by Jim Schooley and David R. Lide, SAA History Committee
"In the summer of 1931", wrote Rexmond C. Cochrane in "Measures for Progress," his history of the first 50 years of NBS, "Harold C. Urey, associate professor of chemistry at Columbia [University]... became convinced that an isotope of hydrogen of mass 2, though unknown, could be found."
At that time, Urey, at age 38, already had considerable experience as a scientist. He had obtained a PhD degree in chemistry at the University of California at Berkeley in 1923, had spent a year studying at the Niels Bohr Physical Institute in Copenhagen, and had served on the faculty at the Johns Hopkins University in Baltimore. While at Hopkins, he became acquainted with Ferdinand G. Brickwedde, a graduate student in physics. Brickwedde assisted Urey by proofreading "Atoms, Molecules, and Quanta", which Urey had written in collaboration with Arthur E. Ruark.
The existence of nuclear isotopes, suggested as early as 1913, interested Urey very much. He was especially fascinated by the possibility that isotopes of the light elements might exist, particularly in hydrogen.
Urey knew that very precise studies of the atomic weight of hydrogen indicated the likelihood of the existence of a heavier isotope. In collaboration with a Columbia colleague, George M. Murphy, Urey had studied the spectrum of hydrogen and wanted to believe that a faint satellite of the well-known hydrogen-alpha line was in fact due to heavy hydrogen. The very faintness of the satellite line, however, indicated that if there were a heavy isotope, it must exist naturally only in relatively tiny abundance compared to the major isotope of mass one. Guesses based on available evidence indicated an abundance ratio no greater than about 1/5000.
But Urey needed hard evidence even to convince himself that "heavy hydrogen" existed. His thermodynamic calculations indicated that distillation of liquid hydrogen might produce samples that would be enriched in the heavy isotope. In that case, spectroscopy would surely show enhanced intensity of the Balmer lines of the heavy isotope that would settle the question. Urey knew that his friend Brickwedde, by that time head of the Cryogenic Laboratory at the National Bureau of Standards, was currently studying the ortho-para conversion in liquid hydrogen and could produce liquid hydrogen in multi-liter quantities, so he approached Brickwedde with his idea.
Conversations between Urey and Brickwedde convinced the latter that, although a liquid fractionation experiment would be difficult, it was possible that it could succeed. Brickwedde prepared samples for Urey's spectrographic study by two methods, both involving the evaporation of large quantities of liquid hydrogen, on the supposition that hydrogen of mass one would evaporate more readily than a heavier isotope, so that the last portion to evaporate would be enriched in "heavy hydrogen". Brickwedde's process started with some 400 cubic feet of hydrogen gas at room temperature, which he precooled at a pressure of 2500 pounds per square inch in a pumped liquid nitrogen bath (somewhat below 77 K). The pressurized, precooled gas was then allowed to expand to atmospheric pressure through a Joule-Thompson valve. In that process, the gas cooled until it liquefied, and the evaporative step followed.
The first sample that Brickwedde prepared consisted of the last few milliliters left after the evaporation of six liquid liters, with the evaporation taking place at one atmosphere of pressure, at a temperature of about 20 K. Two other samples were each obtained by evaporating four liters of liquid at a pressure only a few millimeters above the triple-point pressure, at a temperature some 6 K lower. Again, the samples consisted of the last 2-3 ml of liquid.
Spectrographic study of the samples prepared by Brickwedde provided the hard evidence that Urey had sought. Each of the samples revealed Balmer-series lines at the wavelengths appropriate for hydrogen of mass two, with the lines recorded from the second and third samples appearing much stronger than those from the first.
Urey, Murphy, and Brickwedde jointly submitted a paper describing the experiments and their results to the Physical Review in April, 1932. They estimated the natural abundance ratio of hydrogen mass two to mass one to be 1/4000, and the ratio in Brickwedde's second and third samples as 5/4000. The experimental proof of the existence of "heavy hydrogen" was definitive, and it stimulated so much interest in the scientific world that the Nobel prize was awarded to Urey only three years after the new findings were reported.
Urey was not through collaborating with Bureau scientists, however. Even while Brickwedde was preparing his experiment, Urey was investigating another method to concentrate the heavy isotope of hydrogen. This time, he discussed the matter with Edward W. Washburn, chief of the NBS Chemistry Division, and the experiment involved the fractional electrolysis of water. The two scientists expected that the cathodic potentials of hydrogen mass one and mass two would be substantially different because of the large ratio of the atomic masses. Thus a carefully performed systematic fractional electrolysis might produce two final fractions, one of nearly pure mass one and a second enriched in "heavy hydrogen". Washburn directed an assistant, Edgar R. Smith, to construct an experiment to test the idea. In the meantime, they found a commercial source of water that had undergone sequential electrolysis for over 2 years. Spectrographic study of samples of that water showed a definite increase in the relative abundance of "heavy hydrogen", thus corroborating the results obtained with liquid hydrogen. Washburn and Urey submitted a description of that experiment in June, 1932 to the National Academy of Sciences. Smith's experiment was also successful, providing samples that corroborated the existence of heavy hydrogen.
Discovery of "heavy hydrogen", given the name "deuterium", earned Urey the Nobel Prize in chemistry in 1934, and the scientific achievements involved in the discovery earned further laurels for Brickwedde, Washburn, Smith, and the entire National Bureau of Standards.
An interesting sidelight on further work by Brickwedde with liquid hydrogen occurred nearly two decades after his collaboration with Urey. As a consequence of the nuclear arms race that dominated the "Cold War", President Harry Truman okayed in 1950 new research on a "superbomb". The idea, conceived earlier by Edward Teller and Enrico Fermi, was to initiate a fusion reaction between hydrogen nuclei, providing explosive power perhaps 1000 times that obtained through uranium fission. It was expected that research on the "super" would involve large quantities of liquid hydrogen. In response to the new mandate, Brickwedde, by then chief of the National Bureau of Standards Heat and Power Division, collaborated with NBS colleagues Russell Scott, William Gifford, and Victor Johnson in designing and building a hydrogen liquefier capable of producing liquid hydrogen at 10 times the previous capacity. The new liquefier was transported to new NBS facilities in Boulder, CO, where it materially contributed to the development of the "hydrogen bomb". The entire group received the U. S. Department of Commerce Gold Medal award in 1953 in recognition of their achievement.
Further information on the Urey-Brickwedde experiments can be obtained from "Measures for Progress", NBS Miscellaneous Publication 275, 1966, by Rexmond C. Cochrane; from "Harold Urey and the discovery of deuterium", Physics Today, September 1982, by Ferdinand G. Brickwedde; in a paper by Urey, et al, Journal of the American Chemical Society, Vol. 51, p 2872, 1931; and in a paper by Washburn and Urey, Proc. of the National Academy of Sciences Vol. 18, p 496, 1932.
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