Raman Micro‐Spectroscopic Study of Sulfate Ion in the System Na2SO4 – H2O

Abstract

This work aims to quantify sulfate ion concentrations in the system Na2SO4‐H2O using Raman micro‐spectroscopy. Raman spectra of sodium sulfate solutions with known concentrations were collected at ambient temperature (293 K) and in the 500 cm−1–4000 cm−1 spectral region. The results indicate that the intensity of the SO42– band increases with increasing concentrations of sulfate ion. A linear correlation was found between the concentration of SO42– (c) and parameter I1, which represents the ratio of the area of the SO42– band to that of the O‐H stretching band of water (As/AW): I1 = –0.00102+0.01538c. Furthermore, we deconvoluted the O‐H stretching band of water (2800 cm−1–3800 cm−1) at 3232 and 3430 cm−1 into two sub‐Gaussian bands, and then defined Raman intensity of the two sub‐bands as AB1 (3232 cm−1) and AB2 (3430 cm−1), defined the full width of half maximum (FWHM) of the two sub‐bands as WB1 (3232 cm−1) and WB2 (3430 cm−1). A linear correlation between the concentration of SO42– (c) and parameter I2, which represents the ratio of Raman intensity of SO42– (As) (in 981 cm−1) to (AB1+AB2), was also established: I2=–0.0111+0.3653c. However, no correlations were found between concentration of SO42–(c) and FWHM ratios, which includes the ratio of FWHM of SO42– (Ws) to WB1, WB2 and WB1+B2 (the sum of WB1 and WB2), suggesting that FWHM is not suitable for quantitative studies of sulfate solutions with Raman spectroscopy. A comparison of Raman spectroscopic studies of mixed Na2SO4 and NaCl solutions with a constant SO42– concentration and variable Cl− concentrations suggest that the I1 parameter is affected by Cl−, whereas the I2 parameter was not Therefore, even if the solution is not purely Na2SO4‐H2O, SO42– concentrations can still be calculated from the Raman spectra if the H2O band is deconvoluted into two sub‐bands, making this method potentially applicable to analysis of natural fluid inclusions.