COMMUNICATIONS TO THE EDITOR AN ATTEMPT TO SEPARATE ISOTOPES BY REVERSIBLE FRACTIONAL DISTILLATION'
This paper shows experimentally that the boiling points of the isotopic modifications of methylene chloride (CH2C12) differ by 0.005"C. or less. S. A. Peoples and L. Newsome (see Evans, Cornish, Lepkovsky, Archibald and Feskov: Ind. Eng. Chem., Anal. Ed., 2, 342 (1930)) found no separation of the isotopes of chlorine by reversible fractional distillation of chlorobenzene, but their technique was improved for the present attempt, for which their 6.09 meter column was available. Twenty liters of methylene chloride (Roessler and Hasslacher Co.) were carefully purified in the 6.09 meter column. Eighteen liters boiled within 0.1"C. or less, and this was returned to the cleaned still. The column was then operated a t total reflux, and all traces of water very carefully removed from the condenser (in which any water collects). A calcium chloride tube was placed on the condenser vent. Water is sufficiently soluble in methylene chloride to reduce appreciably the specific gravity. Thirty cc. samples were now withdrawn a t intervals for specific gravity tests, using a 25 cc. Leroy Weld type pycnometer (Central Scientific Co.), which made it possible to check individual weights of samples to less than 25.10 0.5 mg. The results, each being the mean of two determinations of d25.10, are 1.32043 after 22 hours, 1.32039 after 64 hours, and 1.32046 after 117 hours (all at total reflux). The entire charge was then distilled off (all boiling within 0.1"C. or less), mixed, and found to have d
ii:;:
1.32040.
Values are not corrected for buoyancy of air. Assuming Raoult's law, we write for the fhal limiting steady state at total reflux (Fenske: Ind. Eng. Chein. 24, 483 (1932)): Xp~/Xp = ~an XA/%
where X p ~ / X Pis B the ratio of the mole fractions of components A and B in the product, XA/XB is the sameratio in kettle, a is ratio of vapor pressure 1 Aided by grants from the Committee for Research in Problems of Sex of the National Research Council, and from the Rockefeller Foundation. These funds have been generously augmented by the Board of Research and the College of Agriculture of the University of California. 397
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COMMUNICATIONS TO THE EDITOR
of pure A to that of pure B, and n is the number of theoretical plates. The 6.09 meter column used, packed with 4 mm. X 4 mm. cylindrical glass beads, should give a separation equal to that of at least forty theoretical plates, so that n = 40. (Evans, Cornish, Lepkovsky, Archibald, and Feskov: Ind. Eng. Chem., Anal. Ed. 2,342 (1930) and Peters: Ind. Eng. Chem. 14, 476 (1922).) For methylene chloride a t 39.7"C., its boiling point (Carlisle and Levine: Ind. Eng. Chem. 24, 146 (1932)), dP/dT is 0.035 atmospheres per degree. (Morrison and Duus: Chem. & Met. Eng. 39, 230 (1932).) If both pair of successive isotopic forms of methylene chloride boil yo apart; a = 1 0 . 0 3 5 ~for either pair. The composition of chlorine, Clz, is 57.6 per cent Clz 35--35, 36.6 per cent C1235-37 and 5.8 per cent C1237-37. (Giauque and Overstreet: J. Am, Chem. SOC. 64, 1737 (1932).) Assuming the same ratios for the three isotopic forms of methylene chloride, and assuming their molal volumes in the liquid state to be identical, one calculates 1.32040-0.01621 for limiting specific gravity of distillate. Comparing this with experimental values above given, it is seen that y must be 0.005"C. or less. This result is apparently at variance with the claim (Keesom and van Dijk: Proc. Acad. Sci. Amsterdam 34,42 (1931)) of a recognizable separation of the isotopes of neon in a reversible distilling column of nineteen actual plates. The 6.09 meter column will separate in better than 95 per cent purity (with a single distillation), liquids boiling as little as 5°C. apart. The time for maximum enrichment at total reflux is then about twenty-four hours (one day). Hence even with a difference of 0.005"C. (the maximum possible for methylene chloride isotopes), to get about 95 per cent purity of isotopes, such a column must be 6 X 1000 or 6000 meters high, and the time for maximum enrichment would be increased to 10002 or lo6 days. However, the right set of conditions for such a fractionation may have occurred in some places during the formation of the earth.
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SUMMARY
The boiling points of successive isotopic forms of methylene chloride (due to the chlorine 35 and 37) are shown experimentally to differ by 0.005"C. or less. Hence in a liquid-vapor equilibrium type of fractionating column, the necessary height and particularly the necessary time of distillation, eliminate this method of separation. However this kind of fractionation may have occurred naturally in suitable geological formations. H. J. HENRIQUES. R. E. CORNISH. The Institute of Experimental Biology The University of California Berkeley, California
