STABILITIES OF THE FE(III), GA(III) AND IN(III) CHELATES OF N,N',N''-TRIAZACYCLONONANETRIACETIC ACID

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-degrees-C. The stability constants (K(ML) = [ML]/[L3-][M3+] 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.