Self-diffusion
According to IUPAC definition, self-diffusion coefficient is the diffusion coefficient formula_1 of species formula_2 when the chemical potential gradient equals zero. It is linked to the diffusion coefficient formula_3 by the equation: formula_4 Here, formula_5 is the activity of the species formula_2 in the solution and formula_7 is the concentration of formula_2. 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.