Fluorocarbon Vapor Balance J. H. SIMONS, W. H. PEARLSON, AND W. -4. WILSON Fluorine Laboratories, The Pennsylvania State College, State College, Pa. bottom of the left condenser. As soon as temperatures become constant, the bob is weighed. Weighings are constant for about 15 minutes, after which the diffusion of air into the vapor c a u s a a slight decrease in the measurement. The instrument functions on less $han 1.5 ml. of liquid and the precision of the measurements is within *1%.
OR the determination of molecular weights of fluorocarbons Fboiling below 100' C., the gas density balance previously described (1)is very satisfactory. For substances boiling 100" C. or above, the vapor pressures a t room temperature are too low for the use of this instrument. The device shown in Figure i was designed and found to function well (Table I). I t consists essentially of a glass boiler so arranged that a static body of vapor at constant temperature can be maintained in a container. In this vapor a glass bob is suspended by a h e wire, which is held from above on one side of an analytical balance Weighing the bob in the vapor and determining the temperature with a thermocouple provide the necessary data for the calculation of molecular weight. The balance rests on a metal plate, the horizontal motion of which can be slightly changed by means of setscrews. This is for the purpose of centering the bob. In operation the instrument is mounted securely on a frame. The position of the balance is adjusted by the setscrews until the bob floats freely in the chamber. The weight of the bob in air is determined, the sample is then introduced through the left condenser, and the temperatures of the various sections of the apparatus are adjusted by the amount of current through the heaters. The left and bottom ( R and S ) heaters provide a temperature approximating the boiling point of the liquid. The bob chamber, J , is adjusted to about 50" C. higher and the wire heater, B, about 15' C.higher than the boiling point of the liquid. The pot heater, V , at first provides reflux in both condensers. In a short time air is expelled from the bob chamber, and the current to the pot heater is reduced until the reflux level is about 25 mm. above the
Table I.
JIolecular Weights of Fluorocarbons
Apparent Wt., Bulb Molecular Pressure in Vapor Weight c. C. Mm. Hg Grams 1 101 148 736 4.9670 442 148 736 4.9672 442 735 4 9741 442 187 184 735 4,9748 441 2 99 180 735 4,9802 403 185 735 4.9810 404 185 735 4.9810 404 3 178-188 240 735 4.9615 577 244 733 4.9618 578 245 735 4.9619 579 4 220-238 284 740 4.9515 69 1 285 740 4.9515 693 6 280-300 356 742 4.9418 851 355 742 4.9420 850 355 742 4,9424 848 Samples are fluorocarbons of different boiling ranges. 1 is reasonablg pure CSFII(molecular weight 438). Duplicate determinations a t same and different temperatures for chamber show reproducibility. Bulb volume, 6.78 cc. Bulb weight (in vacuum), 5.0514 grams. Sample No.
fi
Boiling Range
Temp. Chamber
ACKNOWLEDGMENT
c
This work was sponsored by the Minnesota Mining and hlanufacturing Company. The authors wish to express their thanks for the assistance oi C. W. Brouse of the school shops and F. J. Malloy, the school glass blower. LITERATURE CITED
(1) Sirnone, J. H., IND.ENO.CHEM., ANAL.ED.,10,587 (1938).
RECEIVED November 1 , 1947. All figures in millimeters A . Wire support from analytical balance above (not shown) to glass bob KO.40 nickel wire B . Heater to keep wire tree from condensate Nichrome wire winding between two glass cylinders sealed a t bottom. Free space between glass walls filled with Alundum cement C. Prongs on B to center. On lower end (not shown) additional prongs give vertical support by resting on G D. Glass condenser jacket, 38 mm. 0.d. E . Glass reflux tube, F. Glass condenser jacket, 22 mm. 0.d. G. Circular well to catch reflux from condenser N. Return tube to drain liquid from 0, 6 mm. 0.d. I. Glass bob full of air but sealed J . Glass envklope of bpb chamber, 22 mm. 0.d. K . Cylindrical copper jacket to maintain uniform temperature in bob chamber L. Nichrome winding on cylindrical glass envelope, for heating bob chamber M. Glass external jacket N . Thermometer chamber 0. Thermometer. A ring made of short copper tubing to which Chromel-Alumel thermocouples are spot-welded. Temperatures determined by potentiometer readings P . Glass thermocouple wire tube Q. Nichrome winding to heat connecting tubes R. Asbestos insulation covering for connecting tubes 5. Winding Q and insulation R extend around lower connecting tube T. Glass boiling pot U. Glass re-entry heater tube V . Nichrome heater for pot
Figure 1. Fluorocarbon Vapor Balance
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