Thermodynamic investigations using the Pitzer formalism: Extension of the model and its applications

Date of Award




Degree Name

Doctor of Philosophy (Ph.D.)

First Committee Member

Frank J. Millero, Committee Chair


The modeling of ion interactions in concentrated solutions as been of great interest to scientists throughout the century. The limiting laws applicable to dilute solutions were discovered in the 1920's. Advances in statistical mechanics lead Pitzer and co-workers in the 1970's to express the excess Gibbs energy as a virial expansion of terms in increasing powers of molality, defining the so-called "Pitzer equation". Since then, numerous workers have used this semi-empirical model to estimate the thermodynamic properties of solutions. In the following work, I present new measurements over a wide temperature (0--55°C) and molality range (0--3 m) on the interactions of sodium with bisulfate ions, nickel with bromide ions as well as three trivalent ions (samarium, gadolinium and europium) with chloride, which expand the current model. All measurements were made by Dr. Rabindra Roy's group at Drury College, Missouri.A self-consistent model was put together and its validity in estimating the thermodynamics of the carbonate system was tested. The present model considers the ionic interactions of the major seasalt ions (H+, Na +, K+, Mg2+, Ca2+, Sr 2+, Cl-, Br-, OH -, HCO3-, B(OH)4 -, HSO4-, SO4 2-, CO32-, CO2, B(OH) 3, H2O). Coefficients needed to determine the dissociation constants of the acids in seawater (H2CO3, B(OH) 3, H2O,HF, HSO4-, H 3PO4, H2S NH4+) have been added to the model and are valid from 0 to 50°C. The activity and osmotic coefficients determined from the model are in good agreement with the measured values in seawater from 0 to 50°C. To this model, I incorporated data on divalent and trivalent metals; interaction coefficients, dissociation constants for their inorganic complexation, enthalpy and heat capacity data, to examine the speciation of metals in natural waters. The program I developed can easily be adapted to examine the organic speciation of metals. Recommendations for future measurements are mentioned. Finally, I derived Pitzer coefficients for a fictitious 1:1 seasalt to be able to estimate the thermochemical properties of seawater. Seawater densities calculated from pure salt properties are in reasonable agreement with the values given by the equation of state of seawater.


Chemistry, Physical

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