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In 1931 Birge and Menzel suggested that an apparent discrepancy between the atomic weight of hydrogen and the value found with the mass-spectrograph might be explained if ordinary hydrogen contained a small amount of a heavier isotope of hydrogen of mass approximately 2 ("heavy hydrogen"). The existence of this was detected in 1931 by Urey, Brickwedde and Murphy, who found that the gas obtained from the small residue of the evaporation of a large volume of liquid hydrogen showed a spectrum line in the position calculated for hydrogen of mass 2. This was confirmed by Bleakney in 1932 with the mass-spectrograph.
The concentration of the heavier isotope of hydrogen, now called deuterium, was effected in 1932-33 by Washburn and Urey, and by G. N. Lewis and Macdonald, by the prolonged electrolysis of water, when the heavier isotope accumulates in the residue. By suitable modification of this process, nearly pure deuterium oxide, D2O ("heavy water"), was obtained, and this is now a commercial product. It is very similar to ordinary water but has a higher density (about 1.1), freezing point (3.82°), and boiling point (101.42°). The freezing and boiling points of deuterium are also slightly higher than those of hydrogen. Deuterium forms compounds with other elements in the same way as hydrogen, e.g., DCl, D2SO4, ND3, etc., and also replaces water of crystallisation of salts, e.g., CuSO4, 5D2O, the vapour pressure of deuterium oxide over which is smaller than that of water in the corresponding salts, e.g., CuSO4, 5H2O.
Besides the molecule D2 (ortho- and para- forms of which exist), the molecule HD exists, and hydrogen and deuterium may exchange between compounds such as benzene, C6H6, and deutero-benzene, C6D6. Ordinary hydrogen contains very minute amounts of a third isotope of mass approximately 3, called tritium, which is also formed by the collision of deuterium nuclei: D + D = T + H, or 2H + 2H = 3H + 1H.
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