Boric Acid Equilibria in Near-Critical and Supercritical Water

The pH values of aqueous solutions of boric acid and KOH were measured with the optical indicator 2-naphthol at temperatures from 300 to 380 °C. The equilibrium constant K b - 1 for the reaction B(OH) 3 + OH - = B(OH) 4 - was determined from the pH measurements and correlated with a modified Born model. The titration curve for the addition of HCl to sodium borate exhibits strong acid−strong base behavior even at 350 °C and 24.1 MPa. At these conditions, aqueous solutions of sodium borate buffer the pH at 9.6 ± 0.25.