Abstract
This paper presents a near-infrared imaging method to measure the concentrations of acid (HCl and H2SO4), base (NaOH), and produced salt (NaCl and Na2SO4) simultaneously during neutralization in a microfluidic channel. This method is based on spectral variations in the water absorption band observed in the wavelength range 1350–1600 nm. Linear correlation models using three characteristic wavelengths in the band are constructed to convert the absorbance to the concentrations. The images clearly show that at the interface between the acid and base solution streams, the concentrations decrease and salt is produced, diffusing in the transverse direction. The concentration range is 0–2 mol/L and the resolution is <0.1 mol/L. The concentrations were verified by comparison with numerical simulations and quantitative evaluation of influencing factors. From the images, the local production rates of salt were determined, which depended on the flow velocity and temperature. This study indicates the potential of the method for analyses of not only acid–base neutralizations but also other reactions, mixing, and separation between various aqueous components.
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