Thermodynamic and structural studies of aqueous chelating agents and their metal complexes at various temperatures and pressures : diethylenetriaminepentaacetic acid (DTPA) and tartaric acid

摘要

Chelating agents are used in many industrial applications including cleaning conventional boilers, decontaminating nuclear power systems and treating oil wells to remove scales and certain clay minerals. Standard partial molar heat capacities Cp° and volumes V° are important in this context because they define the temperature- and pressure-dependence of the stability constants, respectively. They are also of considerable interest in the development of semi-empirical models and simulations of ionic solvation, because both are sensitive indicators of hydration effects. In this research, we have studied the thermodynamic and structural properties of (i) aqueous diethylenetriamine- pentaacetic acid (DTPA) and its metal complexes, and (ii) aqueous tartaric acid and its sodium salts, to examine the effects of temperature and pressure. -- The apparent molar heat capacities and volumes of aqueous DTPA species, Na₃H₂DTPA (aq) and Na₅DTPA (aq), and metal complexes, Na₃CuDTPA (aq), NaCu₂DTPA (aq), Na₃NiDTPA (aq), Na₂FeDTPA (aq) and NaFeHDTPA (aq) were determined over a range of temperatures between 283 to 328 K by a Sodev Picker flow micro-calorimeter and vibrating-tube densimeter at a pressure of 0.1 MPa. The experimental results were analyzed by means of the Guggenheim form of the extended Debye-Huckel equation to obtain Cp°, V° and expressions for the excess properties. The revised Helgeson-Kirkham-FIowers (HKF) model has been used to represent the temperature-dependence of these standard partial molar properties within the experimental uncertainty. It was found that, while the values of Cp° and V° of DTPA species and metal complexes are charge dependent, the metal complexes show regular behavior despite the complexity in their structures and charge distribution. Widely applicable predictive methods to estimate the "non-Born" contributions to the partial molar heat capacities and volumes of the complexes of metal ions (MZ⁺) with the chelating agents, EDTA⁴⁻ (aq), and DTPA⁵⁻ (aq) over a wide range of temperature were developed using the Cu²- complex as a model system. The results may also be applicable to nitrilotriacetic acid (NTA) complexes. -- The apparent molar heat capacity of L-tartaric acid (H₂Tar, aq) and its sodium salts, NaHTar (aq) and Na₂Tar (aq), at temperatures from 283 to 328 K, and the apparent molar volume of H₂Tar(aq) and sodium tartrate (Na₂Tar, aq) at temperatures 283 to 530 K, and pressures up to lOMPa were determined. The behavior of V2° (Na₂Tar, aq) over the whole temperature range is typical of most electrolytes. Unusual behavior observed for V° (H₂Tar, aq) at temperatures above 473 K reflects the attractive interactions between tartaric acid molecules and water because of the strong hydrogen-bonding. Various semi- empirical solvation models were used to extrapolate standard partial molar properties of tartaric acid and sodium tartrate to elevated temperatures.