sists essentially of two major parts: (1) A flask, Bt with a ground joint, A, inserted in the neck and, (2) fitting joint A, a cap equipped with pressure and delivery tubes. A quantity of oil,
A
MOST useful and interesting property of a hydrocarbon is its molecular weight, and the determination of this property for petroleum hydrocarbons, which has been thoroughly reviewed by Headington (1), has been the object of considerable study by many in
vestigators. The Bureau of Mines has studied the cryoscopic method of determining molecu
lar weights in some detail and has published two papers (4, 5) on this subject. The present report concerns primarily a device that has been used successfully in the bureau’s Petro
leum Experiment Station laboratory at Bar
tlesville, Okla., for handling very viscous oils in cryoscopic molecular weight studies.
In determining the molecular weight of a liquid "some type of weighing pipet is com
monly used to weigh the sample and to intro
duce it into the molecular weight apparatus.
A volumetric pipet with the tubes bent to prevent loss of liquid is usually satisfactory for reasonably fluid materials but is not suitable for liquids of very high viscosity. Such liquids have been handled in some laboratories (2, S) by using thin-walled glass capsules, which are filled with the viscous sample and dropped into the molecular weight apparatus. This procedure seems more suitable for the ebullio- scopic method than the cryoscopic. In the latter method serious loss of solution may attend removal of the capsule, dissolution of the oil from the capsule is more difficult owing to lower temperatures, and the capsule or its fragments if not removed may interfere with the stirring device, particularly the rotary type preferred by the writers.
The flask-pipet described in this paper eliminates the difficulties mentioned above
387
388 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 11, NO.
T a b l e I. M o l e c u l a r W e i g h t D e t e r m i n a t i o n o n F o u r T e s t O i l s , D i l u t e d a n d U n d i l u t e d P r i o r t o D e t e r m i n a t i o n
Sam ple D iluent M olecular
W eight of
0-8 A cetone extract from 1.4833 0.8 768 2.3 67 1.094 1.019
0-17
Cabin Creek, W.
Va;l crude
Burning oil cut from 1.4472 0.8007 0-18 Rodessa, La., crude
its contents and left sealed until needed. To use this solution, tip C is cut off and the cap, earning the delivery and pressure tubes, is quickly put in place as shown in the figure.After the flask and auxiliary equipment are weighed, air pres
sure applied through the sidearm at joint D fills delivery line J with solution and forces it from pipet tip F. Valve E prevents return of liquid to the flask when pressure is removed, thereby avoiding bubbles in the delivery line that are troublesome when the next quantitjr of solution is expelled. Caps G and II prevent evaporation of the solvent. Joints A and I should be lubricated, but D is preferably left dry to avoid complications in weighing.
Evaporation from this apparatus has been found to be negligible, as the average loss of weight was about 0.3 mg. in 16 hours. It is obvious, however, that the apparatus must be protected from wide temperature varia--- tions to avoid loose joints and attendantand glass hooks may be used to keep the joints tight. The particular apparatus “breathing” losses. If necessary, springs described in this paper was constructed of ordinarily available glassware and weighed about 55 grams. An expert glassworker probably could make a lighter and more compact apparatus.
(54.4° C.)
b Too dark for accurate observation.
the molecular weight of which is to be ascertained, is introduced into flask B and its weight determined. Then sufficient solvent to make a fluid solution is added to the oil in the flask, which is again weighed after the neck is sealed at C. From these data the quantity of solvent and the concentration of the solute are determined.The inside of the neck of the flask should be kept free of oil and solvent to prevent difficulties and loss of sample when the flask is sealed. Glass must not be lost in this operation, and the portion of the neck “sealed off” must be weighed with the flask. The flask should be shaken thoroughly to ensure the homogeneity of
To ascertain the precision of this
“method of dilution” the molecular weights of four petroleum oils were de
termined by the cryoscopic method ... (4), using both diluted and undiluted solutes. The solvent used was benzene, dried with magnesium perchlorate. As shown in Table I and in Figure 2, the deviation in the extrapolated molecular weight due to using the “dilution” procedure is in each instance less than 2 per cent. Previous work has shown that this is well within the limits (4) within which two different laboratories would be expected to duplicate cryoscopic molecular weight determinations. It probably also approaches the limit of precision of an individual laboratory using conventional pro
cedures. The points on the curves in Figure 2 were obtained
760
ANALYTICAL EDITION 389 during the course of at least three separate series of two to
six freezing-point determinations on each sample, and the equa
tions given were derived by applying the method of least squares to the accepted data. A critical examination of the procedure and data did not disclose the reason for the small discrepancy between the two methods. Table II shows a few properties of the test samples and gives a brief description of their origin.
Table III and Figure 3 present the results of molecular weight determinations on three highly viscous samples, 0-19, 0-22, and 38. These had been diluted with benzene in the dispensing device described previously, giving oil concentra
tions of 65.32, 61.70, and 56.65 per cent, respectively. An
indication of the tacky nature of these samples is obtained from Table IV where a number of properties including viscos
ity are reported. The amount of solution prepared and the high molecular weight of sample 38, necessitating the use of
T a b l e III. D a t a O b t a in e d i n D e t e r m i n i n g M o l e c u l a r
54.883 55.22155.603 0.024370.01153 0.1 1 00.231 53 6.453 9 .8
55 .998 0.03749 0.3 5 4 54 1.9
54.609 54.913 0.01041 0.1 0 0 53 2.7
55.300 0.02354 0.2 2 5 53 5.3
Extrapolated m olecular weight 55 7.9
55 .062 56 .450 0.03212 0.091 1806.2
58 .102 0.06838 0.1 87 1871.1
59 .740 0.10232 0.2 69 1946.4
61.561 0.13795 0.351 2011.1
53 .590 55.391 0.04249 0.1 19 1827.1
57 .097 0.08027 0.2 1 3 1928.4
58 .706 0.11389 0.2 9 7 1962.2
61 .535 0.16873 0.4 1 9 2060.6
E xtrapolated m olecular weight 1746.3 Fig u r e 3
390 INDUSTRIAL AND ENGINEERING CHEMISTRY VOL. 11, NO. 7
R efractive D ensity K ine K ine
Index a t a t m atic Saybolt m atic Saybolt
20.0° C. 20.0° C. stokes U niversal“ stokes U niversal'
1.5886 1.0225 3.1 84 1,481
1.5596 0.9976 50.05 23,170 1.661 773
1.5020 0.9015 103.4 48,000
Ä C onversion by A. S. T . M . m ethod D 446-37T.
6 T oo dark for accurate observation.
large amounts of the solution in the determination, permitted only two runs to be made on this material. The separation or identification of the data for individual runs (Figure 3) is impossible for this sample as well as for the lower molecular weight samples 0-19 and 0-22 in which three and four runs, respectively, were made. Although the precision of the determination is better than 2 per cent there is no logical reason for believing that these samples behaved differently than did the four test oils. Consequently, if it can be assumed that the cryoscopic method used in this investigation gives correct molecular weights with undiluted samples, then the results obtained on the viscous oils may be considered as being accurate to =•= 2 per cent.