Journal of Solution Chemistry, Vol. 28, No. 7, 1999 Apparent molar heat capacities C p,$ for 71 rare earth chlorides, nitrates, and perchlorates, alkaline earth and transition metal chlorides, nitrates, and perchlo- rates, and alkali metal carbonates and sulfates have been fitted to the Pitzer equation for heat capacities. The apparent molar heat capacities at infinite dilution C o (equal to the standard partial molar heat capacity, C o , 2 ) were used to evaluate a set of "best" ionic heat capacities, from which improved values of C o ,o for the electrolytes were calculated. These were then used in the Pitzer equation to reevaluate the higher Pitzer coefficients. The Pitzer coefficients so evaluated can express, in most cases, the behavior of C p,o within experimental error from infinite dilution to the upper limit of the data. Ionic heat capacities have been correlated with the absolute entropies of the ions by statistically assigning the ionic heat capacities to obtain the best linear fit. 1. INTRODUCTION Aqueous electrolyte solutions play a dominant role in the chemistry of the Earth's biosphere as well as in many industrial processes. While the standard state thermodynamic properties of these solutions have been exten- sively tabulated at 25°C, the temperature dependences, especially as functions 1 Department of Chemistry, University of Miami, Coral Gables, Florida 33124. 2 Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, Florida 33149-1098. f We dedicate this paper to the memory of Kenneth S. Pitzer in recognition of his many invaluable contributions to solution chemistry. Modeling Heat Capacities of High Valence-Type Electrolyte Solutions with Pitzer's Equations f Cecil M. Criss 1, * and Frank J. Millero 2 Received October 23, 1998; revised April 20, 1999 KEY WORDS: Apparent molar heat capacities; partial molar heat capacities; Pitzer's equations; lanthanide elements; transition metals; alkali metal sulfates; magnesium sulfate. 849 0095-9782/99/0700-0849$ 16.00/0 C 1999 Plenum Publishing Corporation