A major barrier for studying many different geologically-significant physicochemical processes in hydrothermal environment is the lack of a reliable and generally applicable method for measuring pH. If pH could be measured accurately at temperatures above 300~ we would significantly increase our knowledge of protolitic phenomena, including the acid-base dissociation, hydrolysis, ion-pairing, solubility processes, and their associated thermodynamic properties. The development of accurate

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... metric and pH measurement techniques would yield information that will greatly improve our understanding of the thermodynamic, kinetic and surface processes in different water-rock interactions. We have developed a flow-through electrochemical cell (FTEC) and demonstrated that it successfully operates in both subcritical and supercritical aqueous environments at temperatures up to 400~ The FTEC consists of a previously developed flowthrough external (AglAgC1) pressure-balanced reference electrode (FTEPBRE) (Lvov et al., 1998a,b) and a modified flow-through platinum hydrogen electrode (FTPHE). The design of the FTEPBRE was similar to one described by Lvov et al. (1998a, Fig.