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