At some level, the macroscopic pK a can be thought of as describing the acid dissociation constant related to the loss of a proton from a molecule regardless of which functional group the proton is dissociating from, but it may be more helpful to think of it (in the case of polyprotic molecules) as a macroscopic observable describing the collective behavior of various tautomeric states as the dominant formal charge of the molecule shifts. In cases where a molecule has only a single location for a titratable proton, the microscopic pK a becomes equal to the macroscopic pK a .