UV-Vis Spectrophotometric Determination of the Dissociation Constants for Monochlorophenols in Aqueous Solution at Elevated Temperatures

摘要

The dissociation constants of monochlorophenols (2-, 3-, 4-chlorophenols) were examined using direct UV-vis spectroscopy at temperaturs from 25 to 175 deg C and at saturated vapor pressures in a high-temperaure, high-pressure cell. The dissociation constant, K_a increased under experimental temperatures in the order: 2-chlorophenol, 3-chlorophenol, and 4-chlorophenol. The dissociation constant of 4-chlorophenol increased with increasing temperature under experimental conditions, while those of 2-and 3-chlorophenol reached maximum values at around 125 deg C, and then decreased with further increase in temperature. The slope of partial deriv(log K)/partial deriv (1/T) was nonconstant and positive, that is, endothermic, below 150 deg C. The data on dissociation constants were analyzed by simultaneous regression to yield a five-term equation that described the Van't Hoff isobar. The magnitude of enthalpy DELTAH increased at 25 deg C in the order: 3-chlorophenol, 4-chorophenol, and 2-chlorophenol. The decrease in enthalpy at the absolute temperature was larger for 3-chlorophenol than for either 2- or 4-chlorophenol. Considering the equilibrium constant K_b for the isocoulomebic reaction of monochlorophenol with OH~-, the nearly linear relationship of log K_b vs. 1/T for temperatures between 25 and 175 deg C indicates that the DELTAC_p values for this isocoulombic reaction are low.