Permanganate index detection method and on-line monitoring method

Abstract

The invention relates to an automatic monitoring technology, and provides a permanganate index detection method and an on-line monitoring method.The permanganate index detection method comprises the steps that a first reagent is prepared, and a mixed solution containing 98% of sulfuric acid, anhydrous manganese sulfate and pure water is prepared into a mixed solution containing 10-25% of sulfuric acid; the method comprises the following steps: adding a quantitative reagent I into a water sample, and heating and digesting to 90-100 DEG C; dropwise adding a potassium permanganate solution, stirring at the speed of 280-350 revolutions per minute, and then keeping at the temperature of 90-100 DEG C for 10 minutes; dropwise adding a sodium oxalate solution, and uniformly stirring; dropwise adding a potassium permanganate solution again and stirring at the same time, and judging a titration end point according to the absorbance of the solution; and calculating the permanganate index according to the drop number at the titration end point. A certain amount of anhydrous manganese sulfate reagent is added into the sulfuric acid reagent, water bath heating is not needed, and the preparation method is simpler and easier to operate than a national standard method. The real drop number of the titration end point is obtained through the equation, so that the titration drop number is more accurate and is accurate to four decimal points.

Description

Technical Field

[0001] The invention relates to an automated monitoring technology, and in particular to a permanganate index detection method and an online monitoring method. Background Art

[0002] The permanganate index is an important indicator for evaluating the degree of water pollution. It can reflect the degree of oxidation of organic and inorganic matter in the water. The oxidizable substances in the water mainly exist in agricultural breeding wastewater, industrial wastewater and urban domestic sewage. If the oxidizable substance index in the water is high for a long time, it will affect the growth of aquatic organisms in breeding and agricultural production, and damage the soil and crops. Therefore, the permanganate index is of great significance for evaluating the degree of organic pollution in water bodies, assessing the effectiveness of sewage treatment and monitoring industrial wastewater discharge.

[0003] The permanganate index of water quality is mostly tested manually in laboratories, which requires complicated manual operations to carry out precise testing. It is necessary to add water samples and various reagents and wait for the mixing reaction to proceed, compare and calculate the results, and transport and store the water samples to the laboratory for testing. The process of taking water samples is long. If the water samples are deteriorating, it will cause large detection errors. This method is not only labor-intensive, time-consuming, and complex in operation process, but also has disadvantages such as a large amount of waste liquid generated after analysis and testing and a low detection frequency. There are two main methods for detecting the permanganate index of water quality: direct photometry and titration. In direct photometry, due to the excessively high acid concentration contained in the reagent, when comparing water samples with the laboratory manual method, the results obtained are quite different from those measured by the laboratory manual method. The titration method can be consistent with the results of the laboratory manual method. The titration method is divided into acidic potassium permanganate method and alkaline potassium permanganate method. The alkaline potassium permanganate method is suitable for water bodies with a chloride ion concentration higher than 300 mg / L, and the acidic potassium permanganate method is suitable for water bodies with a chloride ion concentration lower than 300 mg / L. For the acidic potassium permanganate method, most of them adopt the reagent preparation scheme in the national standard method. However, for online monitoring equipment using the titration method, the accuracy and repeatability of the detection results for low-concentration water samples are poor due to the systematic errors in the sampling caused by the equipment's own pipelines and titration pumps. In addition, since potassium permanganate needs to be added to the reagent preparation in the national standard method, as time goes by and the ambient temperature rises, a large number of bubbles will be generated in the reagent pipeline and attached to the wall of the reagent tube, seriously affecting the quantification of the reagent and thus affecting the measurement results. Summary of the invention

[0004] In order to achieve the above object, the present invention provides the following technical solutions:

[0005] The present invention provides a method for detecting a permanganate index, comprising:

[0006] (1) Prepare reagent 1 by mixing 98% sulfuric acid, anhydrous manganese sulfate and pure water to prepare a mixed solution containing 10-25% sulfuric acid;

[0007] (2) Add a certain amount of reagent 1 to the water sample and heat it to 90-100 degrees Celsius for digestion;

[0008] (3) Add potassium permanganate solution, stir at 280-350 rpm, and then keep at 90-100 degrees Celsius for 10 minutes;

[0009] (4) Add sodium oxalate solution and stir evenly;

[0010] (5) Add potassium permanganate solution again while stirring, and determine the titration endpoint based on the absorbance of the solution;

[0011] (6) Calculate the permanganate index based on the number of drops at the titration endpoint.

[0012] At present, most of the reagents are prepared by the national standard method, that is, potassium permanganate is added to the sulfuric acid reagent to prepare the reagent. In the present invention, the reagent is prepared by adding a certain amount of anhydrous manganese sulfate to the sulfuric acid reagent instead of adding potassium permanganate in the national standard method, so as to determine the permanganate index. The chemical formula is:

[0013]

[0014] in, middle, The concentration is controllable, and It is a catalyst in the entire reaction system, which has the effect of increasing the reaction rate without participating in the reaction, so it can be adjusted The concentration of NH4Cl2O4 controls the overall reaction rate. In the subsequent oxidation reaction, there is sufficient NH4Cl2O4 in the initial oxidation process. , to ensure a higher reaction rate in the initial oxidation stage, and the purpose of adding potassium permanganate to the sulfuric acid reagent in the national standard method is to eliminate The impurities containing trace reducing substances in the mixed solution are reduced by potassium permanganate, so trace amounts of , thus producing a pink color, a trace amount of This means that in the subsequent oxidation reaction, the initial oxidation process will become very slow, and as time goes by, Gradually restored and , and then gradually accelerate the reaction rate. The change rate of the reaction rate of the national standard method is a process from slow to fast; the reaction rate of the present invention is higher than that of the national standard method, and the oxidation is more thorough in the same time.

[0015] In addition, since potassium permanganate needs to be added to the reagent preparation in the national standard method, as time goes by and the ambient temperature rises, a large number of bubbles will be generated in the reagent pipeline and attached to the reagent tube wall, seriously affecting the quantification of the reagent, and thus affecting the measurement results. The present application is used for the acidic potassium permanganate titration method for permanganate index, and there is no problem with the reagents in the national standard method.

[0016] Furthermore, in step (2), the volume ratio of reagent 1 to water sample is 1:10.

[0017] Furthermore, the volume ratio of the potassium permanganate solution in step (3) to the sodium oxalate solution in step (4) is 1:1, and the concentration of the sodium oxalate solution is greater than the concentration of the potassium permanganate solution, ensuring that the potassium permanganate is completely reduced.

[0018] Furthermore, the concentration of potassium permanganate solution in step (3) is the same as that in step (4).

[0019] Furthermore, in step (5), the determination criterion for the titration endpoint is: collecting the light signal of each drop of potassium permanganate titration solution after mixing, and calculating the absorbance; when the absorbance values ​​are all greater than the set determination absorbance, it is determined as the endpoint drop number.

[0020] Further, the judgment standard of the titration end point is: collect the light signal after the solution is mixed during each drop of potassium permanganate titration, and calculate the absorbance. When the absorbance values ​​collected for the first time are greater than the set judgment absorbance, continue to titrate for 5 times, and ensure that the absorbance measured by the subsequent 5 titrations is greater than the set value, determine a linear curve by the absorbance and the number of drops of the first and subsequent 5 drops, and obtain the equation of the curve, and obtain the number of drops at the end point through the equation. According to the titration pump principle, when performing permanganate titration, if the number of titrations is recorded according to the national standard method, it is bound to cause errors in the measurement results, because the titration pump is fixed when quantitatively measuring the liquid, and it is impossible to have half a drop or one-third of a drop. The present invention obtains the true number of drops of the titration end point through the curve equation, making the titration drop number more accurate, and can be accurate to four decimal places.

[0021] Furthermore, the formula for calculating the permanganate index by the number of drops at the titration endpoint is:

[0022] ;

[0023] Wherein, M and P are algorithm coefficients, A is absorbance, b is the number of endpoint drops, and n is the permanganate index;

[0024] When the absorbance A=0, n is the true value of the permanganate index.

[0025] The present invention also provides an online monitoring method for permanganate index water quality, wherein the online detector applicable to the online monitoring method for water quality comprises: an injection pump, a metering unit, a titration pump, a digestion unit and a liquid discharge pump, wherein the injection pump is connected to the metering unit, wherein the metering unit and the titration pump are respectively connected to the digestion unit, and the digestion unit is connected to the liquid discharge pump through a valve 1; wherein the metering unit comprises a multi-way valve group and a liquid level detector, wherein interfaces 1 to 10 of the multi-way valve group are defined as a pure water port, a channel port, a water sample inlet, a standard sample port, a reagent port 1, a reserved port, a reagent port 3, a quality control sample port, a cleaning waste liquid port 1, and a cleaning waste liquid port 2 in sequence;

[0026] The digestion unit includes a signal detector and a colorimetric cell. The colorimetric cell is wound with a heating wire and has a temperature sensor. A fan is provided outside to assist in cooling.

[0027] Furthermore, the online monitoring method includes:

[0028] (1) The sampling pump draws a certain amount of water sample by opening the water sample inlet pipeline on the multi-way valve and then measures it through the metering unit and then injects it into the colorimetric cell through the channel outlet pipeline;

[0029] (2) The sampling pump draws a certain amount of reagent 1 through the reagent inlet pipe and injects it into the colorimetric cell through the channel outlet pipe, and heats and digests it to 90~100 degrees Celsius; the volume ratio of reagent 1 to water sample is 1:10;

[0030] (3) A certain amount of potassium permanganate solution is dripped into the colorimetric cell by a titration pump, and the motor is turned on to stir at 280-350 rpm, and then maintained at 90-100 degrees Celsius for 10 minutes; when the potassium permanganate solution is quantitatively added by the titration pump, its liquid outlet will give the liquid an impact force to ensure that there will be no residual liquid droplets at the liquid outlet, thereby preventing errors in the calculation of the permanganate liquid volume.

[0031] (4) The sampling pump draws a certain amount of sodium oxalate solution through the reagent three-port pipeline and injects it into the colorimetric cell through the channel port pipeline and stirs it evenly; the volume ratio of potassium permanganate solution to sodium oxalate solution is 1:1, and the concentration of sodium oxalate solution is greater than the concentration of potassium permanganate solution to ensure that potassium permanganate is completely reduced;

[0032] (5) Potassium permanganate solution is added dropwise to the colorimetric cell by a titration pump while stirring. The signal is collected by a signal detector and the titration endpoint is determined. The content of the permanganate index in the measured sample is calculated. The content of the permanganate index is obtained by the formula: ;

[0033] Wherein, M and P are algorithm coefficients, A is absorbance, b is the number of endpoint drops, and n is the permanganate index;

[0034] When the absorbance A=0, n is the true value of the permanganate index.

[0035] (6) The liquid in the absorption reaction module is drained by the drain pump and the entire reaction flow path is cleaned.

[0036] Furthermore, the reagent 1 is a mixed solution containing 10-25% sulfuric acid prepared from 98% sulfuric acid, anhydrous manganese sulfate, and pure water.

[0037] The present invention has the following beneficial effects:

[0038] (1) The present invention adopts a novel reagent to detect the permanganate index in water quality through an online monitoring device. The application of the present invention in the online monitoring device for the permanganate index by the acid potassium permanganate titration method can realize real-time monitoring of water quality, replace the complicated manual operation during manual detection, and has good accuracy, which is consistent with the detection results of the laboratory manual method. The data repeatability and stability are very good. Compared with the national standard method, the detection results for low-concentration water samples have higher data accuracy and stability. In addition, since potassium permanganate needs to be added to the reagent preparation in the national standard method, as time goes by and the ambient temperature rises, a large number of bubbles will be generated in the reagent pipeline and attached to the reagent tube wall, which seriously affects the quantification of the reagent and further affects the measurement result. The chemical detection method of the online monitoring device for the permanganate index by the acid potassium permanganate titration method of the present invention does not have the problem of the reagent in the national standard method.

[0039] (2) The present invention adopts the method of adding a certain amount of anhydrous manganese sulfate reagent into sulfuric acid reagent, and does not require water bath heating. The preparation method is simpler and easier to operate than the national standard method.

[0040] (3) When determining the titration endpoint, the present invention continues to titrate for 5 times when the absorbance values ​​collected for the first time are all greater than the set determination absorbance, and ensures that the absorbances measured in the subsequent 5 titrations are all greater than the set value, and then determines a linear curve by the absorbance and number of drops of the first and subsequent 5 drops, and obtains the equation of the curve. The true number of drops at the titration endpoint is obtained by the equation, so that the titration drop number is more accurate and accurate to four decimal places.

[0041] (4) The present invention simplifies the measurement process, combines the steps of the national standard method one by one, and shortens the online monitoring cycle.

[0042] (5) The preparation of reagent 1 of the present invention is simpler, does not require heating, requires less sulfuric acid, and does not generate bubbles. It does not affect the true value of the subsequent online monitoring of the permanganate index. The subsequent water sample oxidation rate is faster, the oxidation rate is higher, and it is more thorough. The data finally measured is more accurate.

[0043] (6) The instrument used in the present invention is cheaper than the national standard method and is maintenance-free during the instrument's life. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] Figure 1 It is a flow chart of the detection method in Example 1.

[0045] Figure 2 It is a graph of potassium permanganate titration drop number-absorbance in Example 1.

[0046] Figure 3 It is a line comparison chart of the permanganate index calculated from 20 tests in Example 1.

[0047] Figure 4 Schematic diagram of the pipeline of the online monitoring instrument in Example 2.

[0048] Figure 5 This is a flow chart of monitoring in Example 3. DETAILED DESCRIPTION

[0049] The specific implementation methods of the present invention are described in detail below in conjunction with the accompanying drawings. It should be pointed out that the embodiments are only specific explanations of the invention and should not be regarded as limitations of the invention. The purpose of the embodiments is to enable those skilled in the art to better understand and reproduce the technical solutions of the present invention. The protection scope of the present invention shall still be based on the scope defined by the claims.

[0050] Example 1

[0051] like Figure 1 As shown, this embodiment provides a method for detecting a permanganate index, comprising:

[0052] S1, preparing reagent 1, preparing a mixed solution containing 10-25% sulfuric acid by mixing 98% sulfuric acid, anhydrous manganese sulfate and pure water;

[0053] S2, add a certain amount of reagent 1 to the water sample and heat it to 90-100 degrees Celsius for digestion; the volume ratio of reagent 1 to water sample is 1:10;

[0054] S3, adding potassium permanganate solution dropwise, stirring at 280-350 rpm, and then maintaining at 90-100 degrees Celsius for 10 minutes;

[0055] S4, drop sodium oxalate solution and stir evenly; wherein the volume ratio of potassium permanganate solution to sodium oxalate solution is 1:1, and the concentration of sodium oxalate solution is greater than the concentration of potassium permanganate solution, ensuring that potassium permanganate is completely reduced.

[0056] S5, adding potassium permanganate solution of the same concentration as in step S3 again and stirring at the same time, and judging the titration end point according to the absorbance of the solution; the judgment standard of the titration end point can be: collecting the light signal after the solution is mixed during each drop of potassium permanganate titration, and calculating the absorbance, when the absorbance values ​​are all greater than the set judgment absorbance, it is determined as the end point drop number. However, this judgment standard is based on the titration pump principle. When performing permanganate titration, if the number of titrations is recorded according to the national standard method, the number of drops of the titration must be an integer, which is bound to cause errors in the measurement results. Therefore, the present invention also provides another judgment standard, which is to collect the light signal after the solution is mixed during each drop of potassium permanganate titration, and calculate the absorbance. When the absorbance values ​​collected for the first time are all greater than the set judgment absorbance, continue to titrate 5 times, and ensure that the absorbance measured by the subsequent 5 titrations is greater than the set value, determine a linear curve based on the absorbance and number of drops of the initial and subsequent 5 drops, and obtain the equation of the curve. The number of drops at the end point is obtained by the equation, such as Figure 2 As shown, the X-axis represents the number of potassium permanganate drops from the titration pump, and the Y-axis represents the absorbance. The higher the concentration of the potassium permanganate solution, the greater the absorbance. The two are linear, so the absorbance can be used to represent the concentration of potassium permanganate. According to the curve and the national standard test method, when the titration pump drops to the 39th drop, the liquid is pink. The number of liquid drops is recorded and converted into the volume of potassium permanganate to obtain the permanganate index. However, in fact, the permanganate index may be balanced at a decimal point between the 38th and 39th drops. In order to obtain an accurate value, an additional 5 drops will be added when the absorbance rises. Because the reducing agent in the reaction tank has been completely oxidized by potassium permanganate, the concentration of potassium permanganate added subsequently will be linearly related to the number of drops added by the titration pump. After recording the absorbance of the next 5 drops, a linear equation is fitted, and based on this fitted straight line, the straight line and the horizontal line generated by the previous titration, that is, Y=0, are calculated as the focus, and X=B / K=0.441905 / 0.0138892=31.8164 is calculated. The error caused by the digits after the decimal point of K and B that are not displayed is only 0.0001. Therefore, the 31.8164th drop is the end point drop number. The true number of drops at the end point of the titration is obtained through the curve equation, making the titration drop number more accurate and accurate to four decimal places.

[0057] S6, calculate the permanganate index by the number of drops at the titration endpoint, the calculation formula is:

[0058]

[0059] Wherein, M and P are algorithm coefficients, A is absorbance, b is the number of endpoint drops, and n is the permanganate index;

[0060] When the absorbance A=0, n is the true value of the permanganate index.

[0061] The following uses the national standard method and the preparation reagent 1 in the present method to conduct 20 consecutive tests on the same water sample, and the true number of drops at the titration endpoint is obtained by the curve equation in the present invention. The true number of drops obtained in the 20 tests is shown in Table 1, and the broken line comparison chart of the permanganate index calculated from the 20 tests is shown.

[0062] Table 1

[0063]

[0064] It can be obtained that the present invention, compared with the national standard method, under the premise of the same titration algorithm and the same titration method, only improves the stability of data fluctuation by 88.2% in the preparation method of sulfuric acid, that is, the range is improved from 6.4% to 3.4%, and the repeatability is relatively improved by 64.4%, that is, 1.94% to 1.18%. The data accuracy and stability of the chemical detection method test results of the acid potassium permanganate titration permanganate index online monitoring equipment of the present invention are good, and for the detection of low-concentration water samples, the data accuracy and repeatability are significantly improved relative to the national standard method, respectively increased by 3% and 0.75%.

[0065] At present, most of the reagents are prepared by the national standard method, that is, potassium permanganate is added to the sulfuric acid reagent to prepare the reagent. In the present invention, the reagent is prepared by adding a certain amount of anhydrous manganese sulfate to the sulfuric acid reagent instead of adding potassium permanganate in the national standard method, so as to determine the permanganate index. The chemical formula is:

[0066]

[0067] in, middle, The concentration is controllable, and It is a catalyst in the entire reaction system, which has the effect of increasing the reaction rate without participating in the reaction, so it can be adjusted The concentration of NH4Cl2O4 controls the overall reaction rate. In the subsequent oxidation reaction, there is sufficient NH4Cl2O4 in the initial oxidation process. , ensuring a higher reaction rate in the initial oxidation stage; and according to the national standard GB 11892-89, the reagent preparation is to heat the sulfuric acid reagent in a water bath to 70°C, add potassium permanganate reagent while hot and stir continuously until the solution turns pink. The chemical formula is as follows:

[0068]

[0069] ;

[0070] The purpose of adding potassium permanganate to sulfuric acid reagent is to eliminate The impurities containing trace reducing substances in the mixed solution are reduced by potassium permanganate, so trace amounts of , thus producing a pink color, a trace amount of This means that in the subsequent oxidation reaction, the initial oxidation process will become very slow, and as time goes by, Gradually restored and , and then gradually accelerate the reaction rate. The change rate of the reaction rate of the national standard method is a process from slow to fast; the reaction rate of the present invention is higher than that of the national standard method, and the oxidation is more thorough in the same time.

[0071] The purpose of adding potassium permanganate solution in the preparation of reagent 1 in the national standard method is to eliminate The reason why the present invention does not need to drip potassium permanganate solution is that The reducing substances in the solution do not affect the measurement results for the following reasons:

[0072] Reducing substances in measurement

[0073] Blank test reducing substances

[0074] The reducing substances in the final water sample are

[0075] It is proved that when the reagent 1 is initially prepared, the trace reducing substance contained therein will not affect the final result, so the present invention does not need to drip the potassium permanganate solution.

[0076] In addition, since potassium permanganate needs to be added to the reagent preparation in the national standard method, as time goes by and the ambient temperature rises, a large number of bubbles will be generated in the reagent pipeline and attached to the reagent tube wall, seriously affecting the quantification of the reagent, and thus affecting the measurement results. The present application is used for the acidic potassium permanganate titration method for permanganate index, and there is no problem with the reagents in the national standard method.

[0077] Example 2

[0078] like Figure 4 As shown, this embodiment provides an online monitor for permanganate index water quality, including a sampling pump 1, a metering unit 2, a titration pump 3, a digestion unit 4 and a liquid discharge pump 5, wherein the sampling pump 1 is connected to the metering unit 2, the metering unit 2 and the titration pump 3 are respectively connected to the digestion unit 4, and the digestion unit 4 is connected to the liquid discharge pump 5 through a valve 6; the metering unit 2 includes a multi-way valve group and a liquid level detector, and interfaces 1 to 10 of the multi-way valve group are defined as a pure water port, a channel port, a water sample inlet, a standard sample port, a reagent port 1, a reserved port, a reagent port 3, a quality control sample port, a cleaning waste liquid port 1, and a cleaning waste liquid port 2 in sequence;

[0079] The digestion unit 4 includes a signal detector and a colorimetric cell. A heating wire is wound around the outside of the colorimetric cell and a temperature sensor is provided. A fan is provided outside to assist in cooling.

[0080] Example 3

[0081] like Figure 5 As shown, this embodiment provides an online detection method of a permanganate index water quality online monitor, comprising:

[0082] S101, the sampling pump draws a certain amount of water sample by opening the water sample inlet pipeline on the multi-way valve, and then measures it through the metering unit and injects it into the colorimetric cell through the channel outlet pipeline;

[0083] S102, the sample injection pump draws a certain amount of reagent 1 through the reagent port pipeline and injects it into the colorimetric cell through the channel port pipeline, and heats and digests it to 90-100 degrees Celsius; wherein the volume ratio of reagent 1 to water sample is 1:10; the reagent 1 is a mixed solution containing 10-25% sulfuric acid prepared from 98% sulfuric acid, anhydrous manganese sulfate, and pure water;

[0084] S103, a certain amount of potassium permanganate solution is dripped into the colorimetric cell by a titration pump, and the motor is turned on to stir at 280-350 revolutions per minute, and then maintained at 90-100 degrees Celsius for 10 minutes; when the potassium permanganate solution is quantitatively measured by the titration pump, its liquid outlet will give the liquid an impact force to ensure that there will be no residual liquid beads at the liquid outlet, and thus no error will be caused in calculating the volume of the permanganate liquid. This application uses a titration pump to drip potassium permanganate solution, which is relatively cheap compared to the national standard method using a syringe pump and a plunger pump, and its service life exceeds the 5-year life of the instrument, that is, maintenance-free.

[0085] S104, the sampling pump draws a certain amount of sodium oxalate solution through the reagent three-port pipeline and injects it into the colorimetric cell through the channel port pipeline, and stirs it evenly; wherein the volume ratio of the potassium permanganate solution to the sodium oxalate solution is 1:1, and the concentration of the sodium oxalate solution is greater than the concentration of the potassium permanganate solution, ensuring that the potassium permanganate is completely reduced;

[0086] S105, adding potassium permanganate solution to the colorimetric cell by a titration pump while stirring, collecting signals by a signal detector and determining the titration end point, and calculating the content of the permanganate index in the measured sample; the content of the permanganate index is obtained by the formula:

[0087] Wherein, M and P are algorithm coefficients, A is absorbance, b is the number of endpoint drops, and n is the permanganate index;

[0088] When the absorbance A=0, n is the true value of the permanganate index.

[0089] S106, draining the liquid in the absorption reaction module by a liquid draining pump and cleaning the entire reaction flow path.

[0090] Although the preferred embodiments of the present application have been described, those skilled in the art may make additional changes and modifications to these embodiments once they have learned the basic creative concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications falling within the scope of the present application.

[0091] It should be noted that the technical features not described in detail in the present invention can be implemented by any existing technology.

Claims

1. A method for detecting a permanganate index, characterized in that: include: (1) Prepare reagent 1 by mixing 98% sulfuric acid, anhydrous manganese sulfate and pure water to prepare a mixed solution containing 10-25% sulfuric acid; (2) Add a certain amount of reagent 1 to the water sample and heat it to 90-100 degrees Celsius for digestion; (3) Add potassium permanganate solution, stir at 280-350 rpm, and then keep at 90-100 degrees Celsius for 10 minutes; (4) Add sodium oxalate solution and stir evenly; (5) Add potassium permanganate solution again while stirring, and determine the titration endpoint based on the absorbance of the solution; (6) Calculate the permanganate index based on the number of drops at the titration endpoint.

2. The method for monitoring the permanganate index according to claim 1, characterized in that: In step (2), the volume ratio of reagent 1 to water sample is 1:

10.

3. The method for monitoring the permanganate index according to claim 1, characterized in that: The volume ratio of the potassium permanganate solution in step (3) to the sodium oxalate solution in step (4) is 1:1, and the concentration of the sodium oxalate solution is greater than the concentration of the potassium permanganate solution, ensuring that the potassium permanganate is completely reduced.

4. The method for monitoring the permanganate index according to claim 1, characterized in that: The concentration of potassium permanganate solution in step (3) is the same as that in step (4).

5. The method for monitoring the permanganate index according to claim 1, characterized in that: In step (5), the determination criterion for the titration endpoint is: collecting the light signal of each drop of potassium permanganate titration solution after mixing, and calculating the absorbance. When the absorbance values ​​are all greater than the set determination absorbance, the drop is determined to be the endpoint drop.

6. The method for monitoring the permanganate index according to claim 1, characterized in that: In step (5), the determination criterion for the titration endpoint is as follows: the light signal after the solution is mixed for each drop of potassium permanganate titration is collected, and the absorbance is calculated. When the absorbance values ​​collected initially are all greater than the set determination absorbance, the titration is continued for 5 more times, and when it is ensured that the absorbance measured in the subsequent 5 titrations is greater than the set value, a linear curve is determined by the absorbance and number of drops of the initial and subsequent 5 drops, and the equation of the curve is obtained, and the number of drops at the endpoint is obtained by the equation.

7. The method for monitoring permanganate index according to claim 1, characterized in that: The formula for calculating the permanganate index by the number of drops at the titration endpoint is: ; Wherein, M and P are algorithm coefficients, A is absorbance, b is the number of endpoint drops, and n is the permanganate index; when absorbance A=0, n is the true value of the permanganate index.

8. A method for online monitoring of water quality by permanganate index, characterized in that: The online detector applicable to the water quality online monitoring method comprises: a sampling pump, a metering unit, a titration pump, a digestion unit and a liquid discharge pump, wherein the sampling pump is connected to the metering unit, the metering unit and the titration pump are respectively connected to the digestion unit, and the digestion unit is connected to the liquid discharge pump through a valve 1; the digestion unit comprises a signal detector and a colorimetric cell, the colorimetric cell is externally wound with a heating wire and is provided with a temperature sensor, and a fan is externally provided for auxiliary cooling; The metering unit includes a multi-way valve group and a liquid level detector. The interfaces 1 to 10 of the multi-way valve group are defined as a pure water port, a channel port, a water sample inlet, a standard sample port, a reagent port 1, a reserved port, a reagent port 3, a quality control sample port, a cleaning waste liquid port 1, and a cleaning waste liquid port 2, respectively.

9. A method for online monitoring of water quality of permanganate index according to claim 8, characterized in that: Online monitoring methods include: (1) The sampling pump draws a certain amount of water sample by opening the water sample inlet pipeline on the multi-way valve and then measures it through the metering unit and then injects it into the colorimetric cell through the channel outlet pipeline; (2) The sample injection pump draws a certain amount of reagent through the reagent inlet pipe and injects it into the colorimetric cell through the channel outlet pipe, and heats and digests it to 90~100 degrees Celsius; (3) Use a titration pump to drip a certain amount of potassium permanganate solution into the colorimetric cell, turn on the motor to stir at 280-350 rpm, and then keep it at 90-100 degrees Celsius for 10 minutes; (4) The sampling pump draws a certain amount of sodium oxalate solution through the reagent three-port pipeline and injects it into the colorimetric cell through the channel port pipeline and stirs it evenly; (5) Potassium permanganate solution is added dropwise to the colorimetric cell by a titration pump while stirring, and a signal detector is used to collect signals and determine the titration endpoint, and the content of the permanganate index in the measured sample is calculated; (6) The liquid in the absorption reaction module is drained by the drain pump and the entire reaction flow path is cleaned.

10. A method for online monitoring of water quality of permanganate index according to claim 9, characterized in that: The reagent 1 is a mixed solution containing 10-25% sulfuric acid prepared from 98% sulfuric acid, anhydrous manganese sulfate and pure water.

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