A method for detecting residual sulfate ion content after preparation of calcium carbonate from phosphogypsum

Abstract

This invention relates to the field of detection and analysis, specifically to a method for detecting the residual sulfate ion content after preparing calcium carbonate from phosphogypsum, comprising: preparing a standard curve; and preparing SO4. 2‑ Standard stock solution; with SO4 2‑ SO4 was prepared by dilution using the standard storage solution as a reference. 2‑ Standard solution; take different amounts of SO4 respectively 2‑ Standard solution, in SO4 2‑ Concentration is plotted on the x-axis and absorbance on the y-axis to construct a standard curve. Sample preparation and measurement: Take the sample to be tested, dissolve it in digesting acid, and boil to digest. Dilute the digested sample to a fixed volume, filter, and collect the filtrate for later use. Add indicator, deionized water, anti-interference agent, dielectric constant adjuster, and barium chloride solution to the filtrate, then make up to a fixed volume and let stand. Begin testing using a standard cuvette and read the absorbance value. Calculate the sulfate content of the test solution by referring to the standard curve. This method can quickly and accurately determine trace amounts of SO4 remaining in large quantities of carbonates. 2‑ The content of ions.

Description

Technical Field

[0001] This invention relates to the field of detection and analysis, and in particular to a method for detecting the residual sulfate ion content after calcium carbonate is prepared from phosphogypsum. Background Technology

[0002] Phosphogypsum is a large-scale industrial solid waste generated by phosphate chemical enterprises during the wet process of decomposing phosphate rock to produce phosphoric acid, phosphate fertilizers, and other products. For every ton of phosphoric acid produced, 4-6 tons of phosphogypsum are typically produced as a byproduct, making it one of the main waste residues in the phosphate chemical industry. The main component of phosphogypsum is calcium sulfate dihydrate (CaSO4·2H2O), with a content generally ranging from 70% to 90%, and in some cases exceeding 80%. It also contains free phosphoric acid, fluorides, phosphorus pentoxide, phosphates, and small amounts of heavy metals such as arsenic, lead, and cadmium, as well as radioactive elements and other impurities. The composition varies depending on the origin of the phosphate rock.

[0003] Due to the complex composition of phosphate rock, and the fact that these impurities ultimately end up in phosphogypsum, the comprehensive utilization of phosphogypsum is greatly limited by these impurities. With the development of the phosphate fertilizer industry, the emission of phosphogypsum is increasing at a rate of over 10% per year, and the utilization rate of less than 15% is leading to a gradual increase in the stockpiling of phosphogypsum, which has seriously constrained the development of the phosphate fertilizer industry.

[0004] The production of light calcium carbonate and co-production of ammonium sulfate from phosphogypsum can simultaneously utilize the calcium and sulfate ions in phosphogypsum, reducing the need for stockpiling at the source and achieving resource-based value-added utilization of waste. Therefore, the production of light calcium carbonate from phosphogypsum has become one of the optimal pathways for the comprehensive utilization of phosphogypsum in most phosphate chemical enterprises. To ensure the quality of light calcium carbonate produced from phosphogypsum, improving the conversion rate of phosphogypsum is crucial, that is, reducing the residual calcium sulfate in the light calcium carbonate product. Therefore, an efficient and accurate method for determining the residual trace sulfate ions in the solid product light calcium carbonate after the reaction is urgently needed.

[0005] Currently, the main methods for determining sulfate ions include barium sulfate precipitation turbidimetric method, barium sulfate precipitation gravimetric method (GB / T13025.8-2012), ICP sulfur elemental analysis method, ultraviolet-visible spectrophotometry (GB / T12690.21-2024), ICP sulfur elemental analysis method, and water quality barium chromate spectrophotometry method for determining sulfate ions in water samples (HJ / T342-2007), etc.

[0006] Due to the complexity of the system in the preparation of calcium carbonate samples from phosphogypsum, the residual trace amounts of SO4... 2- The methods described above are either unsuitable or too costly for content determination. Currently, there is no low-cost and accurate method. Summary of the Invention

[0007] The purpose of this invention is to provide a method for detecting residual sulfate ions after the preparation of calcium carbonate from phosphogypsum. This method is a UV-Vis spectrophotometric technique with advantages such as high accuracy and precision, simple operation, and low cost. It can rapidly and accurately determine trace amounts of SO4 remaining in large quantities of carbonates. 2- content.

[0008] The specific technical solution of the present invention is as follows: A method for detecting the residual sulfate ion content after preparing calcium carbonate from phosphogypsum, comprising: Creating a standard curve: Configure SO4 2- Standard stock solution; with SO4 2- SO4 was prepared by dilution using the standard storage solution as a reference. 2- Standard solution; take different amounts of SO4 respectively 2- Standard solution; with SO4 2- Concentration is plotted on the x-axis and absorbance on the y-axis to create a standard curve. Sample preparation and measurement: Take the sample to be tested, dissolve it with digesting acid, and boil it to digest. Dilute the digested sample to a fixed volume, filter it, and keep the filtrate for later use. Add indicator, deionized water, anti-interference agent, dielectric constant adjuster and barium chloride solution to the filtrate to be tested, make up the volume, and let it stand. Start the test using a standard cuvette and read the absorbance value. Calculate the sulfate content of the test solution by referring to the standard curve.

[0009] As a preferred option, SO4 2- Standard stock solution and SO4 2- The reference reagents used in the preparation of standard solutions include one or more of anhydrous sodium sulfate, anhydrous potassium sulfate, rubidium sulfate, cesium sulfate, and magnesium sulfate.

[0010] As a preferred option, a standard curve of 2-10 μg / mL is prepared, wherein the absorbance value obtained by different concentrations is less than 0.7.

[0011] As a preferred method, the digestion is carried out by heating and boiling for 1-2 minutes, and the digesting acid includes 10-15 mL of hydrochloric acid, hydroiodic acid, or hydrobromic acid.

[0012] As a preferred option, the volume after digestion is between 50 mL and 1000 mL.

[0013] As a preferred embodiment, the indicator includes 1-2 drops of p-nitrophenol, phenolphthalein, or thymolphthalein; the anti-interference agent includes 1-2 mL of hydrochloric acid, nitric acid, or EDTA solution or several thereof; and the dielectric constant adjuster includes 4-5 mL of a mixture of ethanol, glycerol, or methanol, wherein the amount of glycerol used is greater than the amount of ethanol or methanol.

[0014] As a preferred option, the sulfate content of the test solution is: Where c represents the absorbance of SO4 obtained by comparison. 2- The concentration is expressed in μg / mL; £ represents the dilution factor; V represents the test volume; and m represents the mass of the sample taken.

[0015] As a preferred option, different amounts of SO4 were taken. 2- Standard solution, in SO4 2- The standard curve is constructed with concentration on the x-axis and absorbance on the y-axis. The specific steps are as follows: Transfer different amounts of sulfate standard solution into colorimetric tubes, dilute with water, and mix well. Add 1-2 mL of anti-interference agent and 4-5 mL of dielectric constant adjuster, mixing well after each addition. Let stand, add 4-5 mL of barium chloride solution, dilute with water to the mark, and let stand for 10-15 minutes. Transfer the above well-mixed solution into a cuvette, using the reagent blank solution as a reference, and measure its absorbance with a spectrophotometer. Plot a working curve with sulfate amount as the abscissa and absorbance as the ordinate. Attached Figure Description

[0016] Figure 1 Standard curves obtained from sulfate samples with different sulfate contents in Example 1 of this invention. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased.

[0018] Unless otherwise stated, all percentages in this invention represent mass fractions. Ratios are mass percentages, and concentrations are mass concentrations.

[0019] Unless otherwise specified, all materials, instruments, and equipment used below are conventional materials, instruments, and equipment or obtained through commercial channels; all testing methods used are existing methods unless otherwise specified.

[0020] In existing technologies, the barium sulfate precipitation turbidimetric method and the barium sulfate precipitation gravimetric method have low accuracy and are subject to random errors, and are only suitable for salt chemical products containing a large amount of sulfate. The ICP sulfur elemental analysis method is a plasma emission spectroscopy method, which is expensive and has high maintenance costs, but has high detection sensitivity and is suitable for ppm-level trace analysis. Meanwhile, the ultraviolet-visible spectrophotometry method is an ultraviolet absorption spectroscopy method, and its detection principle is similar to that of ICP. However, its sensitivity is lower than that of ICP, but its operation is relatively simple and fast, and it is suitable for rapid analysis of a large number of samples.

[0021] Turbidimetric and gravimetric methods cannot accurately determine trace amounts of SO4. 2- The high content is mainly due to the large amount of Ca present in the calcium carbonate samples prepared from phosphogypsum. 2+ Yi and Ba 2+ Competition SO4 2- PO4 residue in phosphogypsum 3- Will be with Ba 2+ The formation of Ba3(PO4)2 precipitate readily adsorbs onto barium sulfate seed crystals, hindering crystal growth and significantly increasing the measurement errors of turbidimetric and gravimetric methods. Existing UV-Vis spectrophotometric methods do not employ metal ion masking agents to conceal the large amounts of Ca that may interfere with detection. 2+ Interference and weakly polar solvents such as alcohols alter the dielectric constant of the system, leading to significant measurement errors. Furthermore, conventional standard methods are not optimized for this composite matrix: gravimetric methods are prone to co-precipitation, turbidimetric methods are greatly affected by color and particle size, and ultraviolet spectrophotometry suffers from significant matrix interference; none of these methods can be directly applied.

[0022] Therefore, this invention uses ultraviolet-visible spectrophotometry to detect residual sulfate in the preparation of light calcium carbonate from phosphogypsum and compares the results with those from ICP detection. The relative standard deviation is compared to ensure the accuracy of the detection results and to provide a reference for optimizing the process of preparing light calcium carbonate from phosphogypsum.

[0023] Furthermore, this invention provides a method for detecting residual sulfate ions after preparing calcium carbonate from phosphogypsum, comprising: Creating a standard curve: Configure SO4 2- Standard stock solution; with SO4 2- SO4 was prepared by dilution using the standard storage solution as a reference. 2- Standard solution; take different amounts of SO4 respectively 2- Standard solution, in SO4 2- Concentration is plotted on the x-axis and absorbance on the y-axis to create a standard curve. Sample preparation and measurement: Take the sample to be tested, dissolve it with digesting acid, and boil it to digest. Dilute the digested sample to a fixed volume, filter it, and keep the filtrate for later use. Add indicator, deionized water, anti-interference agent, dielectric constant adjuster and barium chloride solution to the filtrate to be tested, make up the volume, and let it stand. Start the test using a standard cuvette and read the absorbance value. Calculate the sulfate content of the test solution by referring to the standard curve.

[0024] This method is applicable to the rapid and accurate monitoring of the reaction process in the preparation of calcium carbonate from phosphogypsum. By using anti-interference reagents and alcohols, such as ethanol and glycerol, the dielectric constant of the overall solvent is altered, causing the barium sulfate precipitate to be uniformly dispersed into fine particles. Anti-interference reagents HCl, HNO3, and HClO4 are used to adjust the pH, converting residual P in the sample into a form that is less reactive with Ba. 2+ The precipitated phosphoric acid and EDTA solution complex a large number of calcium ions, preventing calcium sulfate from being encapsulated. This two-step process, combined into one, achieves a lower relative standard deviation and higher accuracy compared to traditional methods that do not require anti-interference agents or alcohol reagents. This method is the first to apply Lambert-Beer's law to the monitoring of trace sulfate ions in calcium carbonate samples prepared from phosphogypsum.

[0025] The reaction principle of this invention includes: As one implementation method, SO4 2- Standard stock solution and SO4 2- The primary standard reagents used in the preparation of standard solutions include one or more of anhydrous sodium sulfate, anhydrous potassium sulfate, rubidium sulfate, cesium sulfate, and magnesium sulfate. For example, anhydrous sodium sulfate or anhydrous potassium sulfate. Primary standard reagents mainly consist of sulfates that are soluble, completely ionized, and stable in aqueous solution. SO4 2- The standard stock solution can be prepared at a concentration of 1 mg / mL; it can be prepared at a concentration of 1 mg / mL of SO4. 2- Using the standard stock solution as a reference, SO42- was prepared by dilution to a concentration of 100 μg / mL. 2- Standard solution.

[0026] As one implementation method, a standard curve of 2-10 μg / mL is prepared, wherein the absorbance values ​​obtained from different concentrations are set to be below 0.7 to ensure a good correlation between content and concentration.

[0027] In one implementation method, the digestion is carried out by heating and boiling for 1-2 minutes. The digesting acid, consisting of 10-15 mL of hydrochloric acid, hydroiodic acid, or hydrobromic acid, is used. For example, the digesting acid may be 10 mL, 12 mL, or 15 mL of hydrochloric acid or hydrobromic acid. An acid capable of reacting fully with the carbonate to form a soluble salt is selected to dissociate trace amounts of sulfate from the calcium carbonate solid.

[0028] In one implementation method, the volume after digestion is between 50 mL and 1000 mL, preferably 250 mL. The content can be preliminarily determined based on the process of preparing calcium carbonate from phosphogypsum. For high concentrations, the volume can be adjusted to 1 L, and for low concentrations, it can be adjusted to 50 mL, or between 50 mL and 1000 mL.

[0029] In one embodiment, the 1-2 drops of indicator include one of p-nitrophenol, phenolphthalein, and thymolphthalein, for example, p-nitrophenol or phenolphthalein can be selected; the anti-interference agent includes one or more of 1-2 mL of hydrochloric acid, nitric acid, and EDTA solution; the dielectric constant adjuster includes a mixture of several of 4-5 mL of ethanol, glycerol, and methanol, wherein the amount of glycerol used is greater than the amount of ethanol or methanol, for example, a mixture of ethanol and glycerol in a ratio of 2:3 or 1:3.

[0030] Indicators are generally chosen to be colorless under acidic conditions and capable of developing color under alkaline conditions. Anti-interference agents are used to eliminate interference from carbonate, sulfite, and phosphate ions. Alcohol reagents are used to lower the dielectric constant of the aqueous solution, preventing uneven precipitation and dispersion that could cause measurement errors.

[0031] As one implementation method, the sulfate content of the test solution is: Where c represents the absorbance of SO4 obtained by comparison. 2- The concentration is expressed in μg / mL; £ represents the dilution factor; V represents the test volume; and m represents the mass of the sample. This formula is mainly based on the absorbance corresponding to the sulfate content data, the dilution factor, and the volume, etc., to continuously calculate and convert the sulfate content in the sample to be tested.

[0032] As one implementation method, different amounts of SO4 were taken respectively. 2- Standard solution, in SO4 2- The standard curve is constructed with concentration on the x-axis and absorbance on the y-axis. The specific steps are as follows: Transfer different amounts of sulfate standard solution into colorimetric tubes, dilute with water, and mix well. Add 1-2 mL of anti-interference agent and 4-5 mL of dielectric constant adjuster, mixing well after each addition. Let stand, then add 4-5 mL of barium chloride solution, dilute to the mark with water, and let stand. Transfer the thoroughly mixed solution into a cuvette, using a reagent blank solution as a reference, and measure its absorbance using a spectrophotometer. Plot a working curve with sulfate amount on the x-axis and absorbance on the y-axis. This process eliminates interference from carbonate, sulfite, and phosphate ions, reduces the dielectric constant of the aqueous solution, and avoids measurement errors caused by uneven precipitation and dispersion, laying the foundation for accurate subsequent determinations.

[0033] To further illustrate the present invention, the following describes in detail, with reference to embodiments, a method for detecting residual sulfate ions after preparing calcium carbonate from phosphogypsum according to the present invention.

[0034] Measuring instruments: N2S type UV-Vis spectrophotometer; Analytical balance: maximum weighing capacity 220 g, minimum weighing capacity 10 mg; Small beaker: 100 mL; Volumetric flask: 250 mL; Resistance furnace: Rated temperature 380 ℃; Pipette: 10 mL; Pipettes: 5 mL, 1 mL; Anhydrous sodium sulfate: AR, purity 99%; Acids to be dissolved: 1+1 hydrochloric acid (a mixture of 1 part 37.5% concentrated hydrochloric acid and 1 part deionized water), 1+1 hydroiodic acid (a mixture of 1 part 57% hydroiodic acid and 1 part deionized water), 1+1 hydrobromic acid (a mixture of 1 part 48% hydrobromic acid and 19 parts deionized water), 1+1 nitric acid (a mixture of 1 part 65% concentrated hydrochloric acid and 1 part deionized water). Dielectric constant modifiers: anhydrous ethanol, 1+1 glycerol (a mixed solution of 1 part analytical grade glycerol and 1 part deionized water), anhydrous methanol; Anti-interference agents: 1+19 hydrochloric acid (a mixture of 1 part 37.5% concentrated hydrochloric acid and 19 parts deionized water), 1+19 nitric acid (a mixture of 1 part 65% concentrated hydrochloric acid and 19 parts deionized water), 1+19 perchloric acid (a mixture of 1 part 70% perchloric acid and 19 parts deionized water), 0.02MEDTA.

[0035] The specific source of the samples to be tested: several calcium carbonate samples prepared by reacting desilicationized phosphogypsum with ammonium carbonate by Tianan Chemical. Example 1

[0036] The method for detecting the residual sulfate ion content after preparing calcium carbonate from phosphogypsum in this embodiment includes the following steps: Standard curve creation: Step (1) SO4 2- Standard stock solution: Weigh 1.4786 g of sodium sulfate that has been dried at 110 °C for 1 h and cooled to room temperature in a desiccator, place it in a 100 mL beaker, add 50 mL of water to dissolve and clarify it, then transfer it to a 1 L volumetric flask, dilute to the mark with water, and mix well.

[0037] Step (2) SO4 2-Standard solution: Transfer 10 mL of the sulfate standard stock solution from step (1) into a 100 mL volumetric flask, dilute to the mark with water, mix well, and obtain 100 μg / mL SO4. 2- Standard solution.

[0038] Step (3) Reagent preparation: Transfer 0.00 mL, 0.50 mL, 1.00 mL, 1.50 mL, 2.00 mL, and 2.50 mL of sulfate standard solution into 25 mL colorimetric tubes, dilute with water to 15 mL, and mix well. Add 1 mL of 1+19 hydrochloric acid, 0.5 mL of EDTA, 3 mL of 1+1 glycerol solution, and 1 mL of anhydrous ethanol. Mix well after each addition and let stand for 1 min. Add 4 mL of barium chloride solution, dilute with water to the mark, and let stand for 10 min.

[0039] Step (4) Measurement: Transfer the thoroughly mixed solution from step (3) into a 1 cm cuvette. Using the reagent blank solution as a reference, measure its absorbance at a wavelength of 400 nm using a spectrophotometer. Plot a working curve with sulfate content as the x-axis and absorbance as the y-axis. The standard curve is shown below. Figure 1 As shown.

[0040] Sample preparation and measurement: Step (1) Weigh about 0.6 g of the sample to be tested (accuracy 0.001 g) using an analytical balance, dissolve it in 10 mL of 1+1 hydrochloric acid, and heat to boil for 1.5 min to digest.

[0041] The component content of sample 1 is shown in Table 1: Table 1. Content of each element in the calcium carbonate sample to be tested Step (2) Dilute the digested sample to a volumetric flask containing 250 mL, filter, and take 10.00 mL of the filtrate for later use.

[0042] Step (3): Add 1 drop of p-nitrophenol indicator, 4 mL of deionized water, 0.5 mL of EDTA solution, 1 mL of 1+19 hydrochloric acid, 3 mL of 1+1 glycerol, 1 mL of anhydrous ethanol, and 4 mL of barium chloride solution to the filtrate to be tested, and then make up to 25 mL and let stand for 10 min. Step (4): Start the test using a 1 cm cuvette and read the absorbance (Abs) value. Calculate the sulfate content of the test solution by referring to the standard curve.

[0043] According to the calculation formula SO4 was found in the calcium carbonate sample. 2- The content of.

[0044] In the formula: C------------Standard curve concentration, μg / mL; £-------------Dilution factor (25 mL); V-------------Sample volume (25 mL); m-------------Weigh out the sample amount (0.6±0.001 g).

[0045] The conditions for Examples 2-8 are shown in Table 2 below: Table 2 Conditions of Examples 2-8 The samples to be tested correspond to samples 2-8, and the other conditions are the same as in Example 1.

[0046] The conditions for Comparative Examples 1-10 are shown in Table 3 below: Table 3 Conditions for Comparative Examples 1-10 Comparative Example S11: GB / T13025.8-2012.

[0047] Comparative Example S12: GB / T12690.21-2024.

[0048] Comparative Example S13: HJ / T342-2007.

[0049] The test sample corresponds to sample 9-21, and the other conditions are the same as in Example 1.

[0050] Verification against ICP: The measurements were performed according to Tables 2 and 3, and the results were compared with the ICP measurement results, as shown in Table 4: Table 4. Detection results for different embodiments As shown in Table 4, in the embodiments of the present invention, the interference of phosphate ions was eliminated by HCl and the large amount of Ca was eliminated by EDTA complexation in the preparation of calcium carbonate samples from phosphogypsum. 2+ After interference, its trace amounts of SO4 2- The accuracy of the test was significantly improved, with SO42- showing a significant improvement after treatment with 1 mL of 1+1 HCl and (0.5 mL, 0.02 M) EDTA. 2- The measurement accuracy is only 0.105 relative standard deviations compared to ICP determination, making it the most accurate method. Its accuracy surpasses that of existing barium chromate methods for determining trace SO4 in water. 2- The industry standard HJ / T342-2007 and the national standard GB / T12690.21-2024 for the determination of sulfate content in rare earth oxides.

[0051] Comparative Examples S2 and S3 illustrate the crucial importance of the type of digesting acid; choosing other acids leads to a higher relative standard deviation, which is inferior to existing techniques. Comparative Examples S1 and S4-S7 show that equal amounts of ethanol and glycerol also result in a higher relative standard deviation. Comparative Examples S8-9 demonstrate that a single dielectric constant modifier is also ineffective; the choice of alcohol type and dosage is critical. Example S10 shows that choosing other acids as the anti-interference agent also leads to a higher relative standard deviation; the selection of the appropriate type is very important.

[0052] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for detecting the residual sulfate ion content after preparing calcium carbonate from phosphogypsum, characterized in that, include: Creating a standard curve: Configure SO4 2- Standard stock solution; with SO4 2- SO4 was prepared by dilution using the standard storage solution as a reference. 2- Standard solution; take different amounts of SO4 respectively 2- Standard solution; with SO4 2- Concentration is plotted on the x-axis and absorbance on the y-axis to create a standard curve. Sample preparation and measurement: Take the sample to be tested, dissolve it with digesting acid, and boil it to digest. Dilute the digested sample to a fixed volume, filter it, and keep the filtrate for later use. Add indicator, deionized water, anti-interference agent, dielectric constant adjuster and barium chloride solution to the filtrate to be tested, make up the volume, and let it stand. Start the test using a standard cuvette and read the absorbance value. Calculate the sulfate content of the test solution by referring to the standard curve.

2. The detection method according to claim 1, characterized in that: SO4 2- Standard stock solution and SO4 2- The reference reagents used in the preparation of standard solutions include one or more of anhydrous sodium sulfate, anhydrous potassium sulfate, rubidium sulfate, cesium sulfate, and magnesium sulfate.

3. The detection method according to claim 1, characterized in that: Prepare standard curves for concentrations of 2-10 μg / mL, where the absorbance values ​​obtained at different concentrations are below 0.

7.

4. The detection method according to claim 1, characterized in that: Heat and boil for 1-2 minutes to digest the acid, which includes 10-15 mL of hydrochloric acid, hydroiodic acid, or hydrobromic acid.

5. The detection method according to claim 1, characterized in that: After digestion, the final volume should be between 50 mL and 1000 mL.

6. The detection method according to claim 1, characterized in that: The indicator includes 1-2 drops of one of p-nitrophenol, phenolphthalein, or thymolphthalein; the anti-interference agent includes one or more of hydrochloric acid, nitric acid, or EDTA solution in 1-2 mL; the dielectric constant adjuster includes a mixture of several of ethanol, glycerol, and methanol in 4-5 mL, wherein the amount of glycerol used is greater than the amount of ethanol or methanol.

7. The detection method according to claim 1, characterized in that: Sulfate content in the test solution: Where c represents the absorbance of SO4 obtained by comparison. 2- The concentration is expressed in μg / mL; £ represents the dilution factor; V represents the test volume; and m represents the mass of the sample taken.

8. The detection method according to claim 1, characterized in that: Take different amounts of SO4 2- Standard solution, in SO4 2- The standard curve is constructed with concentration on the x-axis and absorbance on the y-axis. The specific steps are as follows: Transfer different amounts of sulfate standard solution into colorimetric tubes, dilute with water, and mix well. Add 1-2 mL of anti-interference agent and 4-5 mL of dielectric constant adjuster, mixing well after each addition. Let stand, add 4-5 mL of barium chloride solution, dilute with water to the mark, and let stand for 10-15 minutes. Transfer the above well-mixed solution into a cuvette, using the reagent blank solution as a reference, and measure its absorbance with a spectrophotometer. Plot a working curve with sulfate amount as the abscissa and absorbance as the ordinate.

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