POTENTIOMETRIC PH MEASUREMENTS IN HIGH SUBCRITICAL AND SUPERCRITICAL AQUEOUS-SOLUTIONS

An operational electrochemical thermocell is proposed for potentiometr ic pH measurements in high subcritical and supercritical aqueous solut ions. The thermocell consists of the silver-silver chloride flow-throu gh external pressure-balanced reference electrode (FTEPBRE) and the fl ow-through platinum-hydrogen pH sensor. Using this thermocell, we have measured the potentials of the thermocell for a few HCl(aq) and NaOH( aq) solutions of different concentrations and derived the correspondin g pH differences (Delta pH) over a wide range of temperatures from 25 to 400 degrees C at pressures between 27.5 and 33.8 MPa. Comparison of the experimentally derived and theoretically calculated Delta pH valu es clearly demonstrates an ability of the thermocell to measure pH wit h accuracy better than +/-0.1 units, and this result in turn demonstra tes the viability of the FTEPBRE as a versatile reference electrode fo r use at high subcritical and supercritical temperatures. (C) 1998 Els evier Science B.V. All rights reserved.