Stabilities of the Fe(III), Ga(III) and In(III) chelates of N, N′, N″-triazacyclononanetriacetic acid

Abstract

The stability constants of the complexes of Fe(III), Ga(III) and In(III) ions with the sexadentate macrocyclic ligand 1,4,7-triazacyclonane- N, N′, N″-triacetic acid (NOTA) have been determined in KCl supporting electrolyte (0.100 M) at 25.0 °C. The stability constants (K ML=[ML]/[L 3−][M 3+] of the Ga(III) and Fe(III) complexes with NOTA are high, 10 30.98 and 10 28.3, respectively, and indicate preferential interaction of these small metal ions with NOTA. The stability constant of the In(III)NOTA complex is somewhat lower (10 26.2). Species distribution curves are presented to illustrate the conversion of the Ga(III) and In(III) chelates to the monohydroxo forms at p[H] 9.3 and 6.5, respectively. The Fe(III)NOTA complex dissociates to form ferric hydroxide at and above p[H] 7.5, while the hydroxo In(III)NOTA complex is converted to indium hydroxide at and above p[H] 7.5. The Ga(III)NOTA system is soluble at all p[H] values, as a consequence of conversion of the hydroxo complex to the tetrahydroxo gallate ion at p[H] 10.4 and above.