Chloride isotope separation

Figure 26-31 Separation of natural isotopes of 0.56 mM Cl by capillary electrophoresis with indirect spectrophotometrlc detection at 254 nm. Background electrolyte contains 5 mM CrOJ to provide absorbance at 254 nm and 2 mM borate buffer, pH 9.2. The capillary had a diameter of 75 m, a total length of 47 cm (length to detector = 40 cm), and an applied voltage of 20 kV. The difference in electrophoretic mobility of 36C and 37CI is just 0.12%. Conditions were adjusted so that electroosmotlc flow was nearly equal to and opposite electrophoretic flow. The resulting near-zero net velocity gave the two isotopes maximum time to be separated by their slightly different mobilties. [From C. A Lucy and T. L McDonald, "Separation of Chloride Isotopes by Capillary 35 40 45 Electrophoresis Based on the Isotope Effect on Ion Mobility"Anal.

Up to now, isotopic separations in extraction systems have been investigated for calcium by Jepson and DeWitt as well as by Heumann and Schieferfor lithium by Jepson and Cairns and for sodium by Knochel and Wilken In these investigations the system chloroform/water was mainly used. Jepson and DeWitt have also carried out experiments with methylene chloride but no detailed results have been submitted about this system. 

In Table 12 the distribution of sodium ions between water and chloroform (referred to g HjO/g CHCI3) is presented in dependence on the different polyethers. All results were obtained under analogous conditions with 0.1 mmol Na and 0.1 mmol polyether in the system where the pH-value was established to be 8 by adding 10 mmol of tetraethylammonium chloride. The establishment of the equilibrium requires less than 60 min in all systems and was followed by the y-activity of the sodium isotopes and the P-activity of C-labeled polyethers. The enrichment of one of the sodium isotopes in a practical scale from a Na/ Na-mixture can only be achieved in a system where the distribution ratio (Na ),/(Na ) g is not too high. However, in contrast to the enrichment of stable isotopes from a sample with natural isotope abundance, the enrichment of Na or of Na from an isotopic mixture is not of great importance because these two isotopes can be produced by nuclear reactions. On the other hand, the investigations on sodium isotopic separations are of common interest in respect to further knowledge about isotopic effects. 

The alkali elution curves of the displacement chromatography are shown in Fig. 17, the ratio Li/ Li dependent on the effluent volume is given in Fig. 18. As one can see from Fig. 18, an increase of the Li/ Li ratio from 0.07 to 0.09 is found within the lithium elution band which corresponds to a column length of 91 cm. The relative enrichment of the heavy lithium isotope Li in the first fractions — that is in the methanolic phase — agrees with isotopic separations of calcium using a condensation resin with dibenzo(18]crown-6 and [2b.2.2], respectively (Chap. 4.3.2.3 and Chap. 4.3.2.4). Fujine and coworkers have also carried out one breakthrough experiment with methanolic solutions of cesium chloride and lithium acetate The evaluation of the front analysis with Spedding and coworkers method resulted in an isotopic separation factor of a = 1.014. 

Separation of isotopes is an extremely challenging analytical problem. Lucy and McDonald [4] demonstrated the extraordinary separation power of CE by obtaining a baseline resolution of two chloride isotopes, and Counter migration was... 

Figure 10.5. Separation of chloride isotopes by CE using counter migration (Courtesy of Youchun Shi ).

C. A. Lucy and T. L. McDonald, Separation of chloride isotopes by capillary electrophoresis based on the isotope effect on ion mobility, Anal. Chem., 67,1074,1995. 

Such a reaction mechanism would be fostered by the pres ce of anions that form complexes more readily than perchlorate ion. If HCl solutions are used rather than HCIO4 it is found that the exchange takes place more rapidly and the half time of exchange is only 2 min, which agrees with the proposed mechanism since chloride ions are known to be more favorable to complex formation than perchlorate ions. Without the use of radioactive (or isotope separated) chromium to label one of the original oxidation states there would be no means of identifying the exchange. 

The principle of the double-isotope dansyl assay is as follows A sample containing ammo acids m unknown quantities is added to a mixture of C-labeled ammo acids, and the resulting mixture is dansylated using [ H]-dansyl chloride. Following separation of the dansylated amino acids, generally by thin-layer chromatography, the ratio dpm [ H].dpm ( C) for each dansyl ammo acid is measured. This ratio bears a linear relation to the amount of unlabeled amino acid present m the original sample see section 2.4). 

The neutron absorption cross-sections of any liquid salt for reactor applications must be low to avoid excessive parasitic capture of neutrons. For thermal and intermediate neutron spectrum reactors, this probably eliminates chloride salts with their higher nuclear cross sections, even if the high cross section Cl is removed. Only fluoride salts are candidates. A wide variety of atoms have low cross sections however, the realistic candidates are also restricted by the requirements of thermodynamic stability to ensure viable materials of construction for the container. Table XXVI-5 shows the primary salt options and their cross sections. If either lithium or boron is used as a salt component, isotopically separated lithium and boron are required to have a salt with a low absorption cross section. 

Cl is activated to Cl, a long lived radio nuclide with a half life of 300 000 years. Most chlorides are soluble in water, which makes it more difficult to avoid long term release of Cl from a repository to the environment. In contrast, fluorides have low neutron cross sections and do not activate to radionuclides that create major challenges in waste management systems and form many insoluble waste forms. If a chloride salt was used, isotopically separated Cl would probably be required. 

Earlier proposals for fast-spectrum MSRs have used chloride salts [6]. However chloride salts have three major drawbacks (1) a need for isotopically separated chlorine to avoid high-cross-section nuclides (2) the activation product Cl, which presents significant challenges to waste management because of its mobility in the environment and (3) the more corrosive characteristics of chloride systems relative to fluoride systems. 

A major advantage of this hydride approach lies in the separation of the remaining elements of the analyte solution from the element to be determined. Because the volatile hydrides are swept out of the analyte solution, the latter can be simply diverted to waste and not sent through the plasma flame Itself. Consequently potential interference from. sample-preparation constituents and by-products is reduced to very low levels. For example, a major interference for arsenic analysis arises from ions ArCE having m/z 75,77, which have the same integral m/z value as that of As+ ions themselves. Thus, any chlorides in the analyte solution (for example, from sea water) could produce serious interference in the accurate analysis of arsenic. The option of diverting the used analyte solution away from the plasma flame facilitates accurate, sensitive analysis of isotope concentrations. Inlet systems for generation of volatile hydrides can operate continuously or batchwise. 

Stabilization of a carbocation intermediate by benzylic conjugation, as in the 1-phenylethyl system shown in entry 8, leads to substitution with diminished stereosped-ficity. A thorough analysis of stereochemical, kinetic, and isotope effect data on solvolysis reactions of 1-phenylethyl chloride has been carried out. The system has been analyzed in terms of the fate of the intimate ion-pair and solvent-separated ion-pair intermediates. From this analysis, it has been estimated that for every 100 molecules of 1-phenylethyl chloride that undergo ionization to an intimate ion pair (in trifluoroethanol), 80 return to starting material of retained configuration, 7 return to inverted starting material, and 13 go on to the solvent-separated ion pair. 

Peschanski, using the isotopic method ( ° Hg), has found complete exchange (0 °C) in methanol and various other non-aqueous media. The separation methods used were, (a) paper and column chromatography, (b) paper electrophoresis, and (c) precipitation of Hg(I) with chloride. In the presence of cyanide ions, however, less than complete exchange could be observed. Zero-time exchange was again found to vary in the same manner as for aqueous media. Similar effects were observed in the presence of chloride ions. 

In 1961, Roig and Dodson carried out a further study of the exchange in perchlorate media under identical conditions (25 °C, fi = 3.0 M) to those in the Tl(III) hydrolysis studyThe isotope was used, with a separation procedure based on extracting TI(III) from reaction mixtures with either methyl isobutyl ketone or diethyl ether. The exchange was examined in the absence of light, and a correction procedure to eliminate the catalytic effects of traces of chloride ions was used since Tl(III) concentrations of 10 M were necessary at the very low acidities employed. Using the known values of the first and second hydrolysis constants of Tl(III) (K2 and K3)... 

In aqueous solution this exchange has been studied, in the absence of oxygen, in chloride and sulphate media. The isotope Sn was used as the indicator and the separation of Sn(IV) and Sn(II) was achieved by the formation of the insoluble salts caesium hexachlorostannate(IV) and stannous oxalate . 

The effect of chloride ion on the exchange was found by these workers to be very small, whereas Plane and Taube had estimated a rate coefficient about five times larger in the presence of 10 M chloride ion than in perchlorate solution. Van der Straaten and Aten have studied the exchange in media 1 M with respect to HCl and have estimated a rate coefficient 3.0 x 10 l.mole". sec . The isotopic method ( Cr) and a separation procedure based on the precipitation of Cr(II) as the acetate complex was used. 

The exchange of Mo between the anions Mo(CN)g and Mo(CN)g has been investigated by the isotopic method ( Mo) and the separation methods (a) precipitation of Mo(CN)g with either ethanol or cadmium ions, and (b) precipitation of Mo(CN) with tetraphenylarsonium chloride. Complete exchange was observed by Wolfgang even with reactant concentrations 5x10 M. An estimate of the rate coefficient at 2 °C of >10 l.mole . sec has been sug-geMd. 

Silverman and Dodson made the first detailed isotopic study of this exchange system using the separation afforded by the addition of 2,2 -dipyridyl at pH 5, followed by the precipitation of the ferric iron with either ammonia or 8-hydro-xyquinoline. Dodson , using this separation method, had previously obtained an overall rate coefficient of 16 l.mole" sec at 23 °C for 0.4 M perchloric acid media. The exchange in perchlorate and perchlorate-chloride media was found to conform to a rate law, first order with respect to both total ferrous and ferric ion concentrations, with an observed rate constant (k bs) dependent on the hydrogen-ion concentration, viz. 

The isotopic method has been used in conjunction with a flow apparatus by Stranks, to measure the exchange between the cyclopentadienyl complexes of iron (III) and iron (II) in methanol. Separation was based on the insolubility of Fe(C5H5) in petroleum ether at —80 °C. Using Fe(II) and Fe(III) 10 M and short reaction times ( msec), a rate coefficient 8.7 x 10 l.mole .sec at — 75 °C was obtained. The rate of exchange in the presence of chloride ions and inert electrolytes was found to be more rapid. Calculations using Marcus Theory showed reasonable agreement with the experimental observations. In deuterated acetone, line broadening measurements have led to an estimate of this rate coefficient of > 10 l.mole . sec at 26 °C. 

In this method, each gas is produced in a separate compartment so they have high purity. In this process, deuterium oxide, D20, is electrolyzed more slowly so the water becomes enriched in the heavier isotope. The other electrolytic process that produces hydrogen is the electrolysis of a solution of sodium chloride. 

Crystallisation was one of the earliest methods used for separation of radioactive microcomponents from a mass of inert material. Uranium X, a thorium isotope, is readily concentrated in good yield in the mother liquors of crystallisation of uranyl nitrate (11), (33), (108). A similar method has been used to separate sulphur-35 [produced by the (n, p) reaction on chlorine-35] from pile irradiated sodium ot potassium chloride (54), (133). Advantage is taken of the low solubility of the target materials in concentrated ice-cold hydrochloric acid, when the sulphur-35 as sulphate remains in the mother-liquors. Subsequent purification of the sulphur-35 from small amounts of phosphorus-32 produced by the (n, a) reaction on the chlorine is, of course, required. Other examples are the precipitation of barium chloride containing barium-1 from concentrated hydrochloric acid solution, leaving the daughter product, carrier-free caesium-131, in solution (21) and a similar separation of calcium-45 from added barium carrier has been used (60). 

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