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constant with a known pure solute. In addition, a drying agent should always be used. Anhydrous magnesium perchlorate has been found to be satisfactory for this purpose. Three possible causes for the high values of the bureau results have been mentioned. Of these, the presence of impurities and moisture in the solvent has been studied. These probably are contributing causes of the high molecular weight values previously noted. The lowest experimental value that could be obtained, with solvent A-4 (wet), is above almost all values reported by the cooperating laboratories. It is obvious that some other factor, presumably either the more effective stirring or the higher bath temperature, or both, is a contributing influence. The term “association” has been used by many writers to explain the increase in molecular weight with increase in concentrations of solute in solvent. However, the authors have found several cases of what, in the same terminology, shouId be called “dissociation.” Cyclohexane gives results of this type. It is difficult if not impossible to find a satisfactory conception of association or dissociation as applied to nonelectrolytes. However, if the slope of the molecular weightConcentration curve is assumed to be the result of abnormal solute characteristics rather than the result of some solvent peculiarities, then oils must be considered as mixtures of molecules of different sizes and kinds existing in some type of molecular aggregation even a t infinite dilution. This assumption permits a t least of a partial explanation of the concentration effect. From this point of view, although the term “molecular weight” has been used in this paper, it probably has little meaning as such. Rather, a variable is determined which is called “molecular weight” and which represents the state of molecular aggregation of the oil in the solvent a t the concentration in effect a t the time. Generally it is found that in dilute solution the value of this “molecular aggregate” changes uniformly with changes in concentration, and a value a t infinite dilution may be obtained by extrapolation. This is the value of the molecular aggregate a t infinite dilution in a given solvent, and might be termed the “apparent molecular weight in benzene, cyclohexane, etc.” Under ideal conditions such extrapolated points may coincide for the solvents under investigation, and this point may be called the most probable “molecular aggregate a t infinite dilution.” Whether the entire explanation of the slope of the molecular weight-concentration curve can justly be placed upon association or dissociation of the solute appears uncertain and remains for further research. It is hoped that the manner in which different oils behave in the same and different solvents will furnish information as to their possible type, methods of separation, and relation to other physical and chemical characteristics.
Acknowledgment The authors wish to express their appreciation to the laboratories who cooperated with them by providing data used in this report and for their suggestions and criticisms.
Literature Cited Adams, L. H., J. Am. Chem. Soc., 37, 481 (1915). Bridgeman, Soc. Automotive Engrs., 23, 4186 (1928). Chemical Rubber Co., “Handbook of Chemistry and Physics,” 17th ed., 1932. Devine, J. M., Okla. Acad. Sci. BUZZ.,9, 131 (1929). Eastman Kodak Co., Synthetic Organic Chemicals, Vol. 5, Oct., 1931. Epperson, E. R., and Dunlap, H. L., IND. ENO.CHEM.,24, 1369 (1932). Fenske, M. R., McCluer, mi. B., and Cannon, M. R., Ibid., 26, 976 (1934). Findlay, A., “Practical Physical Chemistry,” New York, Longmans, Green & Co., 1931.
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FitzSimons, O., and Thiele, E. W., IND. ENO.CHEM.. Anal. Ed., 7, 11 (1935). Getman, F. H., “Physical Chemistry,” New York, John Wiley & Sons. 1922. Gullick, N. G., J. Inst. Petroleum Tech., 17, 541 (1931). International Critical Tables, New York, McGraw-Hill Book Co., 1928. Kraus, C. 9., and Vingee, R . A, J. Am. Chem. Soc., 56, 511 (1934). Landolt-Bernstein, “Physikalisch-chernischeTabellen,” p. 1429, Berlin, Julius Springer, 1923. Menzies, W. C., and Wright, S. L., J . Am. Chem. Soc., 43, 2314 (1921). Normann, W., Chem.-Ztg., 31, 211 (1907). Rall, H. T . , and Smith, H. M., IND.ENG.CHEM.,Anal. Ed., 6, 373 (1934). Smith, H. M., et al., Bur. Mines Tech. Paper 477 (1930). Steed, A. H., J. Inst. Petroleum Tech., 16, 799 (1930). Wilson, R. E., and Wylde, E. P., IXD. ENQ.CREM..15, 801 (1923). RECEIVED June 27, 1936. Presented before the Division of Petroleum Chemistry at the 9lst Meeting of the American Chemical Society, Kansas City, Mo., April 13 to 17, 1936. Published by permission of the Direotor, U. 8 . Bureau of Mines. (Not subject to copyright.)
Nesslerization Applied Directly to a Macro-Kjeldahl Nitrogen Determination SERGIUS MORGULIS AND HOWARD C. SPENCER University of Nebraska College of Medicine, Omaha, Neb.
I
N STUDYING the nitrogen content of tissues the authors were confronted with the need of finding a simple procedure for accurately weighing small samples of wet material. They found that samples of ground fresh material, containing about 15 mg. of nitrogen, can best be weighed on tin foil. The sample is placed on a weighed piece of foil, which is quickly folded in such a way as to prevent any loss of moisture, and weighed. The weight of the sample is determined by difference. The wrapped material is dropped into a Kjeldahl flask containing 5 cc. of concentrated sulfuric acid, without possibility of loss at any stage of the operation. The digestion is carried out as usual with the aid of a few drops of superoxol. The tin foil serves also as a catalyst and the digestion proceeds rapidly to completion. A further simplification of the procedure consists in the substitution of direct nesslerization for the usual distillation. The clear, colorless digestion mixture is transferred quantitatively to a 250-cc. volumetric flask and diluted with water to the mark. The tin is precipitated upon dilution and may be either centrifuged off or allowed to settle out spontaneously. An aliquot of the clear solution, corresponding to 0.7 to 1.5 mg. of nitrogen, is measured into a 100-cc. volumetric flask, while a standard containing an appropriate amount of nitrogen (usually 1.0 mg.) and a similar concentration of sulfuric acid is prepared in another flask. The contents are diluted to about 60 cc. with distilled water, Nessler solution is added (30 cc. for 1 cc. of sulfuric acid) and made up to volume. The colors are matched in a colorimeter as usual. The authors have checked the accuracy of this procedure by comparing the results of the direct nesslerization with the distillation results, also by analyzing the nitrogen content of pure compounds (urea), and have found this procedure absolutely reliable. Although their primary objective was to devise a way of handling wet tissue, the method for nitrogen determination as here outlined has wider application and can be used successfully whenever it is desired to eliminate the distillation. The authors used selenized granules as catalyst in some digestions and found that the direct nesslerization could also be employed here. RBC~IYE April D 30, 1936.
