Migration experiments

If we neglect migration, experiments can be performed under conditions of minimal convection, which are thus dominated by diffusion. Since S increases with time t in such a case, nonstationary conditions exist. On the other hand, if convection dominates in the electrolyte bulk, S 7 /( ), and we approach stationary conditions, as far as diffusion is concerned. 

Migration experiments have shown that the hydrated cations not only carry with them the water in the inner coordination sphere, but also one or more shells of additional water molecules, for typical total values of 10-15. When the metal ion leaves the aqueous phase in the solvent extraction step, this ordered coordinated water returns to the bulk water structure, contributing an additional factor to consider in evaluating the thermodynamics of extraction. 

Vilks, P. Bachinski, D. B. 1996. Colloid and suspended particle migration experiment in a granite fracture. Journal of Contaminant Hydrology, 21, 269-279. 

Vilks, P. Baik, M. H. 2001. Laboratory migration experiments with radionuclides and natural colloids in a granitic fracture. Journal of Contaminant Hydrology, 47, 197-210. 

A portion of the solution was subjected to a qualitative electrical migration experiment using a three-component cell similar to that described by Hardwick and Robertson (79). The solution under investigation was placed in the center compartment, and in the outer compartments were placed sulfuric acid solutions of concentrations similar to that of the test solution. No diffusion occurred over a period of 24 hours. Application of about 50 volts d.c. across the electrodes caused migration of violet color to the cathode and no violet color to the anode. The results indicate a positively charged chromium species. 

First, with no exception, in all classical diffusion models one or more adjustable parameters enter in the formula of D. To calculate the magnitude of this/these param-eter/s a number of diffusion experiments must be performed with the very penetrant polymer system which one intends to simulate theoretically. In practice such experiments most often require quite sophisticated equipment to obtain the experimental data, and often non-trivial theoretical schemes to evaluate them. The attempt to save experimental work by using the adjustable parameters determined for a certain penetrant polymer system in order to estimate/predict Ds in a related system is generally not recomendable. Hence, in a first step, in order to use one or other of the classical diffusion models, one is forced to replace migration experiments with diffusion ones. 

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