Exchange diffusion affecting

For a zoned zircon crystal with a core and a mantle, consider how oxygen isotope exchange would affect the center isotope composition of the core under dry conditions. The diffusivity of diffusivity in zircon under dry conditions is given as (Watson and Cherniak, 1997)... 

Even though the Nernst-Planck equations work well in many instances and their theoretical basis is sound, Helfferich (19B3) mentions several cases where they are not statisfactory. For example, they may not work well in situations where other processes besides mass transfer occur. This could occur if ion mobilities increase or decrease such that diffusion coefficients in the Nernst-Planck equations are not constant and thus particle size of the ion exchanger is affected. In zeolites, for example, which are rigid, the exchange of one counterion for another of different size affects the ease of motion and creates diffusion coefficient differences. 

Ail interpretations based on the assumptions with respect to use of the Nernst-Planck relationships are, however, subject to sizable uncertainty because the constancy attributed to the diffusion coefficients used in these relationships is susceptible to sizable variability. Bead volume variations are ignored. This leads to variations in ion mobilities which dictate changes in the diffusion coefficients of the Nemst-Planck relationships. Sizable divergence of measurements firom prediction can be expected on this basis. Even in rigid ion exchangers such as zeolites the difference in size of the counterions exchanged usually affects their mobility, and so leads to variations of the diffusivities [3]. 

Depending on the nature of the introduced electrolyte, the ion exchange can affect different regions of the EDL the diffuse and adsorption regions, and even the layer of potential-determining ions (in which case it is, however, more appropriate for one to talk about the build-up of the crystal lattice of the solid phase with the constituent ions of introduced electrolyte). The diffuse layers of counter-ions are the ones that undergo exchange most easily. 

The kinetic dependences observed might be caused by the exchange diffusion hindrance. It has been shown experimentally that the exchange is not limited by outer diffusion the results obtained from the experiments with various rates of gas circulation coincide. Exchange probably is not limited by diffusion in zeolite pores. In fact, with as little as 14% of sodium replaced by copper, which does not practically affect the porous structure, the exchange rate constant of zeolite Y becomes more than 6 times as high. 

These effects of differential vapor pressures on isotope ratios are important for gases and liquids at near-ambient temperatures. As temperature rises, the differences for volatile materials become less and less. However, diffusion processes are also important, and these increase in importance as temperature rises, particularly in rocks and similar natural materials. Minerals can exchange oxygen with the atmosphere, or rocks can affect each other by diffusion of ions from one type into another and vice versa. Such changes can be used to interpret the temperatures to which rocks have been subjected during or after their formation. 

Catalytic Properties. In zeoHtes, catalysis takes place preferentially within the intracrystaUine voids. Catalytic reactions are affected by aperture size and type of channel system, through which reactants and products must diffuse. Modification techniques include ion exchange, variation of Si/A1 ratio, hydrothermal dealumination or stabilization, which produces Lewis acidity, introduction of acidic groups such as bridging Si(OH)Al, which impart Briimsted acidity, and introducing dispersed metal phases such as noble metals. In addition, the zeoHte framework stmcture determines shape-selective effects. Several types have been demonstrated including reactant selectivity, product selectivity, and restricted transition-state selectivity (28). Nonshape-selective surface activity is observed on very small crystals, and it may be desirable to poison these sites selectively, eg, with bulky heterocycHc compounds unable to penetrate the channel apertures, or by surface sdation. 

This assumes that the gas-solid exchange kinetics at the interface is rapid. When this process affects the exchange kinetics significantly dieii analysis of concentrations layer by layer in die diffused sample is necessaty. This can be done by the use of SIMS (secondary ion mass spectrometry) and the equation used by Kihier, Steele and co-workers for this diffusion study employs a surface exchange component. 

The effect of temperature on distribution ratios has already been mentioned on page 91. Although the separation proceeds more quickly at elevated temperatures, resolution suffers because of increased rates of diffusion. However, in adsorption TLC only small increases in Rt values are observed even with a 20°C rise. Strict temperature control is not necessary if samples and standards are run at the same time, although large fluctuations should be avoided. The quality of the thin-layer materials, and in particular the presence of impurities in them, determine the extent to which partition, adsorption, ion-exchange and exclusion participate in the sorption process. These factors affect Rr values in an unpredictable manner. Thin layers should be of uniform thickness, between 0.2 and 0.3 mm with thinner layers, local variations in thickness can result in appreciable variations in Rf values. 

The immobilization of dissolved chemical species by adsorption and ion exchange onto mineral surfaces is an important process affecting both natural and environmentally perturbed geochemical systems. However, sorption of even chemically simple alkali elements such as Cs and Sr onto common rocks often does not achieve equilibrium nor is experimentally reversible (l). Penetration or diffusion of sorbed species into the underlying matrix has been proposed as a concurrent non-equilibration process (2). However, matrix or solid state diffusion is most often considered extremely slow at ambient temperature based on extrapolated data from high tem-... 

Diffusion through liquid films is usually better understood than that through porous bodies. In ion exchange, however, there is an additional flux through the him of mobile co-ions which are not present in the resin. The co-ions will be affected by the relative mobilities of the counter-ions. 

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