Raman spectroscopic quantification of sulfur species in aqueous fluids: Ratios of relative molar scattering factors of Raman bands of H2S, HS−, SO2, HSO4−, SO42−, S2O32−, S3− and H2O at ambient conditions and information on changes with pressure

We determined ratios of relative molar scattering factors J (or j) of Raman bands of sulfur species in aqueous solutions at 22 °C, 0.1 MPa, specifically those of H2S(aq) at ~ 2590 cm−1 (J2590), HS−(aq) at ~ 2570 cm−1 (J2570), SO2(aq) at ~ 1150 cm−1 (J1150), HSO4−(aq) at ~ 1050 cm−1 (J1050), SO42−(aq) at ~ 980 cm−1 (J980), and S2O32−(aq) at ~ 445 cm−1 (J445) and of water (i.e. the bending mode at ~ 1640 cm−1 (J1640) and the O-H stretching band at ~ 3400 cm−1 (J3400)). From experiments at elevated temperatures using a hydrothermal diamond-anvil cell, we further estimated ratios of the relative molar scattering factor of the Raman band of S3−(aq) at ~ 535 cm−1 (J535) for excitation at wavelengths of 473 and 532 nm, and discussed the effect of the blue coloration caused by S3− on Raman spectroscopic analysis of sulfur speciation. Another experiment suggested a significant decrease in the Raman scattering cross section of the band of HSO4−(aq) at ~ 1050 cm−1 with temperature. A systematic study on integrated ν1(SO4) and νS(OH) intensities and their ratio as function of temperature to 600 °C and pressure to 2.02 GPa for a 2.33 m Na2SO4 solution revealed a relationship between hydrogen bonding and νS(OH) Raman scattering efficiency. Moreover, it demonstrated that the integrated νS(OH) intensity can be used with reasonable accuracy as internal standard but only if corrected for the response function of the spectrometer, the frequency and scattering factor and the theoretical temperature dependence of the Raman scattering efficiency from the Bose-Einstein factor. The results of our study can be used to obtain the sulfur speciation in Raman spectroscopic studies on aqueous fluids in natural samples and experiments.