Equilibrium condition for an identity pair

Hi.

I am new in COMSOL and I am checking for the software abilities. Currently I am using the identity pair in a mass transfer model. The idea is a fluid flowing on the surface of a slab, which is transfering any specie to the fluid. I would like to know if is possible to define an equilibrium condition for the specie (c_slab=K*c_fluid) on the surface in contact with the fluid?

By default the pair has Continuity, Concentration, Discontinuity, No-flux boundary conditions but I don´t know how to specify the equilibrium condition.

I appreciate any help in this topic.

Thanks a lot.

Jorge