Metal-Dependent UV-Vis Response of Resorcinol in Alkaline Media: Comparative Study of Mg<sup>2+</sup>, Ni<sup>2+</sup>, Cu<sup>2+</sup>, Zn<sup>2+</sup>, and Fe<sup>2+</sup>

The time-dependent UV-Vis response of resorcinol in alkaline solution (NH4OH) was investigated in the presence of divalent metal ions Mg2+, Ni2+, Cu2+, Zn2+, and Fe2+ in order to elucidate the origin of metal-specific optical behavior and assess intrinsic selectivity. Distinct spectral responses were observed depending on the electronic structure of the metal ion. Mg2+ produced a pronounced, time-dependent visible absorption band accompanied by a bathochromic shift, indicative of ligand-centered chromophore formation. Ni2+ exhibited characteristic metal-centered d-d transitions with a small red shift attributed to gradual ligand-field modulation. In contrast, Cu2+ and Zn2+showed no measurable red shift, displaying only intensity changes consistent with Jahn-Teller-dominated (Cu2+) or spectroscopically silent (Zn2+, d10) behavior. Fe2+ showed no discernible absorption features beyond the blank, indicating the absence of either ligand-centered chromophore formation or stable metal-centered transitions under the experimental conditions. These results demonstrate that metal selectivity in the resorcinol-NH4OH system is governed primarily by electronic structure rather than coordination alone, enabling clear differentiation among metal ions based on their spectral signatures.