POTENTIOMETRIC STUDY OF THE EQUILIBRIA BETWEEN TRIETHYLENETETRAMINEHEXAACETIC ACID AND H(I), MN(II), LA(III) AND GA(III) IN AQUEOUS-SOLUTION

The equilibria of triethylenetetraminehexaacetic acid (ttha), an H6L t ype ligand, with Mn(II), La(III) and Ga(III) as well as with Ga(III) i n combination with Ca(II) were studied potentiometrically in aqueous s olution at constant ionic strength I = 0.1(KNO3) and 25 degrees C. The binary mononuclear complexes identified with Mn(II), La(III) and Ga(I II) are: MnL4(-), LaL3(-) GaL3(-), (L is the fully deprotonated ligand ttha(6-)), 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 s pecies 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(LII), La2HL+ and Ga2HL+. Only Ga(III) is able to form a series of soluble bi nuclear hydroxo complexes of the form Ga-2(OH)(p)Lp- (p = 1,2,3). The reaction of Ga(III) and excess of Ca(II) with ttha resulted in the for mation of heterobinuclear mixed-valence complexes GaCaL-, the correspo nding protonated species GaCaHpL(p-1+) (p = 1,2) and the major hydroxo species CaGa(OH)L2-. A comparison with the literature data is given w henever possible. (C) 1998 Elsevier Science B.V.