Formation and stability of proton-amine-inorganic anion complexes in aqueous solution

Interaction of some amines with inorganic ligands was studied potentiometrically in aqueous solutions at 25°C at different ionic strengths. The systems taken into account were: meta-PP, meta-TPP, en-PP, en-TPP, en-HPO₄^2–, dap-SO₄^2–, tetren-HPO₄^2–, 6da-PP and 8da-TPP (meta=methylamine, en=ethylenediamine, tetren=tetraethylenepentamine, dap=1,2-diaminopropane, 6da=1,6-hexanediamine, 8da=1,8-octanediamine, PP=pyrophosphate, TPP=tripolyphosphate). Several ALH_r species are formed for all the studied mixed systems (A=amine, L =inorganic ligand), and in some cases A_pL_qH_r species were also found. The stability of mixed proton-ligand-ligand species is proportional to the charges involved in the complexation reaction and, to some extent, it depends on steric characteristic of the ligands. A simple relationship has been found between formation constants of mixed ligand species and charges involved in the complexation reaction. The ionic strength dependence of formation constants has been taken into account, and it was found that, also for these types of mixed complexes, a general equation independent of the nature of the reactants can be used.