## 3 Experiment and Results Discussion
### Effect of the Influence of Environmental Factors on UV-Vis Spectroscopy
#### 3.1.2 Effect of the Influence of Temperature on UV-Vis Spectroscopy
Given that temperatures in common surface waters, such as domestic sewage, rivers, and lakes, vary widely with seasonal and diurnal changes, compensating for the influence of temperature is essential to enhance the accuracy of COD detection. Figure 6: The influence of temperature on the UV-Vis spectra of real water samples. (**a**) The influence of temperature on the UV-Vis spectra of real water samples. (**b**) The change in absorbance at different wavelengths with the temperature of real water samples. 3.1.3. Effect of the Influence of Conductivity on UV-Vis Spectroscopy  
The conductivity of water is directly related to the concentration of dissolved salts or other inorganic substances capable of dissociating into electrolytes. The purer the water, the lower its conductivity. On the contrary, the higher the concentrations of compounds such as hydrochloric acid, nitrates, and sodium chloride, the higher the conductivity. Conductivity is influenced by several factors, including temperature, pH levels, and the concentration of inorganic pollutants. While pH indirectly influences the absorbance in UV-Vis spectra, certain inorganic ions exhibit strong absorption in the ultraviolet region, which can influence the accuracy of COD detection based on UV-Vis spectroscopy. This research specifically examines the influence of conductivity on UV-Vis spectra, using nitrate \(NO_{3}^{-}\) as a representative example. 1. The influence of \(NO_{3}^{-}\) on the standard solution. Nitrite (\(NO_{2}^{-}\)) and nitrate (\(NO_{3}^{-}\)) in water are byproducts of the oxidation and decomposition of nitrogen-containing organic compounds and are eventually converted into \(NO_{3}^{-}\). To investigate this, a COD concentration of 50 mg/L was prepared by mixing sodium nitrate (NaNO\({}_{3}\)) solution with potassium hydrogen phthalate solution and adjusting the NaNO\({}_{3}\) concentration from 1 to 19 mg/L. The UV-Vis spectra of these solutions are illustrated in Figure 7a. The absorbance curves of the characteristic peaks and valleys (as identified in the COD standard solution) at different NaNO\({}_{3}\) concentrations are illustrated in Figure 7b. Figure 7: The influence of \(NO_{3}^{-}\) on the UV-Vis spectra of the standard solution. (**a**) The influence of \(NO_{3}^{-}\) on the spectra of the standard solution. (**b**) The change in absorbance at different wavelengths with the \(NO_{3}^{-}\) of the standard solution. It can be seen from the spectroscopy of the COD standard solution in Figure 7a that \(NO_{3}^{-}\) shows obvious absorption peaks at around 280 nm and overlaps with one absorption peak of the COD standard solution. There is almost no absorption at the first absorption peak of the COD (236.09 nm). It can be observed that the absorbance increases by about 0.15 at the wavelength of 280 nm as the \(NO_{3}^{-}\) concentration gradually increases from 1 to 19 mg/L. It can also be observed that as the concentration of \(NO_{3}^{-}\) increases, the absorbance also increases (except at 236.09 nm), showing a good linear relationship in Figure 7b. However, the linear relationship corresponding to the absorbance at different wavelengths is inconsistent. Therefore, \(NO_{3}^{-}\) has a significant influence on UV-Vis spectroscopy. In order to eliminate its influence, a compensation method must be used in COD detection based on UV-Vis spectroscopy. 2. The influence of \(NO_{3}^{-}\) on real water samples. The composition of real water samples is far more complex than that of standard solutions. Therefore, in this section, we continue to focus on the lake water sample with a COD concentration of 50.2 mg/L, as discussed in Section 3.1.1. The nitrate (\(NO_{3}^{-}\)) concentration in this water sample was adjusted from 0 to 19 mg/L. The UV-Vis spectra of these water samples are illustrated in Figure 8a. The absorbance curves at the characteristic peaks and valleys (as identified in the COD standard solution) for different NaNO\({}_{3}\) concentrations are illustrated in Figure 8b.