The separation of isotopes

The chemical elements occurring in nature consist for the greater part of mixtures of isotopes. They may be regarded as mixtures, the individual isotopes representing pure components. Isotopes of one element occupy the same position in the periodic system and have the same nuclear charge, but different masses. The carbon isotopes and for instance, merely differ by having 6 and 7 neutrons in the 

Elements having an odd nuclear charge in general consist of only one or two isotopes with an odd mass number exceptions are H, Li, B and N. The element Hg, on the other hand, comprises seven stable isotopes with mass numbers of 196 to 

Some stable isotopes, their relative differences in mass and relative natural abundance 

Symbol Atomic number Mass number Relative difference in mass (%) Relative natural abundance 

For the concentration or isolation of isotopes the methods of diffusion, thermal diffusion, electrolysis and exchange reactions are employed. An enrichment can also be attained with the aid of the mass spectrometer and the centrifuge. Distillation procedures are used for obtaining He, (D), B, 0, Ne, Cland Ar. 

---

The phenomenon of multiphoton dissociation finds a possible application in the separation of isotopes. For this purpose it is not only the high power of the laser that is important but the fact that it is highly monochromatic. This latter property makes it possible, in favourable circumstances, for the laser radiation to be absorbed selectively by a single isotopic molecular species. This species is then selectively dissociated resulting in isotopic enrichment both in the dissociation products and in the undissociated material. 

If a temperature gradient is maintained in a binary gaseous mixture, a concentration gradient is established with the light component collecting preferentially at the hot end and the heavier one at the cold end. This phenomenon, known as the Soret effect, may be used as the basis of a separation technique of commercial significance in the separation of isotopes. 

For difficult separations, requiring many stages such as the separation of isotopes. 

Mulliken, R.S. and Harkins, W.D. (1922). The separation of isotopes. Theory of resolution of isotopic mixtures by diffusion and similar processes. Experimental separation of mercury by evaporation in a vacuum. Journal of the American Chemical Society 44 37-65. 

Similar results have been obtained for methane 12) and for ethane 19). The values quoted in Table II also illustrate the point that the distribution of deuterium between hydrogen and propane differs from the value expected for a random distribution. With the ratio of pressures used, the expected percentage for the mean deuterium content of the hydrocarbon would be 33.3, which is substantially less than the experimental value of 40.9 %. This type of deviation is also found with other hydrocarbons, but it does not affect the validity of using classical theory for the calculation of the interconversion equilibrium constants in studies of mechanism of exchange reactions. More accurate values for these equilibrium constants are necessary, however, if one is interested in the separation of isotopes by chemical processes. 

The separation of isotopes can be used to show the efficiency of separation by capillary electrophoresis. Moreover, the interfacing of a mass spectrometer to the capillary can be used for the study of biological substances (cf. Fig. 16.7). 

HEVESY, GEORG de (1885-1966). A Hungarian chemist who won the Nobel prize in chemistry in 1943, for his work on (he use of isotopes as tracers in the study of chemical processes. He discovered the element hafnium in 192.7. One of his interesting projects involved the calculation of ihc percentages of chemical elements in the universe. He also was involved in research using radioactive lead and phosphorus traces. His work included the separation of isotopes by physical means. His Ph D was granted ai Freiburg in 1908. 

T nterest in the separation of isotopes started as a scientific curiosity. The question arose as to whether it was indeed at all feasible or possible to separate isotopes. After this question was answered in the affirmative (24), it became of interest to separate isotopes on a laboratory scale for use in scientific research. A few examples show the range of utility of separated isotopes. Deuterium has attained widespread use as a biochemical and chemical tracer. It is now abundantly available and is used as freely as any cheap chemical reagent. He has opened up an entirely new field of research in low temperature physics and has important applications in the production of temperatures below 1°K. with a thermal neutron cross section of 4,000 barns, has found wide use in nuclear particle detectors—neutron proportional counters. still finds use as a tracer, but in recent years its most frequent use has been in electron spin and nuclear magnetic resonance spectroscopy. occupies a unique position as the only usable tracer for nitrogen. finds application as a... 

The four principles of photochemical separation apply to the separation of other chemical entities, elements and compounds, as much as they do to the separation of isotopes. Since the spectra of elements and compounds differ to a much greater degree than the spectra of isotopes, the problem is simpler. 

Sabau, C., Calusaru, A. A Literature Survey on the Separation of Isotopes by Ion Exchange,... 

In exchange reactions chemical reaction gives appreciable exchange of isotopes between the reactants. These reactions can be used for the separation of isotopes. Obviously, only partial separations can be achieved through exchange reactions, because these are equilibrium reactions. Some examples of application of exchange reactions are given below. 

The method has been successfully employed for the separation of isotopes of argon and nitrogen. The Hertz procedure has brought about separation of deuterium (spectroscopically pure) from ordinary hydrogen. 

Cryptands have been used for the separation of isotopes Ca and Ca have been separated, using [2.2.1] and [2.2.2], the lighter isotope being enriched in the organic phase.Similarly, preliminary results on the separation of Na and " Na, using [2.2.1 ], are promising. ... 

Although the isotopes of an element have very similar chemical properties, they behave as completely different substances in nuclear reactions. Consequently, the separation of isotopes of certain elements, notably from U and deuterium from hydrogen, is of great importance in nuclear technology. Table 1.5 lists isotopes important in nuclear power applications, together with their natural abundance and processes that have been used or proposed for their separation. In addition to applications mentioned earlier in this chapter. Table 1.5 includes the use of D and Li as fuel for fusion power, a topic treated briefly in Sec. 9, following. 

Lewis made additional valuable contributions to the theory of colored substances, radiation, relativity, the separation of isotopes, heavy water, photochemistry, phosphorescence, and fluorescence. As a major in the U.S. Army Chemical Warfare Service during World War I, he worked on defense systems against poison gases. From 1922 to 1935 he was nominated numerous times for the Nobel Prize in chemistry. Lewis s death, while measuring the dielectric constant of hydrogen cyanide on March 23, 1946, precluded his receiving the prize, which is not awarded posthumously, see also Acid-Base Chemistry Lewis Structures. 

As for the separation of isotopic olefin isomers long columns are required (several tens of meters), so it is advisable to apply the circular chromatography method in this instance [70-73]. 

The electrolytes used here are mainly nitrates and chlorates of alkaline metals or their eutectics in order to achieve low melting points. The procedure is applied mostly to the estimation of electrophoretic mobilities of inorganic anions and cations and for the separation of isotopes. Of theoretical interest is the fact that electro-osmotic flow in molten salts is practically negligible because of the small electric double layer at the temperatures used. 

Countercurrent distillation enables components to be separated having differences in boiling point of about 0.5 deg C, whilst this figure can be as low as 0.05 °C if extremely efficient columns are employed, as in the separation of isotopes. By the use of selective methods and, in difficult cases, by combination with other methods of separation such as extraction, countercurrent distribution and gas chromatography, separations have been performed with mixtures previously r arded as inseparable. 

The maximum gas rotation velocity v a is referred to as the Alfven velocity for plasma centrifuges. The rotation velocities in plasma centrifuges reach 2-3 10 cm/s in the case of light atoms, which exceed those of mechanical centrifuges by about 50-fold. The fast gas rotation in a plasma centrifuge can be applied for the separation of isotopes with small differences in atomic masses - M2) ... 

Graham s law (of diffusion) The principle that gases diffuse at a rate that is inversely proportional to the square root of their density. Light molecules diffuse faster than heavy molecules. The principle is used in the separation of isotopes. The law is named for the Scottish chemist Thomas Graham (1805-69). 

Herold et al. have suggested that the insertion of elements into graphite could be applied to the separation of isotopes. In a study of the equilibrium between rubidium and the first-stage intercalation compound RbCg, it has been shown that the inserted metal is more rich in the heavier isotope than the free metal. A coefficient of enrichment, K, has been estimated from the data ... 

The separation of isotopes of alkaline earth metals by ion-exchange chromatography (Be and Ca), using the band elution technique, and by chemical exchange reactions (Ca), using macrocyclic polyether complexes, has been assessed. The separation factors for Be and Ca decrease with increase in mass of the isotopes, and were found to be of the same order as those determined previously. Enrichment of the heavier isotopes of Ca by reaction (1), where L represents a macrocyclic polyether (c.g. DCH18C6, DB18C6), has also been shown to be effective. ... 

---