PH-Driven Coordination Behaviour and Speciation Modelling of Copper and Zinc Ions in a Mixed Citric–Succinic Acid System

Abstract

The protonation and complexation behaviour of the mixed-ligand system composed of citric (Cit) and succinic (Succ) acids with mixed-metal ions of Cu²⁺ and Zn²⁺ was investigated potentiometrically at 25 °C and an ionic strength of 0.1 M KCl. The combined ligand system (Cit + Succ) exhibited apparent pKa values (3.6, 4.2, 5.2, 6.0), reflecting mutual protonation shifts arising from overlapping carboxylate equilibria. Complexation results showed that Cu²⁺ forms significantly more stable binary complexes (log β = 11.7) than Zn²⁺ (log β = 9.4). When both metal ions coexist, the overall stability of the mixed-metal system reached a remarkably high value (log β = 22.9). Calculation of the synergism index (Δlogβ) revealed strong antagonism for the ternary complexes involving a single metal bound simultaneously to Cit and Succ, particularly for citrate, which exhibited pronounced destabilisation in the presence of both Cu²⁺ and Zn²⁺. In contrast, succinate showed mild positive synergism, especially under conditional pH conditions. Most notably, the full mixed system (Cu + Zn with Cit + Succ) displayed a substantial positive Δlogβ (+1.8), corresponding to a 63-fold enhancement in stability. This indicates that the observed cooperative behaviour arises not from citrate acting as a central binding platform, but from a heterometallic–heteroligand organisation in which succinate favours complementary coordination while citrate contributes only selectively. Species-distribution analysis supports the presence of stabilised heterometallic arrangements rather than classical central–peripheral ligand partitioning.