Time independet boundary with pdede solver

Hello Matlab community!

Im using Matlab the first time in a student project for the calculation of diffusionprofiles in metals.

In my problem I have a gasphase with a higher gas molecule concentration then the maximum solubility of the gas in the metal at this temperature. Also the pressure is regulated and therefore every diffusion flux into the material will be compensated.

Therefore my boundary conditions are:

-concentration at metal/atmosphere boundary c(x=0, t)= c0 with c0 as solubility dependent on the Temperature.

-concentration at a depth d c(x=d,t)=0 where d is big compared to the diffusionlength.

My problem is now that I don't understand how to define a constant bc with the terms pl,ql,pr and qr when it must satisfy the equation "p+q*f=0"

Thank you for your help!!!