Isotopes separative effect

Figure 2. Lithium isotope separation effected during ion exchange with synthetic zeolite (Taylor and Urey 1938). As has been demonstrated repeatedly since that study, in both natural and synthetic experiments, Li is fixed more effectively in the exchanger than Li.

A second type of apparatus based on the pressure diffusion effect is the separation nozzle. Pressure gradients in a curved expanding jet produce an isotopic separation similar to that in a centrifuge. The separation effect obtained with a single jet is relatively small, and separation nozzle stages, similar to gaseous diffusion stages, must be used in a cascade to realize most of the desired separations. 

Other reasons for investigating plutonium photochemistry in the mid-seventies included the widely known uranyl photochemistry and the similarities of the actinyl species, the exciting possibilities of isotope separation or enrichment, the potential for chemical separation or interference in separation processes for nuclear fuel reprocessing, the possible photoredox effects on plutonium in the environment, and the desire to expand the fundamental knowledge of plutonium chemistry. 

Four observation were thought to be in disagreement with the diffusion model (1) the lack of a proportional relationship between the electron scavenging product and the decrease of H2 yield (2) the lack of significant acid effect on the molecular yield of H2 (3) the relative independence from pH of the isotope separation factor for H2 yield and (4) the fact that with certain solutes the scavenging curves for H2 are about the same for neutral and acid solutions. Schwarz s reconciliation follows. 

Secondary isotope effects are small. In fact, most of the secondary deuterium KIEs that have been reported are less than 20% and many of them are only a few per cent. In spite of the small size, the same techniques that are used for other kinetic measurements are usually satisfactory for measuring these KIEs. Both competitive methods where both isotopic compounds are present in the same reaction mixture (Westaway and Ali, 1979) and absolute rate measurements, i.e. the separate determination of the rate constant for the single isotopic species (Fang and Westaway, 1991), are employed (Parkin, 1991). Most competitive methods (Melander and Saunders, 1980e) utilize isotope ratio measurements based on mass spectrometry (Shine et al., 1984) or radioactivity measurements by liquid scintillation (Ando et al., 1984 Axelsson et al., 1991). However, some special methods, which are particularly useful for the accurate determination of secondary KIEs, have been developed. These newer methods, which are based on polarimetry, nmr spectroscopy, chromatographic isotopic separation and liquid scintillation, respectively, are described in this section. The accurate measurement of small heavy-atom KIEs is discussed in a recent review by Paneth (1992). 

The understanding of isotope effects on chemical equilibria, condensed phase equilibria, isotope separation, rates of reaction, and geochemical and meteorological phenomena, share a common foundation, which is the statistical thermodynamic treatment of isotopic differences on the properties of equilibrating species. For that reason the theory of isotope effects on equilibrium constants will be explored in considerable detail in this chapter. The results will carry over to later chapters which treat kinetic isotope effects, condensed phase phenomena, isotope separation, geochemical and biological fractionation, etc. 

The thermodynamic connection between IE s on gas solubility, infinite dilution Henry s law constants, and transfer free energy IE s, implies that gas-liquid chromatography should be a convenient way to study solvent effect IE s. That in fact is the case, and many authors have reported on chromatographic isotope separations and on the interpretation of the separation factors in terms of the transfer free energy IE s (Section 8.5). 

The extension of the ideas presented in Sections 5.8 and 5.10 to the theoretical treatment of isotope separation by gas chromatography is straightforward. The isotope effects observed in chromatography are governed by the isotopic ratio of Henry s Law constants (for gas-liquid separations), or adsorption constants (for... 

At low temperatures and light frequencies, the separative effect per unit shift (the terms in brackets in eq. 11.44 and 11.45) approaches (figure 11.9) and the Helmholtz free energy difference approaches the differences in zero-point energies. At high T (low frequencies), the separative effect per unit shift approaches zero and the total separative effect sis ")/approaches 1, so that no isotopic fractionation is observed. 

Table 11.4 Separative effect / and isotopic fractionation constant K for heavy isotopes, computed through equation 11.47. is angular totally symmetric stretching frequency derived from Raman spectra (see Bigeleisen and Mayer, 1947 for references).

Note that the separative effect is quite significant, even for the relatively heavy isotopes of tin. Note also the large cancellation effect on the isotopic fractionation constant, arising from superimposed partial reactions such as... 

In this paper, we apply the three algorithms to the determination of the V effect of the torsional levels of hydrogen peroxide. The size of this molecule is optimum for the comparison of the three methods. In addition, a previous study shows that torsion of the central bond can be separated from the rest of the vibrational modes, thereby simplifying the problem. Finally, the torsional pseudopotential ofD202 is also calculated for determining the isotopic substitution effects. The experimental data are from Camy-Peiret et al [15]. 

The thermal diffusion method of isotope separation has broad application to liquid-phase as well as gaseous-phase separations. The apparatus widely used for this purpose consists of a vertical tube provided with an electrically heated central wire. The gaseous or liquid mixture containing the isotopes to be separated is placed in the tube, and heated by means of the wire. In such an apparatus two effects act to separate the isotopes. Thermal diffusion tends to concentrate the heavier isotopes in the cooler outer portions of the system, while the portions near the hot wire are enriched in die lighter isotopes. At the same time, thermal convection causes the hotter fluid near the hot wire to rise, while the cooler fluid in the outer portions of the system tends to fell. The overall result of these two effects causes die heavier isotopes to collect at the bottom of the tube and the lighter at the lop, whereby both fractions may be withdrawn... 

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