Metal-Dependent UV-Vis Response of Resorcinol in Alkaline Media: Comparative Study of Mg²⁺, Ni²⁺, Cu²⁺, Zn²⁺, and Fe²⁺

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.