Potentiometric study of the equilibria between triethylenetetraminehexaacetic acid and H(I), Mn(II), La(III) and Ga(III) in aqueous solution Original Research Article

The equilibria of triethylenetetraminehexaacetic acid (ttha), an H6L type ligand, with Mn(II), La(III) and Ga(III) as well as with Ga(III) in combination with Ca(II) were studied potentiometrically in aqueous solution at constant ionic strength I=0.1(KNO3) and 25°C. The binary mononuclear complexes identified with Mn(II), La(III) and Ga(III) are: MnL4−, LaL3−, GaL3− (L is the fully deprotonated ligand ttha6−), their corresponding protonated species MnHpL(4−p)− (p=1,2,3), LaHpL(3−p)− (p=1,2), GaHpL(3−p)− (p=1,2,3) and the hydroxo species Ga(OH)L4− with Ga(III) only. The binuclear complexes identified with Mn(II), La(III) and Ga(III) are: Mn2L2−, La2L and Ga2L together with the corresponding monoprotonated species of La(III) and Ga(III), La2HL+ and Ga2HL+. Only Ga(III) is able to form a series of soluble binuclear hydroxo complexes of the form Ga2(OH)pLp− (p=1,2,3). The reaction of Ga(III) and excess of Ca(II) with ttha resulted in the formation of heterobinuclear mixed-valence complexes GaCaL−, the corresponding protonated species GaCaHpL(p−1)+ (p=1,2) and the major hydroxo species CaGa(OH)L2−. A comparison with the literature data is given whenever possible.