Isotopes separation factors for

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. 

Knyazev and Sklenskaya (9) calculated the isotopic separation factors for exchange reactions between lithium complexes of nitrilotriacetic acid, ethylenediaminetetraacetic acid and aminobarbituric-2V,N -diacetic acid, and aqueous lithium ions. These reactions were postulated for a single phase system therefore, the separations cannot be observed experi-... 

The chemical enrichment of non-metal isotopes on a technical scale has been carried out for a long time whereas it has not been possible to use chemical separation of metal isotopes technically until today because of very small fractionation effects in those chemical systems. This is the main reason why metal isotopes are usually enriched by means of expensive physical methods instead of chemical exchange reactions. However, for a number of years chemical systems which could be used for the separation of metal isotopes in a technical scale have been intensively sought. Therefore, it is not surprising that in this connexion chemical systems with crown ethers and cryptands became of interest after an experiment by Jepson and DeWitt in 1976 They found a significantly higher isotopic separation factor for calcium in an extraction system with crown ethers than had been achieved by other chemical reactions up to that time. 

Table 19. Single-stage isotopic separation factors for Li/ Li with a [2b.2. 1] resin and an extraction system with benzo[15]crown-5...

Uranium concentration profile and isotope separation factors for a U(VI)-U(IV) sample eluted from an ion-exchange column see text (Nomura et al. 1996)... 

Fig. 4.12. Dependence of the isotope separation factor for hydrogen (H and T) on activation energy. (1) Hg, acidic solutions (2) Hg, alkaline solutions (3) Ga, acidic solutions (4) Ga, alkaline solutions.

Electrolysis. For reasons not fiiUy understood (76), the isotope separation factor commonly observed in the electrolysis of water is between 7 and 8. Because of the high separation factor and the ease with which it can be operated on the small scale, electrolysis has been the method of choice for the further enrichment of moderately enriched H2O—D2O mixtures. Its usefiilness for the production of heavy water from natural water is limited by the large amounts of water that must be handled, the relatively high unit costs of electrolysis, and the low recovery. 

It should be noted that the separation factor for the centrifuge process is a function of the difference in the mol wts of the components being separated rather than, as is the case in gaseous diffusion, a function of their ratio. The gas centrifuge process would therefore be expected to be relatively more suitable for the separation of heavy molecules. As an example of the equiUbrium separation factor of a gas centrifuge, consider the Zippe centrifuge, operating at 60°C with a peripheral velocity of 350 m/s. From equation 68, OC is calculated to be 1.0686 for uranium isotopes in the form of UF. ... 

Until the advent of modem physical methods for surface studies and computer control of experiments, our knowledge of electrode processes was derived mostly from electrochemical measurements (Chapter 12). By clever use of these measurements, together with electrocapillary studies, it was possible to derive considerable information on processes in the inner Helmholtz plane. Other important tools were the use of radioactive isotopes to study adsorption processes and the derivation of mechanisms for hydrogen evolution from isotope separation factors. Early on, extensive use was made of optical microscopy and X-ray diffraction (XRD) in the study of electrocrystallization of metals. In the past 30 years enormous progress has been made in the development and application of new physical methods for study of electrode processes at the molecular and atomic level. 

FIGURE 1. Separation factors for specified germanium isotopes with repect to 74Ge (reproduced from Reference 2)... 

Fig. 4.6 Chromatographic separation factors for various uranium isotopes vs. 238U as a function of mass at 433 K. s = ln[(238U/1U)iv,aq/(238U/1U)vi,resin]- The field shift (FS) and vibrational (BM) contributions are of opposite sign. Triangles = calculated vibrational (Bigeleisen-Mayer) contribution, diamonds = calculated FS contribution, circles = measured effects, open squares = calculated effects. Note that agreement between calculation and experiment is quantitative. The correlation lines are drawn through even/even data points only (Data from Bigeleisen, J., J. Am. Chem. Soc., 118, 3676 (1996))...

The primed or unprimed symbols refer to isotope fractions. The single stage separation factor for infinitesimal product removal (yi/zj 0), (rj)0, is given the symbol a . The i s index the stage number. Most often a is close to unity and it is convenient to define the isotope enrichment factor (the single stage enrichment factor), e, between the i th and (i + l) th stage... 

To separate isotopes by this process, they must be in the gaseous form. Therefore, the separation of isolopes of uranium required the conversion of die metallic uranium into a gaseous compound, for which purpose the hexafluoride. UF. was chosen. Since the atomic weight of fluorine is 19, die molecular weight of the hexafluoride of 235 LI is 235 + (6 19) = 349, and the molecular weight of die hexafluoride of 23SU is 238 + (6 x 19) = 352. Since the rate of diffusion of a gas is inversely proportional to die square root of its density (mass per unit volume), the maximum separation factor for one diffusion process of the uranium isotopes is V352/349 = 1.0043, Since only part of the gas can be allowed to diffuse, the, actual separation factoi is even less dian this theoretical maximum. 

The theory of the isotopic separation factor and its dependence on mechanism is rather involved and is of no interest to us here particularly since the argument is still open, the different views have not been unified, and the subject is not ripe for introduction in textbooks. It is generally accepted, though, that the separation factor depends on mechanism, equal values of the separation factor implying identical mechanism and vice versa. Thus, the measurement of the separation factor can serve as an important tool in mechanistic studies, even if the theory behind it is not entirely clear. 

The domain of the search for an improved separation process was defined by certain criteria (a) isotopic fractionation should be achieved by means of a two-phase, chemical exchange reaction which was amenable to countercurrent operation in a multistage contactor at ambient temperature and pressure (b) the single-stage isotopic fractionation factor for the reaction should be appreciably larger than that for the distillation of Me20 BF3 (c) the molecular species in each process stream should be thermally refluxable—i,e, convertible from one species to the other by the addition or removal of heat alone (d) process materials should be more stable with respect to irreversible decomposition than those used in the (CH3)20 process and (e) the chemical form of the product should permit a ready, quantitative conversion of the separated isotopes to the elemental state. 

The influence of the hydrating tendency of cations co-sorbed with lithium isotopes on an ion exchange column was investigated in a series of experiments summarized in Table I. As the heat of hydration of the co-sorbed cation increased, the isotopic separation factor increased (13). The nature of the anion in the solution phase had very little effect upon the separation factor. However, for systems involving complexes of lithium with ethylenediaminetetraacetic acid, there was a reversal of the isotope effect. That is, Li concentrated in the resin phase instead of the aqueous phase as it usually did in ion exchange resin systems. 

The influence of eluent concentration in ion exchange systems on the separation factor for lithium isotopes was examined by several workers (2, 6, 2i). Lee and Drury 14) found that the separation factors decreased with increasing eluent concentration when Dowex-50 and Zeo Karb resins were used. Comparable results were obtained with either chloride or acetate eluents. 

For a fixed temperature So.Do.p.Hs and the ortho-para separation factors Sp-D2.o Doy S0.H2.p-H2 are given by Equation 29 and are constants for a given surface. The dependence of the isotope separation factor on the gas phase ortho-para concentrations is shown in Figure 5. The maximum isotope separation factor is obtained when para-hydrogen is separated from para-deuterium. The minimum is obtained when ortho-hydrogen is separated from ortho-deuterium. 

In Table IV, separation factors for several of the isotopic hydrogens at considerably lower temperatures are given. The values of y and D in this Table were chosen to fit the experimental ortho-para separation factor of hydrogen at the lowest temperature, 27 °K. Even though fair agreement between theory and experiment is obtained for all of the... 

The reduced partition functions of isotopic molecules determine the isotope separation factors in all equilibrium and many non-equilibrium processes. Power series expansion of the function in terms of even powers of the molecular vibrations has given explicit relationships between the separation factor and molecular structure and molecular forces. A significant extension to the Bernoulli expansion, developed previously, which has the restriction u = hv/kT < 2n, is developed through truncated series, derived from the hyper-geometric function. The finite expansion can be written in the Bernoulli form with determinable modulating coefficients for each term. They are convergent for all values of u and yield better approximations to the reduced partition function than the Bernoulli expansion. The utility of the present method is illustrated through calcidations on numerous molecular systems. 

A heterogeneous system where two phases with different isotopic compositions can be separated is the precondition for the application of an isotopic fractionation to enrich isotopes. As mentioned before, the isotopic separation factor a for an exchange reaction corresponding to Eq. (10) can be determined by measuring the isotope ratio in phase A and phase B. 

The characteristic value for a single-stage separation system is the isotopic separation factor a which is defined by Eq. (12). Because a deviates not very much from one, a number of separation stages must be connected in series for a sufficient total isotopic enrichment. The total isotopic separation factor S, using N equilibrium stages, is given by ... 

The results for the isotopic separation of Na/ Na which were obtained by Knochel and Wilken in the system Dowex 50/aqueous or methanolic solution of cryptands are summarized in Table 13 (explanation for Krl and Kr see Chap. 4.3.1.2). To reach a high total enrichment compared with one equilibrium stage, the batch experiments were carried out as a cascade (Chap. 2.5.2). Then Eq. (20) was used for the calculation of a-values. To determine the isotopic separation factor Mr for the complex formation as well, the Kr-vuIucs were analyzed in the same system without cryptands " .iss) see Chap. 4.3.1.2). In all experiments 30 mg cation exchanger resin (Li - or Cs -form) were equilibrated with a 10M Na -solution where a lithium or cesium salt, which corresponds to the counterion of the resin, was added up to a total cation concentration of 10 M. If one has used a complexing ageftt, the initial cryptand concentration has been established to be 10 M (pH = 8). For most of the systems, the standard deviations given in Table 13 correspond to seven parallel experiments. The measurement of the radionuclides Na and Na was carried out as described in Chap. 4.2.4. 

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