Acid–Base Titration Utilizing Ion Diffusion at a Biaqueous Interface Formed with Multiphase Laminar Flows with Simply‐Designed Microfluidic Channel

In this study, a new methodology for microfluidic acid–base titration is developed using a cross‐shaped microfluidic channel. Multiphase laminar flow forms within the channel, where the sample solution is sandwiched between the two laminar flows of titrant solutions. Lateral diffusion between the sample and titrant creates two equivalent points across the channel, and the distance between these points is defined as the titration parameter. Simulations based on Fick's diffusion equation show that this parameter increases with higher sample concentration. Compared with conventional microfluidic‐based titration systems, this approach features a simple channel design that enables multiple titrations on a single microfluidic channel. A computational study of the diffusion process of protons and hydroxide ions in a microfluidic channel is conducted based on a diffusion equation, along with experimental studies on the titration of both strong and weak acids using a sodium hydroxide solution. The acid determination of hot spring samples and vinegar proves the robustness and feasibility of the present methodology. The findings demonstrate a simple and portable titration method that enables accurate, instrument‐free concentration analysis, with potential for on‐site, inline, and point‐of‐care applications.