The flux of a dissolved species

The flux of each dissolved species i - defined as the number of ions moving in a certain direction per unit of surface and time - is due to convection, diffusion and migration. 

It is important to note that normally speaking we would write v - the mean velocity of species i - instead of 

Since we assumed a dilute solution, it is supposed that the velocity of the bulk of the solution is determined by the solvent only. 

When a particle with charge is subjected to an electric field E, an electrical force acts upon it, resulting in a migration velocity. Per mole that force equals, 

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Diffusion is the net flux of matter resulting from the random motion of a species in a concentration gradient. The flux of a dissolved species i due to diffusion, Tdjff,... 

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