Rapid permanganate index monitoring method based on forward difference method end point judgment

Abstract

The invention provides a fast permanganate index monitoring method based on forward difference method end point judgment, which comprises the following steps: adding a stock solution device on the basis of not changing the structure of a permanganate index monitor, and arranging the stock solution device on the rear side of a peristaltic pump and communicating with the peristaltic pump, storing pure water for cleaning the digestion cuvette in advance by using a stock solution device; sequentially adding a sample to be detected and a reagent into the digestion cuvette by using a peristaltic pump and a metering module; digesting in the digestion cuvette, cooling, injecting sodium oxalate into the digestion cuvette, electrifying the double platinum electrodes, and adding a potassium permanganate titrating solution into the digestion cuvette at a low speed by adopting an injection pump; in the titration process, the real-time voltage signal value of the reaction liquid in the digestion cuvette is obtained by utilizing the double platinum electrodes, and the end point is judged by adopting a forward difference method. According to the present invention, the adjustable continuous microtitration and the titration end point determination through the forward differential method are combined based on the existing permanganate index, and the permanganate index monitoring method with characteristics of short monitoring period, high accuracy, good repeatability and large measurement range is provided.

Description

Technical Field

[0001] The invention relates to the technical field of online automatic monitoring of water quality, and in particular to a rapid permanganate index monitoring method based on forward difference method endpoint judgment. Background Art

[0002] Permanganate index refers to the amount of oxygen consumed by potassium permanganate in an acidic or alkaline environment to oxidize organic matter and inorganic reducing substances in water samples, which is expressed by calculation in mg / L of oxygen. The common permanganate index monitors on the market generally use photometry or titration. Among them, the permanganate index monitor using photometry has the advantages of high sensitivity, simple structure, and fast analysis speed. However, this method is easily affected by turbidity and color environmental factors and reagent stability, and its standard curve stability is poor. This method is more suitable for low-concentration permanganate index testing, and multiple dilutions are required when testing high-concentration water samples. The permanganate index monitor using titration is further distinguished according to different titration end point determination methods: judging the titration end point by the change in absorbance in the reaction cell, judging the titration end point by the change in oxygen reduction potential of the ORP (oxidation-reduction potential) electrode, and judging the titration end point by the microcurrent caused by the change in the oxidation-reduction potential of the double platinum electrode. When using photometry to determine the titration end point, the influence of turbidity and color of the sample to be tested cannot be avoided. The use of ORP electrode titration can effectively reduce the influence of turbidity and color of the sample to be tested, but the life of the ORP electrode is generally 2-3 years. The permanganate index monitor using double platinum electrode titration can not only effectively avoid the influence of turbidity and color of the sample to be tested, but also its service life is generally more than 6 years.

[0003] With the obvious improvement of the current surface water environment, the permanganate index value of some high-quality surface water can be below 1 mg / L, requiring the permanganate index monitor to have higher and higher accuracy for low-concentration samples. The basic detection range of the manual permanganate index test of the national standard is (0.5-4.5) mg / L, and the minimum required test range of the permanganate index monitor commonly used in the market includes at least (2-20) mg / L. This requires the permanganate index monitor to ensure the accuracy of low-concentration sample testing and meet the measurement range. To meet this demand, the permanganate index monitor usually adopts a range switching function, that is, the range of the first range set to (0.5-10) mg / L without dilution of the sample, and the range of the second range of the sample diluted twice is (10-20) mg / L, thereby ensuring the test range of the permanganate index monitor by using the range switching function. However, there are certain disadvantages in practical applications of range switching, such as high requirements for instrument functions for range switching, and range switching will also increase the test cycle of the monitor during switching.

[0004] Currently, the titration method of permanganate index automatic monitoring instruments occupy a large proportion in the market. However, they also have the problems of large consumption of water samples and reagents, relatively large volume of digestion tank, and large amount of cleaning liquid. If the cleaning liquid is taken by a metering device during the test, it needs to be repeatedly extracted many times to meet the cleaning liquid usage, resulting in a long monitoring cycle.

[0005] The permanganate index titration reaction process is a relatively complex chemical reaction. For example, in the determination of the titration endpoint by the double platinum electrode titration method, there are many factors affecting the change of the voltage value in the titration reaction system. The fluctuation of the voltage value and the appearance of abnormal values ​​directly affect the accuracy and stability of the determination of the titration endpoint. Therefore, the abnormal fluctuation of the voltage value in the actual titration process ultimately affects the accuracy and stability of the measured value by affecting the determination of the titration endpoint.

[0006] The invention patent with application number 202210505428.6 discloses a titration endpoint analysis method and system based on an automatic permanganate index analyzer, and the method includes: S1, titration starts; S2, in the first titration stage, the potential of the digestion solution after each drop of potassium permanganate is collected during the titration process, and it is determined whether it is greater than the potential threshold; if so, go to step S3; S3, reduce the amount of each drop of potassium permanganate during the titration process, and enter the potassium permanganate titration in the second titration stage; S4, in the second titration stage, the potential of the digestion solution after each drop of potassium permanganate is collected during the titration process, and a dynamic titration curve corresponding to the potential of the digestion solution and the response time of the electrode, the titration reaction temperature, the stirring speed of the digestion solution, the volume of the reducing agent, and the drop volume of potassium permanganate after the start of the titration is established; S5, judging the titration endpoint based on the dynamic titration curve. The dynamic titration curve established by the above invention takes into account the influence of titration environmental factors, and further improves the judgment accuracy of the titration endpoint. The titration method in this invention uses an ORP electrode to measure the potential of the digestion solution, and the titration endpoint judgment module implemented therein judges the point with the largest slope based on the dynamic titration curve, which is the titration endpoint. However, this invention does not elaborate on the way in which the ORP electrode obtains the potential of the digestion solution during the titration process. The potential is continuously obtained in real time. Since the real-time potential is affected by factors such as the titration speed, titration reaction temperature, stirring speed, and titration process, the real-time potential is prone to fluctuations, thereby affecting the accuracy of the endpoint judgment; if the potential value is obtained after completing a unit titration and the voltage is stable, the titration time of the height test sample will be significantly increased. Summary of the invention

[0007] Aiming at the technical problem that the voltage value in the existing permanganate index titration method is easy to change, which affects the accuracy and stability of the titration endpoint judgment, the present invention proposes a rapid permanganate index monitoring method based on the forward difference method endpoint judgment. In a permanganate index monitor using a double platinum electrode method, a titration method for accurate forward difference endpoint judgment is provided. On the basis of adjustable continuous microtitration, the forward difference method is used to reduce the influence of accidental voltage fluctuations, thereby improving the stability and accuracy of the titration endpoint judgment.

[0008] In order to achieve the above object, the technical solution of the present invention is implemented as follows: a rapid permanganate index monitoring method based on forward difference method endpoint judgment comprises the following steps:

[0009] A reserve liquid device is added on the basis of not changing the structure of the permanganate index monitor, the reserve liquid device is arranged at the rear side of the peristaltic pump and is connected to the peristaltic pump, and the reserve liquid device is used to reserve pure water for cleaning the digestion tank in advance;

[0010] Use a peristaltic pump and a metering module to sequentially add samples and reagents to the digestion tank;

[0011] After digestion in the digestion tank, the temperature is lowered and sodium oxalate is injected into the digestion tank, the double platinum electrode is powered on, and potassium permanganate titration solution is added to the digestion tank at a low speed using a syringe pump;

[0012] During the titration process, a double platinum electrode was used to obtain the real-time voltage signal value of the reaction solution in the digestion cell, and the forward difference method was used to determine the endpoint.

[0013] Preferably, the bottom of the reserve liquid device is connected to the front side of the peristaltic pump, and the top of the reserve liquid device is connected to the air; by controlling the peristaltic pump and the metering device to reserve pure water for cleaning in advance in the reserve liquid device, during the cleaning process of the digestion tank, the drainage pump is controlled to discharge the liquid synchronously with other monitoring steps.

[0014] Preferably, when the reserve liquid device reserves pure water for cleaning, the liquid level sensor that the pure water reaches the metering module is used as a signal. When the metering module has a liquid signal, the timing starts, and the volume of liquid extracted from the reserve liquid device is measured by the peristaltic pump extraction time. When the peristaltic pump extraction time reaches the set time, the extraction stops.

[0015] Preferably, before adding the sample to be tested and the reagent into the digestion tank, the liquid remaining in the digestion tank is emptied by the liquid discharge pump, and the process of drawing enough pure water for cleaning the digestion tank once into the reserve liquid device by the peristaltic pump and the metering module is carried out simultaneously;

[0016] After the liquid retained in the digestion tank is emptied by the liquid discharge pump, the pure water in the reserve liquid device is discharged into the digestion tank through the metering module by the peristaltic pump as the cleaning liquid; while the cleaning liquid is stirred by the stirring motor and the liquid discharge pump is discharged from the digestion tank, the peristaltic pump is started to extract a quantitative sample to be tested into the metering module to rinse the metering tube and then extract a quantitative sample to be tested into the metering module.

[0017] During the heating and digestion process, the following are carried out simultaneously: using a peristaltic pump to extract pure water to clean the metering tube and discharge the cleaning liquid, extracting sodium oxalate to rinse the metering tube, and then extracting a certain amount of sodium oxalate to reserve in the metering tube;

[0018] While adding potassium permanganate titration solution to the digestion tank at a low speed using a syringe pump, the following is carried out simultaneously: pure water sufficient for cleaning the digestion tank once is again drawn from the reserve liquid device through the peristaltic pump and the metering module;

[0019] After the endpoint judgment is completed and the potassium permanganate index value of the sample to be tested is calculated, the reaction liquid in the digestion tank is discharged by the drainage pump, and then the pure water in the reserve liquid device is discharged into the digestion tank by the peristaltic pump. In the process of cleaning the digestion tank and discharging the cleaning liquid, the peristaltic pump and the metering module are used to draw enough pure water from the reserve liquid device to clean the digestion tank once again, waiting for the next instruction.

[0020] Preferably, the digestion method in the digestion tank is to heat the mixed solution in the digestion tank to 95° C. and digest for 15-20 minutes. After the digestion is completed, the temperature of the digestion tank is cooled to 80° C., and a peristaltic pump is used to discharge a quantitative amount of sodium oxalate into the digestion tank;

[0021] The reagents are sulfuric acid and potassium permanganate; the sample to be tested is the standard solution to be tested or a water sample of the monitor.

[0022] Preferably, an adjustable continuous microtitration is achieved by adjusting the speed of continuously discharging potassium permanganate titration solution using a syringe pump. The titration speed during the titration of the adjustable continuous microtitration is synchronously adjusted according to the titration time and the real-time voltage signal value: when the cumulative titration time is greater than the initial titration time, the real-time voltage value measured by the double platinum electrode is less than the preset voltage threshold for at least 5 consecutive times, and the titration speed of the syringe pump is switched to fast titration; when the real-time voltage value measured by the double platinum electrode is greater than or equal to the preset voltage threshold for at least 5 consecutive times, the titration speed of the syringe pump is switched to low-speed titration, and the forward difference method is started to determine the titration end point.

[0023] Preferably, the initial titration time t0 is in the range of 15s≤t0≤30s; the minimum interval time t2 for obtaining the real-time voltage value twice is min The range is: 0.5s≤t min ≤1s;

[0024] At the beginning of titration, the titration speed of slow titration is α; if the real-time voltage values ​​for at least 5 consecutive times are all less than the preset voltage threshold A and the cumulative titration time t>t0, the titration speed is automatically adjusted to a fast titration of β for titration; if the real-time voltage values ​​for at least 5 consecutive times are all greater than or equal to the preset voltage threshold A, the titration speed of slow titration near the end point is γ; the titration speeds α, β, and γ are fixed values ​​and are related to the concentration of potassium permanganate reagent for titration, the sample injection volume, the digestion reagent injection volume, and the stirring speed.

[0025] Preferably, the method of using the forward difference method to judge the endpoint is: performing multiple forward differences on the real-time voltage value, and when the results of multiple consecutive forward differences are less than a fixed value, the titration ends;

[0026] At the end of the titration, the current cumulative titration volume is recorded, the theoretical endpoint titration volume is calculated, and the permanganate index value of the sample to be tested is calculated based on the theoretical endpoint titration volume.

[0027] Preferably, the method of multiple forward differences is:

[0028] Every minimum interval time t min Get a real-time voltage value v x , the voltage difference between the current real-time voltage value and the previous real-time voltage value Δf(x) = v x -v x-1 ;

[0029] The voltage difference at the current time point and the average of the voltage differences of the previous four times are:

[0030]

[0031] The difference between the average value Δμ(x) at the current time point and the previous average value Δμ(x-1) is:

[0032] δ(x)=Δμ(x)-Δμ(x-1);

[0033] If the values ​​of the difference δ(x) for multiple consecutive times are all less than or equal to the fixed value κ, where κ is a fixed value, it is determined that the titration has reached the titration endpoint, the injection pump stops working, and the titration is completed.

[0034] Preferably, the method for calculating the permanganate index value of the sample to be tested is: after the titration is completed, the volume of potassium permanganate solution discharged by the injection pump from the start of the titration is read to obtain the current real-time cumulative titration volume V x , the theoretical titration endpoint volume is calculated by real-time cumulative titration:

[0035] V EPV =V x -aV min ;

[0036] Where a is a constant, V min is the minimum time interval t min Internal drop volume;

[0037] The calculated permanganate index value of the sample to be tested is: y = kV EPV ; Where k is a constant;

[0038] According to the minimum interval time t after the theoretical titration point min The minimum reduction amount B of the real-time voltage value is set within the range of -B≤κ≤0.

[0039] Compared with the prior art, the invention has the following beneficial effects: the invention provides a novel rapid monitoring method that can shorten the monitoring period of the permanganate index, adds a simple reserve liquid device to the monitoring device, and controls different unit components to act simultaneously through the monitor, and the synchronization of different components greatly shortens the monitoring period of the permanganate index monitor while ensuring the stability and accuracy of the test data of the permanganate index monitor. The invention shortens the monitoring period to 30 minutes in a simpler way, which plays an important role in the practical application of emergency monitoring.

[0040] The present invention adopts a double platinum electrode titration method on the basis of adding a reserve solution device to reduce the monitoring cycle. It mainly involves upgrading the titration method and endpoint determination method based on the traditional titration system, and adopts adjustable continuous microtitration and titration endpoint determination to accurately determine the titration endpoint value using multiple forward difference methods, thereby increasing the stability and accuracy of the titration endpoint. At the same time, the present invention fully fits the slow-fast-slow trend of the titration reaction during the titration process, and adopts multiple forward difference methods and excessive titration to determine the titration endpoint, eliminating the influence of abnormal fluctuations in real-time voltage values ​​on the titration endpoint judgment, ensuring that the monitor can guarantee accuracy and stability for low-concentration and high-concentration sample tests, and the lowest concentration that can be accurately monitored can be lower than 0.5 mg / L, and the upper limit of the monitoring range of 20 mg / L can be met without switching the range.

[0041] The present invention uses a double platinum electrode to continuously obtain the voltage signal value of the reaction solution during the titration process in real time, and proposes a rapid permanganate index monitoring method based on the forward difference method endpoint judgment, which effectively avoids titration misjudgment caused by abnormal fluctuations in voltage values. On the basis of adjustable continuous microtitration, the voltage value obtained in real time is processed by multiple forward difference methods to obtain a dynamic titration curve of titration time and voltage value. The endpoint judgment of the present invention not only ensures the accuracy and stability of the titration endpoint judgment, improves the accuracy of low-concentration samples, but also can effectively reduce the titration time and shorten the measurement time difference between low-concentration and high-concentration samples. The present invention combines adjustable continuous microtitration with the forward difference method to judge the titration endpoint, and provides a permanganate index monitoring method of a double platinum electrode titration method with a short monitoring period, high accuracy, good repeatability, and a large measurement range. BRIEF DESCRIPTION OF THE DRAWINGS

[0042] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0043] Figure 1 It is a flow chart of the present invention.

[0044] Figure 2 This is a flow diagram of the permanganate index monitor used in the present invention.

[0045] Figure 3 It is a titration curve diagram of the signal value of samples with different concentrations changing with time in the present invention. DETAILED DESCRIPTION

[0046] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. 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.

[0047] like Figure 1 As shown, a rapid permanganate index monitoring method based on forward difference method endpoint judgment, the steps are as follows:

[0048] Step 1: Connect a reserve liquid device to the rear side of the peristaltic pump of the permanganate index monitor, drain the liquid remaining in the digestion tank through the drainage pump, and at the same time, the peristaltic pump and the metering module extract pure water sufficient for cleaning the digestion tank once from the reserve liquid device. The above two processes can be carried out simultaneously.

[0049] The present invention adds a storage liquid device 2 after the peristaltic pump 1, such as Figure 2 The permanganate index monitor flow diagram is shown in the figure. Figure 2 In the flow diagram, except for the reserve liquid device 2 added after the peristaltic pump 1, the metering module 3 is installed in front of the peristaltic pump 1, and the metering module 3 is directly connected to the peristaltic pump 1. The overall structure of the monitor has not changed much from the traditional monitor. Here, the reserve liquid device is mainly described, and other devices are not described in detail. The upper part of the reserve liquid device 2 is connected to the air to ensure the pressure balance of the peristaltic pump 1 during the metering extraction process, and at the same time plays an overflow role when the reserve liquid device 2 extracts the cleaning liquid. The reserve liquid device 2 is a general glass device that can store pure water, and the reserve liquid device does not need to be equipped with metering devices such as liquid level sensors. Start the peristaltic pump 1, extract enough pure water for one cleaning of the digestion tank, and store it in the reserve liquid device 2. When the reserve liquid device 2 reserves pure water for cleaning, the liquid level sensor that the pure water reaches the metering module 3 is used as a signal. When the metering module 3 has a liquid signal, the timing starts, and the volume of liquid extracted from the reserve liquid device 2 is measured by the extraction time of the peristaltic pump 1, which can save the use of sensors and ensure that an appropriate amount of pure water can be extracted from the reserve liquid. When the extraction time of the peristaltic pump 1 reaches the set time, the extraction stops, and it is left to stand and wait for the signal that the pure water is discharged to the digestion tank 4.

[0050] The present invention starts the stirring motor in the digestion tank 4 and the drainage pump 5 connected to the digestion tank 4 to discharge the retained liquid in the digestion tank 4. The stirring motor 4 can continuously stir the liquid in the digestion tank through a magnetic stirrer. The drainage pump 5 is used to discharge the cleaning liquid and the reaction liquid in the digestion tank 4.

[0051] After the reserve liquid device 2 in the present invention is installed on the peristaltic pump 1, it can effectively avoid the problem of inaccurate metering of the metering module caused by the excessive space between the peristaltic pump and the metering module 3 and the pressure imbalance of the metering module 3 during the discharge valve switching process. If the reserve liquid device 2 is installed between the peristaltic pump 1 and the metering module 3, when the peristaltic pump 1 extracts the liquid into the metering module 3, it is necessary to first compress the space volume between the peristaltic pump 1 and the liquid to be extracted, and the reserve liquid device 2 due to its relatively large space volume will significantly increase its pressure during the extraction process of the peristaltic pump 1, and the excessive pressure will directly affect the precise metering of the metering module 3 in the incoming liquid. After placing the reserve liquid device 2 on the peristaltic pump 1, the problem of excessive pressure between the peristaltic pump 1 and the metering device can be avoided. The present invention realizes rapid extraction and discharge of cleaning liquid on the basis of adding the reserve liquid device 2, and other monitoring steps can be carried out simultaneously, thereby reducing the monitoring cycle. During the process of accurate metering of the samples and reagents by the metering module 3, the reserve liquid device 2 is in an empty state, the upper end of the reserve liquid is connected to the air port, and the pressure during the pumping process of the peristaltic pump 1 does not change much compared with the state without the reserve liquid device 2 installed.

[0052] Step 2: Use a peristaltic pump to discharge the pure water in the reserve liquid device as a cleaning liquid into the digestion tank through the metering module to clean the digestion tank and then discharge it; at the same time, use the sample to be tested to rinse the metering tube, then extract a certain amount of the sample to be tested from the metering module, and discharge the sample to be tested in the metering module into the digestion tank; then inject a certain amount of sulfuric acid and potassium permanganate into the digestion tank in turn. During the cleaning process of the digestion tank, the metering tube can be rinsed and a certain amount of the sample to be tested can be extracted into the metering module at the same time.

[0053] Start the peristaltic pump 1, and discharge all the pure water in the reserve liquid device 2 into the digestion tank through the metering module 3 and the digestion tank inlet, start the stirring motor, and clean the digestion tank 4. An overflow port is provided on the digestion tank 4 to ensure that when an abnormality occurs during the monitoring process, the excess liquid in the digestion tank 4 can be discharged to the waste liquid collection point through the overflow port. After cleaning for 15 seconds, start the drainage pump 5 to discharge the cleaning liquid in the digestion tank 4. While the drainage pump 5 is started, start the peristaltic pump 1, extract the sample to be tested into the metering module, quantitatively extract through the liquid level sensor, rinse the metering tube, remove the memory effect to ensure the metering accuracy, start the peristaltic pump 1 again, extract the quantitative sample to be tested into the metering module, start the peristaltic pump 1, and discharge the sample to be tested in the metering module 3 into the digestion tank 4. Start the peristaltic pump, and inject quantitative sulfuric acid and potassium permanganate into the digestion tank 4 in sequence, and wait for heating and digestion. The sample to be tested mentioned in the present invention is a standard liquid or water sample to be tested by the monitor.

[0054] Step 3: Heat and digest the mixed solution in the digestion tank; at the same time, use the metering module to extract a certain amount of sodium oxalate and discharge it into the digestion tank cooled to 80°C; again, use the peristaltic pump and the metering module to extract enough pure water to clean the digestion tank once into the reserve liquid device.

[0055] Heat the digestion tank to 95℃ for 15min, keep stirring during the digestion process, and perform oxidation reaction on the sample to be tested. At the same time, start the peristaltic pump, draw pure water to clean the metering tube, and discharge the cleaning liquid, then draw sodium oxalate to rinse the metering tube of the metering module; start the peristaltic pump, draw quantitative sodium oxalate and reserve it in the metering module. After the digestion is completed, cool down to 80℃, start the peristaltic pump, and discharge quantitative sodium oxalate into the digestion tank 4 to reduce all the remaining potassium permanganate in the digestion tank 4. At this time, the liquid level of the reaction liquid in the digestion tank just covers the mouth of the burette.

[0056] Step 4: The double platinum electrode 6 on the digestion cell 4 is powered on, and the double platinum electrode obtains the voltage value in the digestion cell 4 in real time, and simultaneously starts the titration endpoint determination.

[0057] The burette is fixed to the upper cover of the digestion tank 4. The mouth of the burette is just immersed in the reaction liquid in the digestion tank at the beginning of the titration. The injection pump 7 connected to the potassium permanganate solution for titration begins to discharge potassium permanganate into the digestion tank through the burette at a low speed. The stirring motor continues to stir during the titration process. During the power-on process of the double platinum electrode 6 on the digestion tank 4, the peristaltic pump can be started to extract pure water sufficient for cleaning the digestion tank once and store it in the reserve solution device.

[0058] The present invention is based on the reaction speed in the permanganate index titration process, and conforms to the "slow-fast-slow" titration reaction speed during the titration process. The syringe pump 7 is used to adjust the speed of continuous discharge of potassium permanganate to realize an adjustable continuous microtitration method. The titration reaction of the permanganate index monitor is a process in which the oxide potassium permanganate is used to oxidize the reducing substance sodium oxalate. Formula (1) is the ionic reaction equation of sodium oxalate and potassium permanganate. The permanganate index value of the sample to be tested is calculated by the total volume of potassium permanganate dripped in. At the beginning of the titration reaction, the reaction speed of potassium permanganate and sodium oxalate is very slow. As the reaction proceeds, the Mn generated by the reaction 2+ When it is used as a catalyst, the titration reaction gradually accelerates. As the reaction speed increases, the titration speed can be appropriately increased. Before the reaction reaches the titration end point, in order to ensure the accuracy of the titration end point judgment, the titration speed can be reduced.

[0059] 2MnO4 2- +5C2O4 2- +16H + →2Mn 2+ +10CO2+8H2O (1)

[0060] Step 5: When the cumulative titration time is greater than the initial titration duration, the real-time voltage value measured by the double platinum electrode is less than the preset voltage threshold for at least 5 consecutive times, and the titration speed of the injection pump is switched to fast titration; when the real-time voltage value measured by the double platinum electrode is greater than or equal to the preset voltage threshold for at least 5 consecutive times, the titration speed of the injection pump is switched to low-speed titration, and the forward difference method is started to determine the titration end point in step 6. The initial titration duration ranges from 15s≤t0≤30s, which can ensure the accuracy of the test of low-concentration samples to be tested, and can be switched to fast titration in time to shorten the titration reaction time.

[0061] The present invention is based on adjustable continuous microtitration, and the titration endpoint is determined by using a method of multiple forward difference methods to accurately determine the titration endpoint value to automatically monitor the permanganate index value in water. Continuous microtitration means that the titration reagent is not dripping during the titration process, and the height of the burette mouth is just immersed in the reaction liquid in the digestion tank at the beginning of the titration. The uniform speed of the injection pump at different stages allows the titration reagent at the burette mouth to diffuse into the reaction liquid in the digestion tank and react. The titration process can be referred to Figure 1Flow chart, at the beginning of titration, the syringe pump first performs micro titration at a low speed for a time t0 (15s≤t0≤30s), and at the beginning of titration, the adaptive titration speed function is started, wherein the titration speed is adjusted every certain time t min Get a real-time voltage value, time t min is a fixed value, t min The general setting range is: 0.5s≤t min ≤1s.

[0062] The titration speed of continuous microtitration during the titration process is adjusted synchronously according to the titration time and the real-time voltage signal value: the real-time voltage signal value v during the titration process is adjusted synchronously according to the titration time and the real-time voltage signal value v x The titration speed is determined by the total titration time t. At the beginning of titration, slow titration is performed and the titration speed is α. If the real-time voltage signal value v x 、v x-1 、v x-2 、v x-3 、v x-4 When both are < A, the reaction liquid in the digestion tank reacts at a low speed. A can be set as a fixed value through the human-computer exchange screen input. The total titration time is t≤t0. The titration speed is currently maintained at a low-speed titration of the titration speed α. If the real-time voltage signal value v x 、v x-1 、v x-2 、v x-3 、v x-4 When both are < A, and the total titration time t> t0, the titration speed is automatically adjusted to a fast titration with a titration speed of β for titration; if the real-time voltage signal value v x 、v x-1 、v x-2 、v x-3 、v x-4 When all are ≥A, the titration speed is switched to slow titration γ near the endpoint. Among them, the titration speeds α, β, and γ are related to the concentration of potassium permanganate reagent for titration, the sample injection volume, the digestion reagent injection volume, and the stirring speed. The relevant influencing factors of the permanganate index monitor in the present invention are fixed values ​​during the monitoring process. Therefore, the titration speeds α, β, and γ are fixed values ​​in the program control of the permanganate index monitor. Figure 3 The relationship between titration time and voltage signal value during the titration of samples with different concentrations is shown in the figure. Figure 3It can be seen that the voltage values ​​of the theoretical titration endpoint of samples with different concentrations have slight differences but are basically the same, and the voltage signal value of the actual titration endpoint increases with the increase of sample concentration (the smaller the voltage signal value decreases), which is consistent with the determination method of the titration endpoint, that is, the difference between the titration volume of the actual titration endpoint and the titration volume of the theoretical endpoint is basically the same, and as the concentration of the sample to be tested increases, the addition of an equal amount of potassium permanganate after the titration endpoint has a greater impact on the voltage signal value of the reaction solution in the digestion cell within a certain period of time; Figure 3 This indicates that the voltage signal value of the low-concentration sample in the titration method of the present invention changes smoothly over time, and as the sample concentration increases, the curve becomes smoother, which plays an important role in improving the accuracy and stability of endpoint determination. Figure 3 It can be seen that the voltage signal value of the low-concentration sample is higher than the A value before time t0, that is, the titration speed is converted to γ, and there is no large difference between the titration time of the low concentration and the titration time of the high concentration. The titration method of the invention has a positive effect on shortening the monitoring period of the permanganate index monitor.

[0063] Among them, ν x is the real-time voltage value of the xth titration point; A is a constant value; α is the slow titration speed at the beginning of the titration; β is the fast titration speed during the titration process; γ is the slow titration speed near the titration endpoint. t is the total titration time; t0 is the initial titration time, i.e., the slow titration time that can be set at the beginning of the titration, which is a constant. t min The minimum interval between two voltage values.

[0064] The variation trends of the titration reaction time and the titration signal value during the titration of samples with different concentrations of the present invention are as follows: Figure 3 As shown. Figure 3 The titration curve of different concentrations changing with time during the titration process shows that the titration reaction time of different concentrations is not much different, and the fluctuation of the real-time signal value is small during the continuous microtitration process. The abnormal fluctuation value can be eliminated by the differential method, thereby improving the accuracy and stability of the endpoint titration judgment.

[0065] Step 6: The double platinum electrode collects the voltage value of the solution in the digestion tank in real time, and performs multiple forward differentials on the voltage value. When the results of multiple consecutive forward differentials are less than the fixed empirical value, the titration ends, and the current cumulative titration volume is recorded, the theoretical endpoint titration volume is calculated, and the permanganate index value of the sample to be tested is calculated.

[0066] The present invention immediately starts the titration endpoint determination function when the titration process reaches a slow titration speed near the titration endpoint during the titration process. The titration endpoint determination of the present invention adopts the forward difference method, and the titration endpoint determination is performed in real time according to multiple forward difference methods during the titration process. The determination process is as follows:

[0067] The real-time voltage value is v x , where every certain time t min , get the real-time voltage value, real-time voltage value ν x The corresponding current cumulative titration volume is V x .

[0068] Δf(x) is the difference between the current voltage value and the previous voltage value (the time difference between the two voltage values ​​is t min ),and

[0069] Δf(x)=v x -v x-1 (2)

[0070] Δμ(x) is the voltage difference at the current time point and the average of the voltage differences of the previous four times, and

[0071]

[0072] δ(x) is the difference between the current Δμ(x) and the previous Δμ(x-1), and

[0073] δ(x)=Δμ(x)-Δμ(x-1) (4)

[0074] Right now:

[0075]

[0076] By min Real-time acquisition of the analog voltage signal value ν of the double platinum electrode x , and the obtained signal value ν x The current difference δ(x) is calculated by the CPU. If the values ​​of the difference δ(x) for 20 consecutive times, i.e., δ(x), δ(x-1), δ(x-2), ..., δ(x-19) are all ≤κ, where κ is a fixed value, the titration is immediately judged to have reached the titration endpoint, the injection pump stops working, and the titration is completed. The current real-time cumulative titration volume V is obtained by reading the volume of potassium permanganate solution discharged by the injection pump at the start of the titration. x According to the theoretical titration point after one t min The minimum reduction of the voltage signal value within the time is B, and the κ value range is set to -B≤κ≤0. The κ range in the present invention is -10≤κ≤0. At this time, the actual titration endpoint value has exceeded the required titration endpoint. According to the currently acquired real-time cumulative titration volume V x By accumulating a large amount of data on the endpoint of the double platinum electrode titration, the voltage value obtained in real time is a fixed time, and the titration speed near the endpoint is a fixed value. Therefore, after the overtitration, the theoretical titration endpoint volume can be calculated by real-time cumulative titration, as shown in formula (6). The final titration volume V is calculated.EPV .

[0077] V EPV =V x -aV min (6)

[0078] Among them, V EPV V is the theoretical endpoint titration volume; x is the actual titration endpoint; a is a constant (the present invention adopts the endpoint determination method of 20 forward differences, and a is set to 20); V min is the minimum time interval t min The volume added during the time.

[0079] The final titration volume V EPV Substitute into calculation formula (7) to calculate the permanganate index value y of the sample to be tested.

[0080] y=kV EPV (7)

[0081] Where k is a constant; V EPV The titration volume is the theoretical endpoint.

[0082] Step seven: Start the drainage pump to drain the reaction liquid in the digestion tank; start the peristaltic pump to drain the pure water in the reserve liquid device into the digestion tank, stir with the stirring motor and start the drainage pump to drain the reaction liquid in the digestion tank, so as to clean the digestion tank; at the same time, start the peristaltic pump to extract pure water sufficient for cleaning the digestion tank once, and store it in the reserve liquid device; start the peristaltic pump again to drain the pure water in the reserve liquid device into the digestion tank, extract the pure water cleaning liquid and clean the digestion tank and drain the cleaning liquid simultaneously, which significantly shortens the monitoring period of the permanganate index monitor.

[0083] The present invention adds a reserve liquid device to a conventional permanganate index monitor system without affecting other structural components of the monitor, and significantly reduces the monitoring cycle of the detector through the synchronous operation of multiple components in the monitor operation process, thereby realizing rapid testing of the monitor. At the same time, on the basis of using adjustable continuous microtitration, the influence of abnormal real-time voltage values ​​on the determination of the titration endpoint is reduced by multiple differential methods, thereby improving the stability and accuracy of the monitor measurement value, and realizing accurate measurement of (0.5-20) mg / L in the same range.

[0084] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A rapid permanganate index monitoring method based on forward difference endpoint judgment, characterized in that: The steps include: A reserve liquid device is added on the basis of not changing the structure of the permanganate index monitor, the reserve liquid device is arranged at the rear side of the peristaltic pump and is connected to the peristaltic pump, and the reserve liquid device is used to reserve pure water for cleaning the digestion tank in advance; Use a peristaltic pump and a metering module to sequentially add samples and reagents to the digestion tank; After digestion in the digestion tank, the temperature is lowered and sodium oxalate is injected into the digestion tank, the double platinum electrode is powered on, and potassium permanganate titration solution is added to the digestion tank at a low speed using a syringe pump; During the titration process, a double platinum electrode was used to obtain the real-time voltage signal value of the reaction solution in the digestion cell, and the forward difference method was used to determine the endpoint.

2. The rapid permanganate index monitoring method based on forward difference method endpoint judgment according to claim 1 is characterized in that: The bottom of the reserve liquid device is connected to the front side of the peristaltic pump, and the top of the reserve liquid device is connected to the air; by controlling the peristaltic pump and the metering device to reserve pure water for cleaning in advance in the reserve liquid device, during the cleaning process of the digestion tank, the drainage pump is controlled to discharge the liquid synchronously with other monitoring steps.

3. The rapid permanganate index monitoring method based on forward difference endpoint judgment according to claim 2 is characterized in that: When pure water for cleaning is stored in the reserve liquid device, the liquid level sensor at which pure water reaches the metering module is used as a signal. When the metering module receives a liquid signal, timing starts, and the volume of liquid extracted from the reserve liquid device is measured by the peristaltic pump extraction time. When the peristaltic pump extraction time reaches the set time, extraction stops.

4. The rapid permanganate index monitoring method based on forward difference method endpoint judgment according to claim 2, characterized in that: Before adding the sample and reagent to be tested into the digestion tank, the liquid remaining in the digestion tank is emptied by the liquid discharge pump, and the process of drawing enough pure water for cleaning the digestion tank once into the reserve liquid device is carried out simultaneously by the peristaltic pump and the metering module; After the liquid remaining in the digestion tank is emptied by the liquid discharge pump, the pure water in the reserve liquid device is discharged into the digestion tank through the metering module by the peristaltic pump as the cleaning liquid; While the stirring motor is stirring the cleaning liquid and the drainage pump is discharging the cleaning liquid in the digestion tank, the peristaltic pump is started to extract a quantitative sample to be tested into the metering module to rinse the metering tube and then extract a quantitative sample to be tested into the metering module. During the heating and digestion process, the following are carried out simultaneously: using a peristaltic pump to extract pure water to clean the metering tube and discharge the cleaning liquid, extracting sodium oxalate to rinse the metering tube, and then extracting a certain amount of sodium oxalate to reserve in the metering tube; While adding potassium permanganate titration solution to the digestion tank at a low speed using a syringe pump, the following is carried out simultaneously: pure water sufficient for cleaning the digestion tank once is again drawn from the reserve liquid device through the peristaltic pump and the metering module; After the endpoint judgment is completed and the potassium permanganate index value of the sample to be tested is calculated, the reaction liquid in the digestion tank is discharged by the drainage pump, and then the pure water in the reserve liquid device is discharged into the digestion tank by the peristaltic pump. In the process of cleaning the digestion tank and discharging the cleaning liquid, the peristaltic pump and the metering module are used to draw enough pure water from the reserve liquid device to clean the digestion tank once again, waiting for the next instruction.

5. The rapid permanganate index monitoring method based on forward difference endpoint judgment according to any one of claims 1 to 4, characterized in that: The method of digestion in the digestion tank is to heat the mixed solution in the digestion tank to 95°C and digest for 15-20 minutes. After the digestion is completed, the temperature of the digestion tank is cooled to 80°C, and a peristaltic pump is used to discharge a quantitative amount of sodium oxalate into the digestion tank; The reagents are sulfuric acid and potassium permanganate; the sample to be tested is the standard solution to be tested or a water sample of the monitor.

6. The rapid permanganate index monitoring method based on forward difference endpoint judgment according to claim 1, characterized in that: The adjustable continuous microtitration is realized by adjusting the speed of continuously discharging potassium permanganate titration solution by using a syringe pump. The titration speed in the titration process of the adjustable continuous microtitration is synchronously adjusted according to the titration time and the real-time voltage signal value: when the cumulative titration time is greater than the initial titration time, and the real-time voltage value measured by the double platinum electrode is less than the preset voltage threshold for at least 5 consecutive times, the titration speed of the syringe pump is switched to fast titration; When the real-time voltage value measured by the double platinum electrode is greater than or equal to the preset voltage threshold for at least 5 consecutive times, the titration speed of the syringe pump is switched to low-speed titration, and the forward difference method is started to determine the titration endpoint.

7. The rapid permanganate index monitoring method based on forward difference endpoint judgment according to claim 6, characterized in that: The initial titration duration t0 is in the range of 15s≤t0≤30s; the minimum interval time t2 for obtaining the real-time voltage value twice is min The range is: 0.5s≤t min ≤1s; At the beginning of titration, the titration speed of slow titration is α; if the real-time voltage values ​​for at least 5 consecutive times are all less than the preset voltage threshold A and the cumulative titration time t>t0, the titration speed is automatically adjusted to a fast titration of β for titration; if the real-time voltage values ​​for at least 5 consecutive times are all greater than or equal to the preset voltage threshold A, the titration speed of slow titration near the end point is γ; the titration speeds α, β, and γ are fixed values ​​and are related to the concentration of potassium permanganate reagent for titration, the sample injection volume, the digestion reagent injection volume, and the stirring speed.

8. The rapid permanganate index monitoring method based on forward difference endpoint judgment according to any one of claims 1-4, 6 or 7, characterized in that: The method of using the forward difference method to judge the endpoint is: multiple forward differences are performed on the real-time voltage value, and when the results of multiple consecutive forward differences are less than a fixed value, the titration ends; At the end of the titration, the current cumulative titration volume is recorded, the theoretical endpoint titration volume is calculated, and the permanganate index value of the sample to be tested is calculated based on the theoretical endpoint titration volume.

9. The rapid permanganate index monitoring method based on forward difference endpoint judgment according to claim 8, characterized in that: The method of multiple forward differences is: Every minimum interval time t min Get a real-time voltage value v x , the voltage difference between the current real-time voltage value and the previous real-time voltage value Δf(x) = v x -v x-1 ; The voltage difference at the current time point and the average of the voltage differences of the previous four times are: The difference between the average value Δμ(x) at the current time point and the previous average value Δμ(x-1) is: δ(x)=Δμ(x)-Δμ(x-1); If the values ​​of the difference δ(x) for multiple consecutive times are all less than or equal to the fixed value κ, where κ is a fixed value, it is determined that the titration has reached the titration endpoint, the injection pump stops working, and the titration is completed.

10. The rapid permanganate index monitoring method based on forward difference endpoint judgment according to claim 9, characterized in that: The method for calculating the permanganate index value of the sample to be tested is: after the titration is completed, the volume of potassium permanganate solution discharged by the injection pump from the start of the titration is read to obtain the current real-time cumulative titration volume V x , the theoretical titration endpoint volume is calculated by real-time cumulative titration: V EPV =V x -of min ; Where a is a constant, V min is the minimum time interval t min Internal drop volume; The calculated permanganate index value of the sample to be tested is: y = kV EPV ; Where k is a constant; According to the minimum interval time t after the theoretical titration point min The minimum reduction amount B of the real-time voltage value is set within the range of -B≤κ≤0.

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