The Binary Diffusion Equations

For a binary system, the conservation equations (Eq. 1.3.9) may be written in terms of the molar fluxes A and as 

In most cases the general Eq. 5.1.2 is simplified somewhat before being solved. It is common, for example, to assume constant molar density and Fick diffusion coefficient D. Unless these (or other) assumptions are made it may be difficult or impossible to solve 

---

When one refers to the diffusion equation, it is usually the binary diffusion equation. Although theories for multicomponent diffusion have been extensively developed, experimental studies of multicomponent diffusion are limited because of instrumental analytical error and theoretical complexity, and there are yet no reliable diffusivity matrix data for practical applications in geology. Multicomponent diffusion is hence often treated as effective binary diffusion by treating the component under consideration as one component and combining all the other components as the second component. 

---