Collected by B. Garretl Ohio State University
Columbus
1
1
The f l ~ of ~ ~Genius, h 70
Deuterium: Harold C. Urey
Here was a discovery made soon after a careful research was performed by another very reputable scientist. F. W. Aston concluded from s. mass spectrum ana1.ysis of hydrogen that ". . from these figures i t is safe to conclude that hydrogen is a, 'pure' element and that its atomic weight determined with such consistency and aoouracy by chemical methods (1.008) is the true mass of its atom."': In other words his observations had led him t o believe that hydrogen contained no isotopes. In 1913 Soddy had first used the concept of isotopes t o explain the much larger number of "elements" obtained in the radioactive processes than there were places for these "elements" in the periodic table. By 1930 the isotopes of a number of elements were known; even the isotopes of oxygen, "0, "0, and 'W discovered in 1929 by Giauque and Johnston. What was the possibility of finding an isotope of hydrogen? Several groups of researchers were trying to find the answer, even though in I927 Aston had made the statement quoted above. I n 1931 Birge and M e n d using data of Aston, which later proved to be in error, predicted the possibility of an isotope of hydrogen Hz in a ratio of 1 :4500 of 'H to $Ha. Birge snd Menzel concluded "Of the elements thrtt permit an accurate comparison of the chemical and mass-spectrograph results there remains only hydrogen. The chemical value is 1.00777 rt 0.00002 (probable error) as compared with Aston's 1.00778 zt 0.00015 (limit of error). Aston's value, reduced t o the chemical scale, is 1.00756 and the discrepancy appears to be It could be removed by postulating outside the limit of error. the existence of an isotope of hydrogen of mass 2, with a relative abundance of H/*H = 4500." Urey reasoned that it should be possible to separate 'H from 2H by fraction4 distillation of liquid hydrogen. With the help of one of his graduate students, George Murphy, and of F. G. Brickwedde of the National Bureau of Standards he separated and identified the isotope of hydrogen ZH called deuterium. Urey tells the story this way4:
.
...
"Along in July or August of 1931 I read a paper by Menzel and Birge predicting that heavy should exist to the extent of about one part in five thousand. This was based briefly upon the following argument. The atomic weight of hydrogen had been determined by chemical means, and then Aston determined it by means of a mass spectrometer. In the one case the average of the hydrogen isotopes was compared with the average of the oxygen isotopes, and in the other case hydrogen of atomic weight 1 was compared with oxygen of atomic weight 16. At the time no one knew anything about isotopes of either hydrogen or oxygen. Then Giauque discovered oxygen-18 and this showed that the standards were not the same and hence one must expect that there were heavy hydrogen isotopes. It occurred to me that there should be a difference in vapor pressure of liquid hydrogen and liquid deuterium and that this was a good method for separating them and concentrating the very small amount of 'ASTON,F. W., Proc. Roy. SOC. (London), A 115, 487 (1927). P B r r r o ~R. , T., AND MENZEL,D. H.,Phw. Rev., 37, 1669 (1931). ~-~~ a ALLISON, F., Ind. Eng. Chem., 4 . 9 (1932). Personal eommunic&m.
.
'
deuterium to be expected. The cooperation of Dr. Brickwedde and Dr. Murphy is a matter of record." "The interesting thing about this is that the prediction of Birge and LMenzel was based on an error. Aston made a m i s t a k e a s far as I know it is the only mistake he ever made--and when the mistake was corrected, Birge and Menzel should have predicted a light isotope of hydrogen and not a heavy one. Of course there must be an error also in the chemical determination. It is probable that hydrogen very carefully prepared by electrolytic cells was used and the hydrogen isotopes were fractionated in the process, since we knew nothing about the differences in chemical properties of isotopes a t the time. "At least it can be said that the discovery of heavy hydrogen was an accident, and I have often thought that it would have made a great deal of difference in my fortunes if the heavy hydrogen had not been present. I think important discoveries are mostly made by intelligent people, but they should keep in mind that there is a substantial element of luck as well. We found the heavy hydrogen, but only Nature put it there, and if it had not been present, we would have done all of the same work in the same way but would have found no heavy hydrogen. "Dr. Murphy and I worked night and day on this, and we were first certain that heavy hydrogen existed on Thanksgiving Day 1931. My wife was a scientific widow during those months. I am quite sure that Murphy made no progress in wooing his wife-to-be during the same time." The first published results6 indicated that the ratio of 'H to PHin nature is 4000: 1. Abstracts from the paper me as follows:
"In a recent paper Birge and Menzel pointed out that if hydrogen had an isotope with mass number two present to the extent of one part in 4500, it would explain the discrepancy which exists between the atomic weights of hydrogen as determined chemically and with the mass spectrograph, when reduced to the same standard. Systematic arrangements of atomic nuclei require the existence of isotopes of hydrogen 2H and xHand helium We to give them a completed appearance when they are extrapolated to the limit of nuclei with small proton and electron numbers. An isotope of hydrogen with mass number two has been found present to the extent of one part in about 4000 in ordinary hydrogen; no evidence of 3H was obtained. The vapor pressures of pure crystals containing only a single species of the isotopic molecules 'H'H, 'H2H, 'H3H were calculated. UREY,H. C., BRICKWEDDE, F. G., and MURPHY,G. M., Phpieal Reuim, 40, 1-15 (1932). Volume 39, Number 1 1 , November 1962
/
583
" The calculated vapor pressures of the three isotopic molecules in equilibrium with their solids at the triple point for ordinary hydrogen arein the ratio p11 : p ~ l : p , a= l:O. 37:0.29. The isotope was concentrated in three samples of gas by evaporating large quantities of liquid hydrogen and collecting the gas which evaporated from the last two or three cubic centimeters. Sample I was collected from the end portion of six liters evaporated a t atmospheric pressure and samples I1 and 111 from four liters, each, evaporated a t a pressure only a few millimeters above the triple point. "These samples and ordinary hydrogen were investigated for the visible, atomic BaImer series spectra of =H and 3H from a hydrogen discharge tube run in the condition favorable for the enhancement of the atomic
584
/
Journd of Chemical Education
spectrum and for the repression of the molecular spec. trum, using the se~ondorder of a 21 foot grating with a dispersion of 1.31A per mm. When with ordinary hydrogen, the times of exposure required to just record the strong ' H lines were increased 4000 times, very faint lines appeared a t the calculated positions for the 2H lines accompanying 'HP, 'Hr and 'Ha on the short wavelength side and separated from them by between 1 and 2A. These lines do not agree in wavelength with any known molecular lines. "Relative abundances were estimated by comparing ,the times required to just record photographically the corresponding ' H and lines. The relative abundance of 2H and 'H in natural hydrogen is estimated to be about 1 :4000 and in the concentrated samples about five times as great."
