When surface reactions that result in etching of a bulk phase are included in the surface reaction mechanism, the user must supply the initial composition of that phase (unless there is only one bulk species in the phase). The time-dependent equation for the bulk-phase species for phases that etch is then:

(8–20)

where is the user-supplied initial estimate for the mole fraction of species in bulk phase , normalized so that their sum over all bulk-phase species is equal to one. Here, the residence time in Equation 8–20 is replaced with the time-step in transient simulations.