Self-diffusion

${\displaystyle D_{i}^{*}=D_{i}{\frac {\partial \ln c_{i}}{\partial \ln a_{i}}}.}$

Here, ${\displaystyle a_{i}}$ is the activity of the species ${\displaystyle i}$ in the solution and ${\displaystyle c_{i}}$ is the concentration of ${\displaystyle i}$. 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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