How to Detect COD in the Field Environment by UV Spectroscopy
author: Susan
2022-01-04
Challenge
COD represents the degree of pollution of water bodies by reducing substances. Compared with the traditional method for detecting COD, it has the disadvantages of long detection time and complicated operation. UV spectroscopy has become the mainstream detection method due to its fast detection speed and no need to use chemical reagents.
However, the previous COD detection article used standard water samples. In fact, it did not take into account the use of UV spectroscopy to detect COD accuracy under low-temperature environments and the turbidity of water samples.
Solution
1. The experimental instrument is composed of a Xenon scintillation lamp from Xiamen Optosky, a dimmable optical fiber sensing probe, a full-spectrum optical fiber spectrometer, and a central controller. The absorption spectrum of the solution is measured by the optical fiber sensing system. The system includes a light source, a The dimming program is composed of optical fiber sensor probe, spectrometer, optical fiber, central controller, and display. The total wavelength range of the test is 200-1000nm, the resolution is 2.3nm, and the signal-to-noise ratio is >500:1. The system is shown in Figure 1.
Fig. 1 Physical diagram of experimental divece
The experimental samples are the COD standard solution prepared in accordance with the potassium dichromate method specified in the national standard and the actual water sample collected from surface water in a certain area of Changchun City. The actual water sample concentration was adopted by an environmental research institute in Qingdao, Shandong Province to adopt the national standard of potassium dichromate Method detection.
Fig. 2 COD spectrum of standtard water sample with different concentration
Figure 3 COD spectrum of actual water sample and standard water sample (20℃)
Figure 4 80mg·L-1 COD spectrum standard water sample with different temperature
Figure 5 COD spectrum of the actual water sample changes with different temperature
Figure 6 COD temperature compensation model
COD represents the degree of pollution of water bodies by reducing substances. Compared with the traditional method for detecting COD, it has the disadvantages of long detection time and complicated operation. UV spectroscopy has become the mainstream detection method due to its fast detection speed and no need to use chemical reagents.
However, the previous COD detection article used standard water samples. In fact, it did not take into account the use of UV spectroscopy to detect COD accuracy under low-temperature environments and the turbidity of water samples.
Solution
1. The experimental instrument is composed of a Xenon scintillation lamp from Xiamen Optosky, a dimmable optical fiber sensing probe, a full-spectrum optical fiber spectrometer, and a central controller. The absorption spectrum of the solution is measured by the optical fiber sensing system. The system includes a light source, a The dimming program is composed of optical fiber sensor probe, spectrometer, optical fiber, central controller, and display. The total wavelength range of the test is 200-1000nm, the resolution is 2.3nm, and the signal-to-noise ratio is >500:1. The system is shown in Figure 1.
Fig. 1 Physical diagram of experimental divece
The experimental samples are the COD standard solution prepared in accordance with the potassium dichromate method specified in the national standard and the actual water sample collected from surface water in a certain area of Changchun City. The actual water sample concentration was adopted by an environmental research institute in Qingdao, Shandong Province to adopt the national standard of potassium dichromate Method detection.
2. Spectral collection
In order to determine the best wavelength for COD detection under UV spectroscopy, prepare COD standard solutions with concentrations of 20, 50, 125, 160, 250 and 350 mg·L-1, and collect their absorption spectra at full wavelengths, see Figure 2.
In order to determine the best wavelength for COD detection under UV spectroscopy, prepare COD standard solutions with concentrations of 20, 50, 125, 160, 250 and 350 mg·L-1, and collect their absorption spectra at full wavelengths, see Figure 2.
Fig. 2 COD spectrum of standtard water sample with different concentration
In order to verify the applicability of the best wavelength in the actual water sample, the 8mg·L-1COD actual water sample and the 8mg·L-1 COD standard water sample were collected separately, as shown in Figure 3.
Figure 3 COD spectrum of actual water sample and standard water sample (20℃)
In order to study whether the temperature has an effect on the measured value of COD in water measured by UV spectroscopy, COD standard water samples with concentrations of 10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 mg.L-1 were selected respectively , The COD detection values at the temperature of 0, 5, 10, 15, 20, 25 and 30℃ are shown in Table 2. Take the temperature-dependent spectrum of a standard water sample with a COD concentration of 80mg.L-1 as an example, Figure 4.
The effect of temperature in actual water samples on COD measurement was studied, and COD was collected the UV absorption spectra of actual water samples at different temperatures from 0 to 30 ℃, the figure 5 is the spectrum of the COD of the actual water sample changing with temperature.
Test Result
The effect of temperature in actual water samples on COD measurement was studied, and COD was collected the UV absorption spectra of actual water samples at different temperatures from 0 to 30 ℃, the figure 5 is the spectrum of the COD of the actual water sample changing with temperature.
Test Result
1. Selection of the best wavelength of the spectrum
According to Lambert-Beer's law, the absorbance of several groups of solution samples with known concentrations can be measured by using a full-spectrum spectrometer. In this way, the matching curve of the concentration of the COD solution and the absorbance is calculated, and then the concentration of the unknown COD solution is calculated.
According to Lambert-Beer's law, the absorbance of several groups of solution samples with known concentrations can be measured by using a full-spectrum spectrometer. In this way, the matching curve of the concentration of the COD solution and the absorbance is calculated, and then the concentration of the unknown COD solution is calculated.
Figure 4 80mg·L-1 COD spectrum standard water sample with different temperature
Collect the UV absorption spectra of COD standard water samples with different concentrations, the absorbance values at wavelengths of 256, 267, 276, 286, and 296 nm were collected, and a unitary linear regression model was established based on the obtained data, and linear fitting was performed.
Figure 5 COD spectrum of the actual water sample changes with different temperature
2. The influence of temperature on the accuracy of COD detection
To verify the effect of temperature on the COD of the actual water sample under the spectroscopic method, the UV absorption spectra of the actual COD water sample at different temperatures from 0 to 30 ℃ were collected. See Figure 5. It can be seen from Figure 5 that temperature has a relatively large impact on its detection value. As the temperature increases, the COD absorbance curve moves upward as a whole, which is consistent with the upward trend of the spectrum in Figure 4, so a COD temperature compensation should be established. Model and correct the influence of temperature on the detection of COD by UV spectroscopy. The actual water sample is used to analyze the error of the temperature compensation model.
To verify the effect of temperature on the COD of the actual water sample under the spectroscopic method, the UV absorption spectra of the actual COD water sample at different temperatures from 0 to 30 ℃ were collected. See Figure 5. It can be seen from Figure 5 that temperature has a relatively large impact on its detection value. As the temperature increases, the COD absorbance curve moves upward as a whole, which is consistent with the upward trend of the spectrum in Figure 4, so a COD temperature compensation should be established. Model and correct the influence of temperature on the detection of COD by UV spectroscopy. The actual water sample is used to analyze the error of the temperature compensation model.
Figure 6 COD temperature compensation model
Conclusion
The experiment selects 286 nm as the best wavelength for COD detection, which verifies that the best wavelength is also applicable to the COD value of the actual water samples in the detection field. According to the principle of least squares, a COD temperature compensation model between absorbance, temperature, and COD concentration is established. The actual water sample is used as the research object, and the error analysis of the temperature compensation model is carried out. The results show that the actual value of COD is basically consistent with the compensated value. The maximum relative error is 6.38%, the minimum relative error is 0.63%, and most of the relative errors are concentrated. In; 4% . It can be seen that the compensation value of the COD temperature compensation model is within the error range, the compensation accuracy is high, and the effect is good. This study can provide an effective basis for the use of UV spectroscopy to detect COD in the field at low temperature.
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The experiment selects 286 nm as the best wavelength for COD detection, which verifies that the best wavelength is also applicable to the COD value of the actual water samples in the detection field. According to the principle of least squares, a COD temperature compensation model between absorbance, temperature, and COD concentration is established. The actual water sample is used as the research object, and the error analysis of the temperature compensation model is carried out. The results show that the actual value of COD is basically consistent with the compensated value. The maximum relative error is 6.38%, the minimum relative error is 0.63%, and most of the relative errors are concentrated. In; 4% . It can be seen that the compensation value of the COD temperature compensation model is within the error range, the compensation accuracy is high, and the effect is good. This study can provide an effective basis for the use of UV spectroscopy to detect COD in the field at low temperature.
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