Fundamental baseline variations in aqueous near-infrared analysis

Abstract

Distinctive, relatively small, analyte extinction coefficients in the near-infrared (NIR) spectral region, have led to NIR methodologies to probe the composition of concentrated solutions. However, as exemplified with alkaline hydroxide solutions, it is shown that the analyte strongly affects the conventional aqueous baseline of these measurements, and this baseline behavior must be addressed for effective NIR analysis. In the domain of molal and higher analyte concentration, the water baseline changes drastically as solute interacts and confines bulk solvent. At 967 nm an (hydroxide) absorption peak increases smoothly with increasing hydroxide activity in LiOH and NaOH electrolytes. Yet at this same wavelength, there is an unexpected absorption decrease in concentrated cesium (>5 m CsOH) and potassium (>13 m KOH) electrolytes, evidently due to solvent bridged anion–cation association. A peak for bulk water occurs nearby at 976 nm, with a unit pathlength absorbance of 0.24. This absorbance diminishes by an order of magnitude at high analyte concentration, as water activity decreases. A broad strong (0.53 cm −1) series of absorbances centered near 1200 nm diminish smoothly with decreasing bulk water activity. However, a broad strong absorption (14 cm −1) centered near 1450 nm, is resolved into two peaks, one disappearing, the other increasing at high hydroxide concentrations. Recognition of these variations permits effective NIR determination of analyte (in this case hydroxide). Second-order differentiation, ∂ 2 A/ ∂λ 2, at either 967 or 1421 nm, minimizes the effects of the near-lying bulk water absorption, and of baseline shift, and provides a linear variation with hydroxide concentration.