Development of a rapid analytical method for salinity and carbonate species in water samples using near-infrared and infrared absorption spectroscopy.

Abstract

This study aims to develop a new analytical method for the rapid and simultaneous quantification of both salinity and carbonate species (CO₂, HCO₃^-, CO₃^2-) in aqueous solutions such as seawater, hot spring water, and fluid inclusions. Conventional approaches require complex procedures involving chemical equilibrium calculations to determine carbonate species concentrations. In contrast, the proposed method employs near-infrared and infrared absorption spectroscopy to directly quantify component concentrations without relying on equilibrium assumptions.Using NaHCO₃solutions with a total carbonate concentration set to approximately 10 times that of seawater, spectral measurements were conducted while varying the pH. The intensities of peaks corresponding to HCO₃^- (1303 and 1364 cm^-1) and CO₃^2- (1396 cm^-1) changed in response to pH. At low pH, the asymmetric stretching vibration of CO₂(aq) (2350 cm^-1) was observed, and a correlation was obtained between estimated concentrations and absorbance variations. Moreover, spectra of carbonate species were also successfully observed in more dilute carbonate solutions.