Ni Pei

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

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.