07402 Potentiometric pH Measurement of Supercritical Aqueous Solutions with an Electrochemical Microcell

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Product Number: 51300-07402-SG

ISBN: 07402 2007 CP

Author: Kiwamu Sue, Toshihiko Hiaki, Fumiaki Ouchi, and Kunio Arai

Publication Date: 2007

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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.