Buffering Techniques for Hydrostatic Systems at Elevated Pressures

Abstract

Many chemical reactions of general interest and of specific geologic significance evolve or consume volatiles at elevated temperatures and pressures. Experimental techniques are needed to study such reactions as functions of temperature, pressure, and either the chemical potentials of the volatile species participating in the reaction or the composition of the non-condensed phase. One approach to the problem, used successfully by Greenwood (1961, 1962, 1967), Gorden and Greenwood (1970), Metz (1966), Johannes and Metz (1968), and Holloway et al. (1968), is the analysis of the fluid phase, yielding the proportions of the species at the time of the analysis. The proportions of gas species in the fluid will change continually as the reaction progresses. The fluid composition may be determined after the experiment, or before the experiment if the composition is adjusted to compensate for changes that occurred during the run. This chapter will treat a different approach that was developed by Eugster (1957). The fugacities of gas species are fixed by equilibration of fluid with one or more solid phases. The fugacity values are maintained constant (or closely approach equilibrium) for the duration of an experiment while the phases under investigation react. For example, the mole fraction ratio, X CO2/X H2O, in the fluid phase does not vary during a run, even though these species may react with the solids to consume CO2 and liberate H2O as in the reaction Mg(OH)2 + CO2 → MgCO3 + H2O. This method also has the advantage that a reaction can be studied in terms of a variable that cannot be measured directly by chemical analysis. For instance, a redox reaction can be studied as a function of oxygen fugacity, even though the concentration (mole fraction) of molecular oxygen may be 10−15, a value below the limits of analytical detection.