V O L U M E 2 3 , NO. 3, M A R C H 1 9 5 1 (21) Kuck, J., College of the City of New York, and American Cyansmid Co., Stamford, Conn., report during discussion. (22) Lamb, A. B., Bray, W.C., and Geldard, W., J . A m . Chem. Soc., 42,1636-48 (1920). (23) Lundqvist. D., Acta Chem. Scand., 2, 177 (1948). (24) Maylott, A. O., and Lewis, J. B., Ax.4~.CHEM., 22, 1051-4 (1950). (25) Mellor, J. W.,“Comprehensive Treatise on Inorganic and Theoretical Chemistry,” 5-01, 2, p. 294, London, Longmans, Green and Co., 1922. 12fi lleyer, Hans, “Analyse and Konstitutionsermittlung Organischer Verbindungen,” 6th ed., p. 252, Vienna, Julius Springer, 1938. (27 1 Meyer, H. H., M i t t . Kaisei.-WilheZm Inst. Eisenforsch. Diisseld O T f , 9, 273-7 (1927). (28) Orning, A. A., Carnegie Institute of Technology, Pittsburgh, Pa., report during discussion.
533 (29) Schutse, M., 2. anal. Chem., 118,241-5 (1939). (30) Ibid.,pp. 245-58. (31) Shepherd, ANAL.CHEM.,19,77 (1947). (32) Smith, W.H., Organic Section, X’ational Bureau of Standards Washington, D. C., report during discussion. (33) Untersauchcr, J., Ber., 73B, 391-404 (1940). (34) Untereaucher, J., Chemie Ing. Tech., 22, KO.2, 39-40 (1950). (35) Walker, A. P., and Patrick, K.H., J . I n d . Eng. Chem., 4, 799801 (1912). (36) Walton, W.TI‘., McCulloch, I-. IT-., and Smith, I\-. H., J . Research S a t l . RZU.Standards, 40,443-7 (1948). (37) JJ-elton, W.M., and Drake, N. L., IKD.EXG.CHEM.,ANAL.ED., 1, 21 (1929). (38) Zimmermann, W., 2 . anal. Chem., 118,258-63 (1939). RECEIVED January 19, 1951.
Apparatus for Microdetermination of Molecular Weights by the Vapor Density Method JOHN J . HAWKINS
AND
PAUL ARTHUR, Oklahoma A . and M . College, Stillwater, Okla.
7kIE; method of detrimining molecular weights described by Siederl and Niederl ( 1 ) proved accurate for structural work done in the authors’ laboratory on low boiling substances. For high boiling liquids, hoJ\ ever, they found it necessary to design their own apparatus, because no commercial equipment of this type was available. T h e apparatus developed works equally well for liquids of high and low boiling point and has the added adyantage that the techniques involved in its use are much simpler thaii those required for other such apparatus. A determinationneighing the sample, assembling and filling the apparatus, and 1n:rLing the run itself-requires only 10 minutes.
itself being 1 to 3 cm. long. The sample is introduced into the weighed container by warming the bulb, dipping the capillary tip into the liquid, and cooling to draw liquid into the bulb. The bulb is set upright, shaken or centrifuged, then reweighed. Extremely volatile liquids are sealed in by touching a flame momentarily to the capillary tip, which is broken off just before the bulb is placed in the apparatus. The sample tube is then dropped through B , and joint A is assembled and clamped with the usual spring clamp. The stopcock, H , which is lubricated sparingly (preferably with Dow-Corning silicone grease), remains open during the filling operation. The mercury dispenser, F , is next filled and cleared of all air bv flushing mercury through it into a clean beaker. The dispenser is then attached to the vaporizer unit at joint E. Stopcock J is opened slowly and mercury is allowed to flow slowly into the vaporizer unit.
APPARATUS AND PROCEDURE
The principle is the same as that used by Niederl-namely, the determination of the volume of vapor obtained from a weighed sample of the unknown, by means of the weight of mercury it displaces. The apparatus (see illustration) consists essentially of three parts: tube G, the constant-temperature bath; thc vaporizer, A , B, C, D ; and the mercury dispenser, F , which is used in filling the vaporizer with mercury. G is constructed from a 35 X 300 mm. borosilicate glass test tube by sealing on one side tube to admit a thermometer and a second side tube for use in attaching a reflux condenser. A threenecked round-bott,omed flask has been used with equal success. Dispenser F is constructed from a borosilicate glass thistle tube fuwd to a stopcock, J , to the other side of which is attached a 12,;2 standard socket of 2-mm. capillary bore. It is essential th:it this last be no larger in diameter, if mercury spillage is to be avoided. The vaporizer tube, C, is made from a 15 X 125 mm. borosilirste glass test tube. The flattening of its top and the offset position of inlet tube B are essential to prevent the lifting of the sample tube into B and the trapping of air in C when the appiiratus is being filled with mercury. Tube D is a capillary of not more than 2-mm. bore; B is 100 mm. long and must be as near 2-mm. bore as possible, in order just to admit the sample tube. The bore must be no larger, as the mercury retained within it during the vaporization of the sample prevents vapor from reaching the ball joint, A . The top o f D must be a little higher than A ; otherwise, when the mercury dispenser is removed after the apparatus has been fillrd with mercury, air may leak through A and alloJy the mercury level in B to fall. The preparation of the mercury and the n-eighing of the sample in a capillary tube container small enough to pass through B are essentially as described by Niederl. The sample bulblets are made of ordinary melting point capillary. One end is sealed and drawn to a slender rod about 3 cm. long t o serve as handle; the other end is drawn to a fine capillary 4 to 5 cm. long, the bulb
TVERMOMETER WELL
REFLUX CONDENSER
CONNECTION
If joint E has not been greased adequately, air will be sucked into the tube intermittently, making the filling difficult: homever, if this condition is detected early enough, the filling can proceed normally after the joint has been regreased. As the mercury rises in C the weighing capillary usually moves over to the side wall and adheres there, but occasionally it may be necessary to ensure its entrapment a t the flattened top by tilting the apparatus slightly-. When the mercury has risen past stopcock H , the latter is closed. Stopcock J is then closed, the dispenser is disconnected, and a weighed vial is placed a t E to collect the expelled mercury.
ANALYTICAL CHEMISTRY
534 Table I.
Compounds Suitable for Vapor Bath B.P., O C. 100 110 142 174 197 223 262 302 323 340
Compounds Water Toluene Isoamyl acetate ichlorobenzene thylene glycol Methyl salioylate Isoamyl benzoate Diphen lamine Benzyl genzoste n-Butyl phthalate
P
cedure. A suction-flask assembly that connects with ball joint E via a socket joint is clamped into place. Suction is applied, stopcock H is opened, and the mercury in the vaporizer unit is pulled into the suction flask. The constant-temperature bath is operated during this process so that the sample remains vaporized and may be evacuated as the mercury is being drawn off. By successively opening and closing stopcock H, it is possible to flush the vapors of the sample from the vaporizer quantitatively. If need be, a solvent can be flushed through the vaporizer by introduction through joint A . The old weighing capillary may then be removed and the apparatus is ready for the next run.
Table 11. Performance Data Sample Scetone Benzene Toluene Chlorobenzene Ethyl benzoate Aniline
B.P., C. 56.5 80.1 110.8 132.0 212.6 184.4
RESULTS
hloleculrr Weight Theoretical Experimental 58 58 78 79 92 95 113 114 150 153 93 91
After the apparatus is assembled and checked, a liquid of suitable boiling point (see Table I ) is placed in G and heated to boilmg. This boiling is continued until no more mercury is expelled. If the tem erature tends to climb slowly, a final temperature may be ariitrarily chosen, provided the flame is removed immediately after the reading is taken. When the final temperature is noted, the height of the vertical column of mercury as measured from the meniscus in the vaporizer to the level of the orifice of the capillary outlet a t E i also recorded. The usual corrections for the expansion of the mercury and the air error are made by means of a blank run, using an em ty weighing capillary. It has been the practice to run a b l a d for each bath liquid used. The average correction degree change in temperature can be calculated from one suc run and applied in the way described by Niederl. Other corrections also are handied as in Niederl’s method. After completion of the run, the apparatus may be quickly cleaned out and made ready for the next run by the following pro-
The excellent performance of this apparatus is illustrated by the results in Table 11,which were obtained by a senior chemistry student who had had no previous experience with this apparatus. These results were obtained on samples of 10 to 15 mg. weighed on a good analytical balance. Although the use of a microbalance might have given closer results in some cases, those given here are typical of the many determinations run. The capacity of the apparatus described is such that larger samples might readily be used. With water as the liquid used in the vapor bath, an 11-mg. sample of acetone displaced only one third of the available capacity. Because the primary utility of such an apparatus as thia is in the straightforward and rapid determination of molecular weights, the vapor bath has been employed throughout. A liquid bath can be easily substituted, if the boiling and condensation points of a liquid are desired. LITERATURE CITED
( 1 ) Niederl and Yiederl, “Organic Microquantitative Anslysis,” 2nd ed., New York. John Wiley & Sons, 1942. RECEIVED April 10,1950.
Quantitative Determination of Bromine in Terminal Bromodinitromethyl Groups ICARL KLAGER, Aerojet Engineering Corp., Asusa, Calif.
HE study of compounds with terminal dinitromethyl groups rmade it desirable to find a facile laboratory method for their characterization. Compounds with terminal dinitromethyl groups are readily brominated in basic solution to form the corre-
sponding bromodinitro derivative. Because the bromodinitro derivatives are easily prepared and purified, halogen analysis of these compounds seemed to offer a convenient method for the quantitative determination of terminal dinitromethyl groups.
NO2 Table I. Determination of Bromine Compound 1
Br-b-CHzCHzCOOCHa
hI 02
% Dinitromethyl
3%.
7’ Br Ftund
29.49
29.58 29.92
38.75
38.88 39.33
31.10
31.06 31.29
40.87
40.83 41.12
37.17
36.85
48.86
48.43
60.57
60.19 59.89
...
...
40.16
40.11 40.08
52.79
52.72 52.68
Group Calcd. Found
NO? BP-&-CHZCHZCOOH
I
NOz NOn
3 Br-L-CHzOH
I
NO2
yo2 4 Br-6-Br
I
NO2 NO1 5
CHd-Br LO2
I R-C-H-RI
NO*
(OH) -
lo”
Br,
~
I
NO,
NO*
I + R-C-Br I
NO2
Two methods of halogen analysis appeared worthy of consideration: treatment of the compound with an alkaline reagent, followed by titration of the ionic halogen with silver nitrate; and treatment of the compound with potassium iodide in the manner reported by IMeisenheimer (1) for 2-bromo-2,Zdinitro-lethoxyethane, followed by titration of the free iodine with sodium thiosulfate. The first method was found unsatisfactory. In order to obtain the halogen in ionic form, it was necessary to treat the compound with an alkaline reagent in an organic solvent. Colored solutions resulted from this operation, which obscured the end point of the silver nitrate titration. The reaction of compounds with terminal bromodinitromethyl groups and potassium iodide proceeded quantitatively. For every compound investigated, the amount of iodine formed corresponded to the theoretical amount of bromine present in the molecule according to the following equation:
