Near-infrared imaging-based diffusion coefficient mapping for acid-base reactions

This study describes a method based on near-infrared concentration imaging and advection-diffusion-reaction modeling for the simultaneous determination of the diffusion coefficient distributions of acids, bases, and salts during the neutralization reaction. The reactions between parallel streams of aqueous solutions of H2SO4 and NaOH, and HCl and KOH through a microfluidic channel are investigated. The measured concentration profiles were approximated by a Gaussian or error function, to calculate the advection, diffusion, and reaction terms analytically and robustly. The diffusion coefficients differ significantly within the channel; those at the reaction interface are the largest than the literature values measured under non-reactive conditions, indicating temperature and concentration dependence, and anomalous features. The proposed method facilitates the in-situ visual and quantitative analysis of diffusion reactions.