##### Wikipedia

**Self-diffusion**

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

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

Here, *a* is the activity of the species *i* in the solution and *c* 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.