A method for determining the content of chloride ions in a complex matrix

Abstract

The present application relates to a kind of methods for determining the content of chloride ion in complex matrix, solve the problems existing in prior art.The method comprises the following steps: a) the sample containing interfering ions is pretreated, the interfering ions include sulfate and metal ions;b) the pretreated sample is placed in optical titration device, with silver nitrate standard solution as titrant for titration, while real-time monitoring the change of transmission light signal and / or scattering light signal of solution with titration volume;c) the light signal-volume data collected in step b) is smoothed and derived, and the titration end point volume is automatically determined by identifying the characteristic points of derivative curve;d) according to the titration end point volume, the concentration of silver nitrate standard solution and the sample amount, the content of chloride ion in sample is calculated.Compared with prior art, the present application has the advantages of strong anti-interference ability, accurate and objective end point interpretation, automation and intelligentization, wide linear range and high cost-effectiveness, simple device, lower development and use cost.

Description

Technical Field

[0001] This invention relates to the fields of analytical chemistry and automated detection technology, and in particular to a method for accurately quantifying the chloride ion content in complex matrix solutions by combining optical signal detection with digital signal processing. Background Technology

[0002] Chloride ion content is a crucial monitoring indicator in the metallurgical, power, chemical, and environmental protection industries. In complex systems such as copper electrolytes and acidic iron-containing wastewater, excessively high chloride ion concentrations can cause equipment corrosion or process abnormalities. Existing measurement methods include:

[0003] 1. Titration method for detecting chloride ions

[0004] 1) Morfa:

[0005] Principle: Chloride ions are titrated with silver nitrate standard solution, and the endpoint is determined by potassium chromate indicator (forming a brick-red Ag2CrO4 precipitate).

[0006] Features: Easy to operate, but the pH needs to be controlled between 6.5 and 10, otherwise there will be significant interference.

[0007] 2) Volhard Law

[0008] Principle: First, excess AgNO3 is used to precipitate Cl. - Then, the excess Ag was titrated with ammonium thiocyanate. + Ferric ammonium was used as an indicator (red Fe(SCN)). 3+ The appearance of the complex is the endpoint.

[0009] Features: Suitable for contents containing Cl - It can handle a large number of samples and is suitable for acidic media, but the operation steps are relatively complicated.

[0010] 3) Fyansfa

[0011] Principle: Fluorescein-based adsorption indicators change color when the surface charge of AgCl reverses at the titration endpoint.

[0012] Features: High sensitivity, but requires strict operating conditions.

[0013] 2. Instrumental analysis method

[0014] 1) Ion Selective Electrode Method (ISE)

[0015] Principle: Chloride ion selective electrode is used to measure potential and the concentration is converted by Nernst equation.

[0016] Features: Fast and can be monitored online, but susceptible to interference from bromine, iodine, etc.

[0017] 2) Spectrophotometry / Colorimetry

[0018] Principle: Chloride ions react with specific reagents to generate colored compounds or affect the color development system, and then the absorbance is measured with a spectrophotometer.

[0019] Features: Suitable for low concentrations of Cl - It has high sensitivity but poor specificity.

[0020] 3) Ion chromatography (IC)

[0021] Principle: Separation is achieved through an ion exchange column, and quantification is performed using a conductivity detector.

[0022] Features: High accuracy, suitable for simultaneous determination of multiple anions, but the equipment is expensive.

[0023] Existing measurement methods have the following shortcomings:

[0024] 1) Potentiometric titration: In high acid and high Cu environments... 2+ Fe 3+ In such substrates, electrodes are prone to poisoning or drift, resulting in unclear endpoints and poor accuracy and reproducibility.

[0025] 2) Spectrophotometry: requires an additional colorimetric reaction, is cumbersome to operate, and is easily affected by impurities.

[0026] 3) Ion chromatography: The instruments are expensive, the pretreatment is complicated, high-concentration samples need to be diluted in large quantities, and errors are easily introduced.

[0027] In summary, chloride ion detection in complex solution systems is often affected by interference from various anions and cations (such as sulfate, copper, and iron cations), leading to results that deviate from the true values. Therefore, there is an urgent need for a chloride ion determination method with strong anti-interference capabilities, accurate endpoint determination, and applicability to complex systems. Summary of the Invention

[0028] The purpose of this invention is to provide an integrated method combining preprocessing, optical signal detection, and digital algorithm determination, which can rapidly and accurately determine the chloride ion content in complex matrices.

[0029] This invention is achieved through the following technical solutions:

[0030] A method for determining the chloride ion content in a complex matrix includes the following steps:

[0031] 1. Sample pretreatment (for matrices with high sulfate and high metal ion content):

[0032] 1) Desulfate removal: After sampling, add excess barium salt solution (such as barium nitrate), react fully, and then centrifuge or filter to completely remove barium sulfate precipitate and avoid interference from silver sulfate precipitate in subsequent titration.

[0033] 2) Removal of metal ions: After sampling, pass the sample through a strongly acidic cation exchange resin column and collect the effluent. Alternatively, adjust the pH to alkaline using an alkali such as ammonia to cause the metal ions to form hydroxide precipitates. After centrifugation, collect the supernatant and acidify it with nitric acid.

[0034] 2. Optical titration analysis:

[0035] a. Titration apparatus: including an automatic burette, a transparent titration container holding the test liquid, a light source (LED or laser) of a specific wavelength (e.g., 550 nm), a photodetector opposite the light source (for transmitted light), and / or a photodetector at a certain angle (e.g., 90°) to the incident light path (for scattered light).

[0036] b. Titration reaction: Using a known concentration of silver nitrate (AgNO3) standard solution as the titrant, react with Cl in the test solution. - Reaction occurs: Ag + + Cl - → AgCl↓

[0037] c. Signal Acquisition: During the addition of the titrant, the transmitted light intensity (T) and / or scattered light intensity (I_s) signals of the solution are acquired synchronously and at high frequency. As AgCl precipitate forms, nucleates, and grows, the turbidity of the solution increases, leading to a monotonically decreasing transmitted light intensity and a monotonically increasing scattered light intensity.

[0038] d. Endpoint determination: The acquired light intensity signal (T or I_s) and titration volume (V) data are processed by computer as follows:

[0039] i. Smoothing and Denoising: The Savitzky-Golay convolutional smoothing algorithm is used to process the original light intensity-volume curve to filter out high-frequency random noise.

[0040] ii. Derivative calculation: Calculate the first derivative (dT / dV or dI_s / dV) and / or the second derivative (d...) of the smoothed curve. 2 T / dV 2 or d 2 I_s / dV 2 ).

[0041] iii. Automatic identification: The titration endpoint (Ve) corresponds to the point where the light intensity changes most drastically, i.e., the extreme point (valley or peak) of the first derivative or the zero-crossing point of the second derivative. The system sets a threshold algorithm to automatically search for and confirm this feature point, and outputs the endpoint volume Ve.

[0042] 3. Result Calculation: Calculate the chloride ion content in the sample according to the formula: ρ(Cl - ) = (C(AgNO3)×Ve ×M(Cl -)) / Vsample where, ρ(Cl - C(AgNO3) is the concentration of chloride ions (mg / L), C(AgNO3) is the concentration of silver nitrate titrant (mol / L), Ve is the endpoint volume (L), and M(Cl) is the concentration of chloride ions (mg / L). - ) represents the molar mass of chloride ions (35.45 g / mol), and Vsample represents the volume of the sample taken (L).

[0043] 4. Range Coverage Scheme: By combining different titrant concentrations (C_AgNO3, such as 0.01 mol / L, 0.1 mol / L, 1.0 mol / L) and sample volumes (V_sample, such as 1 mL, 10 mL, 100 mL), the detection range of the method can cover an ultra-wide concentration range from 1 mg / L to 10000 mg / L.

[0044] Compared with the prior art, the present invention has the following advantages or beneficial effects:

[0045] 1. Strong anti-interference ability: Pretreatment (sulfate removal, metal removal) effectively eliminates high concentrations of SO4. 2- Fe 3+ Cu 2+ The main interfering ions are eliminated, which solves the problem that traditional potentiometric methods cannot accurately measure in such matrices.

[0046] 2. Accurate and objective endpoint interpretation: Utilizing the extremely high sensitivity of optical signals to changes in turbidity, combined with Savitzky-Golay smoothing and derivative algorithms for endpoint identification, the subjective and drift errors of human visual interpretation (Mohr method) or electrode interpretation (potential method) are completely eliminated, resulting in excellent reproducibility.

[0047] 3. Automation and intelligence: The entire process can be automated or semi-automated, and the endpoint is automatically determined by the algorithm, which reduces the workload and technical dependence of operators.

[0048] 4. Wide linear range: By flexibly adjusting the titrant concentration and sampling volume, a single method can cover an extremely wide concentration range, making it widely applicable.

[0049] 5. High cost-effectiveness: Compared with large instruments such as ion chromatography, this method has a simpler setup, lower development and usage costs, and is easier to promote and popularize in industrial quality inspection. Attached Figure Description

[0050] Figure 1 This is a flowchart of the method of the present invention. Detailed Implementation

[0051] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0052] Example 1: Determination of chloride ions in copper electrolyte

[0053] Matrix characteristics: Cu 2+ ≈45 g / L, H2SO4≈160 g / L, Fe 3+ ≈5 g / L, Cl - ≈3 g / L (complex high-acid, high-metal system). Instruments and reagents:

[0054] • Automatic titrator (with 0.01 mL step dispensing); LED light source 550 nm; transmission light detector; data acquisition frequency ≥10 Hz.

[0055] • AgNO3 standard solution: 0.1000 mol / L (calibrated with NaCl standard).

[0056] • 5% (mass fraction) barium nitrate solution; 001×7 strong acid cation exchange resin (H + Type); ultrapure water.

[0057] • A transparent glass titration cup (inner diameter ≥ 60 mm) and a magnetic stirrer.

[0058] step:

[0059] 1. Sampling: Transfer 10.00 mL of electrolyte into a 50 mL centrifuge tube.

[0060] 2. Desulfate: Add 10 mL of 5% barium nitrate solution, shake vigorously for 1 min, let stand for 10 min; centrifuge (4000 rpm, 5 min), discard the precipitate, and take the supernatant.

[0061] 3. Removal of metal ions: The supernatant is filled with 001×7 (H) + The solution was processed in a resin column (φ8 mm × 80 mm, linear velocity ≈ 1 mL / min), eluted with 10 mL of ultrapure water, and the eluent and eluent were combined in a 250 mL beaker.

[0062] 4. Titration apparatus: Placed under a magnetic stirrer (approximately 400 rpm), with the light path passing approximately 1 cm below the liquid surface; record the volume of liquid added, V, and the intensity of transmitted light, T.

[0063] 5. Titration: 0.1000 mol / L AgNO3 was added continuously (in 0.02 mL increments), and T–V data were collected in real time.

[0064] 6. Signal Processing and Endpoint Determination: Perform Savitzky-Golay smoothing on the T–V curve (11-point window, cubic polynomial), and calculate the first derivative dT / dV; the minimum value corresponds to the volume of the endpoint. .

[0065] 7. Calculate: ρ(Cl) - ) = (C_AgNO3 × V_e × M_Cl - ) / V_sample Result: ρ(Cl - = 3090 mg / L.

[0066] In complex matrices with high acidity and high metal content, the endpoint mutation is clear and reproducible, demonstrating the synergistic effect of the "pretreatment + optical determination" of this invention.

[0067] Example 2 (without any pretreatment): the same copper electrolyte

[0068] Differences: The optical titration and algorithm are exactly the same as in Example 1, but the sulfate / demetallization step is skipped, and the titration is performed directly.

[0069] Endpoint and Calculation: Principal extreme points identified by the automatic algorithm

[0070] Result: According to the formula, ρ(Cl) - = 4,120 mg / L. The positive deviation from Example 1 (3,090 mg / L) is approximately +33%, and the curve morphology is unstable between different batches.

[0071] Conclusion: Without preprocessing, significant systematic positive bias and poor reproducibility are introduced, verifying the necessity of the "three-stage preprocessing" method of this invention.

[0072] Example 3 (Comparative method, non-optical titration): Potentiometric titration

[0073] Method selection: Potentiometric titration was performed using an Ag electrode-reference electrode (Ag / AgCl or saturated calomel) system (routine data processing: E–V first derivative / Gran plot).

[0074] Sample: The same batch of copper electrolyte as in Example 1 (to ensure electrode response, minimize dilution: take 2.00 mL of the original sample and bring the volume to 20.0 mL).

[0075] Reagents and instruments:

[0076] • 0.1000 mol / L AgNO3 standard solution; pH maintained as strongly acidic (original acidity was H2SO4≈160 g / L).

[0077] • Potentiometric titrator, data acquisition ≥2 Hz; stirring approximately 300 rpm.

[0078] step:

[0079] 1. Take 20.0 mL of the diluted sample into a titration cup, insert the Ag indicator electrode and the reference electrode, and record E–V.

[0080] 2. Titrate with 0.1000 mol / L AgNO3 in 0.02 mL increments; calibrate the counter electrode with 0.01 mol / L NaCl before and after titration.

[0081] 3. Calculate dE / dV using numerical differentiation or extrapolate using Gran's method; due to the presence of high acidity and metal ions, this sample exhibits electrode hysteresis and potential drift, resulting in a non-sharp inflection point.

[0082] 4. Take the volume V corresponding to the peak value of the first derivative. e (diluted) = 1.94 mL. Equivalent volume (converted back to original value (×10 dilution factor)) V e (eq) = 19.4 mL, which corresponds to a volume equivalent of 9.70 mL when the original sample is 10.00 mL (based on the conversion between titration equivalent and volume).

[0083] Result: ρ(Cl - = 3440 mg / L; compared with Example 1 (3090 mg / L), it is positively biased by about +11%, and the slope of the curve inflection point is smaller, and the data processing is sensitive to the selection of threshold.

[0084] In this complex matrix, potentiometric titration is affected by electrode poisoning / drift, resulting in lower accuracy and reproducibility compared to the optical method of this invention; it also requires more manual intervention and experience-based judgment.

[0085] These three sets of examples complement each other: Example 2 demonstrates that pretreatment is crucial against interference; Example 3 demonstrates that the optical determination of relative potential is more robust in such high-acid, high-metal matrices.

[0086] The embodiments described above are some, but not all, of the embodiments of this application. The detailed description of the embodiments of this application is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

Claims

1. A method for determining the chloride ion content in a complex matrix, characterized in that: Includes the following steps: a) Pretreatment of samples containing interfering ions, including sulfate and metal ions; b) Place the pretreated sample in an optical titration apparatus and titrate with silver nitrate standard solution as the titrant, while monitoring the changes in transmitted light signal and / or scattered light signal of the solution with the titration volume in real time; c) Smooth and differentiate the light signal-volume data collected in step b), and automatically determine the titration endpoint volume by identifying the characteristic points of the derivative curve; d) Calculate the chloride ion content in the sample based on the titration endpoint volume, the concentration of silver nitrate standard solution, and the sample amount.

2. The method for determining the chloride ion content in a complex matrix according to claim 1, characterized in that: In step a), the sulfate pretreatment involves adding a barium salt solution to the sample to generate barium sulfate precipitate, which is then removed after separation.

3. The method for determining the chloride ion content in a complex matrix according to claim 1, characterized in that: In step a), the metal ion pretreatment involves passing the sample through a cation exchange resin column or adjusting the pH to precipitate the metal ions as hydroxides before separation.

4. The method for determining the chloride ion content in a complex matrix according to claim 1, characterized in that: In step b), the optical signal is the intensity of transmitted light at a wavelength of 550 nm.

5. The method for determining the chloride ion content in a complex matrix according to claim 1, characterized in that: In step c), the smoothing process uses the Savitzky-Golay algorithm.

6. A method for determining the chloride ion content in a complex matrix according to claim 1 or 5, characterized in that: In step c), the differentiation process includes calculating the first derivative and / or the second derivative, where the feature point is an extreme point of the first derivative or a zero-crossing point of the second derivative.

7. A method for determining the chloride ion content in a complex matrix according to any one of claims 1-6, characterized in that: The system for implementing the method includes an automatic titrator, a light source, a photodetector, a signal converter, and a data processing unit, wherein the data processing unit is programmed to perform signal smoothing, differentiation, and automatic endpoint determination algorithms.

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