Resorcinol solution was prepared by accurately weighing 5.505 g of resorcinol and dissolving it in 100 mL of distilled water in a volumetric flask. The solution was stirred thoroughly using a magnetic stirrer to ensure complete dissolution of the resorcinol. This resulted in a 0.5 M solution, which was stored in a tightly sealed container at room temperature to avoid degradation. The prepared solution was subsequently used in the complexation reactions with zinc ions in all experiments, ensuring consistent reagent quality throughout the study.

Zinc chloride (ZnCl₂) solution was prepared by accurately weighing 1.3629 g of ZnCl₂ and dissolving it in 100 mL of distilled water in a volumetric flask. The solution was stirred thoroughly until the ZnCl₂ was completely dissolved, resulting in a 0.1 M zinc chloride solution. This solution was stored in a clean, sealed container to prevent contamination or evaporation. The prepared Zn²⁺ solution was used in the complexation experiments, providing a reliable and consistent source of zinc ions for the detection and analysis procedures in this study.

The dilution of zinc chloride (ZnCl₂) solutions was carried out systematically to prepare a series of solutions with decreasing concentrations. Initially, 10 mL of the 0.1 M ZnCl₂ stock solution was pipetted into a 100 mL volumetric flask and diluted to the mark with distilled water, resulting in Solution A (0.01 M). Following the same procedure, 10 mL of Solution A was further diluted to 100 mL, yielding Solution B (0.001 M). This process was repeated for each subsequent dilution: 10 mL of Solution B was diluted to prepare Solution C (0.0001 M), 10 mL of Solution C was diluted to prepare Solution D (0.00001 M), and finally, 10 mL of Solution D was diluted to obtain Solution E (0.000001 M or 1 µM). Figure 2 shows the dilution process of the solutions. These diluted solutions were used for the calibration and detection experiments, ensuring a precise range of Zn²⁺ concentrations for analysis. This sequential dilution provides solutions of progressively lower concentrations, ideal for sensitivity analysis or calibration curves.

The standard solutions for the detection of zinc ions were prepared by combining specific volumes of resorcinol, ammonium hydroxide (NH₄OH), zinc chloride (ZnCl₂), and sulfuric acid (H₂SO₄) solutions in a stepwise manner. First, 5 mL of the prepared 0.5 M resorcinol solution was transferred into a clean test tube. Then, 2 mL of 2.3 M NH₄OH was added to create an alkaline environment, and the mixture was allowed to react for 5 minutes. After the waiting period, 5 mL of the desired Zn²⁺ solution (either the stock or one of the diluted solutions) was added, followed by another 5-minute wait to ensure complete complexation. Finally, 3 mL of 2 M H₂SO₄ was added to the mixture to stabilize the formed Zn²⁺-resorcinol complex. The concentrations of NH₄OH and H₂SO₄ were chosen to optimize pH for complex formation and subsequent stabilization. These standard solutions were prepared in triplicate for each zinc concentration and were used for UV-visible spectrophotometric analysis, ensuring consistent and reliable measurements throughout the study.

This procedure allows for the formation of the Zn²⁺-resorcinol complex in the presence of NH₄OH, followed by stabilization with sulfuric acid, which is essential for further analysis.

The UV-visible (UV-vis) spectrophotometric measurements were performed using a standard UV-vis spectrophotometer. Each standard solution, prepared as described earlier, was transferred into a clean quartz cuvette with a path length of 1 cm. The absorbance spectra were recorded over the wavelength range of 200–800 nm, with a specific focus on the absorbance at the maximum wavelength (λₘₐₓ = 478 nm), which corresponds to the Zn²⁺-resorcinol complex ( Figure 4 ). The spectrophotometer was blanked using a solution containing all reagents except Zn²⁺ to account for background absorbance.

For each concentration of Zn²⁺, absorbance values were measured at 478 nm, and the data were recorded. These values were used to construct a calibration curve of absorbance versus Zn²⁺ concentration. All measurements were performed at room temperature, and each sample was measured in triplicate to ensure accuracy and reproducibility. The results demonstrated a linear relationship between absorbance and zinc ion concentration within the tested range.

The UV-visible spectrophotometric analysis of the Zn²⁺-resorcinol complex revealed a distinct absorption maximum (λ_max) at 478 nm ( Figure 4 ). This consistent absorption peak indicates the successful complexation of Zn²⁺ ions with resorcinol under the given conditions. The wavelength of 478 nm suggests that the Zn²⁺-resorcinol complex absorbs strongly in the visible region, making it suitable for detection using standard UV-vis spectrophotometers.

To evaluate the sensitivity of the method, a series of Zn²⁺ solutions with varying concentrations were prepared and analyzed. The absorbance values were recorded at 478 nm for each concentration, and a calibration curve was plotted. Figure 5 shows the calibration curve of the Zn²⁺ solutions. The plot of absorbance versus concentration resulted in a linear relationship, indicating a direct correlation between Zn²⁺ concentration and the absorbance of the Zn²⁺-resorcinol complex. The linearity of the curve suggests that the method follows Beer-Lambert’s law within the tested concentration range, which is critical for quantitative analysis.

The equation of the calibration curve was determined to be:

$A=mC+b$

where A is the absorbance, C is the concentration of Zn²⁺, m is the slope, and b is the y-intercept (close to zero in this case).

The non-zero y-intercept in the calibration curve may result from background absorption by reagents or solvent impurities, which can be mitigated using a reagent blank correction, or from matrix effects due to acidic/alkaline conditions.

The method demonstrated a high sensitivity for zinc detection, with successful quantification down to concentrations as low as 10⁻⁵ M. This level of sensitivity is comparable to more complex analytical techniques, such as atomic absorption spectrometry (AAS), but with the added advantage of simplicity and cost-effectiveness. This makes the technique particularly useful for routine monitoring of zinc levels in food and drinking water, especially in resource-limited settings [5] .

The formation of the Zn²⁺-resorcinol complex [6] [7] was facilitated by the addition of ammonium hydroxide (NH₄OH), which created an alkaline environment conducive to the complexation reaction. The waiting time of 5 minutes before and after the addition of Zn²⁺ ensured sufficient interaction time for the complex to form, stabilizing upon the addition of sulfuric acid (H₂SO₄). The pH-dependent behavior of the reaction is crucial for optimizing the complexation and stabilizing the final product for spectrophotometric analysis [6] [8] .

The ${\lambda }_{\max }$ observed at 478 nm corresponds to the specific electronic transitions within the Zn²⁺-resorcinol complex, likely involving charge transfer between Zn²⁺ and the aromatic system of resorcinol. The strong absorbance at this wavelength provides a clear spectrophotometric signature that can be used for routine detection of zinc in various matrices. Though the experiments were performed with the aqueous Zn²⁺ solutions only, the method requires future validation with real-world samples to assess potential matrix effects in foods and drinking water.