How to Detect COD by UV Spectroscopy

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author: Susan

2022-01-04

Chemical oxygen demand (COD) is a chemical method to measure the amount of reducing substances that need to be oxidized in a water sample. COD indicates the degree of pollution of the water body by reducing substances. Therefore, COD is one of the important indicators of water quality testing.
At present, the main detection methods of COD are chemical method and spectroscopy. The chemical method is mainly based on the potassium dichromate method specified in the national standard. In terms of law, it has high sensitivity, no
Requires chemical reagents and other characteristics, spectroscopy has become the current mainstream detection method. As one of the four major spectra, ultraviolet spectroscopy is mainly used for quantitative analysis of molecules and is an important qualitative tool for identifying organic substances. Such COD values are highly correlated. Therefore, UV spectroscopy is the most representative method for detecting COD in spectroscopy. How to use UV spectroscopy to detect COD more accurately has become one of the current research questions for scholars from all over the world.

Principle: The Lambert - Beer law is the basic law of absorption spectroscopy. When a beam of parallel monochromatic light passes through a uniform, non-scattering light-absorbing solution vertically, its absorbance A is proportional to the concentration of the light-absorbing solution c and the thickness of the liquid layer (optical path) l. This law is also quantitative by ultraviolet spectroscopy. The theoretical basis of the analysis, the schematic diagram of the Lambert-Beer law is shown in Figure 1, which can be expressed as

Figure 1 Schematic diagram of Lambert-Beer law

Spectroscopy Solution:
Absorbance:
The instrument is the pulsed xenon light source (ATG1030), the central controller, the vertical probe and the full-spectrum optical fiber spectrometer (ATP2000P) produced by Optosky, the needle thermometer, and the electromagnetic heating stirrer, as shown in Figure 2. The experimental use The sample is a COD standard solution formulated in accordance with the potassium dichromate method specified in the national standard.

Figure 2 The complete experimental instrument system

Test Result:

COD full wavelength spectrum

In order to study the best detection wavelength of COD, COD standards with concentrations of 20 mg/L, 50 mg/L, 125 mg/L, 160 mg/L, 200 mg/L, 250 mg/L, 325 mg/L, 350 mg/L, and 500 mg/L were collected. The full-wavelength ultraviolet absorption spectrum of the liquid is shown in Fig. 3. The spectrum range of the spectrometer is 200~1100nm, the integration time is 10ms, the sampling interval is 500ms, and the average number of scans is 100 times.

Figure 3 COD absorption spectra of different concentrations

Temperature sample spectrum

In order to study the influence of temperature on COD measurement, COD with concentrations of 20 mg/L, 30 mg/L, 40 mg/L, 50 mg/L, 60 mg/L, 70 mg/L, 80 mg/L, 90 mg/L, and 100 mg/L were collected. The UV absorption spectra of the standard solution at temperatures of 0℃, 5℃, 10℃, 15℃, 20℃, 25℃, 30℃, taking the COD standard solution with a concentration of 70mg/L as an example, Figure 4 shows the 70mg/LCOD Spectrogram of temperature change.

Figure 4 70mg/L COD spectrum with temperature

Conclusion:
Let the wavelength be λ, it can be seen from Figure 3 that the COD absorbance only has obvious absorption at λ between 185~310nm, and there is no obvious absorption at other wavelength bands, and shows an upward trend at 185~230nm, and a downward trend at 230~264nm. There was an upward trend again at ~290nm, a downward trend at 290~310nm, and almost no absorption after 310nm.

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