Isotopes exchange rate tracers

The large Fe isotope fractionations predicted to occur between aqueous ferric and ferrous Fe species (Fig. 3) has been investigated experimentally at two temperatures for hexaquo Fe(III) and Fe(II), as well as the effect of CF substitution (Johnson et al. 2002 Welch et al. 2003). The kinetics of isotopic exchange in these experiments was determined using an enriched Te tracer for the ferric Fe phase. The Fe tracer experiments reported in Johnson et al. (2002) and Welch et al. (2003) indicate that 95% isotopic equilibrium between Fellll) and Fe(II)gq occurs within —60 seconds at room temperature (22°C), or within —5 minutes at 0°C. The relatively slower isotopic exchange rates at lower temperatures are expected. 

With the development of liquid scintillation counters which permit tracer levels of tritium to be measured accurately, isotope exchange reactions are now usually carried out with tritium rather than deuterium. Frequently, however, it is useful to compare rates of exchange for all three isotopes and deuterium exchange may be followed using, for example, mass spectrometry [22]. An example of the application of isotope exchange to proton transfer from carbon is shown in eqns. (8) and (9) for the hydroxide ion catalysed exchange of phenylacetylene in aqueous solution [23], viz. 

In the ionization mechanism exemplified by benzhydryl thiocyanate, the reaction is strictly first order over a wide concentration range. The rate of isomerization increases with increasing solvent polarity Tracer and stereochemical evidence indicates that this involves an internal ion-pair mechanism. Isomerization is faster than isotopic exchange and so it was concluded that the former process occurs via an intimate ion pair which was shown to collapse to thiocyanate and isothiocyanate in the ratio of 5 1. In the case of optically active 4-chlorobenzhydryl thiocyanate in acetonitrile, racemization occurs at a rate comparable with isomerization. With a given solvent the structural effect acts essentially on the energy term and for a given substrate, the solvent effect acts essentially on the entropy term. 

Zelsmann and co-workers [88] have reported tracer diffusion coefficients for water in Nafion membranes exposed to water vapor of controlled activity. These were determined by various techniques, including isotopic exchange across the membrane. They reported apparent self-diffiision coefficients of water much lower than those determined by Zawodzinski et al. [64], with a weaker dependence on water content, varying from 0.5 x 10 cm to 3 x 10 cm /s as the relative humidity is varied from 20 to 100%. It is likely that a different measurement method generates these large differences. In the experiments of Zelsmaim et al., water must permeate into and through the membrane from vapor phase on one side to vapor phase on the other. Since the membrane surface in contact with water vapor is extremely hydrophobic (see Table 7), there is apparently a surface barrier to water uptake from the vapor which dominates the overall rate of water transport in this type of experiment. 

The isotopic oxygen exchange rates between these ions and H2O can be represented by a first-order process provided only tracer quantities of labeled reactants are introduced into the chemically equilibrated solutions. Applied to Equations (81) and (82) this rate law results in Equations (83) and (84) as follows... 

AH of the studies described in earlier chapters were concerned with the observed net velocity of a chemical reaction. In die steady state, the net velocity is a difference between the absolute forward and reverse velocity of any step. The unidirectional velocity of a step may be considerably faster than the observed net velocity. Isotope exch ge is based on the simple fact that, in a chemical reaction, even if it is at equilibrium, when its net rate is zero by definition, the unidirectional rates through steps or groups of steps can be measured by means of isotopic tracers. Therefore, the isotope exchange studies provide a way of measuring the unidirectional rates of individual steps within a reaction sequence. 

If there exists a rate determining step, an isotope tracer method can be used for the determination of a or Urds at equilibrium or near equilibrium conditions. Its basic principle has been expounded systematically by Horiuti, Temkin and Happel. If the exchange rate or rate at equilibrium defined by Wangner is introduced, when the reaction approaches equilibrium, A is close to zero for any elementary step. Consequently, Horiuti-Temkin relation can be deduced ... 

Transport properties (separate determination of electronic and ionic conductivity, oxygen tracer diffusion and chemical diffusion) and surface stages (rate/constant of exchange) parameters of dense ceramics can be studied by several methods, such as electronic blocking polarization methods [32-34], O tracer profile analysis by SIMS [26, 27, 29, 35], study of the isotope exchange kinetics by gas-phase analysis of the isotope composition [27,... 

Keenan has made an investigation of the exchange reaction between Pu(IV) and Pu(in) in perchlorate media. The isotopic method was used with an a energy analyser to separate the tracer activity ( Pu) from that normally present from the major constituent ( Pu). Tributylphosphate extraction of the Pu(IV) formed the basis of the separation method. It was shown that the rate law has the approximate form... 

Figure 12. Extent of dissolution and re-precipitation between aqueous Fe(III) and hematite at 98°C calculated using Fe-enriched tracers. A. Percent Fe exchanged (F values) as calculated for the two enriched- Fe tracer experiments in parts B and C. Large diamonds reflect F values calculated from isotopic compositions of the solution. Small circles reflect F values calculated from isotopic compositions of hematite, which have larger errors due to the relatively small shifts in isotopic composition of the solid (see parts B and C). Curves show third-order rate laws that are fit to the data from the solutions. B. Tracer experiment using Fe-enriched hematite, and isotopically normal Fe(lll). C. Identical experiment as in part B, except that solution Fe(lll) is enriched in Te, and initial hematite had normal isotope compositions. Data from Skulan et al. (2002).

The rate of self-exchange reactions can generally be measured by isotopic tracer methods, but in several cases other techniques (optical rotation, nmr, epr) are more useful. Reactions like (18),... 

A number of other reactions have been studied using orthodox isotopic tracer techniques for which complete exchange in the time of separation was observed. The lower limits for the specific rates that can be calculated are considerably smaller than those to which we have referred, and since in many cases no proof could be adduced that the separation method did not cause the exchange, these results are not reported. The review by Amphlett (5) gives references to many of the literature reports on these and other reactions in the entire field. 

In deeper waters the deficiency of from its uranium precursor is dramatic (Table 5.5) because this thorium isotope has a very long half hfe (c. 75 000 y) and thus particle scavenging is much more effective at removal than the ingrorvth toward secular equilibrium with Bacon and Anderson (1982) showed that depth profiles of dissolved and particulate °Th could be used to demonstrate the dynamic relationship of metal exchange between particulate and dissolve forms. They argued that the thorium-uranium isotope pair could be used as a tracer of particle removal rates for those metals that faU in the category of adsorbed in Fig. 1.3. 

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