The
diffusion coefficient

of species

in the absence of a
chemical potential gradient.
It is related to the
diffusion coefficient

by
where

is the activity of

in the solution,
and

is the concentration of

.
If an
isotopically labelled species
(
)
is used to study
diffusion, the
tracer diffusion coefficient,

,
is practically identical to the self-
diffusion coefficient provided that the
isotope effect is sufficiently small.
Source:
PAC, 1972, 31, 577
(Manual of Symbols and Terminology for Physicochemical Quantities and Units, Appendix II: Definitions, Terminology and Symbols in Colloid and Surface Chemistry)
on page 617
Cite as:
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8.
doi:10.1351/goldbook.