Drift under a Chemical-Potential Gradient Diffusion

IONIC DRIFT UNDER A CHEMICAL-POTENTIAL GRADIENT DIFFUSION 

It has been remarked in the previous section that diffusion occurs when a concentration gradient exists. The theoretical basis of this observation will now be examined. 

Consider that in an electrolytic solution, the concentration of an ionic species i varies in the x direction but is constant in the y and z directions. If desired, one can map equiconcentration surfaces (they will be parallel to the yz plane) (Fig. 4.3). 

The situation pictured in Fig. 4.3 can also be considered in terms of the partial molar free energy, or chemical potential, of the particular species i. This is achieved through the use of the defining equation for the chemical potential [Eq. (3.61)] 

if one transfers a mole of the speeies i from an initial coneentration c, at or, to a final concentration Cf at Xp then the change in free energy, or chemical potential. 

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Remark The ionic species that undergo the electrochemical reactions move under the influence of several transport phenomena ionic drift under an electric field (a transport phenomenon also called conduction or migration), drift under a chemical-potential gradient (diffusion transport), and/or a convection phenomenon. The origin of the transport of electroactive species plays a very important part in the principles of the electrochemical methods of analysis. 

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