protonation constant
The equilibrium constant \(K_{\text{H}n}\) for the addition of the \(n\)th proton to a charged or uncharged ligand. The cumulative protonation constant, \(\beta_{\text{H}n}\), 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