Stability Constants of Fe(III) and Cr(III) Complexes with <scp>dl</scp>-2-(2-Carboxymethyl)nitrilotriacetic Acid (GLDA) and 3-Hydroxy-2,2′-iminodisuccinic acid (HIDS) in Aqueous Solution

Abstract

The complex formation equilibria of Cr3+ and Fe3+ ions in aqueous solution with two biodegradable aminopolycarboxylate chelants (dl-2-(2-carboxymethyl)nitrilotriacetic acid (GLDA) and 3-hydroxy-2,2′-iminodisuccinic acid (HIDS)) were investigated. The potentiometric data obtained at the constant ionic strengths (I) of (0.1 and 1.0) mol·dm–3 KCl and at (25 ± 0.1) °C was processed with the aid of the computer program HYPERQUAD 2008. The formation constants of the proton–chelant and metal–chelant (log KML) species (M = Fe3+ or Cr3+; L = GLDA or HIDS) were determined, and the concentration distributions of complex species in solution were evaluated for both metal ions. In various pH conditions, the interaction between the chelants (L = GLDA or HIDS) and the metal ions (M = Fe3+ or Cr3+) leads to the formation of different complexes formulated as MH2L+, MHL, ML–, M(OH)L2–, and M(OH)2L3–. The log KML values at I = 0.1 mol·dm–3 KCl (T = (25 ± 0.1) °C) were 15.27 (log KFe–GLDA), 14.96 (log KFe–HIDS), 13.77 (log KCr–GLDA), 12.67 (log KCr–HIDS), and at I = 1.0 mol·dm–3 KCl (T = (25 ± 0.1) °C) were 14.79 (log KFe–GLDA), 14.34 (log KFe–HIDS), 12.90 (log KCr–GLDA), 12.09 (log KCr–HIDS). The conditional stability constants (log K′ML) of the ML complexes were calculated in terms of pH in the range of 2 to 12 and compared with the same for EDTA and other biodegradable chelants (NTA and EDDS).