Spectrophotometric Determination of the Ionization Constants of Aqueous Nitrophenols at Temperatures up to 225 °C

The UV-visible spectra of aqueous o -, m -, and p -nitrophenol were measured as a function of pH at temperatures from 50 to 225 °C at a pressure of 7 MPa. These were used to determine equilibrium constants for the acid ionization reaction of each isomer. The new results were combined with literature data on the ionization of nitrophenols and used for parameter optimization in the thermodynamic model of Marshall and Franck (J. Phys. Chem. Ref. Data 10:295–304, [ 1981 ]), to describe the dependence of ionization properties on temperature and pressure. The model yields predictions of the ionization constants for o -, m -, and p -nitrophenol, log 10 K a , to at least 250 °C and 20 MPa with an estimated uncertainty in log 10 K a of less than ±0.06.