Portable microfluidic titration using a cross-shaped microfluidic device and smartphone camera for on-site quantitative analysis

A portable microfluidic titration system using a smartphone camera was developed as a detector for onsite quantitative analysis. Two types of cross-shaped microfluidic devices with different channel geometries were designed and evaluated to improve the visual detectability of the equivalence point under limited spatial resolution. An expanding-channel device was found to promote molecular diffusion downstream, resulting in enhanced visibility of the equivalence point compared to a constant-width device. Using the optimized device, a strong acid-strong base system was subjected to acid-base titration using bromothymol blue as the indicator. The color images captured by the smartphone camera were analyzed by extracting the red channel intensity profiles and used to determine the distance corresponding to the equivalence point. A linear relationship was obtained between the measured distance and hydrochloric acid concentration in the range of 25-150 mM. The applicability of the proposed system was demonstrated by analyzing a real hot-spring water sample. The determined acid concentration was in good agreement with that obtained by conventional volumetric titration, with no statistically significant difference at the 95% confidence level. These results indicate that the proposed portable microfluidic titration system enables reliable quantitative analysis without the use of microscopes or specialized optical instruments, highlighting its potential for onsite analytical applications.