Abstract
The Raman wavenumber of the symmetric stretching vibration of carbonate ion (ν1-CO32−) was measured in three aqueous solutions containing 2.0 mol·L−1 Na2CO3 and 0.20, 0.42, or 0.92 mol·L−1 NaCl, respectively, from 122 to 1538 MPa at 22 °C using a moissanite anvil cell. The ν1 Raman signal linearly shifted to higher wavenumbers with increasing pressure. Most importantly, the slope of ν1-CO32− Raman frequency shift (∂ν1/∂P)I was independent of NaCl concentration. Moreover, elevated ionic strength was found to shift the apparent outline of the carbonate peak toward low wavenumbers, possibly by increasing the proportion of the contact ion pair NaCO3−. Further investigations revealed no cross-interaction between the pressure effect and the ionic strength effect on the Raman spectra, possibly because the distribution of different ion-pair species in the carbonate equilibrium was largely pressure-independent. These results suggested that the ionic strength should be incorporated as an additional constraint for measuring the internal pressure of various solution-based systems. Combining the ν1-CO32− Raman frequency slope with the pressure herein with the values for the temperature or the ionic strength dependencies determined from previous studies, we developed an empirical equation that can be used to estimate the pressure of carbonate-bearing aqueous solutions.
Highlights
Carbonate-bearing hydrothermal fluids are widely present in subduction zones or sedimentary basins[11,12]
The results demonstrated a positive correlation between the Raman wavenumber of CO32− and pressure at ambient temperature for the Na2CO3-NaCl-H2O system (Fig. 2), which was consistent with previous findings obtained from similar high-pressure studies on solutions[14,21], liquid organic matters[25,26], and crystals[27,28]
Both this study and our previous investigation of the binary Na2CO3-H2O system unveiled a positive correlation between pressure and the frequency of the ν1-CO32− Raman line with a slope (∂ν 1/∂P)T of around 5.4 cm−1 ∙ GPa−1 at ambient temperature
Summary
Carbonate-bearing hydrothermal fluids are widely present in subduction zones or sedimentary basins[11,12]. Frantz[15] demonstrated that the speciations of both K2CO3 and KHCO3 solutions varied with temperature by analyzing the characteristic Raman peaks of aqueous CO32− and HCO3− up to 200 MPa and 550 °C These results hinted at the potential value of using the characteristic Raman peak of CO32− as an alternative barometer for determining the pressure of a hydrostatic system, it should be pointed out that carbonate species, in their native geological environments, almost always coexist with a high level of Cl− and various other electrolytes. Their interactions could influence the Raman lines of other Raman-active species[16]. The aim of this study is to examine the dependence of the Raman shift of CO32− on pressure in the presence of different concentrations of Cl− and to develop a pressure equation
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