Migration of cations

Further investigations of spinel formation reactions are to be found in the literature [1], but the above representative selection illustrates a number of typical features of these rate processes. Following migration of cations from one constituent onto the surfaces of the other, the process is limited by the rate of diffusion across a barrier layer. While obedience to a particular kinetic expression is sometimes reported, the data available are not always sufficiently precise to enable the fit found to be positively... 

Since hydrogen ions are six to twelve times more mobile than other cations, there will be a delay between loss of hydrogen ions from solution and migration of glass cations into the aqueous phase. Presumably, this electrical imbalance results in an electric field which acts as a driving force for the migration of cations. Aluminium and fluoride are almost certainly transported as cationic aluminofluoride complexes, AIF and AIFJ, mentioned above. 

The following account is based mainly on the studies of Wilson and coworkers, with some re-interpretation of experimental data. The composition of the cement used is given in Table 6.9. In brief, the reaction takes place in several overlapping stages extraction of ions from the glass, migration of cations into the aqueous phase, precipitation of insoluble salts as pH increases, leading to formation of an aluminium phosphate gel. 

During the migration of cations and anions towards their respective electrodes, each ion tends to carry solvated water along with it. As cations are usually more solvated than anions, a net flow of water towards the cathode occurs during the separation process. This effect, known as electro-osmosis, results in a movement of neutral species which would normally be expected to remain at the point of application of the sample. If required, a correction can be applied to the distances migrated by ionic species by measuring them... 

Fig. 17.5. The migration of cations, anions and neutral compounds in CZE in an ordinary fused silica capillary.

FIGURE 9 Migration of cations, anions, and neutrai compounds in capiiiary zone eiectrophor-esis in an ordinary fused siiica capiiiary. 

These data show that increased rates of migration of cations occur with small applied potentials. One may also extrapolate these data and infer that cation migration, and hence charge flow, is increased by differences in potential at local anodes and cathodes existing at the metal surface in the absence of an applied potential. 

Let us now consider the crystal MO. If the diffusion takes place by migration of cationic vacancies, the number of atoms that undergo the process depends on the vacancy concentration [Vm] and the thermal state of single atoms M (the jump takes place only whenever atom M in the neighborhood of the vacancy has sufficient energy to perform it). The diffusion coefficient associated with the vacancy migration process is given by... 

Magnetite transforms to maghemite (and thence to hematite) in water or alkali under hydrothermal conditions. Conversion to maghemite also involves outward migration of cations via cation vacancies (Swaddle Oltmann, 1980). The hydrothermal transformation is slower than that in air at the same temperature (180 °C) and it has been suggested that this is because the cation vacancies which assist cation diffusion are reduced or eliminated by the large excess of water. 

When the dissolved components are not adsorbed on the solid matrix and do not precipitate, they migrate together with water (e.g., chloride ion). Of course, every part of the solution must be electrically neutral so that the migration of negative chloride ions is followed by the migration of cations, or the migration of cations decreases the migration of anions. In spite of this fact, most literature discusses the diffusion of ions on the basis of chemical potentials, not electrochemical... 

An injection-related artifact can occur in gel buffers with consecutive electroki-netic injections from the same, low volume (10-200 mL) sample progressively smaller amounts of sample are introduced into the capillary, resulting in peak heights or areas that decrease with each injection (Schwartz et al., 1995). This effect is due to the migration of cations (e.g., Tris) from the gel buffer into the sample, changing its relative ionic strength. One solution to this problem is to perform an electrokinetic injection from a water vial prior to sample injection. This water injection generates a zone of ion depletion (i.e., rela-... 

In fact, the decrease of spin concentration observed above 550°C could be better explained by migration of cations in Si position when the cation is the active site (Ce " and Cu ", for example). In the case of LaY, we have no other explanation than that given in our paper. I would like to know if you have another hypothesis which can better explain this decrease. 

There are many redox-active ions that could potentially carry charge including abundant reduced anionic sulphur species and ferrous iron. The migration of cations and anions occurs simultaneously and must be exactly balanced, after accounting for precipitation and other fixation reactions, in order to maintain macroscopic charge... 

The electromigration rate is directly related to the charge and inversely related to the ionic radius of the complex being separated. It means that the direction of migration of cationic species is toward the cathode while the migration of the anionic species moves toward the anode. 

It is well known that the EK process relies on several interacting mechanisms, including (1) advection resulting from electro-osmotic flow and externally applied hydraulic gradients, (2) diffusion of the acid front to the cathode, and (3) the migration of cations and anions toward the respective electrode. The electrolysis of water is the dominant and most important electron transfer reaction that occurs at the electrodes during the EK process. 

A similar treatment for the case where aiuons are more mobile than cations, i.e., the migration of cations may be neglected, yields Equation (3.45),... 

Nickel is an ideal metal for oxidation studies since, under normal temperature and pressure conditions, it forms only one oxide, NiO, which is a p-type semiconductor with cation deflcit. The mechanism by which oxidation of nickel proceeds is, therefore, expected simply to involve the outward migration of cations and electrons forming a single-phase scale. 

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