A very common boundary condition for mass transfer is to know the concentration at the boundaries. This could happen for several physically realistic reasons: you measure the concentration, the boundary is between phases and you have equilibrium data that tells you the concentration (i.e., the boundary (interface) is at the saturation concentration), you have a rapid (infinitely fast) reaction occurring at the boundary so that the concentration at there is zero.

As with heat transfer, it is critical that we realize that we
can only use this condition if we actually know a **value** for
the concentration

We can specify a constant *flux* at the boundary. This is
useful as the symmetry condition (at the center of a shere or
cylinder) where the flux would be zero, if we know that one
boundary is impenetrable so that the flux is zero, if we know what
the value of the rate of reaction is (since we could set the flux
at the boundary equal to this value), or if we simply measure the
flux/flow of mass.

Again, we can combine the two, and specify that the flux is somehow related to the concentration. This would happen at an interphase boundary where we might know that diffusion is equal to convection.

Identify reasonable boundary conditions in a mass transfer problem (explain when each is most useful)