Method for rapidly determining fluorine content in industrial sulfuric acid

Abstract

The invention discloses a method for rapidly determining fluorine content in industrial sulfuric acid, and belongs to the technical field of sulfuric acid analysis and detection. The content of sulfuric acid in industrial sulfuric acid is larger than 92.5%, a large amount of sulfuric acid interferes with fluorine determination, a working curve is drawn in a matrix matching mode to eliminate interference of high-concentration sulfuric acid, an ion-selective electrode method is adopted to determine a potential value, and sodium acetate and a trisodium citrate-potassium nitrate buffer solution are combined to optimize system stability. The rapid determination of the fluorine content in the industrial sulfuric acid is realized. The method has the characteristics of simplicity and rapidness in operation, high sensitivity, less interference, high safety, good accuracy and the like, and is suitable for industrial sulfuric acid production quality control.

Description

Technical Field

[0001] The invention belongs to the technical field of industrial sulfuric acid analysis and detection, and particularly relates to a method for quickly determining the fluorine content in industrial sulfuric acid. Background Art

[0002] During the industrial sulfuric acid production process, fluoride in the raw flue gas is absorbed and converted into fluorinated sulfuric acid. Due to its highly corrosive, oxidizing, and crystalline properties, this fluorinated sulfuric acid poses a serious threat to production equipment such as pipelines, valves, and pumps. This leads to frequent corrosion and crystallization blockages, severely hindering the effective application and market expansion of the final product—industrial sulfuric acid. Therefore, accurate and quantitative determination of the fluoride ion content in industrial sulfuric acid is crucial to ensuring a smooth production process and improving product quality.

[0003] Currently, industrial sulfuric acid analysis complies with the national standard GB / T 534-2014. This standard covers the analysis of multiple items, including sulfuric acid, iron, ash, arsenic, lead, mercury, transparency, and color, but does not include a method for analyzing fluorine. To meet the needs of industrial sulfuric acid production, there is an urgent need to develop an analytical method that can accurately and rapidly determine fluorine content. This method would provide timely and accurate data support for the production process, ensure the stable operation of the entire production system, and promote the continuous optimization and improvement of product quality. Summary of the Invention

[0004] The purpose of this application is to provide an analytical method for rapidly determining the fluorine content in industrial sulfuric acid, in response to the problem that the current national analytical standard for industrial sulfuric acid (GB534-2014) does not contain an analytical method for fluorine.

[0005] The purpose of this application is achieved through the following technical solutions: A method for rapidly determining the fluorine content in industrial sulfuric acid, characterized by comprising the following steps: (1) Draw a matrix matching working curve: add fluorine standard solutions of different concentrations to sulfuric acid, and then add sodium acetate solution, trisodium citrate-potassium nitrate buffer solution and bromocresol green indicator in sequence, adjust the pH to 5.0-6.0, and obtain a series of fluorine-containing sulfuric acid standard solutions with a fluorine concentration gradient of 0.10-1.00 mg / L. Measure the potential of the series of fluorine-containing sulfuric acid standard solutions, and draw a working curve with the negative logarithm of the fluoride ion concentration as the horizontal axis and the potential value as the vertical axis; (2) Sample determination: Take an industrial sulfuric acid sample, add sodium acetate solution, trisodium citrate-potassium nitrate buffer solution and bromocresol green indicator in sequence, adjust the pH to 5.0-6.0 with potassium hydroxide solution, measure the potential value and calculate the fluorine content through the working curve.

[0006] The linear equation of the working curve is: y=54.7901lgx-248.3162, with a correlation coefficient of 0.9999, where y is the potential value of the standard series of fluorine-containing sulfuric acid solutions, in mV, and x is the negative logarithm of the fluoride ion concentration.

[0007] The sampling volume of the industrial sulfuric acid sample is 2 mL; the added amount of the sodium acetate solution is 45-55 mL, with a concentration of 150 g / L, which is used to neutralize part of the sulfuric acid and stabilize the system; the added amount of the trisodium citrate-potassium nitrate buffer solution is 20 mL, which is used to maintain ionic strength and prevent crystallization interference; the concentration of the bromocresol green indicator is 1 g / L.

[0008] Acidity adjustment adopts a step-by-step method: first, sodium acetate and trisodium citrate-potassium nitrate buffer solution are added to neutralize part of the sulfuric acid, and then 500g / L potassium hydroxide solution is used to accurately adjust the pH. When adjusting the pH value, the color of the bromocresol green indicator changes from yellow to blue-green to determine whether the solution pH value has reached the preset range, thus avoiding direct contact with strong acids and frequent pH measurements. The detection limit of the method is 0.0050 mg / L, the lower limit of determination is 0.017 mg / L; the spiked recovery rate is 85.2% to 113.3%, and the relative standard deviation is 3.78% to 7.27%.

[0009] Industrial sulfuric acid contains more than 92.5% sulfuric acid, and large amounts of this interfere with fluorine determination. This invention addresses the interference of high-concentration sulfuric acid (greater than 92.5%) in industrial sulfuric acid by plotting a working curve using a matrix-matching method, effectively eliminating this interference. The advantages of the ion-selective electrode method, namely simple operation, rapid speed, high sensitivity, minimal interference, and high accuracy, enable rapid and accurate determination of the fluorine content in industrial sulfuric acid. Utilizing the color change of the bromocresol green indicator to determine and adjust the pH of the solution avoids the risk of direct contact with strong acids, reduces the number of direct pH measurements, and improves the safety and speed of analysis. Sodium acetate solution and trisodium citrate-potassium nitrate buffer solution are pre-added to neutralize some of the sulfuric acid and maintain the system within an appropriate pH range. Potassium hydroxide solution is then added for accurate pH adjustment, reducing the amount of strong base used, saving costs, while improving operational safety and ensuring the accuracy of the measurement results. When using bromocresol green as an indicator to adjust the pH value, the pH value of the solution can be judged by observing the color of the indicator change from yellow to blue-green, thereby avoiding direct contact with strong acid and frequent pH value measurements.

[0010] Beneficial effects of the present invention: 1. High accuracy and sensitivity A matrix-matching approach was used to draw the working curve, eliminating the interference of high-concentration sulfuric acid (>92.5%) on the fluoride ion determination, stabilizing the recovery rate between 85.2% and 113.3%, and achieving a detection limit as low as 0.0050 mg / L, which is superior to traditional spectrophotometry (usually with a detection limit >0.1 mg / L).

[0011] The use of trisodium citrate-potassium nitrate buffer solution can prevent crystals from adhering to the electrode (Table 4) and ensure the stability of potential measurement, with a relative standard deviation (RSD) of ≤7.27% (Table 7).

[0012] 2. Strong anti-interference ability The step-by-step neutralization strategy (sodium acetate + buffer solution first, then potassium hydroxide) effectively solved the splashing problem when concentrated sulfuric acid was used to adjust the pH directly. At the same time, the buffer system (pH 5.0-6.0) suppressed the competitive response of H⁺ to the fluoride electrode.

[0013] 3. Quickly simplify the process The single measurement time is less than 15 minutes (including sample pretreatment), which is more than 80% more efficient than fluorine analysis processes not covered by the national standard GB / T 534-2014 (such as the distillation-colorimetry method which takes more than 2 hours).

[0014] The bromocresol green indicator can be used to visually adjust pH (the color changes to blue-green) instead of frequent pH test paper measurements, reducing the number of operating steps by 3 to 5.

[0015] 4. Reduced security risks By pre-adding sodium acetate solution to neutralize part of the sulfuric acid, the amount of strong alkali (500 g / L KOH) used can be reduced by more than 50% to avoid violent exotherm and splashing.

[0016] 5. Cost savings Reagents are inexpensive and readily available (e.g., sodium acetate, trisodium citrate), whereas traditional methods require expensive chromatographic columns or specialized colorants (e.g., zirconium-alizarin reagent). Extended electrode life: The buffer system reduces crystal contamination, increasing electrode life by 2-3 times.

[0017] 6. Wide adaptability: Suitable for industrial samples with different fluorine concentrations (0.017-50 mg / L) and sulfuric acid concentrations (92.5%-98%) (see Table 7 for multi-sample test data), covering the needs of smelting, chemical industry and other scenarios. DETAILED DESCRIPTION

[0018] The present invention will be further described below through specific embodiments.

[0019] The instruments required for use in the present invention are: a pH meter and a magnetic stirrer.

[0020] The reagents used are: Sulfuric acid (ρ1.84 g / mL), high purity.

[0021] Glacial acetic acid (ρ1.05 g / mL).

[0022] Nitric acid (1+1).

[0023] Potassium hydroxide (500g / L).

[0024] Sodium acetate (150g / L).

[0025] Trisodium citrate-potassium nitrate buffer solution: Weigh 294.1 g of trisodium citrate and 20 g of potassium nitrate and dissolve them in 700 mL of water. Adjust the pH to 5.0-6.0 with nitric acid (1+1). Transfer the solution to a 1000 mL volumetric flask, dilute to the mark with water, and mix well.

[0026] CDTA-sodium chloride buffer solution: Weigh 58 g of sodium chloride and 4.0 g of cyclohexanediaminetetraacetic acid (CDTA) and dissolve them in 500 mL of water. Add 57 mL of glacial acetic acid (ρ1.05 g / mL), stir to dissolve, and place in a cold water bath. Add potassium hydroxide (500 g / L) while stirring continuously to adjust the pH to 5.0-6.0. Transfer to a 1000 mL volumetric flask, dilute to the scale with water, and mix well.

[0027] Fluorine standard stock solution: Weigh 0.2211 g of standard sodium fluoride (dried in an oven at 105°C–110°C for 2 hours) into a 100 mL beaker. Dissolve in water. Transfer to a 1000 mL volumetric flask, dilute to the mark, and mix thoroughly. Store in a polyethylene bottle. 1 mL of this solution contains 100 μg of fluorine.

[0028] Fluorine standard solution: Pipette 10.00 mL of fluorine standard stock solution into a 100 mL volumetric flask, dilute to the mark with water, and mix thoroughly. This solution contains 10 μg of fluorine per mL. Prepare immediately before use.

[0029] Bromocresol green indicator (1 g / L): Weigh 0.1 g of bromocresol green, dissolve it in ethanol, and dilute to 100 mL with ethanol.

[0030] Fluoride ion selective electrode.

[0031] Saturated calomel electrode.

[0032] 1. Drawing of matrix matching working curve Pipette 2.00 mL of high-purity sulfuric acid into five 100 mL volumetric flasks. Add 1.00 mL, 3.00 mL, 5.00 mL, 7.00 mL, and 10.00 mL of a fluorine standard solution (10 μg / mL), respectively. Dilute with a small amount of water, then add 45 mL of sodium acetate solution (150 g / L), 20 mL of trisodium citrate-potassium nitrate buffer, and 2 drops of bromocresol green indicator (1 g / L). Adjust the pH to 5.0-6.0 with nitric acid (1+1), dilute to the mark with water, and mix thoroughly. Prepare a matrix-matched standard series of solutions with fluorine concentrations of 0.10 mg / L, 0.30 mg / L, 0.50 mg / L, 0.70 mg / L, and 1.00 mg / L, respectively. Pour the solutions into a 150 mL dry polyethylene beaker, place a polytetrafluoroethylene stir bar, and insert the electrode. Under stirring with a magnetic stirrer, the potential was measured on a pH meter. A working curve was plotted with the negative logarithm of the fluoride ion concentration (mg / L) as the abscissa and the potential as the ordinate. The correlation coefficient was greater than 0.999. The measurement data and working curve equation are shown in Table 1.

[0033] 2. Sample determination Transfer 2.00 mL of industrial sulfuric acid sample to a 100 mL volumetric flask, add 45 mL of sodium acetate solution (150 g / L), 20 mL of trisodium citrate-potassium nitrate buffer solution, and 2d bromocresol green indicator (1 g / L). Adjust the pH to 5.0-6.0 with potassium hydroxide solution (500 g / L). Cool, dilute to the mark with water, and mix thoroughly. Pour the sample into a 150 mL dry polyethylene beaker, place a polytetrafluoroethylene stir bar, and insert the electrode. Stir with a magnetic stirrer. Once the potential stabilizes (electrode potential change no greater than 2 mV / min), read the millivolt reading. Before each measurement, rinse the electrode thoroughly with water and dry with filter paper. Calculate the fluoride content based on the measured potential and the working curve.

[0034] 3. Matrix interference test To investigate the effect of the sulfuric acid matrix on the element fluorine to be measured, 0.30 mg / L, 0.50 mg / L, and 1.00 mg / L fluorine standard solutions were used as research objects. Different volumes of sulfuric acid (high purity) were added, and the samples were measured according to the sample determination method. The results are shown in Table 2.

[0035] The experiment showed that when the amount of sulfuric acid added was between 0 mL and 0.5 mL, the fluorine concentration did not change much. As the amount of sulfuric acid added increased, the fluorine concentration decreased, indicating that sulfuric acid interfered with the determination.

[0036] 4. Sample dosage selection Taking into account the acidity of industrial sulfuric acid, experimental operation risks and sample representativeness, the sample sampling volume was selected as 2.00 mL to ensure that the matrix content of the matrix matching standard series solution was consistent with that of the sample. The amount of sulfuric acid matrix added to the matrix matching standard series solution was 2 mL.

[0037] 5. Selection of sodium acetate addition amount Pipette 5.00 mL of a fluorine standard solution (10 μg / mL) into a 100 mL volumetric flask, dilute to the mark with high-purity sulfuric acid, and mix thoroughly. This solution, with a fluorine concentration of 5.00 mg / L, serves as the quality control sample. Pipette 2.00 mL of the quality control sample and seven aliquots of each of three industrial sulfuric acid samples with varying fluorine contents into a pair of 100 mL volumetric flasks. Add varying concentrations of sodium acetate solution and perform the assay according to the sample assay method. The results are shown in Table 3.

[0038] It can be seen from Table 3 that when the amount of sodium acetate added is 40 mL ~ 55 mL, the determination results of fluorine in concentrated sulfuric acid samples tend to be stable. Therefore, the amount of sodium acetate added is selected to be 45 mL.

[0039] 6. Selection of total ionic strength buffer solution Industrial sulfuric acid samples with different fluoride contents were selected, and different types of total ionic strength buffer solutions were added. Parallel sample determination was performed according to the sample determination method. The results are shown in Table 4.

[0040] The test results showed that when CDTA-sodium chloride buffer solution was added to the sample, crystallization occurred in the test solution. These crystals adhered to the electrode during the measurement, resulting in a low result. However, when trisodium citrate-potassium nitrate buffer solution was added to the sample, the test solution became clear and crystal-free, and the measurement results were stable. Therefore, potassium nitrate-trisodium citrate buffer solution was selected.

[0041] 7. Selection of the amount of trisodium citrate-potassium nitrate buffer solution 2.00 mL of quality control sample and 6 portions of 3 industrial sulfuric acid samples with different fluoride contents were respectively transferred into a set of 100 mL volumetric flasks. Different amounts of trisodium citrate-potassium nitrate buffer solution were added and the samples were tested according to the sample test method. The results are shown in Table 5.

[0042] It can be seen from Table 5 that when the amount of trisodium citrate-potassium nitrate buffer solution added is 18 mL~25 mL, the determination results of fluorine in industrial sulfuric acid tend to be stable. Therefore, the amount of trisodium citrate-potassium nitrate buffer solution added is selected to be 20 mL.

[0043] 8. Choice of acidity adjustment method Industrial sulfuric acid contains over 92.5% sulfuric acid. Adding potassium hydroxide solution to adjust the pH can cause severe splashing. To ensure accurate results and safe operation, the pH of the solution is adjusted after adding the reagent. The procedure is as follows: Pipette 2.00 mL of concentrated sulfuric acid sample into a 100 mL volumetric flask, add 45 mL of sodium acetate solution (150 g / L), 20 mL of trisodium citrate-potassium nitrate buffer solution, and 2d bromocresol green indicator (1 g / L). Adjust the pH to 5.0-6.0 with potassium hydroxide solution (500 g / L), cool, dilute to the mark with water, and mix thoroughly. Perform the determination according to the sample determination method.

[0044] 9. Detection limit and determination limit of the method Because the fluorine content in the reagent blank was low, a 0.03 mg / L fluorine standard solution was added to the reagent blank and the sample determination method was used for 11 consecutive measurements. The detection limit was 3 times the standard deviation of the measurement results, and the lower limit of determination was 10 times the standard deviation. The results are shown in Table 6.

[0045] As can be seen from Table 6, the detection limit of this method is 0.0050 mg / L and the lower limit of determination is 0.017 mg / L.

[0046] 10. Precision experiment Industrial sulfuric acid samples with different fluorine contents were selected and precision experiments were carried out according to the sample determination method. The measurements were repeated 11 times and the relative standard deviation was calculated. The results are shown in Table 7.

[0047] As shown in Table 7, the relative standard deviation of fluorine is between 3.78% and 7.27%, with good reproducibility, which meets the requirements of the analytical method.

[0048] 11. Spike recovery test To examine the accuracy of the method, industrial sulfuric acid samples containing varying concentrations of the element to be measured were spiked with different concentrations of the element to be measured and subjected to spike recovery tests. The results of the spike recovery tests were performed according to the sample determination method, and are shown in Table 8.

[0049] As can be seen from Table 8, the recovery rate of fluorine in sulfuric acid determined by the ion selective electrode method is between 85.2% and 113.3%. This method has good accuracy and can meet the requirements for the determination of fluoride ion content in sulfuric acid.

Claims

1. A method for rapidly determining the fluorine content in industrial sulfuric acid, characterized in that: The following steps are involved: (1) Draw a matrix matching working curve: add fluorine standard solutions of different concentrations to sulfuric acid, and then add sodium acetate solution, trisodium citrate-potassium nitrate buffer solution and bromocresol green indicator in sequence, adjust the pH to 5.0-6.0, and obtain a series of fluorine-containing sulfuric acid standard solutions with a fluorine concentration gradient of 0.10-1.00 mg / L. Measure the potential of the series of fluorine-containing sulfuric acid standard solutions, and draw a working curve with the negative logarithm of the fluoride ion concentration as the horizontal axis and the potential value as the vertical axis; (2) Sample determination: Take an industrial sulfuric acid sample, add sodium acetate solution, trisodium citrate-potassium nitrate buffer solution and bromocresol green indicator in sequence, adjust the pH to 5.0-6.0 with potassium hydroxide solution, measure the potential value and calculate the fluorine content in the industrial sulfuric acid through the working curve.

2. The method according to claim 1, wherein: The linear equation of the working curve is: y=54.7901lgx-248.3162, with a correlation coefficient of 0.9999, where y is the potential value of the standard series of fluorine-containing sulfuric acid solutions, in mV, and x is the negative logarithm of the fluoride ion concentration.

3. The method according to claim 1, wherein: The sampling volume of the industrial sulfuric acid sample is 2 mL; the added amount of the sodium acetate solution is 45-55 mL, with a concentration of 150 g / L, which is used to neutralize part of the sulfuric acid and stabilize the system; the added amount of the trisodium citrate-potassium nitrate buffer solution is 20 mL, which is used to maintain ionic strength and prevent crystallization interference; the concentration of the bromocresol green indicator is 1 g / L.

4. The method according to claim 1, wherein: Acidity adjustment was performed using a step-by-step method: sodium acetate and trisodium citrate-potassium nitrate buffer solution were first added to neutralize part of the sulfuric acid, and then 500 g / L potassium hydroxide solution was used to accurately adjust the pH.

5. The method according to claim 1, wherein: The pH adjustment is determined by the color change of the bromocresol green indicator (yellow→blue-green), avoiding direct pH measurement.

6. The method according to any one of claims 1 to 5, characterized in that: The detection limit of the method is 0.0050 mg / L, the lower limit of determination is 0.017 mg / L; the spiked recovery rate is 85.2% to 113.3%, and the relative standard deviation is 3.78% to 7.27%.

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