A Co(II) Adsorption Experiment with Rapid Naked-Eye Visualization of the Microscopic Mechanism

This study presents a Co(II) adsorption experiment with rapid naked-eye visualization of the microscopic mechanism, based on the correlation between coordination states of Co(II) and their associated colors. By observing the rapid and clear visible color change of the adsorption system from pink to blue, the experiment demonstrates a microscopic mechanism in which aqueous [Co(H2O)6]2+ undergoes dehydration during adsorption and is adsorbed as octahedrally coordinated Co2+ bound to O atoms in the [TiO6] framework of the layered titanate adsorbent. A hypothesis-driven verification module, together with X-ray absorption fine structure (XAFS), thermodynamic analysis using a statistical physics model, and optional X-ray diffraction (XRD) characterization, further confirms the evolution of the coordination structure and the microscopic mechanism underlying the color change during Co(II) adsorption. This experimental design improves students’ proficiency in adsorption experiments and deepens their understanding of the correlation between macroscopic phenomena and microscopic mechanisms.