This paper reports methods for obtaining time-dependent reduced isotropic Raman spectra of aqueous species in quartz capillary high-pressure optical cells under hydrothermal conditions, as a means of determining quantitative speciation in hydrothermal fluids. The methods have been used to determine relative Raman scattering coefficients and to examine the thermal decomposition kinetics of the non-complexing anions bisulfate (HSO4−), perchlorate (CIO4−), perrhenate (ReO4−), and trifluoromethanesulfonate, or “triflate” (CF3SO3−) in acidic and neutral solutions at temperatures up to 400 °C and 30 MPa. Arrhenius expressions for calculating the thermal decomposition rate constants are also reported. Thermal stabilities in the acidic solutions followed the order HSO4− (stable) > ReO4− > CIO4− > CF3SO3−, with half-lives (t1/2) > 7 h at 300 °C. In neutral solutions, the order was HSO4− (stable) > CF3SO3− > ReO4− > CIO4−, with t1/2 > 8 h at 350 °C. CF3SO3− was extremely stable in neutral solutions, with t1/2 > 11 h at 400 °C.
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