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

The dissociation constants of monochlorophenols (2-, 3-, 4-chlorophenols) were examined using direct UV-vis spectroscopy at temperatures from 25 to 175°C and at saturated vapor pressures in a high-temperature, high-pressure cell. The dissociation constant, Ka 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°C, and then decreased with further increases in temperature. The slope of ∂(log K)/∂ (1/T) was nonconstant and positive, that is, endothermic, below 150°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 Δ H increased at 25°C in the order: 3-chlorophenol, 4-chlorophenol, 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 Kb for the isocoulombic reaction of monochlorophenol with OH−, the nearly linear relationship of log Kbvs. 1/T for temperatures between 25 and 175°C indicates that the Δ Cp values for this isocoulombic reaction are low.