The diffusion coefficient Di* of species i in the absence of a chemical potential gradient. It is related to the diffusion coefficient Di by \[D_{i}^{*}=D_{i}\ \frac{\partial (\ln c_{i})}{\partial \ln a_{i}}\] where ai is the activity of i in the solution, and ci is the concentration of i. If an isotopically labelled species (i*) is used to study diffusion, the tracer diffusion coefficient, Di*, is practically identical to the self-diffusion coefficient provided that the isotope effect is sufficiently small.