protonation constant
The equilibrium constant KHn for the addition of the nth proton to a charged or uncharged ligand. The cumulative protonation constant, β H n, is the equilibrium constant for the formation of HnL 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.
Orange Book, 2nd ed., p. 13 (