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. I n 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.” I n 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. I n 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. I n almost all instances the bulb may be re-used.
Table 1.
Molecular Weights
Formula.
Compound (CzH&GeBr (C2H,)3GeOCOCHs ( CH&Si( OCOCF& Clz
co2
CeHsCa CsHsC1 c&(cH~h(0) C&OC& CHsCOOH (234' C.) CHICOOH (213' C.) CHSCOOH (193' C.) CHsCOOH (182' C.) CH3COOH (165.5' C.)
Wt. 239.7 218.8 284.2 70.92 44.01 92.13 112.56 106.16 108.13 60.05 60.05 60.05 60.05 00.05
Mol. Wt., Weighing 242.8, 246.0, 246.0 231.3, 224.2, 225.8, 232.0 289.5, 291.9, 292.7 73.2, 73.2, 73.4 44.42, 44.25, 44.39 92.9, 94.5, 93.7, 9 5 . 1 119.6, 120.1, 119.6, 121.5 113.1,110.2,111.3,111.0 113.0,113.5, 115.4, 114.4
Description of Bulbs. All the niodified bulbs used for moderate-boiling liquids-boiling point above 105' and under 200O-have flat bottoms and range in size from 2.7 t o 100 ml.; obseired operational difficulties render a bulb of approximately 1.5 ml. the sniallest ordinarily useful. Figure 1 shons the 4.511-ml. bulb used with acetic acid, which has the following special dimensions: upper neck tapering from 2.0 mm. t o 1.0 mm. in inside diameter a t the tip a t A ; two tiny glass "ears" a t C, for fastening platinum mire beneath, with a loop for holding. A 2.700-ml. bulb, for organometallic compounds, resembles the 4.511-ml. bulb in general proportions. A 24.32-m1. bulb, used with neutral organic compounds, has a comparable construction, with upper neck tapering from 3.5 to 1.5 mm. in inside diameter a t the tip.
...... ......
......
...... ......
Average Mol. Wt., Titration Average 244.8 f 1 . 6 2 5 0 . 9 , 2 5 2 . 4 , 2 4 4 . 9 , 2 5 1 . 3249.5 , 249.8 & 2 . 1 228.3 =k 3 . 3 219.5, 220.1, 222.8, 218.1 220.1 f 1 . 3 291.4 f 1 . 1 ...... 7 3 . 3 f 0 . 1 71.6, 72.4, 72.4, 7 2 . 3 72.2.i' 0 . 3 44.35 & 0.07 ...... ..... 94.0 f 0 . 7 ...... ..... 120.2 f 0 . 7 ...... ..... 111.4 f 0 . 9 ...... ..... 114.1 f 0 . 8 ...... ..... 62.0, 61.6, 62.2, 61.6 61.8'2 0.3 63.9, 63.7, 64.7, 6 3 . 3 ..... 6 3 . 9 j=0 . 4 ..... 67.5, 66.8, 66.6, 66.6 66.9 & 0.3 ..... 69.9, 70.0, 69.6, 6 9 . 9 69.9 iz 0 . 1 ..... 74.5, 7 4 . 7 , 7 4 . 7 , 7 5 . 1 74.8 + 0.2
erol, if previously used in the hot liquid bath. Remove the rubber tubing and glass rod; dry the bulb and then reweigh. An alternative procedure is to remove the bulb from the liquid bath, without any rubber tubing, instantly place the tip of an index finger over the tip of the bulb, then cool the bulb in 1%-ater,dry, and reweigh. Calculate the molecular weight from the equation PV
wRT M
=---
Titration. The method suggested for obtaining molecular weights of acidic or basic compounds by titration and without weighing is a n extension of the earlier method with a selffilling micropipet and a liquid ( 1 , 3).
MODERATE-BOILING LIQUIDS
Weighing. Complete raporization of the liquid compound requires a liquid bath temperature 50" above the boiling point of the compound, such as toluene, chlorobenzene, o-xylene, anisole, triethylgermanium bromide ( 7 ) . trietliylgermanium acetate (4), or dimethylhi!: - (trifluoroacetoxy)silane ( 5 ) . Equilibrium conditions require approximately 2.5 minutes of immersion of the bulb in the liquid bath, while longer immersion promotes secondary gaseous diffusion, with low results. PROCEDURE. Introduce the liquid into the weighed bulb, using a syringe and a transfer micropipet constructed from borosilicate glass tubing 4 mm. in outside diameter-for the compounds in Table I take 0.6 ml. for the 24.32-m1. bulb, 0.3 ml. for the 4.511-ml. bulb, and 0.15 ml. for the 2.700-ml. bulb. Place a 19-nim. length of hemocytometer rubber tubing on the neck to permit deep immersion to the glass ears. Immerse bulb in bath a t constant temperature for 2.5 to 3.0 minutes; next insert a glass rod into the rubber tubing to close; then withdraw the bulb and cool the bulb rapidly with cold water to condense the compound. Wash the outside of the bulb completely; it is easy to remove a writer-soluble compound such as glyc-
e 12mmd Figure 1. Dumas bulb A. Tip 5. Narrowing C.
Ears
PROCEDURE. Follow the method of weighing for moderate-boiling liquids, but instead of weighing the condensed liquid, first introduce 0.1 ml. of phenolphthalein solution with a micropipet of extremely small diameter, and then slowly titrate the available acidity, using 0.0300M sodium hydroxide in ethyl alcohol in a 10-ml. buret with a special adapter. This adapter, made from borosilicate glass tubing 4 mm. in outside diameter, has the following dimensions: 70-mm. length; 10 mm. of tubing 4 mm. in outside diameter, flared a t the end; 50-mm. length of 0.6-mm. average outside diameter and 0.25-mm. average inside diameter. A 14-mm. length of hemocytometer rubber tubing connects the glass adapter and the delivery tip
of the 10-ml. buret. Rotate the glass bulb extremely slowly and carefully with the fingers during the slow titration. Table I lists the temperatures of the liquid bath used with acetic acid, which has considerable association a t temperatures only slightly above its boiling point of 118". These molecular weights for acetic acid are in general agreement with values obtained by Johnson and Nash (6). PERMANENT GASES
Weighing (Titration). Carbon dioxide and chlorine both displace air satisfactorily. Relatively simple apparatus serres for the determination of the molecular weight of carbon dioxide (well suited for a n undergraduate experiment in physical chemistry). Although toxic, chlorine offers the possibility of titration in addition to weighing.
PROCEDURE. Fill a 125-m1. Erlenmeyer flask almost completely with powdered solid carbon dioxide; insert a one-hole rubber stopper through which a long borosilicate glass tube 4 mm. in outside diameter fits flush. This tube has tn.0 90' bends, with constriction on one side to an average of thin-walled capillary 1 mm. in outside diameter and 180 mm. long. The capillary side of the U-tube reaches the bottom of a 102.85ml. round-bottomed flask with a 2-mm. straight-bore stopcock; the niolecular weight flask is 165 mm. long, exclusive of 46 mm. of the length above and outside the stopcock; much of the length is tubing 8 mm. in outside diameter. First weigh the evacuated roundbottomed flask. Next pass gaseous carbon dioxide through the flask for 15 minutes to expel all the air, warming the Erlenmeyer flask with one hand. Remove the long capillary from the molecular weight unit and close the stopcock immediately; let the bulb stand 15 minutes to reach room temperature; then quickly reopen and reclose the stopcock; next evacuate the region outside the closed stopcock and let air replace the carbon dioxide in this zone; finally reweigh the molecular VOL. 29, NO. 5, MAY 1957
853
weight unit to obtain the weight of carbon dioxide. Similarly, us? a long capillary to pass chlorine gas from a cylinder through the molecular weight flask (Table I), Table I also lists the titration of chlorine in a 15-ml. round-bottomed flask with both parts of a 10/30 standard-taPer ground joint and also a 2-mm. straight-bore stopcock. Displace the air with chlorine, shake with excess
potassium iodide solution, and then titrate the liberated iodine with standard sodium thiosulfate solution. LITERATURE CITED
(1) Anderson, H. H., ~ A L CHEni. . 20, 1241 (1948); 24, 5 i 9 (1952). (2) Anderson, H. H., J . Am. Chem. SOC. 67, 223 (1945). (3) Ibid., 71, 1801 (1949).
(4) Ibid., 72, 2089 (1950).
( 5 ) Anderson, H. H., Fischer, H , J . Org.
Chem. 19, 1296 (1954). (6) Johnson, E. W.,Xash, L. K., J . Am. Chem. SOC.72, 547 (1950). (7) Kraus, C. A., Flood, E. A, Ibid., 54, 1635 (1932).
RECEIVED for reviex July 30, 1956. A4ccepted January 30, 1957. First Delaware Valley Regional Meeting, ACS, Philadelphia, Pa., February 16, 1956.
Colorimetric Estimation of Milligram Quantities of Inorganic Azides CHARLES E. ROBERSON' and CALVIN M. AUSTIN Quality Evaluation laboratory, U. S. Naval Ammunition Depof, Crane, lnd. ,The azide is acidified and the resulting hydrazoic acid is distilled into an acidified ferric nitrate solution. The reddish brown color due to formation of ferric azide is suitable for quantitative measurement at 460 mp. The color obeys Beer's law over the range of concentration investigated-i.e., 0.5 to 4 mg. of azide ion per 50 ml. of colored solution. The method is applicable to the determination of lead azide in primer mixtures, except those containing thiocyanates. It is not recommended for purity determinations.
It mas felt that the method would be particularly useful for determination of lead azide in the presence of other materials and when only small samples are available for analysis, as in some primer mixtures. Synthetic primer mixtures were prepared and analyzed with favorable results. The method was also employed successfully in this laboratory to investigate the rate and extent of formation of copper azide on small strips of copper foil, which had been exposed to hydrazoic acid vapor ( 5 ) .
C
Very simple distillation equipment is employed. A 10-ml. micro round-bottomed flask with a neck about 4 cm. long is fitted with a one-hole rubber stopper. The latter supports a glass tube drawn out until a very small orifice (approximately 0.4 mm. in inside diameter) is obtained on the receiver end. The tube is bent to extend nearly to the bottom of a 50-ml. volumetric flask. Absorption measurements are made with a Beckman Model DU spectrophotometer using 1-cm. Corex cells.
APPARATUS
methods for the quantitative estimation of inorganic azides have usually been restricted to determining purity of lead azide. The azide is oxidized by a cerium(1V) salt with subsequent measurement of the volume of nitrogen gas evolved or by titration of a n excess of a measured volume of a standard cerium(1V) salt solution with ferrous perchlorate ( I , 3). Because these methods are not wholly satisfactory for the determination of milligram quantities of azides, either alone or in the presence of other substances, the method described herein was developed. The formation of red ferric azide when hydrazoic acid reacts with ferric chloride, is the basis of the qualitative test described by Feigl ( 2 ) . An attempt to apply this test to quantitative measurement was unsuccessful because of the instability of the ferric chloride. However, use of ferric nitrate proved satisfactory in connection with a simple distillation of the hydrazoic acid formed when a n azide solution is acidified. OXVENTIONAL
Present address, Water Resources Division, U. S. Geological Survey, Menlo Park, Calif. 854
ANALYTICAL CHEMISTRY
REAGENTS
All reagents were reagent grade chemicals. An acidified ferric nitrate solution is prepared by dissolving 2.000 grams of ferric nitrate in about 50 ml. of water, and adding 5 ml. of concentrated nitric acid which is relatively free of nitrogen dioxide as determined by observation. This is filtered and the filtrate is diluted to 1 liter. A standard azide solution is prepared from sodium azide (Fisher Scientific Co., Cat. S-227). PREPARATION OF STANDARD CURVE
Sodium azide solution (1 ml. equal to 1 mg. of azide ion) is used in preparing the standard samples containing up to 4.0 mg. of azide ion. The standard solu-
tion placed in the distilling flask is diluted to about 5 ml. with distilled water. Two or three glass beads are used to help prevent bumping. Twenty milliliters of the ferric nitrate solution are pipetted into a 50-ml. volumetric flask and the stopper assembly is made ready for immediate connection after the addition of 0.6 ml. of 1 to 4 sulfuric acid to the contents of the distilling flask. The orifice of the glass tube is placed well below the surface of the ferric nitrate solution in the receiver. The receiver is cooled in a n ice bath and the distillation is continued for 3.5 minutes after first appearance of the reddish brown ferric azide color. The solution is adjusted in the receiver to approximately 25" C., diluted to the mark with n-ater, and again adjusted to 25" i 1" C. The transmittance is immediately measured a t 460 mp against a reference solution made by diluting 20 ml. of the ferric nitrate solution to 50 ml. PROCEDURE FOR PRIMER MIXTURES
Place a sample containing 0.5 to 4.0 mg. of NBin the distilling flask, add glass beads and distilled mater until the flask is about half filled. Add 2 drops of a 27, solution of sodium hydroxide and 0.3 ml. of a 30% hydrogen peroxide solution, and then boil the mixture gently for 1 minute to remove the excess hydrogen peroxide. Cool the flask and contents to 25' C. or below and proceed as indicated under preparation of standard curve. DISCUSSION A N D RESULTS
The absorption maximum is broad' and measurement is made at 460 mp. The intensity of the color varies with pH, but this variable is satisfactorily controlled by the distillation technique so long as no acid distills over. For this reason sulfuric acid is used to displace the azide ion. The color is stable for a t least an hour if the volumetric receiver is full and stoppered to prevent,
