Spectroscopic Measurement of pH in Aqueous Sulfuric Acid and Ammonia from Sub- to Supercritical Conditions

The pH of aqueous sulfuric acid and sulfuric acid−ammonia mixtures was measured using the optical indicator acridine at temperatures from 200 to 400 °C and pressures from 3500 (24.1) to 6000 psia (41.3 MPa). Because of large changes in the p K a of protonated acridine in supercritical water (SCW), the measurable pH range shifts from 2−4 at a density of 0.60 g/cm 3 to 4.5−7 at a density of 0.24 g/cm 3 . At 3500 psia, the first dissociation constant ( K a1 ) of H 2 SO 4 decreases sharply with increasing temperature above 350 °C, primarily due to a reduction in density and thus the solvation of the bisulfate and hydrogen ions. The acidity of H 2 SO 4 relative to HCl increases with increasing temperature at constant pressure up to the critical point of pure water. Based on titrations of sulfuric acid solutions with ammonia, weak acid−weak base behavior is observed at 380 °C and 5000 psia (34.5 MPa). At these conditions the system H 2 SO 4 −NH 4 HSO 4 may be used as a buffer to maintain pH in the range 3.5 ± 0.25.