CN114720465A - Water quality permanganate index online monitoring method and system - Google Patents
Water quality permanganate index online monitoring method and system Download PDFInfo
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Abstract
The invention provides a water quality permanganate index on-line monitoring method and a system, which have the following beneficial effects: the set absorbance threshold is used as a titration end point judgment condition, the water body adaptability is strong, and the turbidity resistance is improved through the setting of the threshold; compared with an ORP electrode method, the method has the advantages of small reaction system, small waste liquid amount, low whole machine cost and low subsequent operation and maintenance cost.
Description
Technical Field
The invention relates to the technical field of potassium permanganate brine quality analysis, in particular to a water quality permanganate index online monitoring method and system.
Background
The Chemical Oxygen Demand (COD) is the amount of reducing substances needing to be oxidized in a water sample measured by a chemical method. High chemical oxygen demand means that the water contains a large amount of reducing substances, mainly organic pollutants. The higher the chemical oxygen demand, the more serious the organic pollution of the water body. Not only do the organisms harmful to the water body, such as fish, but also enter the human body finally through the enrichment of food chains, causing chronic poisoning. Therefore, COD is an important index in environmental monitoring. The COD detection method comprises a manganese method and a chromium method, and the permanganate index refers to the measured value of the manganese method.
Generally, the oxidizing agent used for measuring the chemical oxygen demand is potassium permanganate or potassium dichromate, and the values obtained by using different oxidizing agents are different, so that the detection method needs to be noted. The permanganate index refers to the amount of an oxidant consumed in treating a water sample by using potassium permanganate as the oxidant in an acidic or alkaline medium, and is mainly applied to measuring surface water, but the surface water generally has the conditions of high turbidity, complex and various water bodies and the like.
Chemical principle of permanganate index online monitor: adding a known amount of potassium permanganate solution into a water sample, heating in a boiling water bath for reaction for a certain time, oxidizing certain organic matters and inorganic reducing substances in a sample by potassium permanganate, adding excessive sodium oxalate to reduce the residual potassium permanganate after the reaction, and back-dripping excessive sodium oxalate by using a potassium permanganate standard solution. The permanganate index value of the sample was calculated.
The measurement mode of the existing permanganate index on-line monitor is as follows: direct colorimetry, titration ORP electrode method.
A grafting colorimetric method: adding a quantitative potassium permanganate solution into a water sample, heating and digesting for a period of time, fully reacting reducing substances in the water sample with potassium permanganate, cooling and detecting the absorbance of the water sample, wherein the absorbance has a certain linear relation with the permanganate index in the water sample. The permanganate index content can be calculated by measuring the absorbance of the component to be measured.
The direct colorimetric method inhibits the generation of manganese dioxide precipitates by improving the acidity, and the solution is clear and transparent after heating and digestion, so that the fading quantity of potassium permanganate can be detected by direct colorimetric to judge the quantity of reducing substances in a water sample, the more reducing substances in the water sample, the higher the reaction quantity of permanganate, the more fading the color of the solution and the lower the absorbance of the solution, and the reduction substance quantity in the water sample, namely CODMn, can be determined by calculating the absorbance value through the direct colorimetric after digestion. Although the method can be linear in the test process of the standard substance, the COD intensity of the water body is reflected to a certain degree. But the direct colorimetric method improves the acidity of the system and reduces the temperature of the system to 80-90 ℃ from 100 ℃ of the standard method compared with the national standard method. The oxidation capacity of permanganate is directly related to acidity and temperature conditions, and the higher the temperature is, the higher the acidity is, the stronger the oxidation capacity is. The direct colorimetric method changes the conditions of temperature and acidity, resulting in inconsistent digestion rates for different reducing standards such as glucose and sodium oxalate. This distorts the permanganate index values in actual water samples, which is inconsistent with the results of the permanganate index test in national standards. Therefore, although the direct colorimetric method has small waste liquid amount, the test method is simple and direct, titration judgment is not needed, and the test speed is high. However, the test result can be frequently distorted in the complex actual water sample comparison process, so that the method can cause misjudgment in the automatic monitoring and judging process, and the characteristic of poor adaptability of the method is increasingly highlighted along with the increasing requirements on accuracy and reliability in the permanganate detection field.
Titration colorimetry: adding a known amount of potassium permanganate solution into a water sample, heating in a boiling water bath for reaction for a certain time, oxidizing certain organic matters and inorganic reducing substances in the sample by potassium permanganate, adding excessive sodium oxalate to reduce the residual potassium permanganate after the reaction, and then back-dripping excessive sodium oxalate by using a potassium permanganate standard solution. The permanganate index value of the sample was calculated. The method judges the reaction endpoint through the luminosity change caused by the color change of the water sample at the titration endpoint. The method is greatly interfered by turbidity, the adaptability of a water sample is poor, the oxidation rates of sodium oxalate standard solutions are inconsistent in testing, and two curves need to be calibrated.
Conventional titration colorimetry: the conventional titration colorimetry is the most common method at the present stage, permanganate is used for digesting a water sample under the acidity and temperature of national standards, manganese dioxide precipitates are generated after reaction, the precipitation interferes with photometry colorimetry, and the reaction amount of potassium permanganate cannot be measured by direct colorimetry at this time, so that a fixed amount of sodium oxalate needs to be added firstly to reduce redundant potassium permanganate and manganese dioxide into a clear solution, at this time, the sodium oxalate is excessive, and then the potassium permanganate is used for titrating the excessive sodium oxalate at this time, so that the higher the reaction amount of potassium permanganate in an initial state is, the higher the CODMn content in water is, the more the residual amount of sodium oxalate after reaction is, the higher the titration amount of potassium permanganate is needed, the titration amount represents the CODMn content in water, the colorless transparent to light red color in the solution can be judged by the absorbance of the colorimetry, when the absorbance is increased in a mutation manner, namely, the light intensity is reduced in a sudden way, which indicates that the solution system is from colorless and transparent to a reddish titration end point of potassium permanganate. Endpoint values were determined by the amount of the titration, i.e., the number of drops. The method simulates a national standard method under an automatic system, and the acidity and the temperature of the method can be kept consistent with those of the national standard method, so that the oxidation efficiency can also be kept consistent with the national standard. In theory, the accuracy and the reliability are greatly improved compared with a direct colorimetric method, and although the time is long, the reagent consumption of the system is basically consistent. However, the scheme has a serious defect which cannot be solved by most of the existing products. Under the condition of a complex water sample, for example, the water sample contains turbidity, in the process of titration and uniform mixing, the solution can cause severe fluctuation of absorbance due to the factor of turbidity contained in the solution, and the conventional titration colorimetric method judges the titration end point by detecting the change of light intensity, so that the judgment of the end point can be failed due to the fluctuation of the light intensity in the titration process, and the conventional turbidity interference removing mode is to remove the absorbance interference caused by the turbidity factor by long-time (1-5 min) settlement waiting. It is clearly very impractical to wait for every 1 drop to stabilize for a long period of time during the titration. Therefore, the titration colorimetry is easy to have numerical value distortion under turbidity interference, and when a fixed light intensity change endpoint range 500 is set (for example, the light intensity changes from 3000 to 2500, the endpoint is obtained). It is often the case that the light intensity reaches 500 due to turbidity disturbance, and the solution itself is colorless and turbid and does not reach the end point. Therefore, the conventional titration colorimetry is very easily interfered by turbidity in the actual water sample testing process to distort the value, and the simple increase of the light intensity end point range is not effective, for example, the light intensity end point range is changed from 500 to 2000. Although the light intensity range caused by the turbidity fluctuation does not reach 2000 under the turbidity of 500ntu, the solution is red when the light intensity range is up to 2000, but is over-end. However, turbidity fluctuation is not deducted, the terminal point of the clear standard solution with the light intensity range of 2000 is still slower than the terminal point arrival speed of a turbid water sample (with turbidity fluctuation superposition) with the same concentration, the test terminal point of the turbid water sample can be advanced, and the test result of the turbid water sample is lower than the standard solution result. The condition that the actual water sample test result with turbidity is lower still is not solved, and the water sample adaptability is still insufficient.
Titration ORP electrode method: adding a known amount of potassium permanganate solution into a water sample, heating in a boiling water bath for reaction for a certain time, oxidizing certain organic matters and inorganic reducing substances in a sample by potassium permanganate, adding excessive sodium oxalate to reduce the residual potassium permanganate after the reaction, and back-dripping excessive sodium oxalate by using a potassium permanganate standard solution. The titration ORP electrode method does not need colorimetric detection, so that the water sample has good adaptability, the measurement process is similar to that of the conventional titration colorimetric method, and the reaction endpoint is judged through the change of the ORP value of the titration endpoint water body. However, the electrode system requires a large measuring cup, so the reaction system is large, the waste liquid amount is large, the electrode cost is high, the waste liquid amount is at least 3 times of that of a titration colorimetry, the operation and maintenance cost is high, and the terminal point is judged through the electrode, and when pollutants aiming at the electrode, such as viscous colloidal substances, exist in a water sample, the electrode fails, so that equipment failure can be caused.
Therefore, how to solve the problem of turbidity interference on light intensity in permanganate monitoring and realize the online monitoring of the permanganate index of water with high precision, low cost and low time consumption is a technical problem to be solved urgently by technical personnel in the field.
Disclosure of Invention
In order to solve the above problems, a first object of the present invention is to provide an online monitoring system for permanganate index in water.
Therefore, the above purpose of the invention is realized by the following technical scheme:
the water quality permanganate index online monitoring method is characterized by comprising the following steps of:
s1, setting an absorbance threshold value X of the water sample;
s2, carrying out digestion reaction on a water sample by permanganate at the acidity and temperature of the water sample under the national standard to generate manganese dioxide precipitate, adding excessive sodium oxalate at the moment, and recording the blank photovoltage A at the moment;
s3, titrating excessive sodium oxalate with potassium permanganate, adding a potassium permanganate solution through a burette, bubbling, uniformly mixing, standing until the photovoltage is stable, judging the solution from colorless transparency to light red through colorimetric absorbance, and recording the photovoltage B when the absorbance is increased in a mutability manner1The absorbance C = log (A/B) was calculated1)；
S4, comparing the absorbance C with a set absorbance threshold X, if C is less than X, using a standard turbidity solution, and circulating step S3 to record the photovoltage B1Then calculate its C1=log(A/B1) Mixing C with1Compared with X, if C1If < X, continuing to loop to step S3;
s5, when the step S3 is repeated, if the photovoltage Bn is recorded, the absorbance C is calculatedn，Cn=log(A/Bn) If C isnIf the number of the titration drops is more than or equal to X, the circulation is stopped, the complete circulation times are recorded, the circulation times are recorded as the number n of the titration drops,
s6, determining the single-titration absorbance value, marked as D, wherein D = (C)n-C)/n, wherein CnStep C is the absorbance value before the circulation of the step S3 is finished, step C is the absorbance value calculated after the standard water sample for calibration passes through the steps S1 to S5 and the titration is finished;
s7, calculating the final absorbance, and recording the final absorbance as CComputing，CCalculating out=n*D-Cn，
Wherein the absorbance threshold value X is the absorbance value of the reaction end point, and the threshold value range is set to be 0.04-0.2.
While adopting the above technical solutions, the present invention can also adopt or combine the following technical solutions:
as a preferred technical scheme of the invention: the absorbance threshold X is the absorbance value of the reaction endpoint obtained by the following steps:
firstly, using permanganate to digest a water sample under the acidity and temperature of national standard, and reacting to generate manganese dioxide precipitate;
step two, adding a fixed amount of sodium oxalate to reduce redundant potassium permanganate and manganese dioxide into clear liquor, recording blank photo-voltage a at the moment, titrating potassium permanganate, judging whether the solution is colorless and transparent to light red through colorimetric absorbance, recording photo-voltage b when the absorbance is increased in a mutational manner, and calculating absorbance c (c = log (a/b));
step three, injecting 500NTU turbidity liquid, repeating step one and step two to calculate absorbance c1，
The threshold value range of the absorbance threshold value X is set at c-c1In the meantime.
As a preferred technical scheme of the invention: the standard turbidity solution is not higher than 500 NTU.
As a preferred technical scheme of the invention: the standing time is reserved for 60 seconds before the start of step S2.
As a preferred technical scheme of the invention: the standing time is reserved for 60 seconds before the start of step S3.
As a preferred technical scheme of the invention: the standard turbidity solution is 500NTU turbidity solution.
It is a further object of the present invention to provide an on-line monitoring of the permanganate index of a water quality as described in the preceding paragraph.
Therefore, the above purpose of the invention is realized by the following technical scheme:
the utility model provides a permanganate index on-line monitoring system which characterized in that: the device comprises a peristaltic pump, a quantitative ring, a multi-channel connecting exhaust valve, a digestion colorimetric pool and a titration pump, wherein each channel of the multi-channel connecting exhaust valve is respectively communicated with a water sample, pure water, a standard sample, a sodium oxalate solution, an acid solution and a sodium hydroxide solution, the water sample, the sodium oxalate solution, the acid solution and the sodium hydroxide solution are respectively extracted through the peristaltic pump, and are quantitatively conveyed to the digestion colorimetric pool through the quantitative ring for detection, and the titration pump is communicated with a permanganate solution and titrates the permanganate solution to the digestion colorimetric pool.
While adopting the technical scheme, the invention can also adopt or combine the following technical scheme:
as a preferred technical scheme of the invention: a cleaning system is arranged, a peristaltic pump is used for pumping a water sample to clean the multi-channel connecting discharge valve and the quantitative ring, and cleaning waste liquid is discharged through inversion;
a titration pump extracts a titration solution to clean the connected pipeline;
the peristaltic pump reversely pumps pure water to perform flow path rinsing on the digestion colorimetric pool, and the pure water is discharged through a cleaning waste liquid/air outlet arranged at the other end of the digestion colorimetric pool.
As a preferred technical scheme of the invention: the multi-channel connecting discharge valve is provided with a cleaning waste liquid discharge port and a reaction waste liquid discharge port for discharging waste.
Compared with the prior art, the technical scheme of the invention has the following advantages:
the water quality permanganate index on-line monitoring method and system are formed by modifying a conventional titration colorimetric method, the defect of the conventional titration colorimetric method that the turbidity of a water sample is greatly interfered is optimized, the influence of the turbidity of the water sample is solved, and the advantages of low reagent consumption, low cost and simplicity in maintenance of the titration colorimetric method are kept. According to the water quality permanganate index online monitoring method and system, influences of turbidity are removed by standing for a long time (1-5 min) at the beginning and the end of titration, the setting of the absorbance threshold at the end of titration is adopted to adapt to different turbidity conditions of water samples, the threshold is improved to skip fluctuation influences of high turbidity, and the influence of absorbance fluctuation caused by turbidity in the process of titration and uniform mixing is weakened as much as possible by standing for a long time at the beginning and the end of titration. Compared with the conventional titration colorimetric method, the method only needs 2-4min more time, has controllable flow, meets the anti-turbidity requirement of the conventional water quality permanganate index on-line monitoring, and realizes the on-line monitoring requirements of high precision, low cost and low time consumption of the water quality permanganate index.
Drawings
FIG. 1 is a schematic diagram of a system according to the present invention;
in the figure: peristaltic pump 1, quantitative ring 2, multichannel connecting exhaust valve 3, washing waste liquid outlet 301, reaction waste liquid outlet 302, digestion colorimetric pool 4, titration pump 5, washing waste liquid/air outlet 6.
Detailed Description
The invention is described in further detail with reference to the figures and specific embodiments.
As shown in figure 1, the water quality permanganate index on-line monitoring method disclosed by the invention comprises the following steps of:
s1, setting an absorbance threshold value X of the water sample;
s2, carrying out digestion reaction on a water sample by permanganate at the acidity and temperature of the water sample under the national standard to generate manganese dioxide precipitate, adding excessive sodium oxalate at the moment, and recording the blank photovoltage A at the moment;
s3, adding excessive sodium oxalate to reduce redundant potassium permanganate and manganese dioxide into clear liquid, adding potassium permanganate solution through a burette, bubbling, uniformly mixing, standing until the photovoltage is stable, judging the solution from colorless transparency to light red through colorimetry absorbance, and recording the photovoltage B when the absorbance is increased in a mutation manner1The absorbance C = log (A/B) was calculated1)；
S4, comparing the absorbance C with a set absorbance threshold X, if C is less than X, using a standard turbidity solution, and circulating step S3 to record the photovoltage B1Then calculate its C1=log(A/B1) Mixing C with1Compared with X, if C1If < X, continuing to loop to step S3;
s5, when the step S3 is repeated, if the photo voltage Bn is recorded, the absorbance C is calculatedn，Cn=log(A/Bn) If C isnIf the number of drops is more than or equal to X, stopping the circulation, recording the complete cycle number, recording the cycle number as the number n of titration drops,
s6, determining the single-titration absorbance value, marked as D, wherein D = (C)n-C)/n, wherein CnStep C is the absorbance value before the circulation of the step S3 is finished, step C is the absorbance value calculated after the standard water sample for calibration passes through the steps S1 to S5 and the titration is finished;
s7, calculating the final absorbance, and recording the final absorbance as CComputing，CComputing=n*D-Cn，
Wherein the absorbance threshold value X is the threshold range of the absorbance value of the reaction endpoint.
The absorbance threshold value X is an absorbance value range of a reaction endpoint obtained by an experiment, which is obtained by:
firstly, a water sample is digested by permanganate at the acidity and temperature of national standards, manganese dioxide precipitate is generated after reaction, and the reaction quantity of potassium permanganate cannot be measured by direct colorimetry, so that a fixed quantity of sodium oxalate needs to be added to reduce redundant potassium permanganate and manganese dioxide into a clear solution, at the moment, the sodium oxalate is excessive, and the blank photovoltage a is recorded as 3000 k.
Step two, in this embodiment, the higher the CODMn content in water is, the more the residual amount of sodium oxalate after the reaction is, the higher the titration amount of potassium permanganate is required, the titration amount represents the CODMn content in water, the colorimetric method absorbance can be used to judge through colorless transparency to light red in the solution, when the absorbance is increased in a mutation manner, that is, the light intensity mutation is reduced, it is indicated that the solution system titrates the endpoint from colorless transparency to light red in potassium permanganate, the recording light voltage b is 2730k, and the absorbance c (c = log (a/b) = 0.04) is calculated.
Step three, using a turbidity solution of 500NTU, and adopting the same method, absorbance c1, c1=0.2 was obtained.
If the turbidity of the standard turbidity liquid is higher and the absorbance threshold exceeds 0.2, more potassium permanganate is added, and the time of the whole machine is longer than 1 hour at the moment and does not meet the requirement of online monitoring time, so that the standard turbidity liquid is selected and limited to a certain extent to be suitable for an online monitoring system, and the absorbance threshold range in the embodiment is set to be 0.04-0.2 and can resist the turbidity.
The absorbance threshold value X in step S1 is determined.
S2, blank detection, namely carrying out digestion reaction on a water sample by permanganate at the acidity and temperature under national standards to generate manganese dioxide precipitate, adding excessive sodium oxalate at the moment, and recording blank photovoltage A at the moment;
s3, color development detection, adding excessive sodium oxalate to reduce redundant potassium permanganate and manganese dioxide into clear liquid, adding potassium permanganate solution through a burette, bubbling for 8S, mixing uniformly, standing for 5S to stabilize photoelectric voltage, judging the solution from colorless transparency to light red through colorimetric method absorbance, and when the absorbance has mutation, reducing the excessive potassium permanganate and manganese dioxide into clear liquidThe property is increased, and the photovoltage B at that time is recorded1The logarithm of the ratio of blank detection to color detection is absorbance, and the absorbance C = log (A/B) is calculated1)；
S4, comparing the absorbance C with a set absorbance threshold X, if C is less than X, using a standard turbidity solution, and circulating step S3 to record the photovoltage B1Then calculate its C1=log(A/B1) Mixing C with1Compared with X, if C1If < X, continuing to loop to step S3;
in the invention, absorbance record calculation in the titration process is used as a judgment value and is not included in the final absorbance calculation, a group of circulation command hooks is taken as an example, and S3 is used as a circulation logic command of the titration pump.
In the invention, 60s of standing time is reserved before blank detection and color development detection to remove turbidity interference, and the numerical value of X is adjusted to serve as an anti-interference function, wherein the larger the X threshold value is, the stronger the anti-turbidity interference is.
Compared with the conventional titration colorimetric method, the titration end point judgment method has the difference that in the end point judgment, the absorbance value obtained by calculating S3 of the titration end point in the conventional method is the absorbance value which is obtained after 8S of mixing operation, the stable photovoltage of 5S of standing is used as the end point judgment value, the value is seriously interfered by turbidity, the judgment light intensity range is small, the end point is abnormally stopped when the end point is not tested, the result shows that the readout is abnormally low, the judgment light intensity range is large, and the readout is also low. And then the absorbance threshold value of the 1634 final circulation step is set to be increased, but the absorbance threshold value is not used as the basis of the absorbance of the end point, but is used as the basis of the titration end point judgment, after the endpoint light intensity is stabilized, the endpoint light intensity is recorded, and the absorbance of the endpoint is calculated. The result here is an improved resistance to turbidity disturbances.
The method separates the absorbance judgment of stopping titration from the calculation of the terminal absorbance, the terminal judgment improves the anti-turbidity interference capability by increasing the dropping number, but the reading of the terminal absorbance ensures the stability by standing for a long time, and the method effectively solves the balance problem of the anti-interference capability and the testing time. In the invention, the endpoint is judged by the absorbance instead of the amount of the number of drops, so that the calculation is smoother, the absorbance is continuous, the number of drops is discrete, and the accuracy of the number indication is higher.
As shown in figure 1, the water quality permanganate online monitoring system comprises a peristaltic pump 1, a quantitative ring 2, a multi-channel connecting exhaust valve 3, a digestion colorimetric pool 4 and a titration pump 5, wherein each channel of the multi-channel connecting exhaust valve 3 is respectively communicated with a water sample, pure water, a standard sample, a sodium oxalate solution, an acid solution and a sodium hydroxide solution, the water sample, the sodium oxalate solution, the acid solution and the sodium hydroxide solution are respectively extracted by the peristaltic pump 1, the quantitative digestion colorimetric pool 4 is quantitatively conveyed by the quantitative ring 2 for detection, and the titration pump 5 is communicated with the permanganate solution and titrates the permanganate solution to the digestion colorimetric pool 4.
The system is provided with a cleaning system, a peristaltic pump 1 extracts a water sample to clean a multi-channel connecting drain valve 3 and a quantitative ring 2, and cleaning waste liquid is discharged through inversion;
the titration pump 5 pumps the titration solution to clean the connected pipeline;
the peristaltic pump 1 reversely pumps pure water to perform flow path rinsing on the digestion colorimetric pool and discharges the pure water through a cleaning waste liquid/air outlet 6 arranged at the other end of the digestion colorimetric pool 4;
the multi-channel connection discharge valve 3 of the present invention is provided with a cleaning waste liquid discharge port 301 and a reaction waste liquid discharge port 302 for discharging waste.
When the water quality permanganate index on-line monitoring system is used,
cleaning is required firstly:
Then, sample injection: extracting a certain amount of water sample to a digestion colorimetric pool 4, dropwise adding a fixed-drop-number titration solution of permanganate to the digestion colorimetric pool 4, adding 1 tube of sodium hydroxide solution, digesting at 100 ℃ for 900S, cooling to 50 ℃, keeping the temperature at about 65 ℃, adding 1 tube of 30% solution, bubbling, adding 1 tube of sodium oxalate solution, gradually clearing in the digestion colorimetric pool 4, carrying out blank detection, recordingAnd (3) continuously dropwise adding a titration solution of permanganate solution in a titration pump for 4 times, judging the absorbance for 1 time every titration 1 time until the fifth judgment shows that red appears, and detecting the color development when the absorbance reaches the requirement of the set absorbance threshold value X, namely the titration end point is reached, and recording the photovoltage BnAnd emptying. The reaction liquid in the digestion tube is discharged to reaction waste liquid by the positive rotation peristaltic pump 1.
And (3) cleaning for the second time: the reverse rotation peristaltic pump 1 pumps pure water to the digestion colorimetric pool 4 for cleaning, and then the peristaltic pump 1 rotates positively to discharge the digestion tube cleaning liquid to the cleaning waste liquid port 302.
The specific implementation data of this example are as follows:
therefore, the water quality permanganate index on-line monitoring method and system provided by the invention have the advantages that the set absorbance threshold is used as a titration end point judgment condition, the water body adaptability is strong, and the anti-turbidity capacity is improved through the setting of the threshold; compared with an ORP electrode method, the method has the advantages of small reaction system, small waste liquid amount, low whole machine cost and low subsequent operation and maintenance cost.
The above-described embodiments are intended to illustrate the present invention, but not to limit the present invention, and any modifications, equivalents, improvements, etc. made within the spirit of the present invention and the scope of the claims fall within the scope of the present invention.
Claims (9)
1. A water quality permanganate index on-line monitoring method is characterized in that: the method comprises the following steps:
s1, setting an absorbance threshold value X of the water sample;
s2, carrying out digestion reaction on a water sample by permanganate at the acidity and temperature of the water sample under the national standard to generate manganese dioxide precipitate, adding excessive sodium oxalate at the moment, and recording the blank photovoltage A at the moment;
s3, adding excessive sodium oxalate to reduce redundant potassium permanganate and manganese dioxide into clear solution, adding potassium permanganate solution through a burette, bubbling and mixing uniformly, standing until the photovoltage is stable, judging the solution from colorless transparency to light red through colorimetry absorbance, and recording the photovoltage B when the absorbance is increased in a mutability manner1The absorbance C = log (A/B) was calculated1)；
S4, comparing the absorbance C with a set absorbance threshold X, if C is less than X, using a standard turbidity solution, and circulating step S3 to record the photovoltage B1Then calculate its C1=log(A/B1) And C is prepared by1Compared with X, if C1If < X, continuing to loop to step S3;
s5, when the step S3 is repeated, if the photovoltage Bn is recorded, the absorbance C is calculatedn，Cn=log(A/Bn) If C isnIf the number of drops is more than or equal to X, stopping the circulation, recording the complete cycle number, recording the cycle number as the number n of titration drops,
s6, determining the single-titration absorbance value, marked as D, wherein D = (C)n-C)/n, wherein CnStep C is the absorbance value before the circulation of the step S3 is finished, step C is the absorbance value calculated after the standard water sample for calibration passes through the steps S1 to S5 and the titration is finished;
s7, calculating the final absorbance, and recording the final absorbance as CComputing，CComputing=n*D-Cn，
Wherein the absorbance threshold value X is the threshold range of the absorbance value of the reaction end point and is set to be 0.04-0.2.
2. The water quality permanganate index on-line monitoring method according to claim 1, which is characterized in that: the absorbance threshold X is the absorbance value of the reaction endpoint obtained by the following steps:
firstly, using permanganate to digest a water sample under the acidity and temperature of national standard, and reacting to generate manganese dioxide precipitate;
step two, adding a fixed amount of sodium oxalate to reduce redundant potassium permanganate and manganese dioxide into clear liquor, recording blank photo-voltage a at the moment, titrating potassium permanganate, judging whether the solution is colorless and transparent to light red through colorimetric absorbance, recording photo-voltage b when the absorbance is increased in a mutational manner, and calculating absorbance c (c = log (a/b));
step three, injecting 500NTU turbidity liquid, repeating step one and step two to calculate absorbance c1，
The threshold value range of the absorbance threshold value X is set at c-c1In the meantime.
3. The water quality permanganate index on-line monitoring method according to claim 2, which is characterized in that: the standard turbidity solution has turbidity not higher than 500 NTU.
4. The water quality permanganate index online monitoring method according to claim 1, characterized in that: the standing time is reserved for 60 seconds before the start of step S2.
5. The method for on-line monitoring of the permanganate index in water according to claim 1, wherein the method comprises the following steps: the standing time is reserved for 60 seconds before the start of step S3.
6. The method for on-line monitoring of the permanganate index in water according to claim 1, wherein the method comprises the following steps: the standard turbidity solution is 500NTU turbidity solution.
7. An online permanganate index monitoring system using the method according to any one of claims 1 to 5, comprising: the device comprises a peristaltic pump, a quantitative ring, a multi-channel connecting exhaust valve, a digestion colorimetric pool and a titration pump, wherein each channel of the multi-channel connecting exhaust valve is respectively communicated with a water sample, pure water, a standard sample, a sodium oxalate solution, an acid solution and a sodium hydroxide solution, the water sample, the sodium oxalate solution, the acid solution and the sodium hydroxide solution are respectively extracted through the peristaltic pump, and are quantitatively conveyed to the digestion colorimetric pool through the quantitative ring for detection, and the titration pump is communicated with a permanganate solution and titrates the permanganate solution to the digestion colorimetric pool.
8. The system for on-line monitoring of permanganate index according to claim 7, wherein: a cleaning system is arranged, a peristaltic pump is used for pumping a water sample to clean the multi-channel connecting discharge valve and the quantitative ring, and cleaning waste liquid is discharged through inversion;
a titration pump extracts a titration solution to clean the connected pipeline;
the peristaltic pump reversely rotates to extract pure water to perform flow path rinsing on the digestion colorimetric pool, and the pure water is discharged through a cleaning waste liquid/air outlet arranged at the other end of the digestion colorimetric pool.
9. The permanganate index on-line monitoring system according to claim 8, wherein: the multi-channel connecting discharge valve is provided with a cleaning waste liquid discharge port and a reaction waste liquid discharge port for discharging waste.
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