A method and system for detecting fluoride in water based on a double masking system

Abstract

This invention discloses a method and system for detecting fluoride in water based on a dual-masking system, belonging to the field of water quality analysis technology. The detection method includes: mixing the water sample to be tested with air at a first confluence point, and then mixing it with a buffer reagent at a second confluence point to obtain a slug flow; the reaction dissociates fluoride... ‑ After degassing, the sample flows into the detection device to measure the electromotive force E, and the fluoride ion concentration is calculated using a cubic equation fitting. The buffer reagent contains 2,6-dimethyl-N-(2-sulfonate ethyl)morpholine inner salt and a dual masking agent of cyclohexanediaminetetraacetic acid and citrate, with a pH of 6.0-7.0. This invention uses a sulfonic acid-based zwitterionic buffer to replace the traditional acetic acid system, avoiding irreversible corrosion of the LaF3 electrode by acetate ions; by increasing the pH and using a dual masking system, the amount of masking agent used is significantly reduced, while effectively suppressing aluminum colloid interference, improving the accuracy and anti-interference ability of low-concentration fluoride ion detection.

Description

Technical Field

[0001] This invention relates to the field of water quality analysis technology, specifically to a method and system for detecting fluoride in water based on a dual-masking system. Background Technology

[0002] Fluoride ions are an important monitoring indicator for drinking water and the environment. Low levels can easily lead to tooth decay, while high levels can cause dental fluorosis, skeletal fluorosis, and even damage to the nervous system. The fluoride ion selective electrode method has become the mainstream method for fluoride determination in water due to its advantages such as ease of operation, rapid response, low cost, and suitability for both on-site and online monitoring. It is widely used in environmental monitoring, drinking water safety, food analysis, and clinical testing.

[0003] For more than half a century since the invention of the LaF3 crystal electrode, the traditional TISAB formulation has been based on an acetate / acetate buffer system, supplemented with cyclohexanediaminetetraacetic acid (CDTA) or citrate as a masking agent. However, this classic system has some drawbacks.

[0004] First, acetic acid has an irreversible corrosive effect on the LaF3 electrode. The irreversible ion exchange reaction between acetate ions and the LaF3 crystal surface forms a LaF-acetic acid complex on the LaF3 surface. The charge transfer resistance R... ct From the initial 1.68×10 4 Ω decreased to 5.64 × 10 3 The sensitivity decreased from an initial 35.5 mV / decade to 15.3 mV / decade.

[0005] Secondly, masking efficiency is limited under low pH conditions (5.0-5.5), requiring large amounts of masking agent and generating additional background noise. At pH 5.5, Al 3+ It mainly exists in the form of free hydrated ions, and has an effect on F - It exhibits extremely strong complexing competition ability. To eliminate aluminum ion interference, traditional formulations require high concentrations of masking agents. However, high concentrations of aminopolycarboxylic acid masking agents are themselves a background electrolyte, and fluctuations in their electrochemical activity introduce additional noise into ISE measurements, thus limiting the detection limit. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by proposing a method and system for detecting fluoride in water based on a dual-masking system.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for detecting fluoride in water based on a dual-masking system includes the following steps: S1. Sample introduction and mixing reaction: The water sample to be tested is mixed with the air introduced at the first confluence to form multiple bubbles with air gaps. The bubble is mixed with the introduced buffer reagent at the second confluence to obtain a slug flow; The slug stream is fed into the reaction apparatus, and the reaction yields the post-reaction slug stream. S2. Debubbling and Potential Detection: The downstream flow of the reaction is introduced into a degassing device to remove bubbles and obtain a continuous liquid flow. The continuous liquid flow is then introduced into a detection device with a built-in fluoride ion selective electrode and a reference electrode to measure the electromotive force and obtain the measured electromotive force E. S3. Concentration Calculation: The fluoride ion concentration can be calculated using the following equation: ; Where C is the fluoride ion concentration in the water sample, in mg / L; E is the measured electromotive force, in mV; and A, B, D, and E0' are the fitting coefficients determined by calibration and fitting using standard solutions. Furthermore, the buffer reagent consists of the following components: a buffer, a masking agent, an ionic strength modifier, and a base; the buffer is 2,6-dimethyl-N-(2-sulfoethyl)morpholine inner salt; the masking agent is cyclohexanediaminetetraacetic acid and trisodium citrate; and the pH of the buffer reagent is 6.0-7.0.

[0008] Preferably, the concentration of 2,6-dimethyl-N-(2-sulfoethyl)morpholine inner salt is 0.1-1.0 mol / L; the concentration of cyclohexanediaminetetraacetic acid is 0.001-0.005 mol / L; the concentration of citrate is 0.005-0.02 mol / L; and the ionic strength of the buffer reagent is 0.5-2.5 mol / L.

[0009] Furthermore, the method for preparing the buffer includes the following steps: 2,6-Dimethylmorpholine and sodium bromoethanesulfonate were dissolved in a mixed solvent of deionized water and anhydrous ethanol in a volume ratio of 1:0.8-1.2, and the mixture was refluxed at 70-80℃ for 8-12 hours. After the reaction was completed, the reaction solution was concentrated under reduced pressure, the pH was adjusted to 6-7, and then cooled to crystallize, filtered and dried under vacuum to obtain 2,6-dimethyl-N-(2-sulfoethyl)morpholine inner salt.

[0010] The reaction equation is as follows: ; In an aqueous-ethanol mixture, the secondary amine nitrogen atom on the 2,6-dimethylmorpholine ring carries a lone pair of electrons, acting as a nucleophile to attack the β-carbon in the sodium bromoethanesulfonate molecule. The strong electron-withdrawing inductive effect of Br in sodium bromoethanesulfonate imparts a high positive charge to this β-carbon, favoring the nucleophile attack. Subsequently, the bromide ion departs as a leaving group, completing SN2 substitution.

[0011] The product exists as an inner salt: the nitrogen atom on the morpholine ring becomes positively charged after quaternization, and the sulfonic acid group becomes negatively charged after dissociation. The positive and negative charges within the molecule neutralize, forming a "betaine" type inner salt structure. This inner salt form makes the compound a neutral molecule, avoiding the introduction of additional counterions that would interfere with the ionic strength.

[0012] Preferably, the molar ratio of 2,6-dimethylmorpholine to sodium bromoethanesulfonate is 1:1-1.2.

[0013] Preferably, the ionic strength adjuster is sodium chloride; the base is sodium hydroxide; and the pH of the buffer reagent is 6.5-6.8.

[0014] Preferably, in step S1, the flow rate of the water sample to be tested is 0.5-2.0 mL / min, the flow rate of the buffer reagent is 0.5-2.0 mL / min, the flow rate of the air is 0.1-0.5 mL / min, and the reaction conditions are 40-60℃ for 1-15 min.

[0015] Preferably, in step S2: the degassing device is a hydrophobic porous membrane degasser, the fluoride ion selective electrode is a lanthanum fluoride single crystal membrane electrode, and the reference electrode is a saturated calomel electrode.

[0016] Preferably, in step S3, the fitting coefficients are determined through the following steps: Prepare at least five fluoride ion solutions of different concentrations as standard solutions, with the concentration range of the standard solutions covering the expected concentration range of the water sample to be tested; under the same conditions as steps S1 and S2, measure the electromotive force of each standard solution; with the logarithm of the fluoride ion concentration of each standard solution lgC as the independent variable and the measured electromotive force E as the dependent variable, fit the equation using the least squares method to determine the fitting coefficient; At pH 6.5, OH - The concentration is 3.16 × 10⁻⁶ -8 M, which produces the equivalent F - The activity is approximately 3.16 × 10⁻⁶. -9 M is negligible. Meanwhile, HF has a pKa of 3.2, and the HF content in the solution is only 0.05%, far below the level that would significantly affect the measurement results (generally, an HF content of <1% is considered negligible). This indicates that raising the pH from 5.5 to 6.5 will not introduce new OH groups. - Or HF interference issues.

[0017] Al 3+ Al exists in solution 3+ AlOH 2+ Al(OH)2 + Al(OH)3, Al(OH)4 - Multi-stage hydrolysis equilibrium. At pH 5.5, free Al 3+ Al(OH)2 accounts for approximately 8% of the total aluminum. + As the main species, it has a strong affinity with F. - Complexing ability. At pH 6.5, free Al 3+ It decreased to 0.03% of total aluminum, with the main species transforming into Al(OH)4. - And Al(OH)3.

[0018] At pH 6.5, the complexation constant of the masking agent for metal ions increases significantly compared to pH 5-5.5. Therefore, the amount of masking agent can be significantly reduced while ensuring sufficient masking capacity. However, it is worth noting that because the main form of aluminum changes, it may adsorb and encapsulate fluoride ions by forming Al(OH)3 colloids, greatly hindering the dissociation of fluoride ions and thus producing unpredictable negative biases in the detection results.

[0019] Therefore, this invention employs a dual-masking system to reduce this bias. A single masking agent cannot simultaneously meet both kinetic and thermodynamic requirements: CDTA has a strong metal complexing ability, but its complexation reaction rate is limited by Al. 3+ The multi-step dissociation process makes it difficult to suppress Al using only one method. 3+ The process of forming colloids; citrate has moderate complexing strength but a fast rate, and can not only form complexes with AlF... 2+ Formation of ternary mixed complexes, reducing F - The activation energy of dissociation increases the amount of F in the Al-F complex. - The release rate, while also being able to rapidly complex Al 3+ They compete for the hydrolysis reaction and avoid the generation of Al(OH)3.

[0020] Furthermore, the above detection method is implemented using the following fluoride detection system, including: The sample introduction unit is used to independently deliver the water sample to be tested, buffer reagent, and air, including: an air sample introduction unit; a water sample introduction unit; and a buffer reagent introduction unit. The first confluencer has its input terminals connected to the water sample injection unit and the air injection unit, respectively, and is used to combine the water sample and air to obtain a bubble. The second confluencer has its input end connected to the output end of the first confluencer and the buffer reagent injection unit, respectively, and is used to combine the bubble and the buffer reagent to obtain a slug flow. A reaction device, the input of which is connected to the output of the second confluence member, is used to provide a reaction site for the slug flow and obtain a post-reacting slug flow. A degassing device, whose input end is connected to the output end of the reaction device, is used to remove bubbles from the plug flow in the later stage of the reaction to form a continuous liquid phase flow; The detection device, whose input end is connected to the output end of the debubbling device, has a built-in fluoride ion selective electrode and a reference electrode, and is used to measure the electromotive force of the continuous liquid phase flow. A data processing device, electrically connected to a fluoride ion selective electrode and a reference electrode, is used to calculate the fluoride ion concentration.

[0021] Compared with the prior art, the beneficial effects of the present invention are: 1. Traditional buffer solutions often use acetic acid as the buffer solution; however, acetic acid has an irreversible corrosive effect on LaF3 electrodes and reduces electrode sensitivity, affecting the detection limit. This invention uses 2,6-dimethyl-N-(2-sulfoethyl)morpholine inner salt instead of the traditional acetic acid buffer system. The sulfonic acid groups are more reactive with LaF3 electrodes. 3+ The coordination ability is extremely weak, and only a reversible electrostatic adsorption-dissociation process occurs on the electrode surface, which fundamentally eliminates the irreversible ion exchange reaction between acetate and the LaF3 crystal surface.

[0022] 2. Traditional acetic acid buffer systems typically maintain a pH of 5-5.5. Within this pH range, aluminum ions exhibit a strong binding affinity for fluoride ions, making the release of fluoride ions difficult and often requiring a high concentration of masking agent. This invention introduces a methyl group into a sulfonic acid zwitterionic buffer, increasing the buffer's pKa and raising the buffer pH to 6-7. Within this range, the primary form of aluminum ions changes, and the deprotonation degree of the masking agent is significantly enhanced, thus drastically reducing the actual amount of masking agent required. Combined with the kinetic and thermodynamic complementarity of the dual-masking system, interference from potential Al(OH)3 colloids is avoided, resulting in improved anti-interference capabilities and detection accuracy.

[0023] 3. Traditional methods for detecting fluoride ions in water are limited by the inherent defects of material interference from traditional buffer reagents and the nonlinear response of electrodes near the detection limit, often resulting in significant deviations from actual values ​​for low-fluoride solutions. This invention achieves accurate measurement near the detection limit by extending the detection limit, reducing detection noise, and employing mathematical fitting, thus broadening its application in low-fluoride ion concentration solutions. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of a fluoride detection system proposed in this invention; Figure 2 This is a fitting graph of a cubic equation fitted using the least squares method under the conditions of Example 1; Figure 1 In the middle: 1: Sample injection unit; 2: Air injection unit; 3: Water sample injection unit; 4: Buffer reagent injection unit; 5: First merging unit; 6: Second merging unit; 7: Reaction device; 8: Degassing device; 9: Detection device; 10: Data processing device. Detailed Implementation

[0025] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0026] Example A. Fluoride Detection System: This embodiment provides a fluoride detection system for implementing the detection method of the present invention, including a sample injection unit 1, a first confluence member 5, a second confluence member 6, a reaction device 7, a degassing device 8, a detection device 9, and a data processing device 10.

[0027] The sample introduction unit 1 is used to independently deliver the water sample to be tested, the buffer reagent, and the air. In this embodiment, the sample introduction unit 1 adopts a multi-channel peristaltic pump and a silicone pump tube connected to it. Each channel of the peristaltic pump independently controls the delivery flow rate of the water sample to be tested, the buffer reagent, and the air.

[0028] The first merging component 5 is a T-type three-way connector. Its first input end is connected to the water sample injection unit 3 and its second input end is connected to the air injection unit 2. It is used to merge the water sample to be tested with air, so that the water sample to be tested is cut by air to form multiple uniform liquid bubbles with air gaps.

[0029] The second connector 6 is a T-type three-way connector. Its first input end is connected to the output end of the first connector 5, and its second input end is connected to the buffer reagent injection unit 4. It is used to combine the bubble and the buffer reagent to form a slug flow.

[0030] The reaction device 7 is a polytetrafluoroethylene coil immersed in a constant-temperature water bath. Its input end is connected to the output end of the second confluence 6 to provide a reaction site for the slug flow. The constant-temperature water bath uses an HX-105 type constant-temperature circulator with a temperature control accuracy of ±0.1℃.

[0031] The degassing device 8 is a hydrophobic porous membrane degasser. In this embodiment, a polytetrafluoroethylene hollow fiber membrane module with a pore size of 0.22 μm is used. Its input end is connected to the output end of the reaction device 7 to remove bubbles from the plug flow in the later stage of the reaction and form a continuous liquid phase flow.

[0032] The detection device 9 is a flow cell with a built-in fluoride ion selective electrode and a reference electrode, and its input end is connected to the output end of the debubbling device 8. The fluoride ion selective electrode is a lanthanum fluoride single crystal film electrode; the reference electrode is a 217 type saturated calomel electrode.

[0033] Data processing device 10 includes a high-impedance potential acquisition card (input impedance ≥ 1 × 10⁻⁶). 12 The electromotive force (EMF) is measured at Ω and the electromotive force is measured at 100 Ω. The potential acquisition card is electrically connected to the fluoride ion selective electrode and the reference electrode, respectively, to acquire the EMF signal and transmit it to the computer. The computer has pre-set calibration equations and calculation programs to calculate the fluoride ion concentration by substituting the measured EMF into the calibration equations, and then displays and outputs the results.

[0034] Preparation of standard stock solutions and working standard solutions: Weigh 0.2210 g of sodium fluoride (analytical grade), dried to constant weight at 105 °C, dissolve in deionized water, transfer to a 1000 mL volumetric flask, and dilute to the mark with deionized water. Mix well to obtain a fluoride ion standard stock solution (100 mg / L, expressed as F). - count).

[0035] The above standard stock solution was serially diluted with deionized water to prepare a series of standard working solutions with concentrations of 0.10 mg / L, 0.50 mg / L, 1.00 mg / L, 5.00 mg / L, 10.00 mg / L, 50.00 mg / L, and 100.00 mg / L (all expressed as F). - count).

[0036] Establishment of the standard curve: Using the above series of working standard solutions, the electromotive force (E) of each standard solution was measured under the same detection conditions as the sample determination. The logarithm of the fluoride ion concentration (lgC) of each standard solution was plotted on the x-axis, and the measured electromotive force (E) was plotted on the y-axis. The cubic equation was then solved using the least squares method. Perform a fitting test and determine the fitting coefficients A, B, D, and E0'.

[0037] Preparation Example 1: Preparation of the buffer 2,6-dimethyl-N-(2-sulfonate ethyl)morpholine inner salt, comprising the following steps: In a reaction flask equipped with a reflux condenser and a magnetic stirrer, 50 mL of deionized water and 50 mL of anhydrous ethanol were added, along with 0.10 mol of 2,6-dimethylmorpholine and 0.11 mol of sodium bromoethanesulfonate. The mixture was stirred until dissolved. The temperature was raised to 75 °C, and the reaction was refluxed for 10 h. After the reaction was complete, the reaction solution was concentrated under reduced pressure to remove most of the solvent, and the pH was adjusted to 6.5 with 1 mol / L NaOH. The solution was transferred to an ice bath for cooling and crystallization for 6 h. The mixture was filtered, and the filter cake was washed three times with anhydrous ethanol and dried under vacuum at 60 °C for 12 h to obtain a white crystalline product.

[0038] Preparation Example 2: 0.1 mol of 2,6-dimethylmorpholine and 0.1 mol of sodium bromoethanesulfonate were added to a mixed solvent of 50 mL deionized water and 40 mL anhydrous ethanol. The reaction temperature was controlled at 70 °C, and the reaction was refluxed for 12 h. In the post-treatment, the pH was adjusted to 6.0, and other preparation conditions were the same as in Preparation Example 1.

[0039] Preparation Example 3: In a mixed solvent of 50 mL deionized water and 60 mL anhydrous ethanol, 0.10 mol of 2,6-dimethylmorpholine and 0.12 mol of sodium bromoethanesulfonate were added. The reaction temperature was controlled at 80 °C, and the reaction was refluxed for 8 h. In the post-treatment, the pH value was adjusted to 7.0, and other preparation conditions were the same as in Preparation Example 1.

[0040] Example 1: 1. Preparation of buffer reagents: For example, to prepare a 1L buffer solution: Weigh 0.5 mol of 2,6-dimethyl-N-(2-sulfoethyl)morpholine inner salt obtained from Preparation Example 1, dissolve it in approximately 800 mL of deionized water, and stir until completely dissolved. Weigh 0.003 mol of CDTA and 0.0125 mol of trisodium citrate, add them to the above solution, and stir until completely dissolved. Weigh 58.5 g of sodium chloride and add it to the solution as an ionic strength adjuster, and stir until dissolved. Adjust the pH to 6.5 with 1 mol / L sodium hydroxide solution, transfer to a 1L volumetric flask, dilute to the mark with deionized water, and mix well.

[0041] 2. Testing process: Using the fluoride detection system of Example A, the multi-channel peristaltic pump was turned on, and the water sample or standard solution to be tested was introduced at a flow rate of 1.0 mL / min, the buffer reagent was introduced at a flow rate of 1.0 mL / min, and air was introduced at a flow rate of 0.3 mL / min.

[0042] The water sample to be tested merges with air at the first confluence 5, where the air cuts through the continuous water flow to form uniform air-spacing bubbles. These bubbles then merge with a buffer reagent at the second confluence 6, forming a slug flow. The slug flow enters a constant-temperature water bath heating coil and reacts at 50°C for 8 minutes.

[0043] After the reaction, the plug flow passes through the degassing device 8 to remove bubbles, forming a continuous liquid flow. This liquid flow enters the flow cell containing the built-in fluoride ion selective electrode and reference electrode, flowing continuously over the electrode surface. After the potential reading stabilizes (potential change rate ≤ 0.1 mV / min), the measured electromotive force E is recorded.

[0044] Example 2: 1. Preparation of buffer reagents: Weigh out 0.1 mol of 2,6-dimethyl-N-(2-sulfoethyl)morpholine inner salt, 0.005 mol of CDTA, 0.005 mol of trisodium citrate, and 0.5 mol of NaCl obtained from Preparation Example 2. Adjust the pH to 6.0 with 1 mol / L NaOH, and bring the volume to 1 L with deionized water. Shake well and set aside.

[0045] 2. Testing process: The water sample was introduced at a flow rate of 0.5 mL / min, the buffer reagent at a flow rate of 2.0 mL / min, and the air flow rate at 0.1 mL / min. The reaction temperature was 40℃, and the reaction time was 15 min. The remaining operations were the same as in Example 1.

[0046] Example 3: 1. Preparation of buffer reagents: Weigh 1.0 mol of 2,6-dimethyl-N-(2-sulfoethyl)morpholine inner salt, 0.001 mol of CDTA, 0.02 mol of trisodium citrate, and 2.0 mol of NaCl obtained from Preparation Example 3. Adjust the pH to 7.0 with 1 mol / L NaOH, and bring the volume to 1 L with deionized water. Shake well and set aside.

[0047] 2. Testing process: The water sample was introduced at a flow rate of 2.0 mL / min, the buffer reagent at a flow rate of 0.5 mL / min, and the air flow rate at 0.5 mL / min. The reaction temperature was 60℃, and the reaction time was 1 min. The remaining operations were the same as in Example 1.

[0048] Establishing the standard curve and determining the fitting coefficient: Under the detection conditions of Example 1 (water sample flow rate 1.0 mL / min, buffer reagent flow rate 1.0 mL / min, air flow rate 0.3 mL / min, reaction temperature 50℃, reaction time 8 min), the electromotive force E of each standard solution was measured, and the average value was taken from three parallel measurements. The measurement results are shown in Table 1, and the fitting results are as follows: Figure 2 As shown: Table 1. Standard solution concentrations and measured electromotive forces under the conditions of Example 1

[0049] Based on the data in Table 1, the following results were obtained: Figure 2 ,according to Figure 2 Based on the fitting results, determine the fitting parameters: A=-0.1987, B=1.4013, D=-34.488, E0'=120.28; The equation of the standard curve is: , The coefficient of determination R of the fit 2 =0.9997.

[0050] The fitting was performed under the same conditions as in Example 1, and the fitting result is as follows: A=-0.203, B=1.567, D=-34.03, E0'=126.85; The equation of the standard curve is: The coefficient of determination R 2 =0.9999.

[0051] The fitting was performed under the same conditions as in Example 1, and the fitting result is as follows: A=-0.181, B=1.372, D=-35.12, E0'=115.42; The equation of the standard curve is: The coefficient of determination R 2 =0.9999.

[0052] Comparative Example 1: The buffer reagent formulation is as follows: 1.0 mol / L glacial acetic acid, 0.5 mol / L sodium acetate, 0.01 mol / L CDTA, 0.02 mol / L trisodium citrate, and 1.0 mol / L NaCl, with the pH adjusted to 5.5 using 1 mol / L NaOH. All other detection conditions and procedures are exactly the same as in Example 1.

[0053] Comparative Example 2: No citrate was added to the buffer reagent; only CDTA (0.0155 mol / L) was used as a single masking agent. The remaining components were the same as in Example 1. The remaining detection conditions and procedures were exactly the same as in Example 1.

[0054] Comparative Example 3: The buffer reagent formulation was as follows: 0.5 mol / L 2-(N-morpholino)ethanesulfonic acid (MES), 0.5 mol / L NaCl, with the addition of 0.003 mol CDTA and 0.0125 mol trisodium citrate, and the pH was adjusted to 5.5 with 1 mol / L NaOH. The remaining detection conditions and procedures were exactly the same as in Example 1.

[0055] Performance testing: Determination of detection limit and lower limit of quantitation: According to the IUPAC definition, the limit of detection (LOD=3σ / S) and the lower limit of quantification (LOQ=10σ / S) are calculated by taking the average value and standard deviation of the electromotive force of 11 consecutive measurements with blank solution (deionized water), where S is the slope of the calibration curve in the low concentration region (0.10-1.00 mg / L).

[0056] Precision experiment: Three fluoride ion standard solutions of different concentrations (0.10 mg / L, 1.00 mg / L, and 10.00 mg / L) were taken and measured n times under the detection conditions of each example and comparative example. The relative standard deviation (RSD) was calculated. The results are shown in Table 2. Table 2. Summary of Detection Limit and Precision Data

[0057] Data Analysis: As shown in Table 2, the detection limits of Examples 1-3 were between 0.015-0.018 mg / L, and the lower limit of determination were between 0.05-0.06 mg / L, which were significantly better than the detection limit of 0.058 mg / L of the traditional acetic acid system. Although Comparative Example 3 used MES zwitterionic buffer, the lower pH resulted in a lower degree of deprotonation of CDTA and citrate, so more masking agent should have been added. The comparative example used the same masking agent as Example 1, and its detection limit was significantly worse than that of the examples of this invention.

[0058] The RSD of Examples 1-3 at all concentration levels was less than 3%, indicating that the method of the present invention has good repeatability. The RSD of Comparative Example 1 was significantly higher than that of the Examples, especially reaching 6.5% in the low concentration region, which may be related to the instability of the response caused by the irreversible erosion of the electrode surface by acetic acid. The precision of Comparative Example 3 was between that of Comparative Example 1 and the Examples, indicating that the MES zwitterionic buffer itself is electrode-friendly, but due to the low pH, the buffer capacity and masking efficiency are insufficient, and the fluctuation in the low concentration region is still greater than that of the present invention.

[0059] Electrode lifespan and sensitivity decay experiments: Three brand-new lanthanum fluoride single-crystal film electrodes of the same model were taken and continuously run for 15 days, 8 hours per day, according to the buffer systems and detection conditions of Example 1, Comparative Example 1, and Comparative Example 3, respectively. The electromotive force response value of the same concentration (1.00 mg / L) of fluoride ion standard solution was measured daily, and the change in sensitivity (mV / decade) over time was calculated. To ensure data comparability, the initial sensitivity of all three electrodes was calibrated to the range of 58.0 ± 0.5 mV / decade.

[0060] Experiment on the effect of interference ion masking: In a standard solution containing 1.00 mg / L of fluoride ions, different concentrations of common coexisting ions (Al₂O₃) were added. 3+ Fe 3+ Ca 2+ Mg 2+The interference of each ion on the determination of fluoride ions was investigated, and the degree of interference was evaluated by measuring the recovery rate. The buffer systems and detection conditions of Examples 1, 2, and 3 were used for the determination. The results are shown in Table 3. Table 3. Summary of experimental data on interfering ion masking and sensitivity attenuation

[0061] Data Analysis: In Example 1, after 15 days of operation, the electrode sensitivity decreased from the initial 58.2 mV / decade to 55.3 mV / decade, with a decay rate of only 5.0%, indicating that the electrode performance remained good. In Comparative Example 1, after 15 days of operation, the electrode sensitivity decreased from the initial 58.0 mV / decade to 12.8 mV / decade, with a decay rate of 77.9%, and it had already decreased to 35.8 mV / decade on the first day, indicating that the irreversible corrosion of the electrode surface by acetic acid had a significant impact in a short period. In Comparative Example 3, after 15 days of operation, the electrode sensitivity decreased from the initial 58.1 mV / decade to 52.6 mV / decade, with a decay rate of 9.5%, which was significantly better than the acetic acid system, but worse than Example 1 of this invention, indicating that the long-term stability of MES under pH 5.5 conditions was slightly lower than the performance of this invention under pH 6.5 conditions.

[0062] To further differentiate the reversibility of electrode surface damage, the electrodes in the three experimental groups were cleaned with deionized water and repolished after 15 days of operation, and then their sensitivity was measured in the same standard solution. The results showed that the electrode in Example 1 recovered to 57.1 mV / decade after cleaning; the electrode in Comparative Example 3 recovered to 56.4 mV / decade after cleaning; while the electrode in Comparative Example 1, in the acetic acid system, only recovered to 15.6 mV / decade after cleaning and polishing, and could not be recovered even after polishing. This indicates that the effect of the sulfonic acid zwitterionic buffer on LaF3 crystals is mainly reversible surface adsorption, while the acetic acid system leads to irreversible chemical damage.

[0063] In the dual-masking system of Example 1, common coexisting ions did not cause significant interference within the conventional concentration range, and the recovery rate was between 97.0% and 102.0%. In the system of Comparative Example 2, Al 3+ and Fe 3+ The interference is significantly enhanced when Al 3+ At a concentration of 5.0 mg / L, the recovery rate was only 91.0%; when Al 3+ When the concentration increased to 10.0 mg / L, the recovery rate further decreased to 85.3%. This may be because CDTA alone cannot rapidly inhibit Al. 3+The process of hydrolysis to form Al(OH)3 colloids causes some fluoride ions to be unable to be responded to by the electrode through adsorption and encapsulation.

[0064] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for detecting fluoride in water based on a dual-masking system, characterized in that, Includes the following steps: S1. Sample introduction and mixing reaction: The water sample to be tested is mixed with the air introduced at the first confluence (5) to form multiple bubbles with air gaps. The bubble is mixed with the introduced buffer reagent at the second confluence (6) to obtain a slug flow; The slug flow is fed into the reaction device (7) and reacted to obtain the post-reaction slug flow; S2. Debubbling and Potential Detection: The reaction downstream flow is introduced into the degassing device (8) to remove bubbles and obtain a continuous liquid phase flow; the continuous liquid phase flow is introduced into the detection device (9) with built-in fluoride ion selective electrode and reference electrode to measure the electromotive force and obtain the measured electromotive force E. S3. Concentration Calculation: The fluoride ion concentration can be calculated using the following equation: ； Where C is the fluoride ion concentration in the water sample, in mg / L; E is the measured electromotive force, in mV; and A, B, D, and E0' are the fitting coefficients determined by calibration and fitting using standard solutions.

2. The method for detecting fluoride in water based on a dual-masking system according to claim 1, characterized in that, The buffer reagent consists of the following components: a buffer, a masking agent, an ionic strength modifier, a base, and water; the buffer is 2,6-dimethyl-N-(2-sulfoethyl)morpholine inner salt; the masking agent is cyclohexanediaminetetraacetic acid and trisodium citrate; and the pH of the buffer reagent is 6.0-7.

0.

3. The method for detecting fluoride in water based on a dual-masking system according to claim 2, characterized in that, Based on the total volume of the buffer reagent, the concentration of the 2,6-dimethyl-N-(2-sulfoethyl)morpholine inner salt is 0.1-1.0 mol / L; the concentration of the cyclohexanediaminetetraacetic acid is 0.001-0.005 mol / L; the concentration of the trisodium citrate is 0.005-0.02 mol / L; and the ionic strength of the buffer reagent is 0.5-2.5 mol / L.

4. The method for detecting fluoride in water based on a dual-masking system according to claim 2, characterized in that, The method for preparing the buffer includes the following steps: 2,6-Dimethylmorpholine and sodium bromoethanesulfonate were dissolved in a mixed solvent of deionized water and anhydrous ethanol in a volume ratio of 1:0.8-1.2, and the mixture was refluxed at 70-80℃ for 8-12 hours. After the reaction was completed, the reaction solution was concentrated under reduced pressure, the pH was adjusted to 6-7, and then cooled to crystallize, filtered and dried under vacuum to obtain 2,6-dimethyl-N-(2-sulfoethyl)morpholine inner salt.

5. The method for detecting fluoride in water based on a dual-masking system according to claim 4, characterized in that, The molar ratio of 2,6-dimethylmorpholine to sodium bromoethanesulfonate is 1:1-1.

2.

6. The method for detecting fluoride in water based on a dual-masking system according to claim 2, characterized in that, The ionic strength adjuster is sodium chloride; the base is sodium hydroxide; and the pH of the buffer reagent is 6.5-6.

8.

7. The method for detecting fluoride in water based on a dual-masking system according to claim 1, characterized in that, In step S1: the flow rate of the water sample to be tested is 0.5-2.0 mL / min, the flow rate of the buffer reagent is 0.5-2.0 mL / min, the flow rate of the air is 0.1-0.5 mL / min, and the reaction conditions are 40-60℃ for 1-15 min.

8. The method for detecting fluoride in water based on a dual-masking system according to claim 1, characterized in that, In step S2: the degassing device (8) is a hydrophobic porous membrane degasser, the fluoride ion selective electrode is a lanthanum fluoride single crystal membrane electrode, and the reference electrode is a saturated calomel electrode.

9. The method for detecting fluoride in water based on a dual-masking system according to claim 1, characterized in that, In step S3: the fitting coefficients are determined through the following steps: Prepare at least five fluoride ion solutions of different concentrations as standard solutions, wherein the concentration range of the standard solutions covers the expected concentration range of the water sample to be tested; Under the same conditions as S1 and S2, the electromotive force of each standard solution was measured. The equation was fitted by least squares method with the logarithm of the fluoride ion concentration of each standard solution (lgC) as the independent variable and the measured electromotive force (E) as the dependent variable, and the fitting coefficient was determined.

10. A water fluoride detection system based on a dual-masking system, used in the detection method according to any one of claims 1-9, characterized in that, The water fluoride detection system includes: The sample introduction unit (1) is used to independently deliver the water sample to be tested, the buffer reagent and the air, including: an air sample introduction unit (2); a water sample introduction unit (3); and a buffer reagent introduction unit (4). The first merging unit (5) has its input end connected to the water sample injection unit (3) and the air injection unit (2) respectively, and is used to combine the water sample to be tested with air to obtain a bubble; The second confluencer (6) has its input end connected to the output end of the first confluencer (5) and the buffer reagent injection unit (4) respectively, and is used to combine the bubble and the buffer reagent to obtain a slug flow; The reaction device (7) has its input end connected to the output end of the second confluence member (6) to provide the reaction site for the slug flow and obtain the post-reaction slug flow; The debubbling device (8) has its input end connected to the output end of the reaction device (7) and is used to remove bubbles from the plug flow in the later stage of the reaction to form a continuous liquid phase flow; The detection device (9) has its input end connected to the output end of the debubbling device (8), and has a built-in fluoride ion selective electrode and reference electrode for measuring the electromotive force of the continuous liquid phase flow. A data processing device (10), which is electrically connected to the fluoride ion selective electrode and the reference electrode, is used to calculate the fluoride ion concentration.

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