Rapid Determination of Nitrate Nitrogen and Nitrite Nitrogen by Second Derivative Spectrophotometry
WANG Jing-min1, ZHANG Jing-chao1*, ZHANG Zun-ju1,2
1. Key Laboratory for Microstructural Material Physics of Hebei Province, School of Science, Yanshan University, Qinhuangdao 066004, China
2. Hebei University of Environmental Engineering, Qinhuangdao 066004, China
Abstract：In the ultraviolet spectrophotometry, we know that the absorption peak of nitrate nitrogen is around 202.0 nm, and the absorption peak of nitrite nitrogen is around 210.0 nm. It can be seen that the absorption peaks of the two are very close, and the absorption curves overlap seriously and interfere with each other badly. It is difficult to use single wavelength to determine the content of the two without the separation, and the national standard method is too complicated and time-consuming. In order to monitor the nitrate nitrogen and nitrite nitrogen in environmental water and drinking water more accurately, rapidly and eco-friendly, and to avoid many shortcomings in the national standard method, in this paper we studied the rapid analysis method of the two in water. This method combines with ultraviolet spectrophotometry and the second derivative spectrometry which are more rapid and accurate. And it does not need any pre-separation treatment. Nitrate nitrogen and nitrite nitrogen series solution were prepared by guarantee reagent. Using deionized water as a reference, UV-Vis spectrophotometer was used to scan the UV absorption spectrum in the range of 195～250 nm. After that, we used the Origin software to do the second derivative spectra of the obtained spectrogram, and used the Savitzky-Golay method in the Origin software to smoothen the second derivative spectra. By observing the above two groups of second derivative spectrogram, we found that the second derivative absorbance at different concentrations of nitrite nitrogen samples at 223.5 nm was 0 and nitrate nitrogen samples at 216.5 nm was also 0. The second derivative absorbance of the UV spectra of the mixed samples were observed by experiments. We found that they conformed to Lambert-Beer law. Then the nitrate nitrogen and nitrite nitrogen mixed samples were prepared. The UV absorption spectra of the mixed samples were scanned. We did the second derivative spectra of the obtained spectrogram and smoothened them. After that, we observed the second derivative spectra of mixed samples. It could be seen that when the concentration of nitrate nitrogen is the same and nitrite nitrogen concentration is different, the concentration of nitrite nitrogen will affect the second derivative absorbance of nitrate nitrogen. However, the second derivative spectra of various samples at 223.5 nm are almost overlapping, indicating that the concentration of nitrite nitrogen at this wavelength will not have any effect on the second derivative absorbance of nitrate nitrogen. At this wavelength, the value of the second derivative absorbance of the mixed samples increase linearly with the increase of nitrate nitrogen concentration. Therefore, 223.5 nm was chosen as the determination wavelength of nitrate nitrogen in the mixed samples. In the same way, the determination wavelength of nitrite nitrogen is 216.5 nm. Linear regression analysis of the nitrate nitrogen single component samples was performed between the second derivative absorbance and the concentration at 223.5 nm. The linear relationship was good, and the regression equation of the obtained standard curve was C=438.69A+0.015, R2=0.995 9. In the same way, we obtained that the regression equation of the nitrite nitrogen at 216.5 nm was C=-657.29A+0.068 8, R2=0.998. In order to test the application of this method in actual water sample measurement, we took three kinds of water samples from New River, Tang River and Dai River in Qinhuangdao to carry out experiments. The results showed that the recovery rate was between 96.7% and 103%, the relative standard deviation was 1.46~3.68. The method is relatively accurate and easy to operate and costs less. The rapid on-line monitoring of nitrate nitrogen and nitrite nitrogen can be realized at the same time.
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