COMPLEXATION OF TRIVALENT CATIONS (AL(III), CR(III), FE(III)) WITH 2 PHOSPHONIC-ACIDS IN THE PH RANGE OF FRESH-WATERS

The complex formation constants of two phosphonic acids, HEDP and ATMP . with three trivalent metallic cations, Al(III), Cr(III) and Fe(III), have been determined by acid-base titration at 25 degrees C and const ant ionic strength (0.1 mol 1(-1), KNO3), using Martell and Motekaitis ' computer programs. Species distribution curves showed that all three cations are in complex form in the pH range of fresh waters (5-9). Th e study of different cation/ligand ratios proved that both ligands mai nly form anionic soluble complexes for systems having an excess of lig and--as protonated and unprotonated forms and especially ternary compl exes with HEDP. For higher metal concentrations (excess of cation), we akly soluble species of HEDP and ATMP were formed with Al(III) and Cr( III). Two insoluble complexes with ATMP have been identified by SEM/ED AX as AlH3X(s) and Cr2X(s). Regarding Fe(III) species, Fe(OH)(3(s)) pr ecipitate seems to predominate in solution. (C) 1998 Elsevier Science B.V. All rights reserved.