## 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  
The structure of molecules changes when the temperature rises, and the movement of molecules is closely related to temperature. The higher the temperature, the faster the movement of molecules in water, which inevitably influences COD detection. Moreover,  
Figure 4: The influence of pH on the UV-Vis spectra of real water samples. (**a**) The influence of pH on the UV-Vis spectra of real water samples. (**b**) The change in absorbance at different wavelengths with the pH of real water samples. some devices in water detection, such as spectrometers and light sources, are also sensitive to temperature, all of which can influence UV-Vis spectroscopy. To investigate the influence of temperature on COD determination, an experiment was conducted on the influence of temperature on UV-Vis spectroscopy. 1. The influence of temperature on the standard solution. The same 50 mg/L potassium hydrogen phthalate standard solution was prepared as that used in Section 3.1.1. The temperature of the solution was adjusted from 0 to 46.7 \({}^{\circ}\)C using a temperature-controlled chamber and ice. The UV-Vis spectra of these solutions are illustrated in Figure 5a. The absorbance curves of the characteristic peaks and valleys (as identified in the COD standard solution) at different temperature ranges are illustrated in Figure 5b. The UV-Vis absorption spectrum of the COD standard solution, as shown in Figure 4a, indicates that temperature changes have a measurable influence, with a variation of approximately 0.1 at the peak position of 236.09 nm. The absorbance demonstrates a positive correlation with increasing temperature. As shown in Figure 4b, the absorbance at different wavelengths also exhibits an upward trend, with a strong linear relationship to temperature. However, it is important to note that the linearity of the absorbance-temperature relationship varies across different wavelengths. 2. The influence of temperature on real water samples. The composition of the water sample is more complex compared to the standard solution. Therefore, in this section, the lake water sample with a COD concentration of  
Figure 5: The influence of temperature on the UV-Vis spectra of standard solution. (**a**) The influence of temperature on the UV-Vis spectra of the standard solution. (**b**) The change in absorbance at different wavelengths with the temperature of the standard solution. 50.2 mg/L, as described in Section 3.1.1, was selected for further analysis. The temperature of the lake water was adjusted from 0 to 47.2 \({}^{\circ}\)C using a temperature-controlled chamber and ice. The UV-Vis spectra of these water samples are illustrated in Figure 6a. The absorbance curves for the characteristic peaks and valleys (as identified in the COD standard solution) in different temperature ranges are illustrated in Figure 6b. As shown in Figure 6a, the UV-Vis absorption spectrum of the real water samples also shows a temperature-induced change in absorbance of approximately 0.1 at the peak wavelength of 236.09 nm. The absorbance increases gradually, showing a positive correlation with rising temperatures. Furthermore, Figure 6b highlights a similar upward absorbance trend, as shown in Figure 5b, with greater clarity and prominence. The above analysis demonstrates that temperature significantly impacts the UV-Vis spectrum of real water samples, generally exhibiting an approximately linear correlation. However, this linearity varies across different wavelengths, making the temperature's overall influence on the spectrum more complex. Consequently, it is challenging to accurately model temperature-induced absorbance changes at each wavelength using a simple curve. For COD detection based on UV-Vis spectroscopy, substantial temperature fluctuations can lead to significant errors.