Multiprotic molecules, and those with multiple tautomeric states, have two types of pK a , microscopic and macroscopic. The microscopic pK a applies to a specific transition or equilibrium between microstates, i.e. for a transition between a specific tautomer at one formal charge and that at another formal charge (e.g. two states at different formal charges in Figure ). It relates to the acid dissociation constant associated with that specific transition. As a special case, a microscopic pK a sometimes refers to the pK a of deprotonation of a single titratable group while all the other titratable and tautomerizable functional groups of the same molecule are held fixed, but this might possibly not reflect the dominant deprotonation pathway of a given acidic tautomer if the base state possesses energetically favored alternate tautomers. There is no pK a between two tautomers with the same formal charge because they have the same number of protons so their relative probability is independent of pH. The pH-independent free energy difference between them determines their relative population .