Visualization of Concentrations of Salts Produced during Neutralization Reactions by using Single Wavelength in the Near-infrared Region

Abstract

This paper proposes a novel imaging method using near infrared light of single wavelength to visualize the concentration distribution of salt generated by neutralization reaction of aqueous solutions of acid and alkali in a microfluidic channel. This method is based on the absorption characteristics of the aqueous solutions at the wavelength of 1520 nm in the ν₁+ν₃ absorption band of water. The wavelength of 1520 nm is an isosbestic point for the aqueous solutions of the acid and alkali with respect to their concentrations while the absorbances of the salt solutions at 1520 nm vary with their salt concentrations. To verify the proposed single-wavelength imaging method, the concentration images of sodium chloride (NaCl) were obtained and analyzed in two cases of the combinations of two liquids flowing into a T-shaped microchannel: (i) water and aqueous solution of NaCl, and (ii) hydrochloric acid and sodium hydroxide. The measured concentrations of NaCl agreed well with the expected distributions of the concentrations and verified the proposed method.