Thermodynamic and kinetic properties of organic acid-base reactions in subcritical and supercritical water solutions have been studied quantitatively by using spectroscopic methods including UV-vis spectroscopy, fluorescence spectroscopy, and single-photon counting. The newly developed pH indicators: acridine, $beta$-naphthoic acid, and $beta$-naphthol can be use to measure the pH values of supercritical water solutions up to 400$spcirc$C and 470 bar. This spectroscopic technique complements measurements by ion conductivity and electrochemical pH sensors.;All of the iso-Coulombic reactions studied in this work exhibit a nearly linear relationship of logK$sb{rm BHA}$ with 1/T over the temperature range from 25$spcirc$C to 300$spcirc$C indicating that $Delta Csbsp{p}{o}$ is small for iso-Coulombic reactions over this temperature range. The Born model agrees with the experimental results up to 400$spcirc$C and 470 bar illustrating the importance of the charge per radius on each ion. Compared with iso-Coulombic reactions, the temperature and pressure effects on the thermodynamic properties for the ionogenic reactions of $beta$-naphthoic acid with ammonia and $beta$-naphthol with ammonia are density dependent.;As for thermodynamic properties, the variation of a chemical reaction rate in water over a large temperature range, where solvent properties such as dielectric and hydrogen bonding vary widely, depends on the role of electric charge in the reaction. The dynamics of iso-Coulombic reactions do not vary significantly with temperature or dielectric constant because the solvation of all parts of the reaction coordinate (reactants, transition state, and products) change in a similar way. Therefore, an extrapolation of low temperature kinetic measurements can be used to approximate the kinetic behavior at high temperatures. In contrast, the dynamics of ionogenic reactions change markedly with temperature leading to dramatically different kinetic behavior at high temperatures, especially near the critical point where water's properties exhibit large variation with pressure and temperature.
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