A flexible method of carbonate determination using an automatic gas analyzer equipped with an FTIR photoacoustic measurement chamber

Abstract

A Fourier transform infrared spectrometer was employed to determine automatically the total inorganic carbonate (TIC) in solids and waters, based on active photoacoustic absorption of infrared light by carbon dioxide. A 2.0 l reactor, connected to the spectrometer, is immersed in water-bath at 20 °C. After purging with pure N₂, 5 ml of 0.5 mol l^–1 HClO₄ are injected into 50 ml of solid suspension or solution with continuous stirring. The specific absorption of infrared light by the CO₂ evolved induces corresponding fluctuations of temperature and pressure in a measurement chamber. Accordingly, photoacoustic signals, with frequencies dependent on the absorbed wavelengths, are generated and measured by the chamber microphones in the form of an absorption spectrum and concentration. For solids, the method exhibits a linear response up to 120 mg of CaCO₃ with a detection limit of 0.02 mg; in the case of waters, these figures of merit are 36.4 mmol l^–1 and 3 µmol l^–1 NaHCO₃, respectively. Since proton consumption by TIC in clay minerals may commonly influence the evaluation of surface acid–base properties, the methodology was applied to determine TIC in three natural illite samples of different origins. In addition, some potential interferences and modifications of this method are discussed.