A method for determining total nitrogen in water quality using an online detector

Abstract

The present invention discloses a method for determining total nitrogen in water quality using an online detector, and relates to the field of water quality detection methods. The method adopts resorcinol spectrophotometry for testing, and includes the following steps: S1, quasi-value acquisition, including injecting distilled water into a digestion chamber, turning on a light-emitting diode, acquiring a reference voltage, and then emptying the distilled water in the digestion chamber; S2: sample digestion, including injecting a sample and a digestion solution into the digestion chamber, then heating up for a digestion reaction, then cooling down, stirring, and then heating up again to continue the digestion reaction, so as to reduce the pressure burden in the digestion chamber; S3: color development reaction. In the digestion reaction, the present invention first performs a temperature increase reaction, and then performs a temperature increase reaction after cooling down, that is, it includes two temperature increase processes, which solves the problem of excessive pressure burden in the digestion chamber while ensuring that the digestion reaction is thorough.

Description

Technical Field

[0001] The invention relates to the field of water quality detection methods, and in particular to a method for determining total nitrogen in water quality using an online detector. Background Art

[0002] Total nitrogen content in water quality is one of the key indicators for assessing the health of water bodies. Exceeding the standard often leads to eutrophication of water bodies, which seriously threatens the balance and diversity of aquatic ecosystems. At present, the online monitoring technology of total nitrogen in water quality mainly relies on the method of measuring absorbance at dual wavelengths (220nm and 270nm). This method is a national standard method. Although the test is accurate, the required xenon lamp or deuterium lamp light source is expensive, which increases the overall operating cost of the monitoring system.

[0003] When seeking a cost-effective online monitoring solution for total nitrogen in water quality, the resorcinol spectrophotometry method is popular due to its easy operation. However, this method requires high-temperature digestion in a closed alkaline environment, which causes the pressure in the digestion chamber to rise. Long-term and rapid temperature rise increases the risk of corrosion and rupture of the digestion chamber. In order to relieve the pressure generated during the digestion process, the commonly used method is to release it through a pressure relief valve. However, this measure undoubtedly increases the cost of the instrument. Therefore, it is particularly important to reasonably set the digestion process to ensure that while maintaining the digestion efficiency, the pressure damage to the digestion chamber is reduced.

[0004] In addition, in the practical application of resorcinol spectrophotometry, there is another significant challenge: when using 365nm light-emitting diodes as light sources, the light decay phenomenon is very serious. This problem directly affects the stability and accuracy of the monitoring results, so it must be taken seriously.

[0005] In summary, although the resorcinol spectrophotometry method has the advantage of simple operation in the online monitoring of total nitrogen in water quality, its problems in digestion chamber pressure and light source stability cannot be ignored. In order to fully realize the potential of this method, it is necessary to further optimize the digestion process settings and explore effective methods to improve the stability of the light source.

[0006] In view of this, this application is filed. Summary of the invention

[0007] The object of the present invention is to provide a method for measuring total nitrogen in water quality using an online detector, by improving the temperature conditions of a digestion reaction during a total nitrogen test of water quality using a resorcinol spectrophotometer, so as to solve the problem in the prior art that a digestion reaction is required to be thorough and the pressure burden in a digestion chamber is too heavy.

[0008] The present invention is implemented by the following technical scheme: a method for determining total nitrogen in water quality using an online detector is provided, comprising the following steps:

[0009] S1, reference value acquisition, including injecting distilled water into the digestion chamber, turning on the light emitting diode, acquiring the reference voltage, and then emptying the distilled water in the digestion chamber;

[0010] S2: Sample digestion, including injecting the sample and digestion solution into the digestion chamber, then heating up for digestion reaction, then cooling down, stirring, and then heating up again to continue digestion reaction, so as to reduce the pressure burden in the digestion chamber;

[0011] S3: Color reaction.

[0012] Preferably, the temperature conditions of the digestion reaction include a first temperature rising stage, a second holding stage, a third temperature falling stage, a fourth temperature rising stage and a fifth holding stage.

[0013] Preferably, the first heating stage is set to heat from room temperature to 110-130°C within 185s, with a heating rate of 0.4-0.5°C / s;

[0014] The second holding stage is configured to maintain the temperature of the first heating stage for a holding time of 300 to 540 seconds;

[0015] The third cooling stage is configured to cool the second holding stage, including cooling to 70-80°C within 105s, with a cooling rate of 0.4-0.5°C / s;

[0016] The fourth heating stage is configured to heat the lowest temperature of the third cooling stage, including heating to 110-130°C within 225s, with a heating rate of 0.15-0.25°C / s;

[0017] The fifth holding stage includes maintaining the highest temperature of the fourth heating stage for a holding time of 280 to 520 seconds.

[0018] Specifically, in the first stage, rapid temperature increase can quickly reach the appropriate temperature for the oxidation reaction. The second stage maintains a high temperature, providing the oxidant with sufficient time to react with various nitrogen-containing compounds in the sample, thereby ensuring the thoroughness of the oxidation reaction. The third stage cools down first to reduce the continuous impact of high temperature on the glass digestion chamber. At the same time, stirring is started during the cooling process to make the digestion solution in the digestion chamber more evenly mixed, thereby improving the uniformity and efficiency of the digestion. The fourth stage heats up again to provide additional oxidation time for the incompletely digested part, thereby ensuring that all nitrogen-containing compounds can be converted into nitrates. The fifth stage maintains a high temperature again to provide final confirmation for the oxidation reaction, thereby ensuring that all nitrogen-containing compounds have been converted into detectable forms.

[0019] Preferably, in the third cooling stage, the sample digestion process further includes stirring the digestion solution and the sample in the digestion chamber when the temperature is cooled to 85-95° C., until the temperature is reduced to the lowest temperature of the third cooling stage.

[0020] Preferably, the digestion solution comprises sodium hydroxide, potassium persulfate and distilled water;

[0021] The mass ratio of the sodium hydroxide, potassium persulfate and distilled water is 1:3-5:70-90.

[0022] Preferably, the volume ratio of the sample and the digestion solution injected into S2 is 4 to 6:1.

[0023] Preferably, the color development reaction includes cooling the reaction liquid after the digestion reaction in S2 to 40-50°C, then discharging part of the reaction liquid and adding resorcinol solution and 98% concentrated sulfuric acid into the digestion chamber, mixing and continuing to cool to 40-50°C, and finally turning on the light emitting diode to obtain the voltage and obtain the test value by conversion.

[0024] Preferably, the concentration of the resorcinol solution is 0.8-1.5wt%, and after part of the post-reaction liquid is discharged in the color development reaction, 1\7-1\5 volume of the post-reaction liquid remains, and the volume ratio of the resorcinol solution, 98% concentrated sulfuric acid and the remaining post-reaction liquid is 1:2-3:0.8-1.3.

[0025] Preferably, the temperature increase process of the sample digestion in S2 is achieved by a resistance heating wire, and the resistance heating wire is arranged in the digestion chamber and below the liquid surface of the reaction liquid of the digestion reaction.

[0026] Preferably, after S3, S4 is further included, and S4 is a cleaning process, which includes emptying the test liquid in the digestion chamber, first cleaning the digestion chamber with 2 to 3 mL of distilled water, and then cleaning the digestion chamber with 7 to 8 mL of distilled water;

[0027] The light emitting diodes are only turned on when the test voltage is applied to S1 and S3.

[0028] Compared with the prior art, the embodiments of the present invention have the following advantages and beneficial effects:

[0029] 1. The embodiment of the present invention adopts the resorcinol spectrophotometry to determine the total nitrogen, avoiding the use of a deuterium lamp or a xenon lamp with a higher cost, thereby reducing the overall cost of the monitor. In the digestion reaction of the present application, a temperature increase reaction is first performed, and then the temperature is lowered and then a temperature increase reaction is performed again, that is, two temperature increase processes are included, which ensures that the digestion reaction is thorough while solving the problem of excessive pressure burden in the digestion chamber.

[0030] 2. The digestion reaction process of the present application includes a first heating stage, a second holding stage, a third cooling stage, a fourth heating stage and a fifth holding stage, which can more accurately control the temperature and time in the digestion process, thereby reducing the pressure in the digestion chamber during the digestion process and improving the digestion efficiency and safety. By setting a cooling stage during the digestion process, the high-pressure environment generated in the closed high-temperature digestion process is depressurized, the pressure in the digestion chamber is reduced, and the adverse effects of the digestion chamber being in a high-pressure alkaline environment for a long time are reduced; by controlling the heating and cooling rates, it is possible to avoid excessive pressure caused by a large amount of gas generated by too fast a reaction, thereby better controlling the progress of the oxidation reaction, alleviating the thermal shock of the glass digestion chamber due to too fast a temperature change, and reducing the risk of cracking and corrosion of the digestion chamber. By accurately controlling the time of heating and constant temperature, it is ensured that the oxidant fully reacts with the nitrogen-containing compounds in the sample, thereby improving the digestion efficiency; by heating in stages, it is ensured that the oxidation reaction can be carried out at an appropriate temperature at different stages, thereby enhancing the oxidation capacity.

[0031] 3. The present invention optimizes the light source usage strategy, that is, the light source is turned on only during testing, which reduces the working time of the light source and thus reduces the light decay phenomenon. The light source voltage is reduced from 2V to 1V, which further extends the service life of the light emitting diode and also ensures the accuracy and efficiency of the detection.

[0032] 4. Changing the winding method of the heating resistor wire so that it is located below the liquid surface of the digestion solution avoids partial empty burning outside the liquid surface and reduces corrosion to the digestion chamber. By optimizing the amount of reagent added, the amount of sulfuric acid used is reduced, the amount of waste liquid generated is reduced, and the impact on the environment is reduced. After the digestion is completed, the volume of waste liquid is further reduced by controlling the discharge of the digestion solution and the amount of reagent added, achieving the dual goals of environmental protection and cost-effectiveness. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] In order to more clearly illustrate the technical solutions of the exemplary embodiments of the present invention, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present invention and should not be regarded as limiting the scope. For those of ordinary skill in the art, other relevant drawings can be obtained based on these drawings without creative work. In the drawings:

[0034] Figure 1 is a process flow chart in an embodiment of the present invention;

[0035] Figure 2 It is a schematic diagram of a digestion chamber after the digestion reaction in the winding mode of the heating resistance wire in the prior art;

[0036] Figure 3 This is a schematic diagram of a digestion chamber after the digestion reaction using the winding method of the heating resistance wire according to an embodiment of the present invention. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0038] Therefore, the detailed description of the embodiments of the present invention provided below is not intended to limit the scope of the claimed invention, but merely represents selected embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] The embodiment of the present invention provides a method for measuring total nitrogen in water quality using an online detector, comprising the following steps:

[0040] S1, reference value acquisition, including injecting distilled water into the digestion chamber, turning on the light emitting diode, acquiring the reference voltage, and then emptying the distilled water in the digestion chamber;

[0041] S2: Sample digestion, including injecting the sample and digestion solution into the digestion chamber, then heating up for digestion reaction, then cooling down, stirring, and then heating up again to continue digestion reaction, so as to reduce the pressure burden in the digestion chamber;

[0042] The temperature conditions of the digestion reaction include a first temperature rising stage, a second holding stage, a third temperature falling stage, a fourth temperature rising stage and a fifth holding stage;

[0043] The first heating stage is set to heat from room temperature to 110-130°C within 185s, with a heating rate of 0.4-0.5°C / s;

[0044] The second holding stage is configured to maintain the temperature of the first heating stage for a holding time of 300 to 540 seconds;

[0045] The third cooling stage is configured to cool the second holding stage, including cooling to 70-80°C within 105s, with a cooling rate of 0.4-0.5°C / s;

[0046] The fourth heating stage is configured to heat the lowest temperature of the third cooling stage, including heating to 110-130°C within 225s, with a heating rate of 0.15-0.25°C / s;

[0047] The fifth holding stage includes maintaining the highest temperature of the fourth heating stage for a holding time of 280 to 520 seconds;

[0048] In the third cooling stage, the sample digestion process further includes stirring the digestion solution and the sample in the digestion chamber when the temperature drops to 85-95° C. until the temperature drops to the lowest temperature of the third cooling stage;

[0049] The digestion solution comprises sodium hydroxide, potassium persulfate and distilled water;

[0050] The mass ratio of the sodium hydroxide, potassium persulfate and distilled water is 1:3-5:70-90;

[0051] The volume ratio of injected sample to digestion solution is 4 to 6:1;

[0052] The temperature rise process of the sample digestion is realized by a resistance heating wire, and the resistance heating wire is arranged in the digestion chamber and below the liquid surface of the reaction liquid of the digestion reaction;

[0053] S3: color development reaction, including cooling the reaction liquid after the digestion reaction in S2 to 40-50°C, then discharging part of the reaction liquid and adding resorcinol solution and 98% concentrated sulfuric acid into the digestion chamber, mixing and continuing to cool to 40-50°C, and finally turning on the light emitting diode to obtain the voltage and obtain the test value by conversion;

[0054] The concentration of the resorcinol solution is 0.8-1.5wt%, and after part of the post-reaction liquid is discharged in the color development reaction, 1\7-1\5 volume of the post-reaction liquid remains. The volume ratio of the resorcinol solution, 98% concentrated sulfuric acid and the remaining post-reaction liquid is 1:2-3:0.8-1.3.

[0055] S4: cleaning process, including emptying the test solution in the digestion chamber, first cleaning the digestion chamber with 2 to 3 mL of distilled water, and then cleaning the digestion chamber with 7 to 8 mL of distilled water;

[0056] The light emitting diodes are only turned on when the test voltage is applied to S1 and S3.

[0057] Example 1

[0058] This embodiment provides a method for measuring total nitrogen in water quality using an online detector. The specific process flow chart is shown in Figure 1 As shown, a light emitting diode at 360 nm is used for the resorcinol spectrophotometry. The light source is turned on only during the test, and the light source voltage is reduced from 2 V to 1 V. The winding method of the heating resistance wire in the glass digestion chamber is changed so that it is located below the surface of the digestion solution.

[0059] Digestion solution formula: weigh 12g sodium hydroxide (superior grade) and 40g potassium persulfate (superior grade) and dissolve them in 800mL distilled water. After dissolution, dilute to 1000mL.

[0060] The formula of the color developer is as follows: Weigh 10g of resorcinol (super pure) and add 800mL of distilled water. After dissolving, dilute to 1000mL.

[0061] The following steps are involved:

[0062] a. Obtaining the reference value: inject 5 mL of distilled water into the digestion chamber, turn on the light-emitting diode next to the digestion chamber (turn on for 60 seconds), obtain the reference voltage, and then empty the distilled water in the digestion chamber.

[0063] b. Sample digestion: Inject 5 mL of sample (2.5 mL of total nitrogen standard solution or total nitrogen water sample or total nitrogen quality control sample within the test range and 2.5 mL of distilled water, a total of 5 mL) into the digestion chamber, and inject 1 mL of digestion solution. The heating digestion process is divided into five stages, including:

[0064] The first stage: the temperature was raised from 35°C to 120°C within 185s, with a heating rate of 0.46°C / s;

[0065] The second stage: maintain 120℃, 355s;

[0066] The third stage: the temperature is reduced from 120℃ to 75℃ within 105s, with a cooling rate of 0.43℃ / s, and stirring is started when the temperature drops to 90℃ to mix the undigested digestion liquid at the bottom with the digested digestion liquid evenly;

[0067] The fourth stage: the temperature is increased from 75°C to 120°C within 225s, and the heating rate is 0.2°C / s;

[0068] Stage 5: Maintain 120°C for 315 seconds, digestion completed.

[0069] c Color development reaction: After digestion, cool to 45°C and drain the excess digestion solution in the digestion chamber, leaving only 1mL. Then inject 1mL of color developer and 2.5mL of 98% concentrated sulfuric acid into the reaction pool, mix well and cool to 45°C. Turn on the light-emitting diode (60s), obtain the voltage, and convert it to get the test value.

[0070] d. Post-test cleaning: After the test, empty the test solution in the digestion chamber, first clean the digestion chamber with 2.5 mL of distilled water, and then clean the digestion chamber with 7.5 mL of distilled water.

[0071] To cooperate with the above process, the resistance heating wire in the glass digestion chamber needs to be kept below the liquid level after adding the sample and the digestion solution, and the light-emitting diode is only turned on when the voltage is obtained.

[0072] In the embodiment of the present invention, the total nitrogen is determined by resorcinol spectrophotometry, which saves instrument costs. The cost of a deuterium lamp or a xenon lamp is tens of thousands of times that of an ordinary light-emitting diode. According to the above operation steps, the same light-emitting diode is repeatedly tested, and the test conditions are shown in Table 2. At the same time, another light-emitting diode is operated according to the existing resorcinol spectrophotometry determination steps (the light-emitting diode always keeps emitting light during the entire process of ac), and the test is repeated. The test conditions are shown in Table 1.

[0073] Table 1

[0074]

[0075] Table 2

[0076]

[0077] The test results of Table 1 and Table 2 were recorded. In the existing resorcinol spectrophotometry step operation, the light source voltage could not be maintained after testing 26 data, and the light decay was serious. The test was performed according to the operating steps of the embodiment of the present invention. The results showed that after 720 tests, the baseline voltage remained stable and there was no obvious light decay.

[0078] Comparative Example 1 provides a method for determining total nitrogen in water quality using an online detector, which differs from Example 1 in that a section of the resistance heating wire is located above the liquid surface, and the digestion method is to increase the temperature from 35°C to 120°C within 185s, with a heating rate of 0.46°C / s, and maintain for 715s. The remaining steps are the same.

[0079] The results are as follows Figure 2 As shown, there are obvious white corrosion marks on the glass digestion chamber. Figure 3 This is a schematic diagram of the digestion chamber after the digestion reaction of an embodiment of the present invention, and no traces of corrosion are observed.

[0080] The results of Example 1 and the national standard method are shown in Table 3, and the total nitrogen quality control sample tested in Example 1 is shown in Table 4:

[0081] Table 3

[0082]

[0083]

[0084] Table 4

[0085] Quality control sample concentration Uncertainty Test value 2.96 0.19 3.08 1.52 0.1 1.44

[0086] It can be seen from Table 3 and Table 4 that Example 1 of the present invention can more accurately test the ammonia nitrogen standard solution and the total nitrogen quality control sample.

[0087] Because the isophthalic acid spectrophotometry method needs to maintain an acidity of about 50% during the determination of total nitrogen. In Example 1 of the present invention, after the digestion is completed, the total volume of the digestion solution is 6mL. After adding 1mL of resorcinol, 7mL of sulfuric acid needs to be added, and the total volume of the waste liquid after the test is completed is 14mL. If 1mL of digestion solution is left in the digestion chamber after 5mL of digestion solution is discharged, after adding 1mL of resorcinol, 2mL of sulfuric acid can be added to meet the test. At this time, the amount of waste liquid is 9mL, which is reduced by 5mL. The amount of sulfuric acid used has been reduced by 71%. Sulfuric acid is highly corrosive and cannot be directly discharged into the environment. It needs to be discharged into the waste liquid for treatment. If the discharge volume is too large, the waste liquid treatment cost will increase. Therefore, it is necessary to reduce the amount of sulfuric acid without affecting the test to reduce the waste liquid treatment cost.

[0088] Example 2

[0089] A method for determining total nitrogen in water quality using an online detector is provided. The total nitrogen standard solution is detected. The difference from Example 1 is that the sample injected into the digestion chamber is a diluted sample, specifically a mixture of the original 2.5 mL sample and 2.5 mL distilled water. In the five stages of the digestion process, the lowest temperature of the third and fourth stages is 70°C, the cooling temperature of the color reaction is 40°C, and the remaining steps are the same. The measurement results are shown in Table 5 below:

[0090] Table 5

[0091]

[0092] As shown in Table 5, the embodiment of the present invention can test the total nitrogen standard solution more accurately.

[0093] Example 3

[0094] A method for determining total nitrogen in water quality using an online detector is provided. The actual water sample is tested. The difference from Example 1 is that the light emitting diode is turned on for 40 seconds in obtaining the reference value, and is turned on for 60 seconds in the color reaction. In the five stages of the digestion process, the lowest temperature of the third and fourth stages is 70°C, the cooling temperature of the color reaction is 50°C, and the color developer component is 15g / L resorcinol. The test results are shown in Table 6:

[0095] Table 6

[0096] Actual water sample test Manual analysis value Instrument test value ① 3.43 3.73 ② 9.43 9.48

[0097] It can be seen from Table 6 that the embodiment of the present invention can accurately test the total nitrogen in actual water samples.

[0098] Example 4

[0099] A method for determining total nitrogen in water quality using an online detector is provided. The quality control sample (No.: 2032106, concentration: 6.22, expanded uncertainty: 0.48) is tested. The difference from Example 1 is that the time and rate of heating in each stage during the digestion process are different. The digestion is performed according to the process of Table 7. The test data are shown in Table 8:

[0100] Table 7

[0101]

[0102] Table 8

[0103] Experimental Group Test Data 1 1.64 2 5.33 3 5.48 4 5.31 5 6.63

[0104] It can be seen from the above table that the total nitrogen quality control sample can be accurately tested only under the conditions of experimental group 5 and the conditions of the above-mentioned S2 sample digestion. In general, in the embodiment of the present invention, the light-emitting diode is turned on only when the absorbance is tested, and the reference voltage is kept at 1V during the test. During heating digestion, heating is performed in sections, and excess digestion solution is discharged after heating to reduce the amount of sulfuric acid used. This not only ensures the function of accurate measurement, but also solves the light decay phenomenon in the prior art, reduces the burden on the digestion chamber, avoids corrosion of the digestion chamber, reduces the use of sulfuric acid, reduces the amount of waste liquid generated, and reduces the impact on the environment.

[0105] The specific implementation methods described above further illustrate the objectives, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific implementation method of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for determining total nitrogen in water quality using an online detector, the method using resorcinol spectrophotometry for testing, characterized in that: The following steps are involved: S1, reference value acquisition, including injecting distilled water into the digestion chamber, turning on the light emitting diode, acquiring the reference voltage, and then draining the distilled water in the digestion chamber; S2: Sample digestion, including injecting the sample and digestion solution into the digestion chamber, then heating up for digestion reaction, then cooling down, stirring, and then heating up again to continue digestion reaction, so as to reduce the pressure burden in the digestion chamber; S3: Color reaction.

2. The method for determining total nitrogen in water quality using an online detector according to claim 1, characterized in that: The temperature conditions of the digestion reaction include a first temperature rising stage, a second holding stage, a third temperature falling stage, a fourth temperature rising stage and a fifth holding stage.

3. The method for determining total nitrogen in water quality using an online detector according to claim 2, characterized in that: The first heating stage is set to heat from room temperature to 110-130°C within 185s, with a heating rate of 0.4-0.5°C / s; The second holding stage is configured to maintain the temperature of the first heating stage for a holding time of 300 to 540 seconds; The third cooling stage is configured to cool the second holding stage to 70-80°C within 105s, with a cooling rate of 0.4-0.5°C / s; The fourth heating stage is configured to heat the lowest temperature of the third cooling stage, including heating to 110-130°C within 225s, with a heating rate of 0.15-0.25°C / s; The fifth holding stage includes maintaining the highest temperature of the fourth heating stage for a holding time of 280 to 520 seconds.

4. The method for determining total nitrogen in water quality using an online detector according to claim 3, characterized in that: In the third cooling stage, the sample digestion process further includes stirring the digestion solution and the sample in the digestion chamber when the temperature drops to 85-95° C. until the temperature drops to the lowest temperature of the third cooling stage.

5. A method for determining total nitrogen in water quality using an online detector according to any one of claims 1 to 4, characterized in that: The digestion solution comprises sodium hydroxide, potassium persulfate and distilled water; The mass ratio of the sodium hydroxide, potassium persulfate and distilled water is 1:3-5:70-90.

6. The method for determining total nitrogen in water quality using an online detector according to claim 5, characterized in that: The volume ratio of sample and digestion solution injected into S2 is 4 to 6:

1.

7. The method for determining total nitrogen in water quality using an online detector according to claim 6, characterized in that: The color development reaction includes cooling the reaction liquid after the digestion reaction in S2 to 40-50° C., then discharging part of the reaction liquid, adding resorcinol solution and 98% concentrated sulfuric acid into the digestion chamber, mixing and continuing to cool to 40-50° C., and finally turning on the light emitting diode to obtain the voltage and obtain the test value by conversion.

8. The method for determining total nitrogen in water quality using an online detector according to claim 7, characterized in that: The concentration of the resorcinol solution is 0.8-1.5wt%, and after part of the post-reaction liquid is discharged in the color development reaction, 1\7-1\5 volume of the post-reaction liquid remains. The volume ratio of the resorcinol solution, 98% concentrated sulfuric acid and the remaining post-reaction liquid is 1:2-3:0.8-1.

3.

9. The method for determining total nitrogen in water quality using an online detector according to claim 1, characterized in that: The temperature increase process of the sample digestion in S2 is achieved by a resistance heating wire, and the resistance heating wire is arranged in the digestion chamber and below the liquid surface of the reaction liquid of the digestion reaction.

10. The method for determining total nitrogen in water quality using an online detector according to claim 1, characterized in that: After S3, S4 is also included. S4 is a cleaning process, which includes emptying the test liquid in the digestion chamber, first cleaning the digestion chamber with 2 to 3 mL of distilled water, and then cleaning the digestion chamber with 7 to 8 mL of distilled water; The light emitting diodes are only turned on when the test voltage is applied to S1 and S3.

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