protonation constant

The equilibrium constant $K H n$ for the addition of the $n$ th proton to a charged or uncharged ligand. The cumulative protonation constant, $β H n$ , is the equilibrium constant for the formation of H_nL from $n$ H⁺ and L. The same equilibrium constant may be described in several ways, depending on how the ligand, L, or the acid, has been defined.

Source:

Orange Book, p. 13