Authors: Seyed Y. S. Lisar, Rouhollah Motafakkerazad, Mosharraf M. Hossain and Ismail M. M. Rahman

The protonation and complex formation equilibria of two biodegradable aminopolycarboxylate chelants {dl-2-(2-carboxymethyl)nitrilotriacetic acid (GLDA) and 3-hydroxy-2,2′-iminodisuccinic acid (HIDS)} with Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ ions were investigated using the potentiometric method at a constant ionic strength of I = 0.10 mol·dm⁻³ (KCl) in aqueous solutions at 25 ± 0.1 °C. The stability constants of the proton–chelant and metal–chelant species for each metal ion were determined, and the concentration distributions of various complex species in solution were evaluated for each ion. The stability constants (log₁₀ K _ML) of the complexes containing Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ ions followed the identical order of log₁₀ K _CuL > log₁₀ K _NiL > log₁₀ K _PbL > log₁₀ K _ZnL > log₁₀ K _CdL for either GLDA (13.03 > 12.74 > 11.60 > 11.52 > 10.31) or HIDS (12.63 > 11.30 > 10.21 > 9.76 > 7.58). In each case, the constants obtained for metal–GLDA complexes were larger than the corresponding constants for metal–HIDS complexes. The conditional stability constants (log₁₀ \( K_{\text{ML}}^{'} \) ) of the metal–chelant complexes containing GLDA and HIDS were calculated in terms of pH, and compared with the stability constants for EDTA and other biodegradable chelants.