the chromatogram, which was placed directly on the fluorescent support. This procedure made possible the detection of less than 5 y of 3,8dinitrobenzoates as dark spots. Traces of a derivative could be recognized as a dark shadow on the fluorescent support. RESULTS
Table I shows the R, values of a typical satisfactory separation of the 3,5dinitrobenzoates of some common primary, aliphatic alcohols obtained in a run of 35 cm. a t 25” C. according to the procedure given above. The R f values should not be taken as absolute. In a two-phase solvent system, as used here, with one polar phase impregnated on the paper and one less polar organic solvent as mobile phase, it is practically impossible to obtain completely reproducible R f values, as the amount of the stationary phase fixed on the paper may easily be variable, even if care is taken to impregnate the paper sheets as uniformly as possible. The humidity of the air during the preparation of the paper,. is also an
important factor, influencing strongly the R/ values of such two-phase systems, as stated by Neher and Wettstein (3). A mixture of the 3,5-dinitrobenzoates of I-pentanol and 1-hexene-3-01 showed only one spot on developing. Such is the case also with a mixture of l-hexene3-01 and 1-hexene-2-01. However, 1pentanol and 1-hexanol could be separated clearly and a mixture of l-hexene2-01 and hexanol also separated nicely into two well defined spots, in spite of the close R f values. All substances were tested with a quantity of 10 y each. A difference in R, value of about 50.06 seemed to be necessary to obtain a clear separation. ACKNOWLEDGMENT
The authom wish to thank R. Sauter and A. Saccardi for their valuable technical assistance. LITERATURE CITED
(1) Horner,. L., Kirmse,
B7., Ann. 597, 50 (1955). (2) Neigh, D. F., Nature 169, 706 (1952).
(3) Keher, R., Rettstein, A,, Hela. Chim. Acta 35, 276 (1952). (4) Rice, R. G., Keller, G. J., Kirchner, J. G., ANAL.CHEM.23, 194 (1951).
RECEIVED for review October 15, 1956. Accepted Dec. 26, 1956.
Determination of Vanadium in Titanium Tetrachloride and Titanium AlloysCorrection
In the article on “Determination of Vanadium in Titanium Tetrachloride and Titanium Alloys” [Owens, W. H., Norton, C. L., Curtis, J. A., ANAL. CHEx 29, 243 (1957)l t4e first sentence under Procedures in the third column should read: “Pipet a Zml. aliquot of titanium tetrachloride into a clean, dry 250-ml. beaker.” In Table IV, first column, “Control,” and “NiSOd 6H20” should both be dropped one line to correspond with the first figurw in the second and third columns. On page 244, second column, the third Line above Interfering Elements should read “from Table 111.”
Improved Dumas Method for Molecular Weight Determination HERBERT H. ANDERSON and LESTER D. SHUBIN’ Chemistry Department, Drexel lnstitufe of Technology, Philadelphia 4 , Pa.
b Careful constriction of the neck and tip of an improved bulb makes possible endless re-use of the same bulb for determining molecular weights of liquids boiling between 105” and 200” C. by the Dumas method. With adequate precautions, titration of the available acidity of compounds such as triethylgermanium bromide or acetic acid can replace weighing.
N
limitations beset the traditional Dumas method of determining the molecular weight of a liquid. Excessive time is required in the determination. Actual sealing of the bulb is difficult and often causes some decomposition of the compound; sealing can be hazardous with flammable compound. It is necessary to calibrate the bulb after sealing i t ; thus each determination requires a new bulb. Present ddress, Westinghouse Electric Corp., Lester, Pa. 852
UMEROUS
ANALYTICAL CHEMISTRY
Three main improvements have been developed on the Dumas method for molecular weights. Vaporization of a liquid of boiling point above 105” and under 200” C. under atmospheric pressure, followed by condensation and then weighing, in an unsealed bulb, yields the satisfactory molecular weight of 120.2 =t0.i for chlorobenzene. In an unsealed bulb, vaporization of a liquid-of boiling point above 105” and under 200 “-under atmospheric pressure, followed by condensation and then by titration of the available acidity, yields the satisfactory molecular weight of 249.8 rt 2.1 for triethylgermanium bromide. Carbon dioxide displaces air, and a molecular weight of 44.35 + 0.07 for carbon dioxide results. Molecular weights here are calculated from the perfect gas law, PV = wRT/M and are 1 to 7% higher than the corresponding formula weights because of absorption of the gaseous compound on the glass surface of the container and deviation from ideality. High values
persist in all gas methods using simple equipment, but disappear in the elaborate method of limiting density in a system with a counterpoise and a gas density balance. METHOD WITH UNSEALED BULB
The altered design of the Dumas bulb resembles equipment used in obtaining molecular weights of 149 and 151 for phosphorus(II1) dichloroisocyanate. which has the formula weight of 144 ( 2 ) . Constriction of the neck and tip of the bulb structurally minimizes diffusion of the gaseous compound out of the bulb into the room after all the liquid compound has vaporized and all the vapors have reached the temperature of the liquid bath. Immersion of the bulb in the hot liquid bath for only 2.5 to 3.0 minutes functionally minimizes diffusion of the gaseous compound out of the bulb into the room. In almost all instances the bulb may be re-used.
