ACID-BASE BEHAVIOR OF ORGANIC-COMPOUNDS IN SUPERCRITICAL WATER

The equilibrium constant K-BHA for a reaction between an organic acid (beta-naphthol) and a base (OH- ion) has been measured for the first t ime in supercritical water (SCW) up to 400 degrees C and 470 bar, by u sing UV-vis spectroscopy. Solvatochromic shifts for the beta-naphthola te anion are used to determine the extent of hydrogen bonding with wat er and ion pairing with the series of cations Na+, K+, and Cs+. The io nization constant for beta-naphthol, K-2, is determined from K-BHA and previous measurements of the ionization constant for pure water, K-w. All of the results are consistent with the Born model. At constant te mperature, density effects are much larger for K-a than K-BHA, since t he latter reaction is iso-Coulombic. At constant water density, K-BHA is exothermic due to the stronger acidity of 2-naphthol versus water, whereas K-a is endothermic due to the energy required for ionization. However, the behavior becomes much more complex at constant pressure, due to large negative values of partial molar enthalpies, entropies, a nd volumes of ions at high temperatures, which are a result of the lar ge isothermal compressibility and volume expansivity of the solvent.