Non-Complexing Anions for Quantitative Speciation Studies Using Raman Spectroscopy in Fused Silica High-Pressure Optical Cells under Hydrothermal Conditions

Abstract

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.

Measurements of Vapor Pressures of Aqueous Solutions in the NaCl–KCl–H2O System from 493.15 to 693.25 K in a Fused Silica Capillary High-Pressure Optical Cell

“…The pressure measuring system was calibrated with the critical pressure of CO 2 (7.38 MPa). A schematic diagram of the "capillary nonmixing" method described in a previous work 40 is shown in Figure 1. CH 4 was chosen instead of liquid water as a pressure-transmitting fluid because water is more likely to change the concentration of the sample solution in the heating zone during our experiments.…”

“…Note that the use of CH 4 to pressurize our sample solutions should have no effect on our measured vapor pressures because, based on the diffusion coefficient of CH 4 in water at room temperature, it will take at least 10 days for CH 4 to diffuse from the CH 4 -solution interface to the sample area under observation, and the duration of our experiments is less than 2 days; CH 4 , which is highly Raman sensitive, has not been detected neither in the solution nor in the coexisting vapor phase in Raman analyses of the samples after vapor pressure measurements. The capillary nonmixing method has been used to determine relative Raman scattering coefficients and to examine the thermal decomposition kinetics of the noncomplexing anions HSO 4 – , ClO 4 – , ReO 4 – , and CF 3 SO 3 – in acidic and neutral solutions at temperatures up to 673.15 K and 30 MPa and was applied in this study to obtain vapor pressure data of aqueous solutions.…”

“…Schematic diagram showing the sample solution in a fused silica capillary, which was pressurized by CH 4 with temperatures controlled by the heating–cooling stage. The capillary nonmixing method in the fused silica capillary high-pressure optical cell (FSC-HPOC) described by Applegarth et al was used in this study (see the text).…”

An Experimental Study of the Solubility and Speciation of MoO3(s) in Hydrothermal Fluids at Temperatures up to 350°C

“…As experiments involving NaCl do not allow us to readily separate the effects of complexes involving Na from those involving Cl, we also evaluated the solubility of MoO3(s) in sodiumtriflate (NaCF3SO3) aqueous solutions. As the triflate ion is non-complexing, i.e., interactions between this ion and metals like Mo 6+ are extremely weak (Fabes and Swaddle, 1975;Palmer and Drummond, 1988;Applegarth et al, 2015), any increase in Mo concentration with increasing concentration of sodium triflate can be attributed to Shang, L., Williams-Jones, A.E., Wang, X., Timofeev, A., Hu, R., Bi, X., 2020. An Experimental Study of the Solubility and Speciation of MoO3(s) in Hydrothermal Fluids at Temperatures up to 350°C.…”

“…ion-pairing of HMoO4or MoO4 2with Na + . However, although the triflate ion is stable to temperatures > 400 °C in solutions with near neutral pH, it begins to decompose at temperatures above 280 °C in highly acidic solutions (Applegarth et al, 2015). Our experiments with sodium-triflate solutions were therefore limited to the 250 °C isotherm.…”