Total nitrogen concentration measurement method for water quality

Abstract

The present invention relates to a total nitrogen concentration measurement method for water quality. The method comprises: (1) adding a water sample, a K2S2O8 solution and a NaOH solution into a digestion tank, and carrying out digestion and oxidation in the digestion tank; (2) adding a H4N2H2SO4 solution and a catalyst into a digestion solution for reduction reaction; (3) adding a mixed reduction solution into a color development reagent, generating a red dye, and obtaining a color development solution; and (4) taking the color development solution, using a light source having a wavelength of 540 nm to carry out ADC signal measurement, recording an ADCx signal value, and calculating the total nitrogen concentration of the water sample to be measured. According to the total nitrogen concentration measurement method for water quality provided by the present invention, measurement conditions are easily realized for an online monitoring instrument; special anti-interference treatment is carried out on an actual water sample, thereby ensuring the accuracy of measured data; and the light source is used for calibration and compensation, thereby improving the accuracy, and further optimizing the online instrument monitoring process for water quality.

Description

A method for detecting total nitrogen concentration in water Technical Field

[0001] The present invention belongs to the technical field of online automatic monitoring of water quality, and in particular relates to a method for detecting the total nitrogen concentration of water quality. Background Art

[0002] Total nitrogen, abbreviated as TN, is a key indicator of water quality. It is defined as the total amount of inorganic and organic nitrogen in all its forms. Excessive nitrogen levels in water can lead to the proliferation of microorganisms, which consume dissolved oxygen and deteriorate water quality.

[0003] Two existing methods are used to measure total nitrogen. One is potassium persulfate oxidation ultraviolet spectrophotometry. However, because the light sources are 220nm and 275nm, respectively, the transmitter requires an imported deuterium lamp, which lacks independent control. Furthermore, the receiver requires a spectrometer, significantly increasing instrument cost. Furthermore, the light source is susceptible to interference from turbidity and color, resulting in poor measurement stability. Another method, resorcinol spectrophotometry, primarily uses concentrated sulfuric acid. Online monitoring equipment often samples sulfuric acid, which can lead to dangerous conditions such as malfunction and overflow. This indirectly results in a high instrument failure rate, making it difficult to meet the testing requirements for online monitoring.

[0004] Therefore, the testing method for total nitrogen needs to be improved and optimized.

[0005] Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the shortcomings of the existing technology and provide a method for detecting the total nitrogen concentration in water quality. The present invention provides a method for detecting the total nitrogen concentration in water quality. The detection conditions provided by the present invention are easy to implement for online monitoring instruments, and special anti-interference processing is performed for actual water samples to ensure the accuracy of test data. The use of light source calibration compensation improves precision and further optimizes the online instrument monitoring process.

[0007] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0008] The present invention provides a method for detecting total nitrogen concentration in water, comprising the following steps:

[0009] (1) Oxidation treatment: Add 2 ml of the water sample to be tested into the digestion tank, then add 1 ml of K2S2O8 solution and 1 ml of NaOH solution into the digestion tank, and heat it to 125℃±3℃ and 2.30-2.40MP pressure in the digestion tank for 15 minutes to oxidize all nitrogen-containing compounds in the water sample into NO - 3. Obtain digestion solution;

[0010] (2) Reduction treatment: Add 1 ml of H4N2·H2SO4 solution to the digestion solution, add 1 ml of a mixture of copper sulfate and zinc sulfate as catalyst, and carry out a reduction reaction at 50°C and normal pressure to reduce NO - 3 is reduced to NO - 2. Obtain a reduced mixed solution; in this process, H4N2·H2SO4 is oxidized, and the nitrogen therein is discharged from the system in the form of N2, which has no effect on the determination of total nitrogen;

[0011] (3) Color development reaction: 3 ml of the reduction mixture obtained in step 2) is measured as the color development reduction solution volume, added to the reaction tank, phosphate buffer and distilled water are used as regulating solutions, and the pH value is adjusted to 1.8±0.3, and 1 ml of the mixture of color development reagents Formula 1 and Formula 2 is added. The compound of Formula 1 and the reduction mixture are reacted at 50°C and 101.325 kPa to form a diazonium salt. The diazonium salt is then coupled with the compound of Formula 2 at 50°C and 101.325 kPa for 5 minutes to generate a stable red dye. The reaction tank is diluted with distilled water to 5 ml as the total color development volume to obtain a color development solution;

[0012] (4) Anti-interference compensation test: the photoelectric detection signal ADC of the water sample reduction liquid to be tested 待测水样 ADC with photoelectric detection signal of standard solution and reducing solution 标液 As a reference signal, the interference compensation signal ADC is calculated according to formula 1 干扰补偿 ;

[0013] Among them, V 显色还原液体积 The volume of color reduction solution measured for the color reaction;

[0014] V 显色总体积 is the total volume of the color reaction cell after constant volume;

[0015] (5) Detection light source calibration:

[0016] ⅰ: Initial calibration of light source: inject 5ml of distilled water into the detection cell, adjust the receiving end light signal to stabilize at 24000±50ADS, record the ADC signal value within 30s after stabilization, and calculate the average value to get ADC 光源初始校准 After the initial calibration of the light source, the instrument must be calibrated at zero point and standard liquid point to obtain ADC0 and ADC 标液 ;

[0017] ⅱ: Measurement before light source test: Before the water sample to be tested is injected into the detection cell, 5 ml of distilled water is first injected into the detection cell. After stabilization, the ADC signal value within 30 seconds is recorded and the average value is calculated to obtain the ADC 光源测量 ;

[0018] iii: According to ADC光源测量 and ADC 光源初始校准 , calculate ADC according to formula 2 光源校准补偿 ; ADC 光源校准补偿 =ADC 光源初始校准 -ADC 光源测量 (2)

[0019] (6) Total nitrogen concentration detection of water quality: Take the color developing solution in the reaction pool to rinse the detection pool and drain the liquid, measure 2.0ml of color developing solution and inject it into the detection pool, use a 540nm wavelength light source to detect the ADC signal, and record it as ADC x Signal value, calculate the total nitrogen concentration of the water sample to be tested according to formula 3;

[0020] Among them, C x is the concentration value of the water sample to be tested, C 标液 is the concentration value of the standard solution, ADC0 is the zero point ADC value, ADC 标液 is the ADC value of the standard solution, ADC x is the ADC value of the water sample to be tested.

[0021] Furthermore, in step (1), the concentration of the K2S2O8 solution is 0.1-0.11 mol / L; the concentration of the NaOH solution is 0.2-0.25 mol / L.

[0022] Furthermore, the concentration of the H4N2·H2SO4 solution in step (2) is 0.015 mol / L.

[0023] Furthermore, the catalyst in step (2) is a mixture of copper sulfate and zinc sulfate, which serves to increase the reaction rate. The concentration of copper sulfate in the mixture is 0.00010 mol / L, and the concentration of zinc sulfate is 0.003 mol / L.

[0024] Furthermore, the reduction reaction time in step (2) is controlled at 3.0 min.

[0025] Furthermore, the concentration of the phosphate buffer solution in step (3) is 1.50 mol / L. In order to adaptively adjust the pH, a pH sensor is used for detection to control the titration speed and titration amount until the pH is controlled within the range of 1.8±0.3.

[0026] Furthermore, the mixed solution of the color developing reagents in step (3) uses 2.5 mol / L phosphoric acid as a solvent, the concentration of color developing reagent formula 1 is 0.28 mol / L, and the concentration of color developing reagent formula 2 is 0.0055 mol / L, and is stored in a brown bottle for later use.

[0027] After calculation, the corresponding reduction of NO -The theoretical value of 3 is 0.000030 mol, corresponding to the reduction of NO - The theoretical mass of NO3 is 1.86 mg. The basic range of the present invention is set to 0-10 mg / L. When 2.0 mL of water sample is added, the corresponding NO - The maximum theoretical mass of 3 is 0.02 mg, which is much larger than the theoretical reduction amount, so it can ensure NO - 3 is completely reduced to NO - 2.

[0028] In colorimetric testing, the turbidity and color of actual water samples can affect ADC signal measurement, resulting in a significant deviation between the actual concentration of the water sample and the measured concentration. Therefore, compensation for the interference of the actual water sample is necessary. Interference compensation is measured in real time for each water sample test, allowing dynamic compensation based on changes in water quality. This effectively eliminates the impact of interference from turbidity and color of the actual water sample on data accuracy. Beneficial effects:

[0029] The present invention provides a method for detecting the total nitrogen concentration in water quality. Compared with the existing technology, the present invention uses a single light source, has low maintenance costs, and solves the problem of autonomous control of the light source. The detection conditions are easy to implement for online monitoring instruments, and there is special anti-interference treatment for actual water samples, which avoids interference from turbidity and chromaticity during measurement, ensures the accuracy of test data, uses light source calibration compensation, improves accuracy, eliminates reference photocell detection, reduces instrument costs, and further optimizes the online instrument monitoring process. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] FIG1 is a flow chart of the process of the detection method of the present invention;

[0031] FIG2 is a standard curve of the water quality total nitrogen detection method of the present invention;

[0032] Figure 3 is the total nitrogen standard curve of HJ 636-2012. DETAILED DESCRIPTION

[0033] The present invention will be described in detail below with reference to specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but are not intended to limit the present invention in any form. It should be noted that those skilled in the art may make several adjustments and improvements without departing from the scope of the present invention. These all fall within the scope of protection of the present invention.

[0034] Example 1

[0035] The detection method of total nitrogen water quality of the present invention:

[0036] As shown in the process flow chart of Figure 1, the specific detection steps are as follows;

[0037] (1) Add 2 ml of the water sample to be tested into the digestion tank, then add 1 ml of 0.1 mol / L K2S2O8 solution and 1 ml of 0.2 NaOH solution into the digestion tank, and heat it to 125℃±3℃ and about 2.35 MPa for 15 minutes to oxidize all nitrogen-containing compounds in the water sample into NO. - 3. Obtain digestion solution;

[0038] (2) Add 1 ml of 0.015 mol / L H4N2·H2SO4 solution to the digestion solution, add 1 ml of a mixture of 0.00010 mol / L copper sulfate and 0.003 mol / L zinc sulfate, and carry out a reduction reaction at 50°C and normal pressure to reduce NO - 3 is reduced to NO - 2. Obtain a reduced mixed solution; in this process, H4N2·H2SO4 is oxidized, and the nitrogen therein is discharged from the system in the form of N2, which has no effect on the determination of total nitrogen;

[0039] (3) 3 ml of the obtained reduction mixture was accurately measured using a measuring tube and added to the reaction cell. Phosphate buffer and distilled water were used as regulating solutions to adjust the pH value to 1.8 ± 0.3. 1 ml of the mixed solution of Formula 1 and Formula 2 was added. The compound of Formula 1 reacted with the reduction solution at 50° C. and 101.325 kPa to form a diazonium salt. The diazonium salt was then coupled with the compound of Formula 2 at 50° C. and 101.325 kPa for 5 min to form a stable red dye. Distilled water was added to the reaction cell to make the volume 5 ml.

[0040] (4) Anti-interference compensation test: the photoelectric detection signal ADC of the water sample reduction liquid to be tested 待测水样 ADC with photoelectric detection signal of standard solution and reducing solution 标液 As a reference signal, the interference compensation signal ADC is calculated according to formula 1 干扰补偿 Interference compensation will be measured every time a water sample is tested. This is real-time and can be dynamically compensated according to changes in water quality, effectively solving the impact of turbidity and color interference in actual water samples on data accuracy.

[0041] (5) Detection light source calibration:

[0042] ⅰ: Initial calibration of light source: Pour 5ml of distilled water into the detection cell to fill the entire detection cell, then automatically adjust the photoelectric signal, and adjust the light source intensity by fine-tuning the current to make the receiving end optical signal stable at 24000±50ADS. After stabilization, record the ADC signal value within 30s and calculate the average value to get ADC 光源初始校准 After the initial calibration of the light source, the instrument must be calibrated at zero point and standard liquid point to obtain ADC0 and ADC 标液The initial calibration cycle of the light source is once every 1-2 weeks, which needs to be synchronized with the standard solution calibration.

[0043] ⅱ: Measurement before light source test: Before the water sample to be tested is injected into the detection cell, 5 ml of distilled water is first injected into the detection cell to fill the entire detection cell. After stabilization, record the ADC signal value within 30 seconds and calculate the average value to obtain ADC 光源测量 ;

[0044] iii: According to ADC 光源测量 and ADC 光源初始校准 , calculate ADC according to formula 2 光源校准补偿 ; ADC 光源校准补偿 =ADC 光源初始校准 -ADC 光源测量 (2)

[0045] (6) Total nitrogen concentration detection of water quality: Accurately measure 2.0 ml of reaction pool color solution to rinse the detection pool and drain the liquid. Then accurately measure 2.0 ml of reaction pool color solution and inject it into the detection pool. Use a 540 nm wavelength light source to detect the ADC signal and record it as ADC x Signal value, calculate the total nitrogen concentration of the water sample to be tested according to formula 3;

[0046] Among them, C x is the concentration value of the water sample to be tested, C 标液 is the concentration value of the standard solution, ADC0 is the zero point ADC value, ADC 标液 is the ADC value of the standard solution, ADC x is the ADC value of the water sample to be tested.

[0047] Example 2

[0048] The standard curve for total nitrogen detection in water quality of the present invention is:

[0049] A nationally certified total nitrogen 1000 mg / L standard solution was purchased and prepared with deionized water to prepare 1.0 L of a graded concentration standard solution for drawing a standard curve.

[0050] Operating Instructions: Total nitrogen standard stock solution: 1000.0 mg / L. Weigh 6.07 g of sodium nitrate (NaNO3, premium grade), dissolve in an appropriate amount of water, transfer to a 1000 ml volumetric flask, and dilute to the mark. This solution will have a total nitrogen concentration of 1000.0 mg / L. Other low-concentration total nitrogen standard solutions can be prepared by serial dilution of this standard stock solution.

[0051] The standard curve can be a two-point setup (zero point and standard solution) or a multi-point setup (zero point, standard solution 1, standard solution 2, or multiple standard solution points). Online monitoring instruments generally use two-point or three-point calibration, but here we use a three-point calibration to establish the standard curve. Two points allow for direct calculation of the standard curve equation, while three points require curve fitting to obtain the standard curve equation.

[0052] According to y=kx+b, the standard curve is finally obtained by fitting, as shown in Figure 2:

[0053] y=-0.1209x+4.356(R 2 =0.9996)

[0054] Example 3

[0055] The total nitrogen test results of the water quality of the present invention are verified:

[0056] Purchase a nationally certified 1000 mg / L total nitrogen standard solution and use deionized water to prepare 1.0 L of a standard solution of approximately 10.00 mg / L. Divide the standard solution into four equal portions, with each portion containing 250 ml of water sample.

[0057] In order to further verify that the detection method of the present invention is accurate and efficient, HJ 636-2012 "Water quality - Determination of total nitrogen - Alkaline potassium persulfate digestion - UV spectrophotometry" was used as a reference for comparison, and the detection was performed according to the steps of this standard.

[0058] Establish the HJ 636-2012 standard curve equation:

[0059] According to y=kx+b, the standard curve is finally obtained by fitting, as shown in Figure 3:

[0060] y=0.0698x+0.0081(R 2 =0.9998)

[0061] Four water samples were tested using the detection method of Example 1 and HJ 636-2012 "Water quality - Determination of total nitrogen - Alkaline potassium persulfate digestion - UV spectrophotometry" respectively, and the test data were recorded. The results are shown in Table 1.

[0062] Table 1 Example 3 water sample detection records and results

[0063] The total nitrogen standard solution detected by the detection method and test system of the total nitrogen water quality online automatic monitoring instrument of the present invention has an indication error of ≤5% compared with the standard test results of HJ 636 "Determination of total nitrogen in water quality - Alkaline potassium persulfate digestion ultraviolet spectrophotometry method", which is qualified; NO3 - Reduction to NO2 -The reduction rate is ≥99.5%, and the reduction efficiency is qualified.

[0064] Therefore, the detection method and test system of the total nitrogen water quality online automatic monitoring instrument meet the working conditions of the total nitrogen standard solution water quality online monitoring.

[0065] Application Example 1

[0066] Water sampling was carried out at the confluence of the Huangpu River and the Dazhi River. Four bottles of water samples, each containing 1.0 L, were collected for use in Example 1 and Comparative Example 2.

[0067] To further verify the accuracy and efficiency of the detection method of the present invention, HJ 636-2012 "Water quality - Determination of total nitrogen - Alkaline potassium persulfate digestion - UV spectrophotometry" was used as a reference for comparison, and detection was performed according to the procedures of this standard. The HJ 636-2012 standard curve equation is shown in Example 3.

[0068] Four water samples were tested using the detection method of Example 1 and HJ 636-2012 "Water quality - Determination of total nitrogen - Alkaline potassium persulfate digestion - UV spectrophotometry" respectively, and the test data were recorded. The results are shown in Table 2.

[0069] Table 2 Example 4 Water Sample Detection Records and Results

[0070] The total nitrogen standard solution detected by the detection method and test system of the total nitrogen water quality online automatic monitoring instrument of the present invention is less than 5% of the standard test results of HJ 636 "Determination of total nitrogen in water quality - Alkaline potassium persulfate digestion ultraviolet spectrophotometry method", and the total nitrogen oxidation and NO3 - Reduction to NO2 - The redox rates are all ≥99.0%, and the redox efficiency is qualified.

[0071] Therefore, the detection method and test system of the total nitrogen water quality online automatic monitoring instrument meet the working conditions of total nitrogen surface water quality online monitoring.

[0072] Application Example 2

[0073] Water sampling was carried out in a wastewater treatment workshop of a chemical plant in Pudong New Area, Shanghai. Four bottles of water samples were collected, each containing 1.0 L, and were used for Application Example 2 and Comparative Example 3, respectively.

[0074] In order to further verify that the detection method of the present invention is accurate and efficient, HJ 636-2012 "Water quality - Determination of total nitrogen - Alkaline potassium persulfate digestion - UV spectrophotometry" was used as a reference for comparison, and the detection was performed according to the steps of this standard.

[0075] Four water samples were tested using the detection method of Example 1 and HJ 636-2012 "Water quality - Determination of total nitrogen - Alkaline potassium persulfate digestion - UV spectrophotometry" respectively, and the test data were recorded. The results are shown in Table 3.

[0076] Table 3 Example water sample detection records and results

[0077] The total nitrogen standard solution detected by the detection method and test system of the total nitrogen water quality online automatic monitoring instrument of the present invention is less than 5% of the standard test results of HJ 636 "Determination of total nitrogen in water quality - Alkaline potassium persulfate digestion ultraviolet spectrophotometry method", and the total nitrogen oxidation and NO3 - Reduction to NO2 - The redox rates are all ≥99.0%, and the redox efficiency is qualified.

[0078] Therefore, the detection method and test system of the total nitrogen water quality online automatic monitoring instrument meet the working conditions of the total nitrogen industrial wastewater online monitoring of water quality.

[0079] The preferred embodiments of the present invention have been specifically described above, but the present invention is not limited to the described embodiments. Those skilled in the art may make various equivalent modifications or substitutions without departing from the spirit of the present invention. These equivalent modifications or substitutions are all included in the scope defined by the claims of this application.

Claims

1. A method for detecting total nitrogen concentration in water quality, comprising the following steps: (1) Oxidation treatment: Add 2 ml of the water sample to be tested into the digestion tank, then add 1 ml of K2S2O8 solution and 1 ml of NaOH solution into the digestion tank, and heat to 125℃±3℃ and 2.30-2.40MP pressure in the digestion tank for 15 minutes to oxidize all nitrogen-containing compounds in the water sample into NO - 3. Obtain digestion solution; (2) Reduction treatment: Add 1 ml of H4N2·H2SO4 solution to the digestion solution, add 1 ml of a mixture of copper sulfate and zinc sulfate as catalyst, and carry out a reduction reaction at 50°C and normal pressure to reduce NO - 3 Reduction to NO - 2. Obtaining a reduced mixed solution; (3) Color development reaction: 3 ml of the reduction mixture obtained in step 2) is taken as the color development reduction solution volume, added to the reaction tank, phosphate buffer and distilled water are used as regulating solutions, and the pH value is adjusted to 1.8±0.3, and 1 ml of the mixture of color development reagents Formula 1 and Formula 2 is added. The compound of Formula 1 and the reduction mixture are reacted at 50°C and 101.325 KPa to generate a diazonium salt, and the diazonium salt is then coupled with the compound of Formula 2 at 50°C and 101.325 KPa for 5 min to generate a stable red dye. The reaction tank is diluted with distilled water to 5 ml as the total color development volume to obtain a color development solution; (4) Anti-interference compensation test: The photoelectric detection signal ADC of the water sample reduction liquid to be tested 待测水样 Photoelectric detection signal ADC with standard solution and reducing solution 标液 As a reference signal, the interference compensation signal ADC is calculated according to formula 1 干扰补偿 ; Among them, V 显色还原液体积 The volume of the color reduction solution measured for the color reaction; V 显色总体积 is the total volume of the color reaction cell after constant volume; (5) Detection light source calibration: ⅰ: Initial calibration of light source: inject 5ml of distilled water into the detection pool, adjust the receiving end optical signal to stabilize at 24000±50ADS, record the ADC signal value within 30s after stabilization, and calculate the average value to get ADC 光源初始校准 After the initial calibration of the light source, the instrument must be calibrated at the zero point and the standard liquid point to obtain ADC0 and ADC 标液 ; ⅱ: Measurement of water samples before light source testing: Before the water sample to be tested is injected into the detection pool, 5 ml of distilled water is first injected into the detection pool. After stabilization, the ADC signal value within 30 seconds is recorded, and the average value is calculated to obtain the ADC 光源测量 ; ⅲ: According to ADC 光源测量 and ADC 光源初始校准 , calculate ADC according to formula 2 光源校准补偿 ; ADC 光源校准补偿 =ADC 光源初始校准 -ADC 光源测量 (2) (6) Total nitrogen concentration detection of water quality: Take the color developing solution in the reaction pool to rinse the detection pool and drain the liquid, measure 2.0ml of color developing solution and inject it into the detection pool, use a 540nm wavelength light source to detect the ADC signal, and record it as ADC x Signal value, calculate the total nitrogen concentration of the water sample to be tested according to formula 3; Among them, C x is the concentration value of the water sample to be tested, C 标液 is the concentration value of the standard solution, ADC0 is the zero-point ADC value, ADC 标液 is the ADC value of the standard solution, ADC x is the ADC value of the water sample to be tested.

2. A method for detecting total nitrogen concentration in water according to claim 1, characterized in that: In the step (1), the concentration of the K2S2O8 solution is 0.1-0.11 mol / L; the concentration of the NaOH solution is 0.2-0.25 mol / L.

3. A method for detecting total nitrogen concentration in water according to claim 1, characterized in that: The concentration of the H4N2·H2SO4 solution in step (2) is 0.015 mol / L.

4. A method for detecting total nitrogen concentration in water according to claim 1, characterized in that: The catalyst in step (2) is a mixture of copper sulfate and zinc sulfate, wherein the concentration of copper sulfate in the mixture is 0.00010 mol / L, and the concentration of zinc sulfate is 0.003 mol / L.

5. The method for detecting total nitrogen concentration in water according to claim 1, characterized in that: The reduction reaction time in step (2) is controlled at 3.0 min.

6. A method for detecting total nitrogen concentration in water according to claim 1, characterized in that: The concentration of the phosphate buffer solution in step (3) is 1.50 mol / L.

7. A method for detecting total nitrogen concentration in water according to claim 1, characterized in that: The mixed solution of the color developing reagents in step (3) uses 2.5 mol / L phosphoric acid as solvent, the concentration of color developing reagent formula 1 is 0.28 mol / L, and the concentration of color developing reagent formula 2 is 0.0055 mol / L.

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