Ion pairing, interactions and speciation of divalent cations with aqueous oxy-anions at high temperatures and pressure

This thesis is an investigation of the thermodynamic properties for ion pairs formed through ion association at high temperature and pressure, in aqueous solutions of bivalent metal sulfates. The apparent molar heat capacities for the aqueous solutions were measured using a differential scanning nano-calorimeter with a temperature range of 273.15 to 433.15 K. Apparent molar volumes were calculated from density measurements made with a commercial densimeter with a temperature range of 278.15 K to 363.15 K and a custom made high-temperature platinum vibrating-tube densimeter with a temperature range of 323.15 to 623.15 K. Ion-pair formation constants were calculated from conductivities obtained from a high temperature AC conductance instrument equipped with a platinum flow cell. Magnesium sulfate was measured first, as a model system followed by the nickel sulfate system.;The thermodynamic equilibrium constants derived from the conductance measurements were used to estimate the concentration of free ions and concentration of all types of ion pairs in solution. The concentration of contact ion pairs in solutions used to measure apparent molar properties was determined by Raman spectroscopy using the isotropic spectra with appropriate peak deconvolution methods. The apparent molar volumes for the contact ion pair and the solvent-separated ion pair were calculated as a function of temperature up to 413.15 K by applying Young's rule to the measured apparent molar properties using these speciation results and literature data for strong electrolytes. The experimental apparent molar heat capacities were corrected for the chemical relaxation effect before calculating the partial molar properties for the ion-pairs. Using these methods we were able to determine for the first time the standard partial molar properties for contact ion pairs MgSO40(aq) and NiSO4 0(aq) as a function of temperature. The Helgeson-Kirkham-Flowers-Tanger (HKF) model for neutral species was fitted to the standard partial molar properties for the contact ion pairs to calculate other standard partial molar properties of formation as a function of temperature. The new model for partial molar properties of contact-ion pairs is the first such model for ion-pairs and allows for prediction of standard partial molar properties up to 423.15 K.