## 3 Experiment and Results Discussion  
### Effect of the Influence of Environmental Factors on UV-Vis Spectroscopy  
#### 3.1.1 Effect of the Influence of pH on UV-Vis Spectroscopy  
The pH value of a water sample is used to indicate the acidity and alkalinity of the water. It is one of the most important physicochemical factors in water. It may lead to a series of water changes when the pH value changes, such as the ionization of some molecules, changes in molecular structure, and, thus, changes in the UV-Vis absorption spectrum of these molecules. In order to understand the influence of pH changes on COD detection based on UV-Vis spectroscopy, experiments were conducted on the influence of pH on COD detection. This section presents an experimental analysis of the influence of pH variations on the UV-Vis spectrum. To adjust the pH, 0.1 mol/L solutions of H\({}_{2}\)SO\({}_{4}\) and NaOH were employed as pH regulators. 1. The influence of pH on the standard solution. A 50 mg/L potassium hydrogen phthalate standard solution (COD standard solution) was prepared, exhibiting acidity with a pH of approximately four. To investigate the effect of pH, the pH of the standard solution was adjusted from 1.6 to 10.6 using an acid-base regulator. The UV-Vis spectra of these solutions are illustrated in Figure 3a. The absorbance curves at specific wavelengths (236.09 nm, 265.22 nm, and 280.46 nm) corresponding to the peaks and valleys in different pH ranges are illustrated in Figure 3b. It can be seen from the spectroscopy of the standard solution in Figure 3a that, as the pH value increased, the absorbance gradually decreased, and a red shift phenomenon occurred in the absorption band. Above a pH of four, the rate of change in absorbance slowed, and the absorption band exhibited a blue shift. This phenomenon occurred because water, as a polar solvent, increases its polarity when the solution's pH rises, and \(OH^{-}\) ions neutralize some \(H^{+}\) ions. This enhanced polarity induces a red shift in the \(\pi\rightarrow\pi^{*}\) absorption band, slightly decreasing the molar absorption coefficient \(\varepsilon_{max}\). Meanwhile, changes in pH may lead to alterations in the internal energy levels of molecules, such as enhanced intermolecular interactions and the increased rigidity of molecular structures. This increases the energy required for \(n\rightarrow\pi^{*}\) transitions, resulting in a blue shift in the absorption band. Figure 3b shows an overall decreasing trend in absorbance as the pH value increases. The absorbance at various wavelengths experiences a sharp decline at pH four. Although the absorbance continues to decrease above pH four, the decline is much slower. Below this pH value, the absorbance shows an overall decreasing trend as the pH value increases. 2. The influence of pH on real water samples. A water sample with a COD concentration of 50.2 mg/L, as described in Section 2.1, was selected for the research. The pH of the water sample was adjusted from 1.3 to 10.8 using an acid-base regulator. The UV-Vis spectra of these water samples are illustrated in Figure 4a. The absorbance curves at the characteristic peaks and valleys (as identified in the COD standard solution) for different pH ranges are illustrated in Figure 4b. Figure 4a shows the absorbance of lake water with a COD concentration of 50.2 mg/L, revealing a general decrease in absorbance as the pH increases. However, when the pH value is above four, a significant decrease in the overall trend is observed. This behavior can be attributed to the complex composition of the water sample, where the ionization of organic molecules is less affected under acidic conditions, leading to a relatively stable internal electronic structure. In contrast, under alkaline conditions, the further ionization of organic molecules occurs, altering their internal electronic structure, which induces polarization within the chromophore system and modifies the electronic states of organic  
Figure 3: The influence of pH on the UV-Vis spectra of the standard solution.