According to IUPAC definition,^[1] self-diffusion coefficient is the diffusion coefficient ${\displaystyle D_{i}^{*))$ of species ${\displaystyle i}$ when the chemical potential gradient equals zero. It is linked to the diffusion coefficient ${\displaystyle D_{i))$ by the equation:

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

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.