Resorcinol (≥99%), copper (II) sulfate pentahydrate (CuSO₄∙5H₂O), ZnSO₄∙7H₂O, NiSO₄∙6H₂O, MgSO₄and FeSO₄∙7H₂O ammonium hydroxide (NH₄OH), and distilled water were used without further purification. All glassware, including flat-bottom volumetric flasks, pipettes, cuvettes, and test tubes, was thoroughly cleaned and rinsed with distilled water prior to use to avoid contamination.

A stock solution of CuSO₄∙5H₂O (0.01 M) was prepared by dissolving 0.2497 g of CuSO₄∙5H₂O (molar mass = 249.685 g∙mol⁻¹) in distilled water and diluting to a final volume of 100 mL in a volumetric flask. The solution was agitated until complete dissolution was achieved.

A 0.001 M CuSO₄∙5H₂O solution was prepared by transferring 10 mL of the 0.01 M stock solution into a 100 mL volumetric flask and diluting to volume with distilled water. Similarly, a 0.0001 M solution was obtained by diluting 10 mL of the 0.001 M solution to 100 mL using distilled water. All solutions were stored in airtight containers and used for subsequent analysis. The same dilution protocol was applied for the preparation of other metal ion solutions when required.

The same solution preparation and serial dilution procedures were followed for other metal ions (Mg²⁺, Ni²⁺, Zn²⁺, and Fe²⁺), with all metal ion solutions prepared from their respective sulfate salts using distilled water.

Colorimetric measurements were performed using a UV-Visible spectrophotometer equipped with matched quartz cuvettes. Distilled water was used as the blank. A resorcinol solution (0.5 M) was prepared separately and used throughout the experiments.

For complex formation, 5 mL of 0.5 M resorcinol solution was transferred into a clean Pyrex test tube, followed by the addition of 1 mL of ammonium hydroxide to establish alkaline conditions. Subsequently, 1 mL of the prepared metal ion solution was added to the mixture. Immediately after addition of the metal ion solution, the reaction mixture was agitated to ensure homogeneous mixing, and the reaction time was recorded using a stopwatch.

An aliquot of the resulting mixture was transferred into a cuvette, sealed with a lid, and used for spectroscopic analysis.

Thermo Scientific-Evolution One UV-Vis spectrophotometer was used for data collection. UV-Visible absorbance measurements were recorded at 10-minute intervals over a period of 70 minutes using distilled water as the blank. The absorbance spectra were collected in the visible region to monitor the formation and time-dependent evolution of the resorcinol-metal ion complexes. The blank cuvette was re-measured as necessary to maintain baseline accuracy. All measurements were performed under identical instrumental conditions to ensure comparability of results. Spectra were collected in the range 400 - 800 nm at regular time intervals after mixing resorcinol with the respective metal ion in NH₄OH. Control experiments were performed using resorcinol in NH₄OH without added metal ions.

Resorcinol in NH₄OH without metal ions exhibits no distinct absorption peak in the visible region. The spectra are shown in Figure 1(a). The spectra show only a weak, featureless absorbance tail that decreases monotonically with increasing wavelength and remains essentially unchanged with time. This confirms that alkaline conditions alone do not generate visible chromophores and establishes a reliable baseline for assessing metal-induced spectral changes.

Fe²⁺ displays a spectral response indistinguishable from the blank with no observable absorption maxima or time-dependent evolution in the visible region. However, the absorption intensity is higher for the Fe²⁺ solution. Figure 1(b) shows the UV-Vis spectrum of the solution containing Fe²⁺ ions. This indicates that Fe²⁺ does not promote ligand-centered chromophore formation under these conditions, nor does it produce stable, detectable metal-centered transitions in the examined wavelength range.

Figure 1. UV-Vis spectra recorded at 10-minute intervals after mixing resorcinol (in NH₄OH) without (a) metal ion and (b) Fe²⁺ ion. No absorption peaks are observed in both cases.

In the presence of Mg²⁺, a broad visible absorption band develops and intensifies with time. Figure 2(a) shows the UV-Vis spectra of the solution containing Mg²⁺ ions. Notably, the absorption envelope undergoes a pronounced bathochromic shift, with the apparent maximum moving progressively toward longer wavelengths. Because Mg²⁺ is a d⁰ ion and lacks intrinsic visible electronic transitions, the observed absorption must be ligand-centered. The spectral evolution is therefore attributed to deprotonation of resorcinol followed by coordination to Mg²⁺ and gradual reorganization of Mg-resorcinolate species, leading to increased electronic delocalization. The absence of an isosbestic point suggests the coexistence of multiple Mg-resorcinolate species rather than a simple two-state interconversion.

Figure 2. UV-Vis spectra recorded at 10-minute intervals after mixing resorcinol (in NH₄OH) with (a) Mg²⁺and (b)Ni²⁺. The absorption peaks shift from lower to higher wavelengths with the time passage and the peak intensity increases monotonically from 20 to 70 min while the shapes remain constant.

Figure 2(b) shows the UV-Vis spectra of the solution containing Ni²⁺ ions. Ni²⁺ exhibits two broad absorption features in the visible region, centered near ~500 nm and ~600 - 630 nm, characteristic of spin-allowed d-d transitions in octahedrally coordinated d⁸ systems. The absorbance of these bands increases with time, accompanied by a small but systematic red shift. This behavior reflects gradual changes in ligand-field strength as resorcinol coordinates to Ni²⁺. Unlike Mg²⁺, the transitions remain metal-centered, limiting the magnitude of the wavelength shift.

Figure 3(a) shows the UV-Vis spectra of the solution containing Cu²⁺ ions. Cu²⁺ displays a broad visible absorption band whose intensity increases with time but whose λmax remains essentially constant. This behavior is consistent with the strong Jahn-Teller distortion inherent to d⁹ Cu²⁺ complexes, which dominates the electronic structure and renders d-d transition energies relatively insensitive to incremental ligand substitution. As a result, no measurable red shift is observed despite coordination with resorcinol. Although Cu²⁺ was examined at a tenfold lower concentration to remain within the linear absorbance range, its lack of bathochromic shift is concentration-independent and reflects the Jahn-Teller-dominated electronic structure of d⁹ Cu²⁺ complexes.

Figure 3(b) shows the UV-Vis spectra of the solution containing Zn²⁺ ions. Zn²⁺ (d¹⁰) shows no discernible red shift and only modest intensity changes relative to the blank. Because Zn²⁺ lacks low-energy electronic transitions, any observed absorption is weak and ligand-based. The absence of a red shift indicates that Zn²⁺ does not induce significant electronic delocalization in resorcinol comparable to that observed for Mg²⁺.

Figure 3. UV-Vis spectra recorded every 10 min after mixing resorcinol (in NH₄OH) with metal ion solutions. (a) Cu²⁺ (initially 0.0001 M) spectra showing much higher molar absorptivity and similar time-dependent intensity growth. (b) Zn²⁺ (0.001 M) spectra showing increasing intensity of the broad band centered at ~620 nm.

The contrasting spectral behaviors of the studied metal ions are summarized in Table 1.

Table 1.Comparative UV-Vis behavior of resorcinol in NH₄OH in the presence of divalent metal ions.

All metal ion solutions were prepared at an initial concentration of 0.001 M, except for Cu²⁺, which was studied at 0.0001 M to avoid detector saturation due to its high absorbance. Spectral trends (presence or absence of red shift, band shape, and mechanistic origin) are therefore compared qualitatively rather than on absolute absorbance intensity measured at 0.0001 M Cu²⁺; higher concentrations exceeded the instrument’s linear absorbance range.

These results demonstrate that metal selectivity in this system is governed primarily by electronic structure rather than coordination alone. Only Mg²⁺ produces a strong, red-shifting ligand-centered response, enabling clear discrimination from transition metals such as Ni²⁺ and Cu²⁺, as well as spectroscopically silent ions such as Zn²⁺ and Fe²⁺.