An electrochemical microcell was recently developed for potentiometric pH measurements of
supercritical aqueous solutions [K. Sue et al., J. Supercrit. Fluids, 39, 271(2006)]. It has an internal
volume of ca. 1.5 cm3 and a surface area of an electrode of 0.04 cm3. These values are around one
order of magnitude lower than that of previous apparatus [K. Sue et al., ibid. 28, 287(2004)]. The
decrease in cell volume allows more reliable pH measurements of several compounds including unstable
organic acids. With the newly-designed cell, pH measurements were carried out for HCl+NaCl aqueous
solutions at flow rates from 1.0 to 4.0 g/min, temperatures from 294.7 to 663.3 K and pressures from
22.0 to 30.1 MPa. The experimentally determined pH showed good agreements with the theoretically
calculated pH over the range of conditions studied.
In this work, pH of 0.01 mol-kg-1 CH3COOH aqueous solutions at supercritical conditions was
measured at temperature of 653 K and pressures from 26 to 34 MPa by a flow-through electrochemical
microcell and it changed from 4.72 at 653.25 K and 35.9 MPa to 5.23 at 653.65 K and 25.9 MPa. The
molal equilibrium constant for the dissociation reaction of CH3COOH was determined on the basis of
the measured pH and the value varied from 3.20 x 10-8 at 25.9 MPa to 1.17 x 10-7 at 35.9 MPa.