Raman spectroscopic study for the determination of Cl− concentration (molarity scale) in aqueous solutions: Application to fluid inclusions

From recent studies on the structure of aqueous alkali and alkaline chloride solutions, it can be deduced that the effects of monovalent and divalent cations are small, and water–chloride interactions are primarily responsible for the solutions. Therefore, Cl− molarity can be treated as an important parameter in describing these systems. In this work, the NaCl–H2O, KCl–H2O, CaCl2–H2O, MgCl2–H2O and NaCl–KCl–CaCl2–H2O systems are synthesized, and their Raman spectra are recorded. From the Raman OH stretching bands of the solutions, the isosbestic points can be discriminated. This means that there exists the equilibrium between various structural species, the Raman OH vibration can be applied to study the salinity of these solutions. In this study, the Raman OH vibration is fitted with two Gaussian sub-bands, the equilibrium constant can be determined, then the calibration equation of Cl− concentration (molarity scale) can be determined. After the effects of the birefringence of the host mineral on the Raman OH stretching bands are discussed, the method is applied to measure the salinity of natural fluid inclusions.