A Visible Light Detection Method and Reagent for Total Nitrogen from Synergistic Digestion

Abstract

This invention relates to the field of water quality testing technology, and in particular to a visible light detection method and reagent for synergistic digestion of total nitrogen. By constructing a ternary system of quartz sand and silver ions synergistically catalyzing the digestion of persulfate, complete digestion of high-organic-content and high-salt wastewater is achieved. The detection reagent includes a digestion reagent and a colorimetric reagent. The digestion reagent contains persulfate, an alkali source, silver salt, and quartz sand, while the colorimetric reagent contains an inorganic acid and a visible light colorimetric agent. The detection method includes three steps: sample digestion, post-digestion treatment, and visible light colorimetric detection. Silver ions react with chloride ions in situ to form silver chloride precipitate, eliminating interference, and simultaneously catalyze the generation of sulfate free radicals to enhance oxidation capacity. Quartz sand, as a physical abrasive, breaks down the structure of macromolecular organic matter, forming a physicochemical synergistic digestion with silver ions. This invention uses visible light spectrophotometry instead of ultraviolet light, reducing equipment costs and significantly improving the detection accuracy of high-organic-content and high-chlorine water samples.

Description

Technical Field

[0001] This invention belongs to the field of water quality testing technology, specifically relating to a method and reagent for the visible light detection of total nitrogen through synergistic digestion. Background Technology

[0002] Total nitrogen (TNI) is a key indicator for evaluating the degree of water pollution. Its content directly reflects the extent of organic pollution in water bodies and has significant application value in environmental monitoring, water resource management, and wastewater treatment. Currently, the alkaline potassium persulfate digestion-ultraviolet spectrophotometry method is commonly used for TNI detection. This method is included in the National Environmental Protection Standard HJ636-2012. Its basic principle is to use potassium persulfate to oxidize and decompose nitrogenous substances in water samples under alkaline conditions, converting various forms of nitrogen into nitrate nitrogen. The absorbance is then measured at a wavelength of 220 nm using ultraviolet spectrophotometry to calculate the TNI content.

[0003] However, this traditional detection method has many shortcomings when dealing with complex actual water samples. For wastewater with high organic matter content, such as chemical wastewater, pharmaceutical wastewater, landfill leachate, and other recalcitrant industrial wastewater, the oxidation capacity of conventional potassium persulfate is relatively limited. It is difficult to completely mineralize the stubborn nitrogen-containing organic matter such as azo compounds, pyridines, and aromatic nitrogen compounds contained in the wastewater, resulting in significantly lower total nitrogen measurement results that cannot accurately reflect the pollution status of the water body.

[0004] More significantly, interference from high-salinity or high-chlorine water samples severely restricts the accuracy of detection results. Chloride ions, which are ubiquitous in water samples, exhibit strong absorption in the 220nm ultraviolet region, directly interfering with absorbance measurements. Furthermore, during digestion, chloride ions react with persulfate oxidants, consuming the oxidant and producing byproducts such as chloramines. These byproducts inhibit subsequent colorimetric reactions, further affecting detection accuracy. Existing techniques typically employ dilution to reduce salinity, but this method not only lowers the detection limit for low-concentration total nitrogen but also introduces additional measurement errors.

[0005] To address the interference from high salt concentrations, existing technologies often require additional pretreatment steps such as pre-distillation, ion exchange, or the addition of multiple masking agents. These methods are cumbersome and complex, and introduce new chemical reagents, increasing the risk of secondary contamination and detection costs. Furthermore, ultraviolet spectrophotometers are significantly more expensive than visible spectrophotometers, and residual absorption of organic matter in the ultraviolet region can easily cause interference, affecting the stability and reliability of the detection results. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a method and reagent for the visible light detection of total nitrogen through synergistic digestion, thereby solving the aforementioned technical problems in the prior art.

[0007] The objective of this invention can be achieved through the following technical solutions: A total nitrogen detection reagent with synergistic digestion comprises two parts: a digestion reagent and a colorimetric reagent; The digestion reagent comprises four components: persulfate, alkali source, silver salt, and quartz sand. The persulfate is used as an oxidant, the alkali source is used to provide an alkaline digestion environment, the silver salt is used as a catalyst and chloride ion removal agent, and the quartz sand is used as a physical aid. Based on mass ratio, the ratio of persulfate:alkali source:silver salt:quartz sand is 0.5~2.0:0.5~2.0:0.05~0.5:2~10; The colorimetric reagent comprises an inorganic acid and a visible light colorimetric agent, wherein the inorganic acid is used to adjust the pH of the colorimetric system to a strongly acidic condition of <1, and the visible light colorimetric agent is used to react with the nitrate nitrogen produced after digestion to form a colorimetric reaction.

[0008] Furthermore, the persulfate is selected from at least one of potassium persulfate or sodium persulfate; The alkali source is selected from at least one of sodium carbonate, potassium carbonate, or calcium carbonate. The silver salt is selected from at least one of silver nitrate and silver sulfate; The quartz sand has a particle size of 100 mesh to 300 mesh.

[0009] Furthermore, the inorganic acid is selected from at least one of sulfuric acid or phosphoric acid; The visible light colorimetric agent is a chromotropic acid-based colorimetric system.

[0010] Furthermore, the digestion reagent is an individually packaged solid mixture, packaged in an aluminum-plastic composite bag or a brown glass bottle.

[0011] A method for visible light detection of total nitrogen through synergistic digestion includes the following steps: S1. Sample digestion: Transfer the water sample to be tested, add the digestion reagent, seal the container, and digest at 100-124℃ for 30-60 minutes; control the molar ratio of silver salt to chloride ions in the water sample to be 1.0-1.2:1; add the components of the digestion reagent according to the mass ratio of persulfate:alkali source:quartz sand = 0.5-2.0:0.5-2.0:2-10. S2. Post-digestion treatment: After digestion, remove the digestion tube and cool it to room temperature. Add a reducing agent to reduce the unreacted persulfate. Then, centrifuge the digestion solution at a speed of 3000-5000 r / min for 5-10 min. Take the supernatant for later use. S3. Visible light colorimetric detection: Take the supernatant into a colorimetric tube, add the colorimetric reagent to carry out the colorimetric reaction. The colorimetric reaction is carried out under strong acid and at room temperature for 5 to 15 minutes. After the reaction is completed, measure the absorbance at a visible light wavelength of 400 to 440 nm or 530 to 550 nm. Calculate the total nitrogen content in the sample through a standard curve.

[0012] Furthermore, the reducing agent is selected from at least one of sodium sulfite, sodium thiosulfate, or ascorbic acid; the mass ratio of the reducing agent to persulfate is 0.5 to 1.0:1.

[0013] Furthermore, the detection wavelength is preferably 400–440 nm.

[0014] Furthermore, the digestion temperature is preferably 105℃, and the digestion time is preferably 30 min.

[0015] Furthermore, the silver ions play a dual role as a catalyst and chloride ion remover during the digestion process. The silver ions catalyze the decomposition of persulfate to generate sulfate free radicals, and at the same time react with chloride ions in the water sample to generate silver chloride precipitate.

[0016] The quartz sand plays a dual role as a physical abrasive and a crystal nucleation promoter during the digestion process. The quartz sand breaks down the structure of macromolecular organic matter through physical friction, while promoting the rapid formation and sedimentation of silver chloride precipitate.

[0017] Furthermore, the application of the synergistic digestion method for total nitrogen detection by visible light is applied to the quantitative detection of total nitrogen in chemical wastewater, pharmaceutical wastewater, landfill leachate, seawater desalination brine with high organic matter and high chloride ion concentration of 1000-15000 mg / L.

[0018] The beneficial effects of this invention are: 1. This invention introduces quartz sand as a physical abrasive to construct a synergistic digestion system combining physical crushing and chemical oxidation. The physical grinding effect of quartz sand disrupts the macromolecular structure of stubborn nitrogen-containing organic matter, increasing the reaction surface area. The silver ion-catalyzed decomposition of persulfate generates highly oxidizing sulfate free radicals, which thoroughly chemically oxidize the crushed organic matter. This synergistic effect significantly improves the oxidation efficiency of recalcitrant nitrogen-containing organic matter in high-organic-content wastewater, solving the technical problem of incomplete digestion of high-organic-content water samples leading to low total nitrogen determination results. Validated with standard materials, the recovery rate of recalcitrant nitrogen-containing compounds such as nitroaniline, pyridine, and quinoline is more than 15% higher than that of the national standard method.

[0019] 2. In this invention, silver ions react in situ with chloride ions in the water sample to form silver chloride precipitate, removing chloride ions from the water sample in solid form. This avoids the problem of chloride ions consuming persulfate oxidant and the generation of byproducts such as chloramines that inhibit the colorimetric reaction. This technical solution allows the method of this invention to be directly used for detection in high-salt, high-chloride water samples without the need for complex pretreatment steps such as pre-distillation, ion exchange, or the addition of masking agents, thus maintaining the accuracy of the detection results. Validated on water samples containing 10,000 mg / L chloride ions, the recovery rate remained above 95%, while the recovery rate of the national standard method under the same conditions was only 60-70%.

[0020] 3. This invention integrates chloride ion removal and organic matter oxidation into a single digestion process, eliminating the need for pre-distillation, ion exchange, or the addition of multiple masking agents. The detection process is simple and rapid, achieving both chloride ion removal and organic matter oxidation in a single digestion. The types and amounts of digestion reagents used are significantly reduced compared to existing technologies, with chemical reagent usage reduced by more than 50%, significantly lowering the risk of secondary contamination and correspondingly reducing detection costs.

[0021] 4. This invention uses visible light spectrophotometry instead of ultraviolet spectrophotometry for determination. The price of a visible light spectrophotometer is only 1 / 3 to 1 / 5 of that of an ultraviolet spectrophotometer. Furthermore, the absorption interference from residual organic matter in the visible light region is significantly less than in the ultraviolet region, resulting in higher stability and repeatability of the detection results. In addition, the requirements for digestion equipment are relatively lower; a common autoclave can meet the digestion requirements, eliminating the need for a dedicated high-temperature, high-pressure digester, further reducing equipment investment costs.

[0022] 5. The method of this invention achieves recoveries of 95-105% for nitrogen-containing compounds in different forms, including potassium nitrate, ammonium nitrate, urea, nitroaniline, pyridine, and quinoline, significantly superior to the national standard method. Particularly for wastewater with high organic matter content, such as chemical wastewater, pharmaceutical wastewater, and landfill leachate, and wastewater containing high concentrations of chloride ions, the oxidation efficiency is significantly higher than the national standard method. The detection results show good consistency with the national standard method, with a correlation coefficient greater than 0.99. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0024] Figure 1 This is a schematic diagram of the test method flow according to an embodiment of the present invention; Figure 2 This is a standard curve for total nitrogen detection in an embodiment of the present invention. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0026] This invention provides a total nitrogen detection reagent for synergistic digestion, the preparation process of which is as follows: Preparation of the digestion reagent: Prepare four components: potassium persulfate, sodium carbonate, silver sulfate, and quartz sand. First, grind the potassium persulfate into a fine powder, ensuring the particle diameter is less than 100 mesh, so that it can dissolve quickly and release oxidizing active species during the digestion process. Weigh out 1.5g of potassium persulfate, 1.0g of sodium carbonate, 0.15g of silver sulfate, and 5g of quartz sand according to the mass ratio. Mix the four solid components evenly and store them in a desiccator for later use.

[0027] Preparation of the colorimetric reagent: Prepare two components: concentrated sulfuric acid and chromotropic acid. Measure 50 mL of concentrated sulfuric acid and slowly add it to 10 mL of deionized water. Stir until homogeneous and cool to room temperature to obtain a sulfuric acid solution. Weigh 0.5 g of the chromotropic acid reagent and add it to the above sulfuric acid solution. Stir until completely dissolved to obtain the colorimetric solution. The colorimetric stock solution should be stored away from light.

[0028] like Figure 1 As shown, a method for the visible light detection of total nitrogen through synergistic digestion specifically includes the following steps: S1. Sample digestion: Transfer 2.0 mL of potassium nitrate standard solution with nitrogen concentrations of 0 mg / L, 10 mg / L, 20 mg / L, 50 mg / L, and 100 mg / L into digestion tubes respectively, add 0.3 g of digestion reagent to each, shake to dissolve the digestion reagent and mix evenly with the water sample.

[0029] S2. Post-digestion treatment: Cover the digestion tubes and place them in an autoclave. Heat to 105℃ and maintain for 30 minutes for digestion. After digestion, remove the digestion tubes and allow them to cool naturally to room temperature. Then, add 0.05g of sodium sulfite reducing agent to each digestion tube, shake well, and let stand for 2 minutes to reduce the remaining potassium persulfate. Then, centrifuge the digestion solution at 4000 rpm for 5 minutes and collect the supernatant for later use.

[0030] S3. Visible Light Colorimetric Detection: Transfer 2.0 mL of supernatant to each colorimetric tube, add 2.0 mL of working colorimetric solution to each, shake well, and let stand for 10 min to allow the colorimetric reaction to proceed. After the colorimetric reaction is complete, measure the absorbance at a visible light wavelength of 400–440 nm or 530–550 nm. The absorbance measurements for each colorimetric tube are shown in Table 1 below. Figure 2As shown in the figure, a standard curve was plotted with nitrogen concentration on the x-axis and absorbance on the y-axis, yielding a linear regression equation of y = 210.25x + 0.0475 and a correlation coefficient R² = 1. This indicates that within the nitrogen concentration range of 0–50 mg / L, absorbance and nitrogen concentration exhibit a good linear relationship.

[0031] Table 1

[0032] Example 1:

[0033] The water sample to be tested was wastewater discharged from a chemical plant. It was dark brown in appearance and contained a large amount of recalcitrant organic matter. First, the water sample was filtered to remove suspended solids, and then 2.0 mL was accurately transferred into a digestion tube. The chloride ion concentration of the wastewater was determined to be 800 mg / L in a preliminary experiment. The amount of silver sulfate to be added needed to be calculated based on 1.1 times the molar amount of chloride ions. The calculated molar amount of chloride ions was 800 ÷ 35.5 = 22.5 μmol. Therefore, the amount of silver sulfate to be added should be 22.5 × 1.1 × 170 ÷ 1000 = 4.2 mg. Considering the convenience of practical operation, 5 mg of silver sulfate was added.

[0034] Subsequently, 0.3g of digestion reagent was added to the digestion tube, including approximately 0.15g of potassium persulfate, 0.1g of sodium carbonate, 0.005g of silver sulfate, and 0.15g of quartz sand. The digestion tube was capped and placed in an autoclave for digestion at 105℃ for 30 minutes. During digestion, the quartz sand physically ground up the large organic molecules in the wastewater under high temperature and pressure, breaking down large azo compounds and pyridine compounds into smaller molecular structures. Simultaneously, the silver ions generated from the decomposition of silver sulfate catalyzed the production of sulfate radicals from potassium persulfate, chemically oxidizing the broken-down organic matter and converting organic nitrogen into nitrate nitrogen. After digestion, the digestion tube was removed, cooled to room temperature, and sodium sulfite was added as a reducing agent to reduce the remaining potassium persulfate. The digestion solution was then centrifuged, and the supernatant was used for colorimetric analysis. The absorbance was measured to be 0.856, and the total nitrogen concentration was calculated to be 18.7 mg / L using the standard curve. The same sample was compared using the national standard method (alkaline potassium persulfate digestion-ultraviolet spectrophotometry), and the total nitrogen concentration was measured to be 12.3 mg / L. There were significant differences in the results of the two methods. The quartz sand-silver ion-persulfate synergistic digestion system used in this invention has a higher oxidation efficiency for recalcitrant organic matter and can convert organic nitrogen that cannot be completely digested by the national standard method into nitrate nitrogen, thus obtaining more accurate measurement results. Therefore, the measured value is significantly higher than that of the national standard method.

[0035] The recovery rates of this invention and the national standard ultraviolet digestion method under various types of nitrogen-containing substrates were compared in Table 2: Table 2

[0036] As shown in Table 2, for inorganic nitrogen sources that are easily digestible, the detection accuracy of this invention is on par with that of the national standard method. However, for organic nitrogen sources containing stable CN bonds (cyclic heterocycles, aromatic amines, melamine, etc.), the national standard method results in low values ​​due to incomplete digestion. The catalytic pre-prepared reagent system of this invention can deeply digest organic nitrogen, and the detection recovery rate is significantly better than that of the existing national standard method.

[0037] Example 2: The water sample to be tested was concentrated brine discharged from a seawater desalination plant, with a chloride ion concentration as high as 15000 mg / L, and also containing a certain amount of organic nitrogen. 2.0 mL of the water sample was accurately transferred into a digestion tube, and silver sulfate was added at 1.15 times the molar amount of chloride ions. The calculated molar amount of chloride ions was 15000 ÷ 35.5 = 422.5 μmol, and the amount of silver sulfate to be added should be 422.5 × 1.15 × 170 ÷ 1000 = 82.6 mg. Considering practical operation, 80 mg of silver sulfate was added.

[0038] 0.3g of digestion reagent was added to the digestion tube and placed in an autoclave at 105℃ for 30 minutes. During digestion, silver sulfate reacted with chloride ions in the water sample to form a white precipitate of silver chloride, removing chloride ions in situ and avoiding interference from chloride ions in the subsequent colorimetric reaction. Simultaneously, silver ions acted as a catalyst, promoting the decomposition of potassium persulfate to generate sulfate free radicals, which oxidized and digested organic matter. After digestion, post-treatment and colorimetric determination were performed, and the total nitrogen concentration was found to be 48.5 mg / L. The same water sample was then measured using the national standard method. Due to interference from high chloride ions, the national standard method required diluting the water sample 10 times before measurement. After dilution, the total nitrogen concentration was measured to be 31.2 mg / L, which, when converted to the original water sample, was 312 mg / L. This result was significantly abnormal because the dilution process introduced a large error, and the absorption interference of high chloride ions at 220 nm severely affected the accuracy of the measurement. The method of this invention eliminates interference by reacting silver ions with chloride ions to form a precipitate, and can be directly measured without dilution, thus obtaining more accurate and reliable results.

[0039] Example 3: In this embodiment, three different types of actual water samples were selected for method verification. The first type of water sample was a provincial station comparison water sample with a nominal total nitrogen concentration of 88 mg / L. The method of this invention was used to perform three parallel measurements, and the measured values ​​were 87.8 mg / L, 88.2 mg / L, and 88.1 mg / L, respectively. The average value was 88.0 mg / L, the relative standard deviation was 0.2%, and the relative error compared with the nominal value was 0.0%.

[0040] The second type of water sample was a provincial station comparison sample with a nominal total nitrogen concentration of 81 mg / L. Three parallel measurements were performed using the method of this invention, and the measured values ​​were 80.8 mg / L, 81.3 mg / L, and 80.9 mg / L, respectively. The average value was 81.0 mg / L, the relative standard deviation was 0.3%, and the relative error compared with the nominal value was 0.0%.

[0041] The third water sample was wastewater from the Qingdao Beer production plant. The total nitrogen concentration measured using the method of this invention was 31 mg / L, while the value measured by the testing agency using the national standard method was 30 mg / L. The results of the two methods were basically consistent, with a relative error of 3.3%, indicating that the method of this invention has high accuracy and reliability in actual water sample testing.

[0042] Example 4: In this embodiment, solutions of eleven standard substances, including potassium nitrate, ammonium chloride, urea, thiourea, guanidine hydrochloride, hydroxylamine hydrochloride, lysine, melamine, aniline, triazole, and tetraazole, with a nitrogen concentration of 100 mg / L, were prepared. The digestion and determination methods of this invention were used to calculate the recovery rates of each standard substance. The recoveries were: potassium nitrate 101%, ammonium chloride 99%, urea 103%, thiourea 76%, guanidine hydrochloride 56%, hydroxylamine hydrochloride 64%, lysine 78%, melamine 57%, aniline 56%, triazole 40%, and tetraazole 9.8%.

[0043] The recovery rate data shows that for simple inorganic nitrogen compounds (such as potassium nitrate and ammonium chloride), the recovery rate of the method of this invention is close to 100%, indicating very thorough digestion. For moderately complex organic nitrogen compounds (such as urea), the recovery rate also reaches 103%, indicating good digestion effect. For recalcitrant organic nitrogen compounds (such as thiourea, melamine, aniline, triazole, and tetraazole), the recovery rate is relatively low, but still significantly higher than that of the national standard method. In particular, the recovery rate of tetraazole reaches 9.8%, while the national standard method is almost undetectable. Compared with the national standard method, the method of this invention significantly improves the recovery rate of various nitrogen compounds, verifying the significant effect of the quartz sand-silver ion synergistic digestion system in improving oxidation efficiency.

[0044] Example 5: The detection reagent prepared in this invention underwent a systematic performance evaluation. Linearity range verification was performed using a series of standard solutions with nitrogen concentrations ranging from 0 to 50 mg / L. The correlation coefficient R² of the standard curve was ≥0.999, meeting the analytical requirements. Detection limit verification was performed using 11 parallel determinations with blank samples. The calculated blank standard deviation was 0.003 mg / L. Based on three times the blank standard deviation, the detection limit was 0.009 mg / L, meeting the sensitivity requirements for water quality detection.

[0045] Precision verification involved ten parallel determinations using a standard solution with a nitrogen concentration of 20 mg / L. The relative standard deviation was 0.8%, indicating that the method of this invention has good precision. Accuracy verification involved spiked recovery experiments using standard substances of different concentrations. The recovery rate ranged from 95% to 105%, indicating that the method of this invention has high accuracy. Stability verification involved storing the test reagent at room temperature for three months and then testing its performance. All indicators were basically consistent with those of the freshly prepared reagent, indicating that the test reagent has good stability.

[0046] Example 6: Single-factor optimization experiments were conducted on digestion temperature, digestion time, and reagent dosage. In this example, the digestion temperature optimization experiment was conducted at five levels: 100℃, 105℃, 110℃, 115℃, and 120℃, with a fixed digestion time of 30 min. The total nitrogen concentration of the same standard water sample was measured. The experimental results showed that the measured total nitrogen value gradually increased with increasing temperature, and tended to stabilize after reaching 105℃. Therefore, 105℃ was selected as the optimal digestion temperature. The digestion time optimization experiment was conducted at five levels: 15 min, 30 min, 45 min, 60 min, and 90 min, with a fixed digestion temperature of 105℃. The total nitrogen concentration of the same standard water sample was measured.

[0047] Experimental results showed that when the digestion time was within 30 minutes, the total nitrogen value increased with increasing time, and the value stabilized after 30 minutes. Therefore, 30 minutes was selected as the optimal digestion time. The reagent dosage optimization experiment set up different ratios of potassium persulfate and sodium carbonate, while fixing the amounts of silver salt and quartz sand, and measured the total nitrogen concentration of the same standard water sample. Experimental results showed that the digestion effect was best when the mass ratio of potassium persulfate to sodium carbonate was 1.5:1.0. Too low a dosage of potassium persulfate would lead to insufficient oxidation capacity, while too high a dosage would increase costs and may produce byproducts.

[0048] Example 7: In this invention, silver salt plays a dual role in catalysis and dechlorination. Its dosage needs to be precisely controlled. An experiment was designed to verify the effect of silver salt dosage on the digestion and chloride ion removal efficiency. In this example, a standard water sample containing 1000 mg / L chloride ions and 20 mg / L total nitrogen was taken, and silver nitrate was added at 0.8, 1.0, 1.1, 1.2, and 1.5 times the molar amount of chloride ions, respectively, for digestion and determination.

[0049] When the amount of silver salt used was 0.8 times the molar amount of chloride ions, chloride ion removal was incomplete, and the total nitrogen was measured at 16.8 mg / L, with a recovery rate of 84%. This was because the amount of silver ions was insufficient to precipitate all chloride ions, and the remaining chloride ions consumed some of the oxidant and caused interference. When the amount of silver salt used was 1.0 to 1.2 times the molar amount of chloride ions, the total nitrogen value stabilized at 19.8 to 20.2 mg / L, with a recovery rate of 99 to 101%. At this point, chloride ions were completely precipitated, and the silver ions also acted as a catalyst to promote the decomposition of potassium persulfate.

[0050] When the amount of silver salt used was 1.5 times the molar amount of chloride ions, the total nitrogen value was 20.1 mg / L, and the recovery rate was 100.5%, which was basically consistent with the results when the amount was 1.0 to 1.2 times, but increased the reagent cost. Considering both digestion effect and cost factors, the amount of silver salt added was selected to be 1.0 to 1.2 times the molar amount of chloride ions.

[0051] Example 8: This embodiment compares the digestion effect by adjusting the particle size and dosage of quartz sand. An optimization experiment was conducted using quartz sand particle size at five levels: 60 mesh, 100 mesh, 200 mesh, 300 mesh, and 400 mesh. The dosage of quartz sand was fixed at 0.15 g / 5 mL water sample, and the total nitrogen concentration of the same standard water sample was measured. The experimental results showed that at a particle size of 60 mesh, the grinding effect of the quartz sand was poor, with a total nitrogen recovery rate of 85%. At a particle size of 100–300 mesh, the grinding effect was good, with a total nitrogen recovery rate of 98–101%. At a particle size of 400 mesh, the quartz sand was too fine, making it difficult to settle in the digestion solution, affecting subsequent centrifugation, and the total nitrogen recovery rate actually decreased to 92%. Therefore, a quartz sand particle size of 100–300 mesh was selected.

[0052] The dosage optimization experiment set up five dosage levels: 0.05g, 0.1g, 0.15g, 0.2g, and 0.3g / 5mL water sample, with a fixed quartz sand particle size of 200 mesh, and measured the total nitrogen concentration of the same standard water sample. The experimental results showed that the digestion effect was optimal when the quartz sand dosage was 0.1–0.2g, with a total nitrogen recovery rate of 98–101%. Dosage below 0.1g resulted in insufficient physical grinding; dosage above 0.2g increased reagent costs and may affect the flowability of the digestion solution. Therefore, the optimal quartz sand dosage was selected as 0.1–0.2g per 5mL water sample.

[0053] Example 9: The method used in this embodiment involves the reaction of silver ions with chloride ions to generate silver chloride precipitate, thereby eliminating chloride ion interference in situ. To verify the elimination effect of this technical solution on chloride ions of different concentrations, the following experiments were conducted. Experimental control variables: The theoretical total nitrogen of the water sample was fixed at 50 mg / L, following the standard procedure of the embodiment; only a single variable was set: the presence or absence of silver salt synergistic components, with a gradient chloride ion concentration covering 10,000~40,000 mg / L. The data are shown in Table 3 below. Table 3 Chloride ion 10000mg / L 41 82 51 102 Chloride ion 15000 mg / L 39 78 50 100 Chloride ion 20000mg / L 37 74 52 104 Chloride ions 40000 mg / L 26 52 54 108 As shown in Table 3, the recovery rate of the silver-free system drops sharply with increasing chloride ion concentration. At 10,000 mg / L, the recovery rate is only 82%, and after increasing to 40,000 mg / L, it drops to only 52%, resulting in completely distorted test results and loss of quantitative value. When chloride ion concentration is 10,000 mg / L, the recovery rate using the national standard method is only 60%–70%, while the recovery rate of the silver-free digestion system in this study is 74%–82%, both far below the acceptable range. This proves that relying solely on physical crushing of quartz sand and silver-free chloride removal catalysis cannot solve the problem of high chloride interference. The improvement in recovery rate of the system of this invention compared to the silver-free system is calculated at the same chloride ion concentration: when Cl… - =10000 mg / L: Recovery rate increased by 20%; when Cl - =15000 mg / L: Recovery rate increased by 22%; Cl - =20000 mg / L: Recovery rate increased by 30%; Cl - =40000mg / L: Recovery rate increased by 56%. The data clearly demonstrates that the silver salt synergistic system can completely eliminate interference from ultra-high concentration chloride ions, while the recovery rate of the silver salt-free system decreases significantly with increasing chloride ion concentration.

[0054] Experimental results show that within the chloride ion concentration range of 0–15000 mg / L, the total nitrogen concentration remained stable at 19.6–20.4 mg / L, with a recovery rate of 98–102%, effectively eliminating chloride ion interference. As a control, the same water sample was measured using the national standard method. Due to chloride ion absorption interference at 220 nm, the measurement results showed significant anomalies, especially when the chloride ion concentration exceeded 5000 mg / L, where the measured values ​​deviated severely from the true values, with a recovery rate of only 60–70%. These experimental data fully demonstrate the significant effect of the method of this invention in eliminating high chloride ion interference.

[0055] As can be seen from the detailed description of the above embodiments, the total nitrogen detection method and reagents provided by the present invention have the advantages of thorough digestion, strong anti-interference ability, simple operation, low cost, accuracy and reliability. They can effectively solve the technical problems existing in the prior art, such as incomplete digestion of water samples with high organic matter, serious interference from water samples with high salt and high chlorine, complex operation and high detection cost. They are suitable for promotion and application in the field of water quality testing.

[0056] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A total nitrogen detection reagent for synergistic digestion, characterized in that, It consists of two parts: digestion reagent and colorimetric reagent; The digestion reagent comprises four components: persulfate, alkali source, silver salt, and quartz sand. The persulfate is used as an oxidant, the alkali source is used to provide an alkaline digestion environment, the silver salt is used as a catalyst and chloride ion removal agent, and the quartz sand is used as a physical aid. Based on mass ratio, the ratio of persulfate:alkali source:silver salt:quartz sand is 0.5~2.0:0.5~2.0:0.05~0.5:2~10; The colorimetric reagent comprises an inorganic acid and a visible light colorimetric agent, wherein the inorganic acid is used to adjust the pH of the colorimetric system to a strongly acidic condition of <1, and the visible light colorimetric agent is used to react with the nitrate nitrogen produced after digestion to form a colorimetric reaction.

2. The total nitrogen detection reagent for synergistic digestion according to claim 1, characterized in that: The persulfate is selected from at least one of potassium persulfate or sodium persulfate; The alkali source is selected from at least one of sodium carbonate, potassium carbonate, or calcium carbonate. The silver salt is selected from at least one of silver nitrate and silver sulfate; The quartz sand has a particle size of 100 mesh to 300 mesh.

3. The total nitrogen detection reagent for synergistic digestion according to claim 1, characterized in that: The inorganic acid is selected from at least one of sulfuric acid or phosphoric acid; The visible light colorimetric agent is a chromotropic acid-based colorimetric system.

4. The total nitrogen detection reagent for synergistic digestion according to claim 1, characterized in that: The digestion reagent is an individually packaged solid mixture, packaged in aluminum-plastic composite bags or brown glass bottles.

5. A method for visible light detection of total nitrogen through synergistic digestion, characterized in that, Includes the following steps: S1. Sample digestion: Transfer the water sample to be tested, add the digestion reagent, seal the container, and digest at 100-124℃ for 30-60 minutes; control the molar ratio of silver salt to chloride ions in the water sample to be 1.0-1.2:1; add the components of the digestion reagent according to the mass ratio of persulfate:alkali source:quartz sand = 0.5-2.0:0.5-2.0:2-10. S2. Post-digestion treatment: After digestion, remove the digestion tube and cool it to room temperature. Add a reducing agent to reduce the unreacted persulfate. Then, centrifuge the digestion solution at a speed of 3000-5000 r / min for 5-10 min. Take the supernatant for later use. S3. Visible light colorimetric detection: Take the supernatant into a colorimetric tube, add the colorimetric reagent to carry out the colorimetric reaction. The colorimetric reaction is carried out under strong acid and at room temperature for 5 to 15 minutes. After the reaction is completed, measure the absorbance at a visible light wavelength of 400 to 440 nm or 530 to 550 nm. Calculate the total nitrogen content in the sample through a standard curve.

6. The method for visible light detection of total nitrogen through synergistic digestion according to claim 5, characterized in that: The reducing agent is selected from at least one of sodium sulfite, sodium thiosulfate, or ascorbic acid; the mass ratio of the reducing agent to persulfate is 0.5 to 1.0:

1.

7. The method for visible light detection of total nitrogen through synergistic digestion according to claim 5, characterized in that: The detection wavelength is 400–440 nm.

8. The method for visible light detection of total nitrogen through synergistic digestion according to claim 5, characterized in that: The digestion temperature was 105℃ and the digestion time was 30 min.

9. The method for visible light detection of total nitrogen through synergistic digestion according to claim 5, characterized in that: The silver ions play a dual role as a catalyst and chloride ion remover during the digestion process. The silver ions catalyze the decomposition of persulfate to produce sulfate free radicals, and at the same time react with chloride ions in the water sample to form silver chloride precipitate. The quartz sand plays a dual role as a physical abrasive and a crystal nucleation promoter during the digestion process. The quartz sand breaks down the structure of macromolecular organic matter through physical friction, while promoting the rapid formation and sedimentation of silver chloride precipitate.

10. The application of the visible light detection method for total nitrogen synergistic digestion according to any one of claims 5 to 9, characterized in that, It is applied to the quantitative detection of total nitrogen in high-salt wastewater with high organic matter and chloride ion concentration of 1000-15000 mg / L, including chemical wastewater, pharmaceutical wastewater, landfill leachate, and concentrated seawater desalination brine.

Images