Ferric trans-1,2-Diaminocyclohexanetetraacetic Acid (Fe3+CDTA4-/Fe3+OH-CDTA4-) in NaCl Aqueous Solutions: Effect of Temperature on the Hydroxy Complex Formation Thermodynamic Constant and on the Activity Coefficient Ion-Interaction Parameters

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 multiplying the equilibrium reaction product (Km) measured from species molal concentrations by the activity coefficient quotient (γ±) predicted by either the Hückel, Bromley, Scatchard, or Pitzer models, all of which required knowledge 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 six temperature sets were used to evaluate the reaction enthalpy (ΔrHm) and entropy (ΔrSm) associated with the above complex formation reaction. ΔrHm was estimated to be (−17.7 ± 0.5) kJ mol-1, whereas ΔrSm was fixed at 23.0 J mol-1 K-1. It was also determined that most of the models ion-interaction empirical constants do not change with temperature in a recognizable way within the studied range. The only exception involves Pitzer's ion-interaction summations (Σ , Σ ) where a slight but definite trend was observed in relation to temperature.