, and 0.6 gram of metallic mercury. Samples of the organic compound used werc 0.3 to 1.0 gram. In titrating the excess acid with standard sodium hydroxide, methyl red-bromocresol green (6) was used as the indicator. This combination indicator was made up for macrotitrations and contained 0.4 gram of methyl red and 0.2 gram of bromocresol green in 100 ml. of 95% ethyl alcohol. It has been found satisfactory.
Table
I. Analyses of Nicotine, Quinoline, and Caffeine Using Various Catalysts Compound0
Catalyat
Found
% Nicotine (theory 1 7 . 2 8 % N)
cuso4 SeOClt Hg Hg
uinoline (theory 1 0 . 8 5 % N)
+ SeOCh
cuso4 SeOClz
COMPOUNDS TESTED
He
The pyridine used was J. T. Baker's C.P. grade. I t s specific gravity at 20/4" C. was 0.9835, and its refractive index at 20" C. was 1.5088; the International Critical Tables give the values a2 0.9832 and 1.5092 respectively. The other samples used were obtained from the hastman Kodak Company, and were analyzed without further purification. Special precautions were taken in weighing the volatile pyridine. The Kjeldahl flask was rotated while 20 ml. of sulfuric acid were poured down the neck. B y means of a Lunge pipet, approximately 1 gram was weighed directly into the flask, which was stoppered immediately. When the sample was ready for
IIg
Caffeine
(theory 28.8552 N)
+ SeOCl?
CUSO, SeOClz Hg Hg f SeOCll
Eacb sample digested 2.5 hours.
437
13.63 12,44 14.61 14.27 17.02 16.98 16.98 17.03 7.96 7.75 6.43 5.30 10.75 IO, 78 10.88 10.87 28.74 28.55 28.51 28.71 28.65
28.58 28.50 28.46
Iiitrogen Recover: 7c
78 0 72 0 84 6 82.6 98.5 98.3 98.3 98.5 73.4 71 4 59 3 47.9 99.1 99.4
:8i : 99 6 99.0 98.8 99.5 99.3 99.1 98.8 98.7
Vol. 17, No. 7
INDUSTRIAL AND ENGINEERING CHEMISTRY
438
CONCLUSIONS Table
11.
Digestion-Time Study of Nicotinic A c i d and Pyridine Analyses Sitrogen Found after Digesting for: 1 2 2 5 3 0 4 0 Compound Catalyat hour hours hours hours hours
Nicotinic acid
(theory 11.38R N:
% .. .. .. ..
CUSO, SCOCIZ Hg
Hg f SeOClz
%
4.33 5.59 8.91 8.41
..
P!-iidine CUSO, (theory 17,72% S ) SeOClz Hg Hg
+ SeOCIz
S
5.73 5.36 3.90 4.08 10.78 1O.li 10.18 10.91 5.49
. . . . . . . . 4.56 . . . . . . . . 14.40 .. 13.49 . . 16.72 . . 17.38
o 5.62
5.05 5.19 4.48 10.83 10.21 10.77 10.97
...
. . ... . . ... ... ...
...
%
%
6.36 6.68 6.34 6.34 11.14 10.00 11.31 11.37 7.49 5.55 7.58 5.13 17.50 17.46 17 46 17.44
,, ,
., ,. .., ,
,
11.33 11.35
Mercury alone or mercury plus selenium oxychloride has been found a satisfactory catalyst for the Kjeldahl determination of the nitrogen content of compounds containing a refractory ringtype nitrogen, as in pyridine, nicotine, nicotinic acid, or quinoline. A digeation-time study showed that 3 to 4 hour3 are required for complete digestion of these compounds by the mercury catalysts. Copper sulfate and selenium oxychloride yielded extremely low resulti, even on prolonged dige-tion
...
...
ACKNOWLEDGMENT
..
The authors wish to thank Beatrice Weaver for perforniing many of the analyses reported in this article.
.. ..
...
17.66 17.69 17.48 17.52
In the case of the mercury plus selenium oxychloride catalytic mixture, the solution became clear in a short time, yet additional digeation for 2 to 3 hours was required. Because the time of digestion ordinarily is not watched too closely, it is believed that the erratic results obtained in the past on pyridine ring-type compounds (even when mercury was used as the catalyst) were probably caused by insufficient digestion time. n'hen running unknown samples, it is advisable to run a known sample, whose structure approximates that of the sample being analyzed, in order to establish the digestion time required.
LITERATURE CITED
Acree, Fred, J . Assoc. Oficial Agr. C h e w , 24, 648-51 (1941). k 3 S O C . Official Agr. Chem., Official and Tentative Methods of Analysis, 5th ed., p. 25, 1940. Belcher, R., and Godbert, A. L., J . SOC.Chem. I n d . , 60, 196-3 (1941).
Bradstreet, R. B., Chem. Reviews, 27, 331-50 (1940). Clark, E. P., J . Assoc. Oficial Agr. Chem., 24, 641-7 (1941). Ma, T. S., and Zuazaga, G., ISD. Eso. CHEM.,- 4 s ~ED., ~ .14, 280-2 (1942).
Osborn, R. A., and Krasnita, -i.,J. Issoc. Oficia2 Bgr. Chem., 17, 339-42 (1934).
Phelm. I. K.. and Daudt. H. W.. Ibid.. 3. 218-20 (1919). Prince. A. L.. Ibid.. 17. 246-50 (19341'. Sreenivasan, 'A., and Sadasivan, V., IND.ENG.CHEJf., . h 4 L . ED.,11, 314-15 (1939). Taylor, L. V., Ibid., 5, 263 (1933). .
Colorimetric Method for Determination of EUGENE L. BAILES
A
AND
MERLE G. PAYNE,
I
DDT
Colorado Agricultural Experiment Station, Fort Collins,
RAPID culvrimetric method for the determination of
l-trichloro-2,2-bi~(p-clilorophenyl)ethane has been developed, using the Friedel-Crafts reaction. This compound, principal ingredient of technical DDT, can be determined in concentrations ranging from 0.001 t o 0.01%. The use of DDT as an insccticide has created an interest in methods for its detection and quantitative determination in such fields as spray residues and pharmacology. The high chloriiie content of D D T has been utilized as a basis for its indirect chemical analysis. Winter (7) has described a method for determining halogens in organic compounds, by burning the compound in a stream of gas and recovering the halogen in a form suitable for determination by standard methods. Hall et al. (3)have employed a modification of the Winter method for the determination of DDT in emulsions and various other materials. Fahey (1) also uses a modification of the Winter method. Gunther ( 2 ) and Neal (4)report a method for determining DDT based upon dehydrohalogenation with alcoholic alkali; a chloride ion is liberated from each molecule of DDT and the quantity of free chloride ion is determined. Schechter (6) reports that a sensitive color test based on nitration to polynitro derivatives will be described in a later paper. The method herein described is a colorimetric procedure using the Friedel-Crafts reaction. The reaction produces a compound with a stable color, orange by transmitted light and greenishorange by reflected light. This colored compound is being investigated and will be described in a later paper. APPARATUS
The Lumetron photoelectric colorimeter, model 400, made by the Photovolt Corporation was used in this study. This colorim-
Colo.
eter has an optical density and per cent trailmission scale and when Beer's law applies the scale readings are proportional to the concentration. Measurements were made in an absorption tube of 1 6 . k n m . inside diameter, with a color filter transmitting about 420 millimicrons. Nissen and Peterson ( 5 ) have given a general discussion of the method; and problems of colorimetric studieq. REAGENTS
Purified DDT, prepared in this laboratory and purified by recrystallization with a n alcohol-ether mixture (25% ether) until it melted a t a constant temperature of 107" C. Technical D D T , Geigy Company product, softening a t 80" C. and completely melting at 96.5" C. Benzene. The benzene used in the procedure was Merck's reagent, thiophene-free. Freezing point minimum, 5.2' C. Anhydrous aluminum chloride, Eimer and -4mend's technical and resublimed product. Ethylene chloride, Eastman 132. PROCEDURE
Purified DDT (0.1 gram) was dissolved and made up to 50 ml. with benzene, and 10 ml. of this solution were heated in a constant-temperature water bath at 66' C. for 5 minutes. Then the solution was treated with 0.5 gram of anhydrous aluminum chloride, and the mixture was heated for 1 hour at 66' C. The complex was decomposed with 3 ml. of water and 30 ml. of benzene were added. This product was treated with 1.5 grams of anhydrous calcium chloride. The mixture was allowed to stand until the turbidity disappeared. The benzene layer was decanted into a 100-ml. volumetric flask. The residue was washed with small portions of benzene to remove the last traces of color, added to the volumetric flask, and diluted to volume with benzene. The per cent transmission was read in the photoelectric colorimeter. If the benzene layer was turbid, it was necessary to allow it to stand before reading. Thi. turbidity often occurred in fruit strippings.
.
