# Self-diffusion

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.