When discussing the properties of acids it is usual to specify equilibrium constants as acid dissociation constants, denoted by K, with numerical values given the symbol pK.
On the other hand, association constants are used for bases. 
However, general purpose computer programs that are used to derive equilibrium constant values from experimental data use association constants for both acids and bases. Because stability constants for a metal-ligand complex are always specified as association constants, ligand protonation must also be specified as an association reaction. The definitions show that the value of an acid dissociation constant is the reciprocal of the value of the corresponding association constant:
Notes
# For a given acid or base in water, , the self-ionization constant of water.
# The association constant for the formation of a supramolecular complex may be denoted as K; in such cases "a" stands for "association", not "acid".
# For polyprotic acids, the numbering of stepwise association constants is the reverse of the numbering of the dissociation constants. For example, for phosphoric acid (details in the polyprotic acids section below):