Isotope exchange reactions kinetic control

Evaluation of kinetic data. Rate constants were determined for 2-H exchange from 3-R-4-methylthiazolium ions, catalyzed by D2O (pseudo first order) and DO- (second order).154 The observed rate constants for the pD-independent exchange reaction were corrected for the solvent isotope effect ( h2o/ d2o = 2.6), and the reverse protonation of the carbene by H30+ was assumed to be diffusion-controlled (k = 2 x 1010 M-1 s-1). A similar analysis was performed for the exchange catalysed by DO-. The results agreed nicely, giving pAfa = 18.9 for 213 and p/sfa = 18.0 for thiamine.154 The thiazolium ion 213 seems to be less acidic in water154 than in DMSO152 (Ap/fa = 2.4). Aside from the... 

An updated and expanded version of PHREEQC (version II) was published by Parkhurst and Appelo (1999). Version II has all of the capabilities of version I, and includes new routines for kinetically controlled reactions, solid-solution equilibria, fixed-volume gas-phase equilibria, variation of the number of exchange or surface sites in proportion to a mineral or kinetic reactant, diffusion or dispersion in one-dimensional (ID) transport, ID transport coupled with diffusion into stagnant zones, and isotope mole balance in inverse modeling. 

The same steps as discussed above for the case of isotope exchange (diffusion in liquid film, surface reaction, intraparticle diffusion) were considered in a kinetic model [771] of metal ion adsorption from solution. This model was presented in a book with diskettes (FORTRAN program, rate controlled by reaction, by transport or mixed control). 

Despite the problems inherent to the mineral synthesis technique, this technique may be the only viable means of obtaining reasonable amounts of isotopic exchange in low temperature experiments (T < 250°C). With care, the technique may indeed provide reliable equilibrium fractionation factors. Criteria that have been used to suggest an approach to equilibrium fractionations in such experiments include controlled precipitation rates, an understanding of reaction pathways and the likelihood of kinetic fractionations associated with particular pathways, similar fractionations obtained in synthesis from different starting materials, and a concurrence of fractionation factors obtained with synthesis and direct exchange techniques (e.g. O Neil and Taylor 1969 Scheele and Hoefs 1992 Bird et al. 1993 Kim and O Neil 1997 Bao and Koch 1999). It should be emphasized, however, that even when these criteria are met, synthesis experiments cannot rigorously demonstrate the attainment of isotopic equilibrium. 

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