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

Abstract

The time-dependent UV-Vis response of resorcinol in alkaline solution (NH 4 OH) was investigated in the presence of divalent metal ions Mg 2+ , Ni 2+ , Cu 2+ , Zn 2+ , and Fe 2+ 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. Mg 2+ produced a pronounced, time-dependent visible absorption band accompanied by a bathochromic shift, indicative of ligand-centered chromophore formation. Ni 2+ exhibited characteristic metal-centered d-d transitions with a small red shift attributed to gradual ligand-field modulation. In contrast, Cu 2+ and Zn 2+ showed no measurable red shift, displaying only intensity changes consistent with Jahn-Teller-dominated (Cu 2+ ) or spectroscopically silent (Zn 2+ , d 10 ) behavior. Fe 2+ 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-NH 4 OH system is governed primarily by electronic structure rather than coordination alone, enabling clear differentiation among metal ions based on their spectral signatures.