If some of the species [PB] was removed from the solution by partitioning on to an adsorbent, the equilibrium of [PB] ionization would shift to the left to compensate for the lost [PB]. This means that H+ ions will be consumed too. In this case, the buffer will have to keep the pH constant by dissociating.
If pH was assumed to remain constant, how can we describe the change in PB- and H2PO4 when PB is changed?
Rearrange and take delta (the change):
If we say that:
Change in H3O+ = change in PB- (1:1 stoichiometry) Change in H3O+ = change in HPO4-2 (1:1 stoichiometry) then: Change in PB-= change in [HPO4-2 ]