A method for determining the titration endpoint during online monitoring of permanganate index

Abstract

The present invention proposes a method for determining the titration endpoint during the online monitoring of the permanganate index. The method comprises the following steps: 1) taking the characteristic wavelength λ1 of potassium permanganate and the turbidity correction wavelength λ2 as the measurement wavelength; 2) before the potassium permanganate titration, continuously measuring the absorbance n times and recording the absorbance as λ1A n and λ2A n , and calculate the absorbance before titration A0; 3) During the potassium permanganate titration process, measure the absorbance of the reaction system every time the titration is performed, and record the absorbance as A x =λ1A x ‑λ2A x , calculate the absorbance change value △A=A x ‑A0; When △A is greater than the fixed value C2, the titration is automatically terminated and the titration volume V is calculated; 4) The points obtained during the titration are plotted on the coordinate axis, and the points C1≤△A≤C2 are linearly fitted. When △A=0, the titration endpoint volume V is calculated. F The present invention eliminates interference, avoids the influence of random errors, and has high accuracy.

Description

Technical Field

[0001] The invention relates to the technical field of water body monitoring, in particular to a method for determining a titration end point in a permanganate index online monitoring process. Background Art

[0002] Permanganate index (COD Mn ) refers to the amount of oxygen consumed per unit of water using permanganate as an oxidant under certain conditions. It is an important indicator used to characterize the degree of water pollution by organic and inorganic oxidizable substances. The permanganate index is measured primarily using the acidic permanganate oxidation-sodium oxalate reduction method specified in the National Standard of the People's Republic of China (GB11892-89), which is currently the primary method for laboratory determination and automatic monitoring at environmental water stations. The permanganate index is closely related to the concentration and volume of the reagent, the volume of the water sample, and the titration volume of the oxidant. The titration volume of the oxidant, determined by the titration endpoint, is a key factor in the accuracy of the final measurement result.

[0003] During the measurement of the permanganate index, there is no color mutation point during the titration stage, which increases the difficulty of determining the titration endpoint. The National Standard Laboratory uses visual titration to determine the permanganate index, and the color changes to light pink as the titration endpoint; however, this determination method has problems such as complex operation, long experimental time, large human error, and inability to achieve online monitoring. The methods for online monitoring of the permanganate index in the existing technology are generally spectrophotometric colorimetry and redox potentiometric titration. The traditional spectrophotometric method is easily interfered by factors such as water color, turbidity, and particulate matter during the actual water sample measurement process. The precipitation of manganese dioxide and other precipitates produced in the intermediate state of the reaction directly affects the measurement results, and the measurement accuracy is low; the redox potentiometric titration method has the problems of high cost, aging and drift of the potential of the electrode after long-term use, and requires regular calibration, and the amount of subsequent maintenance is large. Summary of the Invention

[0004] The present invention aims to provide a titration endpoint determination method during online monitoring of the permanganate index, aiming to solve the problems of low accuracy, high cost, need for regular calibration and large amount of subsequent maintenance in the prior art titration endpoint determination method during online monitoring of the permanganate index.

[0005] In order to solve the above technical problems, the technical solution of the present invention is achieved as follows:

[0006] The present invention provides a titration endpoint determination method in a permanganate index online monitoring process, comprising the following steps:

[0007] 1) The potassium permanganate characteristic wavelength λ1 and the turbidity correction wavelength λ2 are taken as the measurement wavelengths, and the measured absorbances are recorded as λ1A and λ2A respectively;

[0008] 2) Absorbance before titration A0

[0009] After the reduction of sodium oxalate is completed, within 5-20s before the titration of potassium permanganate, continuously measure the absorbance n times, where n is a natural number, n≥5, and record the absorbance as λ1A1 and λ2A1, λ1A2 and λ2A2, ..., λ1A n and λ2A n ,

[0010] but

[0011] 3) Titration end judgment

[0012] During the potassium permanganate titration process, the absorbance of the reaction system is measured each time the titration is performed, λ1A x and λ2A x , record the absorbance as A x =λ1A x -λ2A x , x is a natural number, calculate the absorbance change value △A=A x -A0; when △A is greater than the fixed value C2, C2 is a constant, 0.04≤C2≤0.20, the titration is automatically terminated, the number of titrations is recorded as x, and the single titration volume is V d , the titration volume is recorded as V = V d ×x;

[0013] 4) Calculation of titration endpoint

[0014] With the titration volume V as the horizontal coordinate and the absorbance change value △A as the vertical coordinate, draw the coordinate axis and depict the points obtained during the titration process on the coordinate axis; perform linear fitting on the points C1≤△A≤C2, and record the fitting straight line equation as △A=aV+b. When △A=0, the titration endpoint volume V is calculated. F , where C1 is a constant, C2-C1≥0.03, 0.01≤C1≤0.10.

[0015] The present invention adopts dual wavelength setting, selects the absorbance value before potassium permanganate titration as initial absorbance value, eliminates the influence of water sample turbidity and colorimetry to a great extent, ensures the accuracy of measurement result; Determine titration end-point according to absorbance change value, the titration end-point determined by this method is close to the theoretical titration end-point, and titration end-point judgment is accurate and reliable, and measurement result has high accuracy; Titration volume is dynamically determined by the linear relationship formed by absorbance change value in titration process, selects multi-point to determine titration end-point, and avoids the influence of random error to a great extent; This determination method is simple to operate, and technological process is short, and cost is low, and determination process is fast and convenient, is a kind of new photometric titration end-point determination method applicable to permanganate index online monitoring process, realizes the accurate discrimination of permanganate index titration end-point. The present invention is in potassium permanganate titration process, because the setting of online monitoring device is different, the volume of each titration is also different, therefore, the number of titrations is also uncertain; As long as Δ A is greater than fixed value C2, titration automatically terminates.

[0016] The process of online monitoring of permanganate index consists of three stages, see the attached Figure 1 , absorbance A is the absorbance under characteristic wavelength conditions.

[0017] Stage 1: Potassium permanganate oxidation process

[0018] Add a quantitative water sample, sulfuric acid, and a potassium permanganate solution of known concentration, heat to a certain temperature t1, and keep the reaction at this temperature for a period of time; the reducing substances in the water sample are gradually oxidized by potassium permanganate, the concentration of potassium permanganate decreases continuously, and the absorbance A decreases; as the oxidation reaction continues, it gradually approaches equilibrium, and the absorbance A tends to be stable, at which point the oxidation process is complete;

[0019] Stage 2: Sodium oxalate reduction process

[0020] After the potassium permanganate oxidation reaction is completed, the reaction system is cooled to temperature t2, and a sodium oxalate solution of known concentration is added in a certain amount; the sodium oxalate solution acts as a reducing agent, and the sodium oxalate immediately reacts with the remaining potassium permanganate in the reaction system. At this time, the absorbance A decreases rapidly. After the reduction reaction is completed, the absorbance A remains stable, and the reduction process is complete.

[0021] Stage 3: Potassium Permanganate Titration Process

[0022] Potassium permanganate solution of known concentration is added dropwise to the reaction system after the sodium oxalate reduction reaction. Potassium permanganate immediately reacts with the remaining sodium oxalate in the second stage, and the absorbance A remains unchanged at this time. As the potassium permanganate solution is continuously added, the sodium oxalate reacts completely, and the point of complete reaction is the titration end point. Thereafter, as potassium permanganate is continued to be added, the absorbance A also continues to increase, and the absorbance A is positively correlated with the amount of potassium permanganate added.

[0023] In the titration endpoint determination method during the online monitoring of the permanganate index of the present invention, the titration volume V is used as the horizontal coordinate and the absorbance change value ΔA is used as the vertical coordinate to draw a coordinate axis, and the points obtained during the titration process are depicted on the coordinate axis to obtain a titration curve; and linear fitting is performed on the points where C1≤ΔA≤C2, C1 is a constant, C2-C1≥0.03, 0.01≤C1≤0.10, C1 is selected to avoid the inflection point range of the titration curve, and a point where the slope is stable above the inflection point is selected, and the number of points between C1 and C2 is not less than 10; considering the influence of the titration volume and in order to save the amount of reagents, C2 is not selected too large, thereby ensuring the accuracy of the fitting to the greatest extent; the fitting straight line equation is recorded as ΔA=aV+b, when ΔA=0, the calculated V F This is the titration endpoint volume.

[0024] In the present invention, the potassium permanganate characteristic wavelength λ1 and the turbidity correction wavelength λ2 are selected as the measurement wavelengths, λ1 is between 520nm and 540nm, and λ2 is between 860nm and 900nm, and the measured absorbances are recorded as λ1A and λ2A, respectively. This dual-wavelength setting eliminates the influence of turbidity and color of the water sample, further ensuring the accuracy of the measurement results.

[0025] As a preferred embodiment, in step 3), during the potassium permanganate titration process, the titration temperature is kept constant at 70-85° C. The present invention strictly complies with the online monitoring process of the permanganate index and is fully integrated into the online monitoring process of the permanganate index, thereby ensuring the accuracy of the measurement results.

[0026] As a preferred embodiment, in step 2), the temperature during the absorbance measurement is the titration temperature and is kept constant. The present invention uses the same temperature as the titration temperature during the absorbance A0 measurement before titration, fully eliminating the effect of temperature on absorbance and further eliminating interference.

[0027] As a preferred embodiment, in step 2), n is 5 to 20. The present invention measures the absorbance once approximately every 1 second before potassium permanganate titration, measures the absorbance before potassium permanganate titration multiple times, and calculates the absorbance before titration A0 by arithmetic mean, thereby further improving the accuracy of the absorbance before titration A0 measurement result.

[0028] Compared with the prior art, the present invention has the following beneficial effects: the present invention adopts a dual-wavelength setting and selects the absorbance value before potassium permanganate titration as the initial value, thereby largely eliminating the influence of turbidity and chromaticity of the water sample and ensuring the accuracy of the measurement result; the titration end point is determined according to the absorbance change value, the titration end point determined by this method is close to the theoretical titration end point, the titration end point determination is accurate and reliable, and the measurement result is highly accurate; the titration volume is dynamically determined according to a linear relationship formed by the absorbance change value during the titration process, and multiple points are selected to determine the titration end point, thereby largely avoiding the influence of random errors; the determination method is simple to operate, low in cost, and the determination process is fast and convenient, and is a new photometric titration end point determination method suitable for the online monitoring process of the permanganate index, which realizes accurate determination of the permanganate index titration end point. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a graph showing the absorbance change trend during the online monitoring of the permanganate index of the present invention;

[0030] Figure 2 Schematic diagram for determining the titration endpoint;

[0031] Figure 3 This is a titration curve obtained in Example 1 of the present invention;

[0032] Figure 4 This is the titration curve obtained in Example 2 of the present invention. DETAILED DESCRIPTION

[0033] The technical solutions of the present invention will be clearly and completely described below in conjunction with specific embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0034] A titration endpoint determination method in a permanganate index online monitoring process of the present invention comprises the following steps:

[0035] 1) The potassium permanganate characteristic wavelength λ1 and the turbidity correction wavelength λ2 are taken as the measurement wavelengths, and the measured absorbances are recorded as λ1A and λ2A respectively;

[0036] 2) Absorbance before titration A0

[0037] After the reduction of sodium oxalate is completed, within 5-20s before the titration of potassium permanganate, continuously measure the absorbance n times, where n is a natural number, n≥5, and record the absorbance as λ1A1 and λ2A1, λ1A2 and λ2A2, ..., λ1A n and λ2A n ,

[0038] but

[0039] 3) Titration end judgment

[0040] During the potassium permanganate titration process, the absorbance of the reaction system is measured each time the titration is performed, λ1A x and λ2A x , record the absorbance as A x =λ1A x -λ2A x , x is a natural number, calculate the absorbance change value △A=A x -A0; when △A is greater than the fixed value C2, C2 is a constant, 0.04≤C2≤0.20, the titration is automatically terminated, the number of titrations is recorded as x, and the single titration volume is V d , the titration volume is recorded as V = V d ×x;

[0041] 4) Calculation of titration endpoint

[0042] With the titration volume V as the horizontal coordinate and the absorbance change value △A as the vertical coordinate, draw the coordinate axis and depict the points obtained during the titration process on the coordinate axis; perform linear fitting on the points C1≤△A≤C2, and record the fitting straight line equation as △A=aV+b. When △A=0, the titration endpoint volume V is calculated. F , where C1 is a constant, C2-C1≥0.03, 0.01≤C1≤0.10.

[0043] Preferably, in step 3), during the potassium permanganate titration process, the titration temperature is 70-85° C. and is kept constant.

[0044] Furthermore, in the step 2), during the absorbance measurement, the temperature is the titration temperature and is kept constant.

[0045] Preferably, in step 2), n is 5-20.

[0046] Example 1

[0047] A titration endpoint determination method in a permanganate index online monitoring process of the present invention comprises the following steps:

[0048] 1) The characteristic wavelength of potassium permanganate λ1 = 525 nm and the turbidity correction wavelength λ2 = 880 nm were used as the measurement wavelengths, and the measured absorbances were recorded as λ1A and λ2A, respectively;

[0049] 2) Absorbance before titration A0

[0050] After the reduction of sodium oxalate is completed, within 5 seconds before the titration of potassium permanganate, the absorbance is measured continuously for n times, where n is a natural number, n = 5, and the absorbance is recorded as λ1A1 and λ2A1, λ1A2 and λ2A2, ..., λ1A n and λ2A n , as shown in Table 1,

[0051] but So A0=0.002;

[0052] Table 1 Absorbance measurement results

[0053] Number of measurements 1 2 3 4 5 <![CDATA[λ1A i ]]> 0.004 0.007 0.006 0.004 0.006 <![CDATA[λ2A i ]]> 0.002 0.003 0.003 0.003 0.004

[0054] 3) Titration end judgment

[0055] During the potassium permanganate titration process, the absorbance of the reaction system is measured each time the titration is performed, λ1A x and λ2A x , record the absorbance as A x =λ1A x -λ2A x , x is a natural number, calculate the absorbance change value △A=A x -A0; when △A is greater than the fixed value C2, C2 is a constant, C2 = 0.10, the titration is automatically terminated, the number of titrations is recorded as x = 75, and the single titration volume is V d =0.01mL, the titration volume is recorded as V=V d ×x=0.01×75=0.75mL;

[0056] 4) Calculation of titration endpoint

[0057] With the titration volume V as the horizontal axis and the absorbance change value △A as the vertical axis, draw the coordinate axis and depict the points obtained during the titration process on the coordinate axis; perform linear fitting on the points where C1≤△A≤C2, C1 is a constant, C2-C1≥0.03, C1=0.03, and according to the attached Figure 3 It can be seen that the fitting straight line equation is recorded as △A=0.399V-0.1988. When △A=0, the titration endpoint volume V is calculated. F =0.498mL.

[0058] Example 2

[0059] The present invention provides a titration endpoint determination method in a permanganate index online monitoring process, comprising the following steps:

[0060] 1) The characteristic wavelength of potassium permanganate λ1 = 525 nm and the turbidity correction wavelength λ2 = 880 nm were used as the measurement wavelengths, and the measured absorbances were recorded as λ1A and λ2A, respectively;

[0061] 2) Absorbance before titration A0

[0062] After the reduction of sodium oxalate is completed, within 10 seconds before the titration of potassium permanganate, the absorbance is measured continuously for n times, where n is a natural number, n = 10, and the absorbance is recorded as λ1A1 and λ2A1, λ1A2 and λ2A2, ..., λ1A n and λ2A n , as shown in Table 2,

[0063] but So A0=0.003;

[0064] Table 2 Absorbance measurement results

[0065] Number of measurements 1 2 3 4 5 6 7 8 9 10 <![CDATA[λ1A i ]]> 0.007 0.009 0.007 0.008 0.006 0.009 0.008 0.007 0.005 0.008 <![CDATA[λ2A i ]]> 0.004 0.003 0.004 0.005 0.004 0.003 0.006 0.004 0.003 0.004

[0066] 3) Titration end judgment

[0067] During the potassium permanganate titration process, the absorbance of the reaction system is measured each time the titration is performed, λ1A x and λ2A x , record the absorbance as A x =λ1A x -λ2A x , x is a natural number, calculate the absorbance change value △A=A x -A0; when △A is greater than the fixed value C2, C2 is a constant, C2 = 0.15, the titration is automatically terminated, the number of titrations is recorded as x = 136, and the single titration volume is V d =0.01mL, the titration volume is recorded as V=V d ×x=0.01×136=1.36mL;

[0068] 4) Calculation of titration endpoint

[0069] With the titration volume V as the horizontal axis and the absorbance change value △A as the vertical axis, draw the coordinate axis and depict the points obtained during the titration process on the coordinate axis; perform linear fitting on the points where C1≤△A≤C2, C1 is a constant, C2-C1≥0.03, C1=0.05, and according to the attached Figure 4 It can be seen that the fitting straight line equation is recorded as △A=0.4081V-0.4033. When △A=0, the titration endpoint volume V is calculated. F =0.988mL.

[0070] To verify the feasibility of the method of the present invention, national standard COD solutions were diluted to 4 mg / L and 8 mg / L for measurement. Each concentration was tested three times to determine accuracy and precision. The permanganate index measurement system consists of a multi-channel syringe pump, a precision temperature-controlled reaction-titration cell, a two-way valve, and connecting piping. The volume of water sample and reagent used was approximately 1 / 10 of that of the 11892-89 standard. The metering accuracy of the syringe pump was 0.42 μL / step. The titration endpoint determination method of the present invention was used to test the standard COD water sample.

[0071] The 4 mg / L standard sample was measured three times using the method described in Example 1. The 8 mg / L standard sample was measured three times using the method described in Example 2. The test results were compared with the national standard results, and the accuracy and precision were calculated. The experimental results are listed in Table 3.

[0072] Table 3 Determination results of different permanganate index titration endpoint determination methods

[0073]

[0074] As can be seen from Table 3, the results obtained by the titration endpoint determination method during the online monitoring of the permanganate index of the present invention are basically consistent with the results obtained in the national standard; therefore, the results obtained by the titration endpoint determination method during the online monitoring of the permanganate index of the present invention are highly accurate, and accurate determination of the titration endpoint of the permanganate index is achieved.

[0075] Therefore, compared with the prior art, the present invention has the following beneficial effects: the present invention adopts a dual-wavelength setting and selects the absorbance value before potassium permanganate titration as the initial value, thereby largely eliminating the influence of turbidity and chromaticity of the water sample and ensuring the accuracy of the measurement result; the titration end point is determined according to the absorbance change value, the titration end point determined by this method is close to the theoretical titration end point, the titration end point determination is accurate and reliable, and the measurement result is highly accurate; the titration volume is dynamically determined according to a linear relationship formed by the absorbance change value in the titration process, and multiple points are selected to determine the titration end point, thereby greatly avoiding the influence of random errors; the determination method is simple to operate, low in cost, and the determination process is fast and convenient, and is a new photometric titration end point determination method applicable to the permanganate index online monitoring process, which realizes accurate determination of the permanganate index titration end point.

[0076] 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 principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for determining the titration endpoint during online monitoring of permanganate index, characterized in that: The following steps are involved: 1) The potassium permanganate characteristic wavelength λ1 and the turbidity correction wavelength λ2 are taken as the measurement wavelengths, and the measured absorbances are recorded as λ1A and λ2A respectively; 2) Absorbance before titration A0 After the reduction of sodium oxalate is completed, within 5-20s before the titration of potassium permanganate, continuously measure the absorbance n times, where n is a natural number, n≥5, and record the absorbance as λ1A1 and λ2A1, λ1A2 and λ2A2, ..., λ1A n and λ2A n , but 3) Titration end judgment During the potassium permanganate titration process, the absorbance of the reaction system is measured each time the titration is performed, λ1A x and λ2A x , record the absorbance as A x =λ1A x -λ2A x , x is a natural number, calculate the absorbance change value △A=A x -A0; when △A is greater than the fixed value C2, C2 is a constant, 0.04≤C2≤0.20, the titration is automatically terminated, the number of titrations is recorded as x, and the single titration volume is V d , the titration volume is recorded as V = V d ×x; 4) Calculation of titration endpoint With the titration volume V as the horizontal coordinate and the absorbance change value △A as the vertical coordinate, draw the coordinate axis and depict the points obtained during the titration process on the coordinate axis; perform linear fitting on the points C1≤△A≤C2, and record the fitting straight line equation as △A=aV+b. When △A=0, the titration endpoint volume V is calculated. F , where C1 is a constant, C2-C1≥0.03, 0.01≤C1≤0.

10.

2. The method for determining the titration endpoint in the online monitoring process of permanganate index according to claim 1, wherein: In the step 3), during the potassium permanganate titration process, the titration temperature is 70-85° C. and is kept constant.

3. The method for determining the titration endpoint in the permanganate index online monitoring process according to claim 2, wherein: In the step 2), during the absorbance measurement, the temperature is the titration temperature and is kept constant.

4. The method for determining the titration endpoint during the on-line monitoring of the permanganate index according to any one of claims 1 to 3, wherein: In the step 2), n is 5-20.

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