Conventional pH sensors encounter obstacles when utilized in response to spill accidents involving highly acidic and basic chemicals, since the concentration of chemicals released in the accidents usually exceeds the detection limit of most pH sensors. Practical tools available for the chemical accidents are limited to pH papers and pH-indicating dyes; however, they also suffer from drawbacks such as difficulties in retrieval and removal, leading to secondary environmental contamination. In this study, colorimetric pH-sensing beads (CpH beads) with a core/shell structure were developed to have essential characteristics required for responding to chemical accidents. Magnetic Fe3O4 nanoparticles dispersed in the core allow the CpH bead to be readily recoverable from acidic or basic solutions using a magnet. The shell, which consists of an alginate-based composite with thymol blue-incorporated porous aluminum hydroxide, rapidly indicates the acidity or basicity of the solution by changing color. The response time for color change of the CpH bead decreases as the concentration of the acidic or basic solution increases. The color of CpH indicative of acidity or basicity changes within 5–60 s in 5 M solutions, and within 20–300 s in 0.5 M solutions. Moreover, the CpH bead has a leaching rate of dye approximately 33.5 times slower than the pH paper, and its easy retrieval via a magnet allows for six reuses, minimizing secondary environmental contamination. Therefore, the CpH bead demonstrates a significant potential for effectively responding to leakage accidents involving strong acids and bases in the field.