ANALYTICAL EDITION
260
either aqueous or alcoholic caustic and again liberated in the presence of the Ranger naphtha. In this way, unsaturated hydrocarbons were removed. Phenols or cresols were, however, still present and, although partially corrected for by a blank, may have had some influence on the titration, accounting for the high result obtained on the lower mercaptans, which solution also contained the bulk of the phenols. All the above results are likewise shown in Table IV. It appears that the copper oleate method possesses decided advantages over previously published methods. Its applicability to all types of mercaptans has not yet been established, but since doctor tests a t the end of the various titrations have invariably shown the gasoline to be sweet, even in the case of Cross-cracked gasolines from Coastal crude oil, which would contain most of the mercaptans which should ordinarily be encountered, the method appears sufficiently accurate for normal plant control operation. It is planned to investigate the applicability of this method to the
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determination of disulfides by establishing a method of quantitative reduction to mercaptans. Further work will be undertaken as opportunity affords. ACKNOWLEDGMENT The writer is indebted to S. Comay of the Houdry Process Corporation for several valuable suggestions, and to Russell Lee of the Vacuum Oil Company, Inc., for a number of cooperative titrations in establishing the accuracy of the method between different operators. LITERATURE CITED (1) Borgstrom and Reid, IND. ENG.CHEM.,Anal. Ed., 1, 186 (1929). (2) Faragher, Morrell, and Monroe, IND. ENG.CHDM.,19,1281 (1927). (3) Klason, P., Ber., 20, 3412 (1887). (4) Sampey and Reid, J . Am. Chem. Soc., 54, 3404 (1932). (5) Youts and Perkins, IND. ENG.CHEX,,19, 1250 (1927).
RECEIVED February 7, 1933.
Selenium in the Determination of Phosphorus and Nitrogen in Phospholipides FLOYD ERVINKURTZ Research Laboratories, Bureau of Dairy Industry, United States Department of Agriculture, Washington, D. C.
S
INCE Lauro (2) published his method using selenium as the catalyst in Kjeldahl determinations of nitrogen, several papers have appeared confirming his results and extending their application to additional materials. In this laboratory the method has been applied successfully to the determination of nitrogen in phospholipides. As the digestion of phospholipides with selenium is carried out so smoothly, it was decided to see if this method could be used also in the analysis for phosphorus. It was found that the phosphorus values secured in this way were comparable with those obtained from a nitric acid-sulfuric acid digest. Since both nitrogen and phosphorus values are usually desired in an analysis of phospholipides, the possibility of making both determinations from the same digest was investigated. From Table I it is seen that the phosphorus can be determined satisfactorily from the residue of the nitrogen determination. The digestion, with selenium as the catalyst, takes place much more rapidly than if copper or mercury is used, and is more convenient than digestion with a sulfuric acid-nitric acid mixture. Time is saved in two ways: through the speed of the digestion with selenium, and through the determination of both nitrogen and phosphorus from the same digest. TABLE1. ANALYSISOF PHOSPHOLIPIDES I
PHOSPHORUS NITROGEN % %
HNOa-H&04 digestion
3:51 3.50 3.53
......
.. ..
1.63 1.64
3.54
CuSOa-Kjeldahl digestion Se-Kjeldahl digestion with nitrogen analysis preliminary t o phosphorus determination
..
3.51 3.51 3.51 3.53
1:63 1.65
..
The samples used for the above comparison were prepared by dissolving 8.6393 grams of commercial egg lecithin in carbon tetrachloride, diluting to 100 cc., and then pipetting off 10 cc. of the solution for each analysis. The sample for the selenium-Kjeldahl digestion was run ,into an 800-cc. Kjeldahl flask, 5 grams of potassium sulfate were added,
and the solvent was evaporated off on the steam bath. Twenty cubic centimeters of sulfuric acid and 0.2 gram of selenium were added. On digesting, a clear solution was usually obtained in about 15 minutes, but the heating was continued for an additional 15 to 20 minutes. After cooling the solution, the nitrogen was determined in the usual manner. The residue from this determination was filtered while still alkaline to remove the precipitated selenium. To the filtrate, after acidification, about 30 grams of ammonium nitrate were added. The additional amount of ammonium nitrate over that recommended by the Association of Official Agricultural Chemists (1) was used to counteract the inhibiting effect of the high concentration of sulfate ion on the precipitation of the phosphorus. The phosphorus was precipitated from this solution, first as ammonium phosphomolybdate, and then as magnesium ammonium phosphate. The samples used as a standard of comparison were digested with 10 cc. of sulfuric acid and successive additions of 3-cc. portions of nitric acid until the solution became clear, after which the heating was continued 15 to 20 minutes. The phosphorus was precipitated from the digest in the same manner as outlined above. The chief advantages offered by the determination of both nitrogen and phosphorus from the same digest are a saving of time and of materials. In research work on phospholipides, the progress of the purification of the material is often followed by determining the ratio of nitrogen to phosphorus. Not infrequently a rather small yield of the final pure product is secured. Under these circumstances the saving of the material being investigated would justify the use of this method of analysis. This method of determining bdth nitrogen and phosphorus from a selenium-Kjeldahl digest has been tried only with phospholipides. However, it is likely that its application can be extended to other materials. LITERaTURE CITED (1) Assoc. OfFicial Agr. Chem., “Official and Tentative Methods,” 3rd ed., p. 11 (1930). (2) Lauro, M. F., IND. Em+.CHEM.,Anal. Ed., 3, 401 (1931). RECEIVED March 15, 1933.
