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According to IUPAC definition,[1] self-diffusion coefficient is the diffusion coefficient D_i^* of species i when the chemical potential gradient equals zero. It is linked to the diffusion coefficient D_i by the equation:

D_i^*=D_i\frac{\partial\ln c_i}{\partial\ln a_i}.

Here, a_i is the activity of the species i in the solution and c_i is the concentration of i. This term is commonly assumed to be equal to the tracer diffusion determined by watching the movement of an isotope in the material of interest.

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