Binding of tripolyphosphate by aliphatic amines: Formation, stability and calculation problems

The complex formation between tripolyphosphate (P3O105-) and protonated diethylenetriamine (dien), triethylenetetramine (trien), tetraethylenepentamine (tetren) and pentaethylenehexamine (penten), has been studied potentiometrically, in aqueous solution, at 25 degrees C. It has been found that the species ALH(j) (A = amine, L = P3O105-) are formed, with j = 1...n (n = 6, 7, 8 and 8 for the above amines, respectively). The stability of these species is quite high [e.g., H(3)A(3+) + L(5+) = ALH(3)(2+): logK = 11.1 (A = trien)] and depends strictly on the charges involved in the formation reaction. The simple linear relationships logK = 1.75 z(cation) + 0.58 z(anion) can be used for making a quantitative estimate of the stability of proton-amine-P(3)O10(5-) complexes. The stability trend is: penten > tetren > trien > dien, and the cationic mixed species are more stable than the anionic ones. The effect of ionic medium and of ionic strength has been carefully taken into account and discussed. Relevant calculation problems are met with when dealing with mixed ligand systems: simultaneous formation of several species; strong interferences of Na+ (background salt or NaOH) with polyanions (tripolyphosphate in this work), and of Cl- (HCl) with amines; use of a non constant ionic medium; a high number of mass balance equations. These problems are discussed in detail and a new computer program (STACO, in Quick Basic, MS-DOS) is described.