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

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 Cr 3+; 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 K Fe-HIDS), 13.77 (log KCr-GLDA), 12.67 (log K Cr-HIDS), and at I = 1.0 mol·dm-3 KCl (T = (25 ± 0.1) °C) were 14.79 (log KFe-GLDA), 14.34 (log K Fe-HIDS), 12.90 (log KCr-GLDA), 12.09 (log K Cr-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). © 2012 American Chemical Society