Thermodynamics of Aqueous Diethylenetriaminepentaacetic Acid (DTPA) Systems: Apparent and Partial Molar Heat Capacities and Volumes of Aqueous H2DTPA3−, DTPA5−, CuDTPA3−, and Cu2DTPA− from 10 to 55°C†

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Apparent molar heat capacities and volumes have been determined for aqueous solutions of the mixed electrolytes Na5DTPA + NaOH, Na3CuDTPA + NaOH, and NaCu2DTPA + NaOH, and the single electrolyte Na3H2DTPA (DTPA=diethylenetriaminepentaacetic acid) at temperatures from 10 to 55°C. The experimental results have been analyzed in terms of Young's rule with the Guggenheim form of the extended Debye–Hückel equation and the Pitzer ion-interaction model. These calculations led to standard partial molar heat capacities and volumes for the species H2DTPA3−(aq), DTPA5−(aq), CuDTPA3−(aq), and Cu2DTPA−(aq) at each temperature. The partial molar properties at 0.1 m ionic strength were also calculated. The standard partial molar properties were extrapolated to elevated temperatures with the revised Helgeson–Kirkham–Flowers (HKF) model. Values for the partial molar heat capacities from the HKF model have been combined with the literature data to estimate the ionization constants of H2DTPA3−(aq) and the formation constant of the CuDTPA3−(aq) copper complex at temperatures up to 300°C.

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