Microanalysis of Nitrogen in Certain Pyrimidines by the Dumas Method DOUGLASS F. HAYMAN AND SOL ADLER, Merck & Co., Inc., Rahway, N. J.
T
HE micro-Dumas method of Pregl (3) has an enviable
lyzed by the regular Pregl method gave identical results using the special method, showing that it was sound, It has been found difficultt o explainthecauseof failure of the Dumas method on these pyrimidines. There was considerable evidence that some nitrogenous compound was left behind, a fact that pointed to incomplete combustion as the most likely source of error. It was a common experience for a very low result to be followed by a higher one, occasionally higher than the theoretical. As a rule, the results on known compounds have run high, following a number of failures with the difficult types. The tube as well as the temporary copper oxide filling required a vigorous and lengthy burning before accurate results on known compounds were again obtained.
reputation for producing accurate results on a variety of substances. Properly handled, it was considered an accurate method for any organic nitrogen-containing compound. This method was used in this laboratory on a wide range of compounds without any known discrepancy until a certain series of pyrimidines of known purity failed to give the required amount of nitrogen. The nitrogen values obtained were from 0.4 to 2.0 per cent below theory and were reproducible under uniform treatment of the sample. The carbon and hydrogen values for these compounds agreed with the theoretical values within the limits of experimental error. Several papers have appeared recently in the literature in which references were made to the fact that there were certain types of compounds which could not be analyzed properly by the Dumas method. The nitrogen values obtained were low (1, d , ,$,6). The first step in studying this problem was to make the combustions in a quartz tube, employing the hottest flame obtainable from a Tirrill burner. Utilizing this high temperature, the results obtained for nitrogen present were higher but still were below theory. The addition of potassium chlorate to the samples, as advocated by Pregl, together with high heat showed no improvement over the method of burning a t a high temperature. Potassium dichromate was tried but with no better results. It was thought that the combustion of this particular type of compound must be abnormal or that the combustion was incomplete. Mercuric acetate, a slow-burning compound with a high decomposition point, was mixed with the sample to furnish the normal decomposition products over a longer period of time. The results obtained with this compound were very satisfactory. There was an appreciable amount of metallic mercury produced. Because it appeared that the operator might be harmfully affected by the mercury vapor, especially when burning out the temporary filling between analyses, etc., a trial was made with copper acetate, which also gave excellent results. The metallic copper remaining from the combustion of the copper acetate could not be harmful either to the operator or to the analysis.
TABLEI. TYPICAL ANALYSES Regular Micro-Dumas, Quartz Tube Special Compound and High Heat Method Theory 2-Thio-4-oxy-B-amino26.22 29.53 29.44 pyrimidine 28.28 29.69 27.61 4-Ethoxymethyl-6-methyl 16.04 16.86 16.65 6-oxypyrimidine 16.19 16.92 Vitamin Bt 16.25 16.53 16.62 16.12 16.69 16.075 16.00" 15.83a 2-4-Dimethoxy-5-bromo12.36 12. SO 12.79 pyrimidineb 11.74 12.89 10.710 11.330 Cytosineb 36.67 37.70 37. a4 32.13C 37.74 ~ ni(0 ii _. a Results taken from a paper by Wintersteiner, Williams and Ruehle (6). b Supplied through the courtesy of G. E. Hilbert, Burebu of Chemistry and Soils, U. S. Department of AgricGture. 0 Results taken from a paper by Milner and Sherman ( 8 ) .
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After the copper acetate had been used for some time the results were fairly reasonable if no more copper acetate was used, but the tendency still was for the results to be low so that it did not seem practical to dispense with the new method. Starting with a new combustion tube and an entirely new filling, results will always be low, as shown in the table. It was also found that if the sample was weighed in a copper boat and burned in the combustion tube in contact with the metallic copper, the results would be higher than normal but erratic. This method was not practical for the analysis of such compounds. It would seem that the copper might have some catalytic property that aids the normal combustion. It is difficult to make any generalizations as to what type of pyrimidine may give a low nitrogen by the regular Dumas method. Countless pyrimidines have been analyzed successfully, as a study of the literature will show. As more compounds are found which prove hard to analyze, some satisfactory correlation may be found.
A finely powdered Sam le weighing 1.5 to 3.0 mg. was transferred from the weighing gottle to the mixing bottle, a test tube 10 x 75 mm. About five times as much copper acetate was added to the sample, and the whole was thoroughly mixed by rotating the tube. This mixture was then covered with finely powdered copper oxide to a height of 30 mm., and the mixing tube was closed with a tightly fitting cork. In mixing with the copper oxide, the tube was never shaken, but was rotated at an angle of 15' above the horizontal so that some copper oxide was introduced into the original mixture of unknown and co per acetate. This mixture was transferred to the combustion u.t,! The first rinsing of the mixing bottle was made with about 6 mg. of copper acetate mixed with the usual amount of copper oxide powder. The second rinsing was made with copper oxide powder alone. The combustion was carried out by using a very hot movable flame, which was brought towards the sample very cautiously, as the large concentration of combustible material might result in Loo rapid combustion.
Literature Cited (1) Grewe, R., 2. physiol. Chem., 242,89 (1936). (2) Milner, R. T., and Sherman, M. S., IND. ENG.CHIM.,Anal. Ed., 8,331 (1936). (3) Pregl, F., Fyleman, E., "Quantitative Organio Mioroanalysirr," 2nd ed., Philadelphia, P. Blakiston's Son & Co., 1930. (4) Windaus, A., Tschesche, R., and Ruhkopf, H., Wachschr. Gsr. Wiss. Gattingen 111 Chemie, No.22,342 (1932). (5) Wintersteiner, O., Williams, R. R., and Ruehle, A. E.,J. Am. Chem. Sac., 57, 517 (1935).
Typical analyses before and after using the special method are given in Table I. Blank runs were made with the copper acetate, but only micro bubbles were produced. Compounds previously ana-
RECEIVED January 13, 1937.
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