| Abstract
| - The impact of the variation of temperature from (280 to 323) K on the thermodynamic equilibrium constant(K°) for the Fe3+CDTA4- + OH- ⇄ Fe3+OH-CDTA4- complex formation reaction (CDTA is trans-1,2-diaminocyclohexanetetraacetic acid) was investigated in alkaline solutions. K° is obtained by multiplyingthe equilibrium reaction product (Km) measured from species molal concentrations by the activity coefficientquotient (γ±) predicted by either the Hückel, Bromley, Scatchard, or Pitzer models, all of which requiredknowledge of ion-interaction empirical constants. For each temperature set at (280, 288, 298, 305, 313,323) K ± 1 K, the measured Km values acquired for a multitude of sodium chloride solutions of (1.5 ×10-4 to 0.95) mol kg-1 were fitted on the (K°/γ±) relationship for all four activity coefficient models. K°and ion-interaction empirical constants were obtained from this action. K° values computed from the sixtemperature sets were used to evaluate the reaction enthalpy (ΔrHm) and entropy (ΔrSm) associated withthe above complex formation reaction. ΔrHm was estimated to be (−17.7 ± 0.5) kJ mol-1, whereas ΔrSmwas fixed at 23.0 J mol-1 K-1. It was also determined that most of the models ion-interaction empiricalconstants do not change with temperature in a recognizable way within the studied range. The onlyexception involves Pitzer's ion-interaction summations (Σ, Σ) where a slight but definite trend wasobserved in relation to temperature.
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