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

摘要

This work aims to quantify sulfate ion concentrations in the system Na_2SO_4-H_2O 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 SO_4^(2-) band increases with increasing concentrations of sulfate ion.A linear correlation was found between the concentration of SO_4^(2-)(c) and parameter I_1,which represents the ratio of the area of the SO_4^(2-) band to that of the O-H stretching band of water(A_s/A_w):I_1=-0.00102+0.01538 c.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 A_(Bi)(3232 cm^(-1)) and A_(B2)(3430 cm^(-1)),defined the full width of half maximum(FWHM) of the two sub-bands as W_(B1)(3232 cm^(-1)) and W_(B2)(3430 cm^(-1)).A linear correlation between the concentration of SO_4^(2-)(c) and parameter I_2,which represents the ratio of Raman intensity of SO_4^(2-)(A_s)(in 981 cm^(-1)) to(A_(B1)+A_(B2)),was also established:I_2=-0.0111+0.3653 c.However,no correlations were found between concentration of SO_4^(2-)(c) and FWHM ratios,which includes the ratio of FWHM of SO_4^(2-)(W_s) to W_(B1) W_(B2) and W_(B1+B2)(the sum of W_(B1) and W_(B2)),suggesting that FWHM is not suitable for quantitative studies of sulfate solutions with Raman spectroscopy.A comparison of Raman spectroscopic studies of mixed Na_2SO_4 and NaCI solutions with a constant SO_4^(2-) concentration and variable CI^- concentrations suggest that the I\ parameter is affected by CI^-,whereas the I_2 parameter was not.Therefore,even if the solution is not purely Na_2SO_4-H_2O,SO_4^(2-) concentrations can still be calculated from the Raman spectra if the H_2O band is deconvoluted into two sub-bands,making this method potentially applicable to analysis of natural fluid inclusions.