Water quality total nitrogen detection method

Abstract

The invention discloses a water quality total nitrogen detection method which comprises the following steps: preparing a plurality of groups of total nitrogen standard solutions of which the turbidity changes in a gradient manner; sequentially digesting the plurality of groups of total nitrogen standard solutions, and sequentially obtaining the absorbance of each group of digested total nitrogen standard solutions at 220nm, 275nm and third wavelength; adopting a curve fitting mode to obtain a curve relation between the absorbance of the total nitrogen standard solution at 220nm and 275nm and the absorbance of the total nitrogen standard solution at the third wavelength, and obtaining a correlation coefficient according to the curve relation; obtaining the compensation amount of the absorbance according to the correlation coefficient, and supplementing the compensation amount to the absorbance of the total nitrogen standard solution at 220nm and 275nm to obtain the absorbance of the 220nm and 275nm after calibration; and obtaining the calibrated absorbance according to a spectrophotometric method, and obtaining the calibrated total nitrogen concentration according to the linear relationship between the absorbance and the total nitrogen concentration. According to the invention, the problems of high cost, poor anti-interference performance and the like of the existing spectrograph scheme are solved, and meanwhile, the accuracy of the total nitrogen measurement result of the water sample is improved.

Description

TECHNICAL FIELD

[0001] The present application relates to a water quality detection technology, in particular to a water quality total nitrogen detection method. BACKGROUND

[0002] As a key technology for assessing water eutrophication, the accuracy of water quality total nitrogen detection directly affects environmental governance and drinking water safety decision-making. The detection standard of the current market water quality total nitrogen online analyzer is based on "Determination of Total Nitrogen in Water - Alkaline Persulfate Digestion Ultraviolet Spectrophotometry" (HJ 636-2012). At 120~124℃, the alkaline persulfate solution converts the nitrogen in the nitrogen-containing compounds in the sample into nitrate, and the absorbance A 220 and A 275 are measured at wavelengths of 220nm and 275nm respectively by spectrophotometry. The corrected absorbance is calculated according to the formula A=A 220 -2A 275 , and the total nitrogen content is proportional to the corrected absorbance A. However, the scattering and absorption effect of turbidity substances such as suspended particles and colloids in water on ultraviolet light causes A 220 and A 275 to systematically deviate from the true absorbance, and at this time the corrected absorbance A calculated cannot truly reflect the total nitrogen content in the water body.

[0003] In addition, the existing dual-wavelength total nitrogen detection instrument mostly uses a scheme of full-spectrum lamp source plus spectrometer detection. For example, the application number CN202210710553.0 discloses a water quality total nitrogen online analyzer photoelectric conversion measurement device and method, which realizes water quality total nitrogen detection based on a miniature spectrometer and a flickering xenon lamp. However, the miniature spectrometer is high in cost, and the spectrometer and the cuvette are connected through an optical fiber, which has poor anti-interference performance. SUMMARY

[0004] In order to solve the above problems in the prior art, the purpose of the present application is to provide a water quality total nitrogen detection method, which solves the problems of high cost and poor anti-interference performance of the existing spectrometer scheme, and improves the accuracy of water sample total nitrogen measurement results.

[0005] The technical scheme adopted by the present application to solve its technical problems is: a water quality total nitrogen detection method, comprising the following steps,

[0006] preparing a plurality of groups of total nitrogen standard solutions with gradient changes in turbidity;

[0007] digesting the plurality of groups of total nitrogen standard solutions in turn, and obtaining the absorbance of each group of digested total nitrogen standard solutions at 220nm, 275nm and a third wavelength in turn;

[0008] The curve fitting method is used to obtain the curve relationship of the absorbance of the total nitrogen standard solution at 220nm and 275nm relative to the absorbance of the total nitrogen standard solution at the third wavelength, and the correlation coefficient is obtained according to the curve relationship;

[0009] The compensation amount of the absorbance is obtained according to the correlation coefficient, and the compensation amount is added to the absorbance of the total nitrogen standard solution at 220nm and 275nm, so as to obtain the calibrated absorbance at 220nm and 275nm;

[0010] The calibrated absorbance is obtained according to the spectrophotometry, and the calibrated total nitrogen concentration is obtained according to the linear relationship between the absorbance and the total nitrogen concentration.

[0011] The third wavelength is an infrared wavelength.

[0012] Optionally, when the total nitrogen standard solution is prepared, the turbidity component of the total nitrogen standard solution is kaolin.

[0013] Optionally, the absorbance at 220nm, 275nm and the third wavelength is filtered by a 220nm filter, a 275nm filter and an infrared band-pass filter, respectively.

[0014] Optionally, the 220nm filter and the 275nm filter are symmetrically distributed relative to the optical axis of the light beam emitted by the light source.

[0015] By using the above technical solution, the detection method based on the filter and the photodiode has a lower cost compared with the widely used spectrometer, and the increase of the third wavelength detection light path can eliminate the turbidity interference and the color interference during the measurement of the total nitrogen of the water sample. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 is a structural schematic diagram of the detection system of the present application;

[0017] Figure 2 is a relationship diagram of the absorbance of the total nitrogen standard solution with turbidity at 220nm relative to the absorbance at 860nm;

[0018] Figure 3 is a relationship diagram of the absorbance of the total nitrogen standard solution with turbidity at 275nm relative to the absorbance at 860nm. DETAILED DESCRIPTION

[0019] The present application will be further described in detail below in combination with the drawings and examples. It can be understood that the specific examples described herein are only used to explain the related application, and are not a limitation on the application. In addition, it should be noted that only the parts related to the application are shown in the drawings for the convenience of description.

[0020] It should be noted that the embodiments in the present application and the features in the embodiments can be combined with each other without conflict. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0021] As shown in Figure 1 The present application discloses a water quality total nitrogen detection system, which comprises a digestion bottle 100, a light source 200, filters and photodiodes. The filters are three, which are a 220 nm filter 300, a 275 nm filter 400 and an infrared band-pass filter 500. The photodiodes are also three and correspond to the three filters one by one. When the filters are installed, the three filters are distributed in an up-down manner, and from bottom to top, they are the 220 nm filter 300, the 275 nm filter 400 and the infrared band-pass filter 500. The 220 nm filter 300 and the 275 nm filter 400 are symmetrically distributed relative to the optical axis of the light source 200 to ensure the stability of measurement. The infrared band-pass filter 500 can be a 860 nm filter. In an embodiment of the present application, the center distance between the 220 nm filter 300 and the 275 nm filter 400 is set to 9.5 mm, and the center distance between the infrared band-pass filter 500 and the 275 nm filter 400 is set to 10 mm. By narrowing the distance between the adjacent two filters, the measurement stability can be further ensured.

[0022] In the present application, the light source 200 can adopt a xenon lamp, and the light-emitting wavelength range is 185 nm-2500 nm, which can cover the ultraviolet to infrared waveband.

[0023] Specifically, when the total nitrogen is detected, the steps are as follows.

[0024] S1, several groups of total nitrogen standard solutions with gradient change of turbidity are prepared.

[0025] In an embodiment of the present application, based on the turbidity gradient of 0, 50, 100, 150, 200, 300 and 400 NTU, the total nitrogen standard solutions with concentrations of 1, 2, 3, 4 and 5 mg / L under different turbidity gradients are configured, a total of 35 groups, and the turbidity solution needs to be configured by using kaolin.

[0026] S2, the 35 groups of total nitrogen standard solutions are sequentially digested, and the absorbance A 220 , A 275 and A 860 at 220 nm, 275 nm and the third wavelength (860 nm) of each group of digested total nitrogen standard solutions are sequentially obtained.

[0027] In one embodiment of the present application, the volume of each component in the digestion solution is as follows: 4 ml of water sample, 0.8 ml of alkaline potassium persulfate solution (20 g / L K2S2O8+6 g / L NaOH), and 0.8 ml of dilute hydrochloric acid (1+9), and the digestion condition is 15 min at 120°C.

[0028] S3, a curve relationship of absorbance of the total nitrogen standard solution at 220 nm and 275 nm relative to absorbance of the total nitrogen standard solution at the third wavelength is obtained by linear fitting, and a correlation coefficient is obtained according to the curve relationship.

[0029] In one embodiment of the present application, the measured A 220 and A 275 are fitted by a first order curve or a second order curve. 860 The relationship between A 220 is taken as an example, based on A 220 =k·A 860 +b or A 220 =a·(A 860 ) 2 +b , ·A 860 +c mathematical model fitting, the coefficients (k, b in the first order curve or a, b , , c in the second order curve) in the mathematical model are obtained, and the fitting results are shown in Table 1 and Table 2. Figure 2 Figure 3

[0030] S4, a compensation amount of absorbance is obtained according to the correlation coefficient, and the compensation amount is added to the absorbance of the total nitrogen standard solution at 220 nm and 275 nm to obtain the calibrated absorbance at 220 nm and 275 nm.

[0031] After the correlation coefficient in the first order curve or the second order curve is obtained, the compensation amount is obtained according to the following formula:

[0032] △A=k·A 860 ,

[0033] or

[0034] △A=a·(A 860 ) 2 +b , ·A 860 .

[0035] For example, in one embodiment of the present application, if the first order curve fitting is adopted, the value of k is measured to be 4.3988, and the compensation amount △A is 4.3988×A 860 ; if the second order curve fitting is adopted, the value of a is measured to be 4.7583, and the value of b , ​​is 1.0415, then the compensation amount ΔA is 4.7583·(A 860 ) 2 +1.0415·A 860 .

[0036] S5、According to the spectrophotometry, the calibrated absorbance is obtained, and according to the linear relationship between the absorbance and the total nitrogen concentration, the calibrated total nitrogen concentration is obtained.

[0037] According to the ultraviolet spectrophotometry, the total nitrogen absorbance is obtained by using the compensated absorbance, and the calculation formula is A=A 220 -2A 275 , and the real total nitrogen concentration value of the water sample is calculated based on the absorbance, and in the specific implementation of the present application, the relationship between the calibrated correction absorbance and the total nitrogen concentration is: C=5.4364A-0.0878.

[0038] As shown in Table 1 below, Table 1 shows the calculation process and results of multiple groups of standard solutions with turbidity of 50 NTU based on the above method for turbidity compensation, and it can be found that the compensation method has good effect on measuring the real TN value of the water sample.

[0039] Table 1

[0040]

[0041] The above description is only the preferred embodiment of the present application and the explanation of the applied technical principles. Those skilled in the art should understand that the scope of the application involved in the present application is not limited to the technical solutions formed by the specific combination of the above technical features, and should also cover other technical solutions formed by any combination of the above technical features or their equivalent features without departing from the inventive concept. For example, the above features are replaced with the technical features disclosed in the present application (but not limited to) having similar functions to form technical solutions.

[0042] In addition to the technical features described in the specification, the remaining technical features are known to those skilled in the art, and in order to highlight the innovative features of the present application, the remaining technical features will not be described here.

Claims

1. A method for detecting total nitrogen in water, characterized in that: The following steps are included: Prepare several sets of total nitrogen standard solutions with turbidity gradient changes; Digesting several groups of total nitrogen standard solutions in sequence, and obtaining the absorbance of each group of digested total nitrogen standard solutions at 220 nm, 275 nm, and a third wavelength in sequence; A curve fitting method is used to obtain a curve relationship between the absorbance of the total nitrogen standard solution at 220 nm and 275 nm relative to the absorbance of the total nitrogen standard solution at the third wavelength, and a correlation coefficient is obtained based on the curve relationship; Obtain the compensation amount of absorbance based on the correlation coefficient, and add the compensation amount to the absorbance of the total nitrogen standard solution at 220nm and 275nm to obtain the absorbance at 220nm and 275nm after calibration; Obtaining the calibrated absorbance according to the spectrophotometer, and obtaining the calibrated total nitrogen concentration according to the linear relationship between the absorbance and the total nitrogen concentration; Wherein, the third wavelength is an infrared wavelength.

2. The method for detecting total nitrogen in water according to claim 1, wherein When preparing the total nitrogen standard solution, the turbidity component of the total nitrogen standard solution is kaolin.

3. The method for detecting total nitrogen in water according to claim 2, wherein The absorbance at 220 nm, 275 nm and the third wavelength is filtered through a 220 nm filter, a 275 nm filter and an infrared bandpass filter, respectively.

4. The method for detecting total nitrogen in water according to claim 3, wherein The 220nm filter and the 275nm filter are symmetrically distributed relative to the optical axis of the light beam emitted by the light source.

Images