Cation drift

When two electrodes immersed in an electrolyte solution are connected to a power source, an electrical field of strength E is created between them. In this field a directed mass transport occurs. Anions drift to the positive pole while cations drift to the negative pole. The quantity for the mobility of ions that is independent on the field strength is obtained by dividing the ion velocitity v by the field strength. It is called ion mobility u (cm2 V-1 s 1) ... 

Fig. 1.81 Oxidation of flat surfaces, (a) When cations diffuse the initially formed oxide drifts towards the metal (b) when anions diffuse the oxide drifts in the opposite direction...

Fortunately the oxidation of many metals takes place by the diffusion of the metal cation . This flux is outwards through the oxide layer, and the work of adhesion" enables the loss of metal to be compensated for by a drift of the oxide towards the metal (Fig. 1.81). Thus the stresses set up in the maintenance of oxide/metal contact are compressive and, as such, can be more readily withstood by most oxides. Nevertheless, it is these general movements of the oxide scale which are ultimately responsible for discontinuities in the majority of cases and it is appropriate to discuss transport-induced flows before proceeding any further. 

In the presence of an external electric field E an electron, freed from its geminate cation, quickly attains a stationary drift velocity v. For relatively low fields v is proportional to E ... 

Phase 4 Hyperpolarization occurs before K+ efflux has completely stopped and is followed by a gradual drift towards threshold (pacemaker) potential. This is reflects a Na+ leak, T-type Ca2+ channels and a Na+/Ca2+ pump, which all encourage cations to enter the cell. The slope of your line during phase 4 is altered by sympathetic (increased gradient) and parasympathetic (decreased gradient) nervous system activity. 

As already stated, the diffusion coefficient and mobility of a solvated electron, though much smaller than that of a free electron in a conduction band, are some 4-5 times larger than for the solvated cation. There is some doubt whether the electron carries along a solvated shell with it, and there has been discussion in the literature of mechanisms by which molecules drift across the cavity, changing their orientation as they do so. 

Such a model can clearly account for the drop in the magnetic susceptibility. The drop in the conductivity is to be expected because the molecular dimer should be considerably less mobile than the monomer, the solvation should be stronger, and it will probably be more difficult for the NH3 molecule to drift across the pair of cavities. The alternative explanation is of course the formation of neutral complexes with the cations but if the cations retain their original mobility, this seems ruled out. 

This cannot be solved analytically, even if the functional forms of the drift mobilities dependence on electric field are known. Providing the inter-ion electric field is directed radially (centro-symmetric) and there is no applied electric field, only the radial component of the tensor equation (158) is of interest for recombination of ion-pairs. Furthermore, if electron—cation recombination is of interest, the electron is much more mobile than the cation generally there are exceptions to this statement [352—354]. Equation (158) becomes... 

Any analysis of the recombination probability of the solvated electron with a cation may be further complicated by the possibility that there are two states of the solvated electron, loosely described as a localised and a delocalised state. These are believed [399] to be associated with lower and higher drift mobilities of the solvated electron, respectively. The consequences of such a complex transport behaviour on the recombination probability of solvated electrons has been discussed by Tachiya and Mozumder [328]. 

Equation (251) is very similar to eqn. (249). It represents the change of density of both N — 1 anions and M — 1 cations due to diffusion and drift of any anion, loss by reaction of any pair of anions and cations together and gain by formation of the density from reaction of the k anion and v... 

Figure 15-14 Solid colored circles show the drift in apparent pH of a low-conductivity industrial water supply measured continuously by a single electrode. Individual measurements with a freshly calibrated electrode (black circles) demonstrate that the pH is not drifting. Drift is attributed to slow clogging of the electrode s porous plug with AgCI(s). When a cation-exchange resin was placed inside the reference electrode near the porous plug, Ag(l) was bound by the resin and did not precipitate. This electrode gave the drift-free, continuous reading shown by open diamonds. [From S. Ho, H. Hachlya. K. Baba. Y. Asano. and H. Wada, Improvement of the Ag I AgCt Reference Electrode and Its Application to pH Measurement," talonta 1995,42.1685.]...

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