A method for detecting hexavalent chromium in cement

Abstract

This invention discloses a method for detecting hexavalent chromium in cement, belonging to the field of cement quality testing technology. The method includes: preparation of standard solution, preparation of colorimetric reagent, plotting of standard curve, cement sample processing, sample determination, and waste liquid treatment. This invention solves the problems of insufficient detection stability, non-standardized operation procedures, and poor environmental performance of existing technologies. It has the advantages of accurate detection, simple operation, and safety and environmental protection, and can be widely applied to cement production quality control and environmental testing scenarios.

Description

Technical Field

[0001] This invention belongs to the field of cement quality testing technology, specifically relating to a method for detecting hexavalent chromium in cement. Background Technology

[0002] Cement, as a basic building material, is widely used in various construction projects, roads and bridges, water conservancy facilities, and other fields. Its quality, safety, and environmental performance are directly related to the quality of the project and the ecological environment. Hexavalent chromium is a common harmful heavy metal component in cement, possessing strong toxicity, carcinogenicity, and mobility. If the hexavalent chromium content in cement exceeds the standard, it will leach into water during use, polluting the soil and water sources, endangering human health and the surrounding ecological environment. Therefore, the hexavalent chromium content in cement is an important environmental monitoring indicator that needs to be precisely controlled.

[0003] Currently, the detection of hexavalent chromium in cement is mostly carried out using the diphenylcarbazide spectrophotometric method. This method has become the mainstream detection method in the industry due to its simple operation and high detection sensitivity. However, existing detection methods still have many shortcomings in practical applications: First, conventional colorimetric reagents have poor stability, are easily affected by light and air oxidation, have a short shelf life, and are prone to uneven color development and fading, leading to deviations in detection results. Second, the cement matrix has a complex composition, containing a large number of metal ions, silicates, and other impurities, which can easily interfere with the colorimetric reaction of hexavalent chromium, affecting the accuracy of detection. Third, the sample pretreatment process is not refined enough; the mortar leachate is turbid and contains many impurities, which can interfere with absorbance measurement and reduce detection accuracy. Fourth, if the waste liquid containing hexavalent chromium is discharged arbitrarily after detection, it will cause secondary pollution, and existing technologies lack standardized and efficient waste liquid harmless treatment procedures, making it difficult to meet environmental protection requirements.

[0004] To address the aforementioned issues, existing detection processes fail to simultaneously ensure colorimetric stability, interference resistance, sample pretreatment purity, and environmentally friendly waste disposal, making it difficult to achieve accurate, stable, and environmentally friendly detection of hexavalent chromium in cement. Therefore, developing a method for detecting hexavalent chromium in cement that offers stable colorimetric development, strong interference resistance, accurate detection, and compliant waste disposal has become a pressing technical challenge for those skilled in the art. Summary of the Invention

[0005] Purpose of the invention

[0006] To address the technical problems in existing technologies for detecting hexavalent chromium in cement, such as poor stability of the colorimetric reagent, significant matrix interference, insufficient purity of sample pretreatment, and secondary pollution caused by waste liquid, this invention provides a method for detecting hexavalent chromium in cement. By optimizing the preparation of standard solutions, the colorimetric reagent preparation process, refining sample pretreatment, precisely controlling temperature for color development, and standardizing the harmless disposal of waste liquid, this method achieves accurate and stable detection of hexavalent chromium in cement while eliminating environmental pollution from detection waste liquid.

[0007] Technical solution

[0008] A method for detecting hexavalent chromium in cement includes the following steps:

[0009] (1) Preparation of standard solution: Prepare a hexavalent chromium standard stock solution with a concentration of 50 mg / L, and dilute it stepwise to obtain a hexavalent chromium standard working solution with a concentration of 5 mg / L;

[0010] (2) Preparation of colorimetric reagent: Weigh diphenylcarbazide and add solvent to prepare a colorimetric reagent containing 0.1 to 0.2 g per 50 mL;

[0011] (3) Plotting the standard curve: Transfer different volumes of hexavalent chromium standard working solution into a 50 mL volumetric flask, dilute with water to about 20 mL, add 5 mL of color reagent and 5 mL of 0.04 mol / L hydrochloric acid solution, dilute to the mark of the volumetric flask, shake well, let stand for 8 to 20 min for color development, and measure the absorbance at a wavelength of 540 nm. Plot the standard curve with hexavalent chromium content as the abscissa and absorbance as the ordinate.

[0012] (4) Cement sample preparation: Cement sample, water and standard sand are mixed in the mass ratio to make mortar, the preliminary filtrate is collected by vacuum filtration, and then a second filtration is performed to obtain clear filtrate to be tested;

[0013] (5) Sample determination: Take the filtrate to be tested, dilute it, add color reagent and masking agent, adjust the pH of the solution to 2.1 to 2.5 with 1 mol / L hydrochloric acid, make up the volume and let it stand for 8 to 20 min to develop color, measure the absorbance at 540 nm wavelength, and calculate the hexavalent chromium content in cement according to the standard curve;

[0014] (6) Waste liquid treatment: Collect the waste liquid containing color reagent and hexavalent chromium after the test is completed, and discharge it after harmless treatment. Specifically, first adjust the pH value of the waste liquid to 2.0-3.0 with sulfuric acid, add sodium bisulfite to fully reduce hexavalent chromium to trivalent chromium; after standing for 15-30 minutes, add sodium hydroxide or calcium hydroxide to adjust the pH value to 10.0-10.5, so that chromium ions precipitate in the form of hydroxide; after filtration, the filter residue is disposed of harmlessly; after the supernatant is tested and meets the standards, it is discharged.

[0015] Preferably:

[0016] In the preparation of the standard solution, the hexavalent chromium standard stock solution is prepared by the following steps: 0.1414 g of analytical grade potassium dichromate is weighed, dissolved, and then diluted to 1000 mL. The analytical grade potassium dichromate is then dried at 140 °C for 2 hours.

[0017] Preferably:

[0018] In the cement sample processing, the mass ratio of cement sample, water, and standard sand is 2:1:6, and the mortar degassing and filtration time is 10 minutes.

[0019] Preferably:

[0020] The colorimetric agent described in step (2) is prepared by step a or step b:

[0021] Step a: Weigh 0.125g of diphenylcarbazide, dissolve it in 25mL of anhydrous ethanol, transfer it to a 50mL volumetric flask, dilute it to the mark with anhydrous ethanol, shake well, and store it in the dark to obtain anhydrous ethanol-based colorimetric reagent.

[0022] Step b: Weigh 0.125g of diphenylcarbazide, dissolve it in a compound solvent, add stabilizer, color-developing synergist and anti-interference modifier, stir until completely dissolved, transfer to a 50mL volumetric flask, dilute to the mark with anhydrous ethanol, shake well and store in the dark to obtain the modified color-developing agent.

[0023] Preferably:

[0024] The compound solvent is anhydrous ethanol, glycerol, and glacial acetic acid. Glacial acetic acid accounts for 6% of the total volume of the color developer, glycerol accounts for 2% to 4% of the same volume of the color developer, and the remainder is anhydrous ethanol.

[0025] Preferably:

[0026] The stabilizer is maltitol, and the amount added is 1% to 3% of the mass of diphenylcarbazide.

[0027] Preferably:

[0028] The color-enhancing agent is a compound of o-phenanthroline and bipyridine with a mass ratio of 1:(0.5-1), and the amount added is 2%-3% of the mass of diphenylcarbazide.

[0029] Preferably:

[0030] The anti-interference modifier is a diethylenetriaminepentaacetic acid-sodium gluconate composite system, with a mass ratio of 1:(1-2), and the amount added is 1% to 1.5% of the mass of diphenylcarbazide.

[0031] Preferably:

[0032] The masking agent mentioned in step (5) is aminotrimethylenephosphonic acid, and its volume is 0.2% to 0.5% of the volume of the colorimetric agent.

[0033] Preferably:

[0034] In the waste liquid treatment process, the waste liquid collection tank is made of corrosion-resistant material, and the amount of sodium bisulfite added is 3 to 5 times the mass of hexavalent chromium in the waste liquid. After confirming that the hexavalent chromium content of the supernatant is ≤0.005mg / L and the pH value is between 6.5 and 8.5, it is discharged into the sewage treatment system.

[0035] Beneficial effects

[0036] 1. This invention optimizes the standard solution preparation process by using high-purity potassium dichromate after drying to prepare the standard stock solution, ensuring accurate standard solution concentration, good linearity of the standard curve, and significantly improving the accuracy of the test results.

[0037] 2. This invention refines the pretreatment of cement samples by using a specific ratio of mortar, combined with vacuum filtration and secondary filtration, to effectively remove suspended impurities and colloids from the filtrate, resulting in a clear test solution. This avoids impurities interfering with absorbance measurement and ensures detection accuracy.

[0038] 3. This invention adds a standardized waste liquid harmless treatment step. Through reduction and precipitation, toxic hexavalent chromium is converted into non-toxic precipitate. The discharge index of supernatant is strictly controlled to prevent secondary pollution caused by test waste liquid and meet environmental protection requirements.

[0039] 4. The detection process of this invention is standardized and easy to operate. It has high detection sensitivity and good stability, and is suitable for batch detection of hexavalent chromium in various cement samples, meeting the actual needs of cement production quality control and environmental protection testing. Attached Figure Description

[0040] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0041] Figure 1 This is a flowchart illustrating an embodiment of the present invention.

[0042] Figure 2 This is the standard curve graph for Example 1.

[0043] Figure 3 This is the standard curve graph for Example 2.

[0044] Figure 4 This is a standard curve for comparison. Detailed Implementation

[0045] To enable those skilled in the art to better understand the technical solution of the present invention, the present invention will be described in detail below with reference to the accompanying drawings. The description in this part is only exemplary and explanatory, and should not be used to limit the scope of protection of the present invention in any way.

[0046] This embodiment provides a method for detecting hexavalent chromium in cement, specifically including the following steps:

[0047] (1) Preparation of standard solution: Prepare a hexavalent chromium standard stock solution with a concentration of 50 mg / L, and dilute it stepwise to obtain a hexavalent chromium standard working solution with a concentration of 5 mg / L;

[0048] (2) Preparation of colorimetric reagent: Weigh diphenylcarbazide and add solvent to prepare a colorimetric reagent containing 0.1 to 0.2 g per 50 mL;

[0049] (3) Plotting the standard curve: Transfer different volumes of hexavalent chromium standard working solution into a 50 mL volumetric flask, dilute with water to about 20 mL, add 5 mL of color reagent and 5 mL of 0.04 mol / L hydrochloric acid solution, dilute to the mark of the volumetric flask, shake well, let stand for 8 to 20 min for color development, and measure the absorbance at a wavelength of 540 nm. Plot the standard curve with the mass of hexavalent chromium as the abscissa and the absorbance as the ordinate.

[0050] (4) Cement sample preparation: Cement sample, water and standard sand are mixed in the mass ratio to make mortar, the preliminary filtrate is collected by vacuum filtration, and then a second filtration is performed to obtain clear filtrate to be tested;

[0051] (5) Sample determination: Take the filtrate to be tested, dilute it, add color reagent and masking agent, adjust the pH of the solution to 2.1 to 2.5 with 1 mol / L hydrochloric acid, make up the volume and let it stand for 8 to 20 min to develop color, measure the absorbance at 540 nm wavelength, and calculate the hexavalent chromium content in cement according to the standard curve;

[0052] (6) Waste liquid treatment: Collect the waste liquid containing color reagent and hexavalent chromium after the test is completed, and discharge it after harmless treatment. Specifically, first adjust the pH value of the waste liquid to 2.0-3.0 with sulfuric acid, add sodium bisulfite to fully reduce hexavalent chromium to trivalent chromium; after standing for 15-30 minutes, add sodium hydroxide or calcium hydroxide to adjust the pH value to 10.0-10.5, so that chromium ions precipitate in the form of hydroxide; after filtration, the filter residue is disposed of harmlessly; after the supernatant is tested and meets the standards, it is discharged.

[0053] In step (1), the method of "stock solution + stepwise dilution" is used instead of directly preparing the working solution, which can reduce the error of single weighing and volume adjustment and ensure the accuracy of the standard working solution concentration; in step (2), the concentration of the colorimetric reagent is controlled at 0.1-0.2g per 50mL, which can ensure that the colorimetric reaction intensity is moderate and avoid the color being too dark due to excessive concentration or too light due to insufficient concentration; in step (3), the complex formed by hexavalent chromium and diphenylcarbazide has the highest absorbance at 540nm, so the detection sensitivity is the highest. Adding hydrochloric acid provides an acidic environment, which can promote the stable progress of the complexation reaction. Plotting curves with different volumes of standard working solution can cover the common content range of hexavalent chromium in cement, ensuring good linearity of the curve and accurate detection of actual samples; in step (4), cement, water, and standard are used. The mortar is prepared by mixing the sand according to the mass ratio to simulate the actual working conditions of cement, ensuring that hexavalent chromium is fully leached and avoiding incomplete leaching that leads to low test results. The core of setting up vacuum filtration and secondary filtration is to remove suspended impurities and colloidal particles in the filtrate, avoid interference of light scattering by impurities with absorbance measurement, and improve detection accuracy. In step (5), a masking agent is added to mask the residual calcium, iron and other interfering metal ions in the cement matrix, eliminate false positives and turbidity. The pH value is precisely adjusted to 2.1 to 2.5 to control the acidic environment of the color reaction and avoid incomplete color development or decomposition of the color reagent due to abnormal pH. In step (6), sodium bisulfite is used to reduce hexavalent chromium under acidic conditions, which can convert highly toxic hexavalent chromium into low-toxic trivalent chromium, reducing toxicity. Under alkaline conditions, trivalent chromium is precipitated, which is easy to separate and remove.

[0054] Furthermore:

[0055] In the preparation of the standard solution, the hexavalent chromium standard stock solution is prepared by the following steps: 0.1414 g of analytical grade potassium dichromate is weighed, dissolved, and then diluted to 1000 mL. The analytical grade potassium dichromate is then dried at 140 °C for 2 hours.

[0056] Beneficially, it removes water of crystallization and impurities from reagents, ensuring accurate concentration of standard solutions and improving the linearity of standard curves.

[0057] Furthermore:

[0058] In the cement sample processing, the mass ratio of cement sample, water, and standard sand is 2:1:6, and the mortar degassing and filtration time is 10 minutes.

[0059] Beneficially, the mass ratio of cement sample, water, and standard sand is 2:1:6, which maximizes the leaching of hexavalent chromium in the cement and avoids incomplete leaching leading to lower test results.

[0060] Furthermore:

[0061] The colorimetric agent described in step (2) is prepared by step a or step b:

[0062] Step a: Weigh 0.125g of diphenylcarbazide, dissolve it in 25mL of anhydrous ethanol, transfer it to a 50mL volumetric flask, dilute it to the mark with anhydrous ethanol, shake well, and store it in the dark to obtain anhydrous ethanol-based colorimetric reagent.

[0063] Advantageously, anhydrous ethanol has low toxicity, moderate volatility, safe operation, and can dissolve diphenylcarbazide. The preparation steps are simple and it is suitable for routine rapid detection scenarios.

[0064] Step b: Weigh 0.125g of diphenylcarbazide, dissolve it in a compound solvent, add stabilizer, color-developing synergist and anti-interference modifier, stir until completely dissolved, transfer to a 50mL volumetric flask, dilute to the mark with anhydrous ethanol, shake well and store in the dark to obtain the modified color-developing agent.

[0065] Beneficially, it significantly improves stability, extends shelf life, and avoids errors and troubles caused by frequent preparation; its anti-interference ability is greatly enhanced, effectively resisting the influence of various interfering ions in the cement matrix, resulting in more uniform and stable color development, improving detection sensitivity and accuracy, and making it suitable for scenarios with high detection precision requirements.

[0066] Furthermore:

[0067] The compound solvent is anhydrous ethanol, glycerol, and glacial acetic acid. Glacial acetic acid accounts for 6% of the total volume of the color developer, glycerol accounts for 2% to 4% of the same volume of the color developer, and the remainder is anhydrous ethanol.

[0068] Beneficially, the three solvents work synergistically to further improve the solubility of diphenylcarbazide compared to anhydrous ethanol alone, preventing its precipitation and thus the failure of the color developer. Glycerin can slow down the oxidation rate of the color developer, and glacial acetic acid can inhibit the decomposition of the color developer. The combination of the two can significantly extend the shelf life of the color developer, while gently regulating the color development environment, making the color development reaction more uniform and reducing color development deviation.

[0069] Furthermore:

[0070] The stabilizer is maltitol, and the amount added is 1% to 3% of the mass of diphenylcarbazide.

[0071] Maltitol molecules contain multiple hydroxyl groups, which bind to water molecules through hydrogen bonds, forming a dense hydrated protective layer around the diphenylcarbazide molecule. This protective layer isolates oxygen from the space, inhibits the contact between the color developer molecules and oxygen, prevents oxidative deterioration, and extends the shelf life.

[0072] Furthermore:

[0073] The colorimetric enhancer is a compound of o-phenanthroline and bipyridine with a mass ratio of 1:(0.5-1), and the amount added is 2%-3% of the mass of diphenylcarbazide, which enhances the colorimetric reaction intensity and improves the detection sensitivity.

[0074] Both o-phenanthroline and bipyridine are nitrogen-containing ligand compounds that can form weak coordination with metal ions, slightly activating the colorimetric system and promoting a more complete complexation reaction between diphenylcarbazide and hexavalent chromium. Simultaneously, o-phenanthroline itself possesses auxiliary coordination sensitizing properties, increasing the molar absorptivity of the complex and enhancing colorimetric intensity. The optimal sensitizing effect is achieved when the mass ratio of o-phenanthroline to bipyridine is 1:(0.5–1), and the addition amount is 2%–3% of the mass of diphenylcarbazide. This ensures moderate colorimetric reaction intensity while avoiding solution turbidity or interference with the complexation reaction caused by excessive component addition, thus balancing sensitizing effect and detection accuracy, and preventing reagent waste.

[0075] Furthermore:

[0076] The anti-interference modifier is a diethylenetriaminepentaacetic acid-sodium gluconate composite system, with a mass ratio of 1:(1-2), and the amount added is 1% to 1.5% of the mass of diphenylcarbazide.

[0077] Diethylenetriaminepentaacetic acid (DTPA) contains 8 coordinating atoms and can interact with... , , , , Transition metal ions form ultrastable chelates. If these metal ions are not masked, they will undergo side reactions with diphenylcarbazide (e.g., ...). It forms a yellow complex with the colorimetric reagent or catalyzes the decomposition of the colorimetric product. DTPA "locks" interfering ions through competitive coordination, preventing them from participating in the interfering reaction; sodium gluconate... , Alkaline earth metal ions have moderate chelating ability, which compensates for the effects of DTPA on... The relatively weak chelating ability also prevents DTPA-metal chelates from precipitating under acidic conditions, maintaining the homogeneity of the system. The hydroxyl and carboxylate groups of sodium gluconate can adsorb onto the surface of tiny particles, preventing their aggregation or adsorption of hexavalent chromium.

[0078] The addition amount is 1% to 1.5% of the mass of diphenylcarbazide. This ensures that the amount of anti-interference agent is sufficient to mask the interfering ions remaining in the cement filtrate and avoid interfering with the colorimetric reaction. It also prevents excessive dosage, which could lead to weak complexation between the anti-interference agent and hexavalent chromium, or affect the stability and colorimetric intensity of the colorimetric agent. At the same time, it reduces reagent waste and balances the anti-interference effect with the detection accuracy.

[0079] Furthermore:

[0080] The masking agent mentioned in step (5) is aminotrimethylenephosphonic acid, and its volume is 0.2% to 0.5% of the volume of the colorimetric agent.

[0081] Aminotrimethylenephosphonic acid (ATMP) contains three phosphonic acid groups, which are beneficial for... It has strong chelating ability. In cement leachate At high concentrations, it may form under acidic conditions. It may react with phosphate to form precipitates, which may adsorb hexavalent chromium or encapsulate the colorimetric reagent, leading to lower than expected test results. ATMP forms a soluble Ca-ATMP complex, thereby... It remains stable in solution, preventing precipitation. It further masks interfering ions in the solution, improving the accuracy of detection results.

[0082] Furthermore:

[0083] In the waste liquid treatment process, the waste liquid collection tank is made of corrosion-resistant material to prevent the waste liquid from corroding the container and causing leakage. The amount of sodium bisulfite added is 3 to 5 times the mass of hexavalent chromium in the waste liquid to ensure that hexavalent chromium is completely reduced to non-toxic trivalent chromium. After confirming that the hexavalent chromium content in the supernatant is ≤0.005mg / L and the pH value is between 6.5 and 8.5, it is discharged into the sewage treatment system to achieve zero pollution discharge of waste liquid.

[0084] Beneficially, the corrosion-resistant waste liquid container can effectively prevent waste liquid corrosion and leakage, avoiding secondary pollution; the dosage of 3 to 5 times the amount of sodium bisulfite can ensure the complete reduction of hexavalent chromium, eliminating the residue of highly toxic hexavalent chromium; the filter residue is disposed of in accordance with hazardous waste regulations, and the supernatant is discharged after passing the standard test, forming a complete harmless treatment process, eliminating waste liquid pollution, meeting environmental protection requirements, and at the same time, the operation is standardized, making it easy to implement in the laboratory, taking into account both environmental protection and operability.

[0085] Specific embodiments are provided below. These embodiments are intended to enable those skilled in the art to more fully understand the present invention, but do not limit the present invention in any way.

[0086] Example 1

[0087] This embodiment provides a method for detecting hexavalent chromium in cement, and the specific steps are as follows:

[0088] (1) Preparation of standard solution: Place the analytical grade potassium dichromate in an oven at 140℃ for 2 hours, cool to room temperature, weigh 0.1414 g, dissolve in pure water and transfer to a 1000 mL volumetric flask, dilute with pure water to the mark and shake well to obtain a hexavalent chromium standard stock solution with a concentration of 50 mg / L; take 10 mL of the standard stock solution and place it in a 100 mL volumetric flask, dilute with water to the mark and shake well to obtain a hexavalent chromium standard working solution with a concentration of 5 mg / L.

[0089] (2) Preparation of colorimetric reagent: Anhydrous ethanol-based colorimetric reagent was prepared by step a. 0.125 g of diphenylcarbazide was weighed, dissolved in 25 mL of anhydrous ethanol, transferred to a 50 mL volumetric flask, diluted to the mark with anhydrous ethanol, shaken well, and stored in a brown reagent bottle away from light.

[0090] (3) Standard curve preparation: Transfer 0 mL, 1 mL, 2 mL, 5 mL, 10 mL, and 15 mL of hexavalent chromium standard working solution into six 50 mL volumetric flasks, and dilute each flask with water to approximately 20 mL. Add 5 mL of anhydrous ethanol-based colorimetric reagent and 5 mL of 0.04 mol / L hydrochloric acid solution to each volumetric flask, dilute with water to the mark, shake well, and let stand for 15 minutes. Using the blank solution (0 mL standard working solution group) as a reference, measure the absorbance of each solution at a wavelength of 540 nm. The measurement results are shown in the table below and the attached figure in the instruction manual. Figure 2 :

[0091]

[0092] Note: y is absorbance, and x is the concentration of hexavalent chromium (mg / L). The linear correlation coefficient is used to calculate the hexavalent chromium concentration using the formula: concentration (mg / L) = (volume transferred × concentration of standard working solution) / final volume, i.e. (V × 5mg / L) / 50mL.

[0093] (4) Cement sample preparation: Take 100g cement sample, 50g pure water and 300g standard sand (mass ratio 2:1:6) and mix them. Stir evenly to make mortar. Place the mortar in the vacuum filter device and filter for 10 minutes. Collect the preliminary filtrate and filter again to obtain clear filtrate to be tested.

[0094] (5) Sample determination: Take 10 mL of the filtrate to be tested and put it into a 50 mL volumetric flask. Dilute it with water to about 20 mL. Add 5 mL of anhydrous ethanol-based colorimetric reagent and 0.02 mL of aminotrimethylene phosphonic acid (masking agent, the volume of which is 0.4% of the volume of the colorimetric reagent. Take 1 mL, dissolve it in 50 mL of pure water, and then dilute it to 100 mL. Take 2 mL and add it. Adjust the pH of the solution to 2.3 with 1 mol / L hydrochloric acid. Dilute it with water to the mark. Shake well and let it stand for 15 minutes to develop color. Measure the absorbance at a wavelength of 540 nm. Calculate the hexavalent chromium content in cement according to the regression equation of the standard curve.

[0095] (6) Waste liquid treatment: Collect all waste liquid containing color reagent and hexavalent chromium after testing into a corrosion-resistant PP material waste liquid tank; first adjust the pH value of the waste liquid to 2.0-3.0 with sulfuric acid, add sodium bisulfite (the amount added is 4 times the mass of hexavalent chromium in the waste liquid), stir evenly to reduce hexavalent chromium to trivalent chromium, and let stand for 30 minutes; then add calcium hydroxide to adjust the pH value to 10.2, stir evenly to form a precipitate of heavy metal ions; after filtration, seal and store the filter residue and label it with relevant information of "chromium-containing hazardous waste", and transfer it in strict accordance with the hazardous waste disposal specifications; test the supernatant, and after confirming that the hexavalent chromium content is ≤0.005mg / L and the pH value is 6.5-8.5, discharge it into the sewage treatment system.

[0096] Select cement samples (the actual content of hexavalent chromium is known to be 6 mg / kg), and perform the determination according to step (5). Repeat the experiment 3 times, record the test data and calculate the relative standard deviation (RSD). The test results are shown in the table below:

[0097]

[0098] After anhydrous ethanol-based colorimetric reagent is stored in the dark for 24 hours, the colorimetric sensitivity decreases by 11.8%, and the colorimetric reaction takes 14.5 minutes to reach a stable state. During the detection process, slight colorimetric deviations occur due to interference from trace impurities in the sample.

[0099] Example 2

[0100] The difference between this embodiment and Example 1 is that the colorimetric agent used is the modified colorimetric agent prepared in step b. The remaining steps and parameters are the same as in Example 1. The specific colorimetric agent preparation steps are as follows:

[0101] Weigh 0.125g of diphenylcarbazide and add it to the compound solvent: 30mL of anhydrous ethanol, 3mL of glacial acetic acid, and 2mL of glycerol. Stir until initially dissolved. Add the stabilizer maltitol (2% of the mass of diphenylcarbazide, 2.5g dissolved in 50mL of solvent, then diluted to 1L, and 1mL added), the colorimetric synergist o-phenanthroline-bipyridine complex (2.4% of the mass of diphenylcarbazide, 2g of o-phenanthroline and 1g of bipyridine dissolved in 50mL of solvent, then diluted to 1L, and 1mL added), and the anti-interference modifier diethylenetriaminepentaacetic acid-sodium gluconate complex (1.2% of the mass of diphenylcarbazide, 0.5g of diethylenetriaminepentaacetic acid and 1g of sodium gluconate dissolved in 50mL of solvent, then diluted to 1L, and 1mL added). Continue stirring until completely dissolved, then transfer to 50mL. Dilute to the mark with anhydrous ethanol in a volumetric flask, shake well, and store in a brown reagent bottle away from light.

[0102] The standard curve data are shown in the table below and the attached diagram in the instruction manual. Figure 3 :

[0103]

[0104] Note: y is absorbance, and x is hexavalent chromium content (mg / L). The linear correlation coefficient is used to calculate the hexavalent chromium concentration using the formula: concentration (mg / L) = (volume transferred × concentration of standard working solution) / final volume, i.e. (V × 5mg / L) / 50mL.

[0105] Select cement samples from the same batch as in Example 1 (the actual content of hexavalent chromium is known to be 6 mg / kg), and perform the test according to step (5). Repeat the experiment three times, record the test data, and calculate the relative standard deviation (RSD). The test results are shown in the table below:

[0106]

[0107] After being stored in the dark for 72 hours, the modified colorimetric reagent showed no significant change in colorimetric effect, with a decrease in colorimetric sensitivity of only 2.3%. The colorimetric reaction reached a stable state in 9 minutes, and the anti-interference ability was significantly improved. It can effectively avoid the influence of impurities in the sample on the colorimetric reaction. The detection accuracy and repeatability are better than those of Example 1.

[0108] Comparative Example

[0109] The difference between this comparative example and Example 1 is that acetone is used as the solvent for the colorimetric reagent and no masking agent is added. The remaining steps and parameters are completely consistent with Example 1. The specific colorimetric reagent preparation steps are as follows:

[0110] Weigh 0.125 g of diphenylcarbazide, dissolve it in 25 mL of acetone, transfer it to a 50 mL volumetric flask, dilute it to the mark with acetone, shake well, and store it in a brown reagent bottle away from light to obtain the acetone-based colorimetric reagent.

[0111] The standard curve data are shown in the table below and the attached diagram in the instruction manual. Figure 4 :

[0112]

[0113] Note: y represents absorbance, x represents hexavalent chromium content (mg / L), and R... 2 The linear correlation coefficient is used to calculate the hexavalent chromium concentration using the formula: concentration (mg / L) = (volume transferred × concentration of standard working solution) / final volume, i.e. (V × 5mg / L) / 50mL.

[0114] Cement samples from the same batch as in Example 1 (with a known hexavalent chromium content of 6 mg / kg) were selected and tested according to step (5). The experiment was repeated three times, and the test data were recorded and the relative standard deviation (RSD) was calculated. The test results are as follows:

[0115]

[0116] After 24 hours of storage in the dark with the acetone-based colorimetric reagent, the colorimetric sensitivity decreased by 11.2%, and the colorimetric reaction took 14 minutes to reach a stable state. The degree of interference from trace impurities in the sample during the detection process was similar to that in Example 1. The overall detection effect was comparable to that in Example 1.

[0117] Results Analysis

[0118] 1. Accuracy comparison: Example 2 (RSD=0.33%) < Example 1 (RSD=1.23%) < Comparative example (RSD=1.35%), which meets the requirements (the accuracy of Example 1 and the comparative example is close, while the accuracy of Example 2 is significantly higher).

[0119] 2. Cause Analysis:

[0120] Example 2 uses a modified colorimetric agent, employing three solvents in synergistic effect. Compared to anhydrous ethanol alone, this further enhances the solubility of diphenylcarbazide. Furthermore, the addition of stabilizers, colorimetric enhancers, and anti-interference modifiers improves colorimetric stability and anti-interference capabilities, reducing detection errors.

[0121] Example 1 uses anhydrous ethanol as a solvent and adds a masking agent, resulting in general detection accuracy;

[0122] The comparative example uses acetone as a solvent. Acetone is volatile and has poor stability as a colorimetric reagent. The detection accuracy is comparable to that of Example 1. Moreover, acetone is toxic, poses safety hazards during use, and is not environmentally friendly, which does not meet the requirements of green detection.

[0123] 3. Linearity: Standard curves for the three schemes All values ​​are ≥0.997, indicating good linearity and meeting the detection requirements, particularly in Example 2. The highest value (0.9995) further demonstrates its advantage in detection accuracy.

[0124] This article uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. The above are merely preferred embodiments 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 technical scope disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be within the scope of protection of the present invention.

Claims

1. A method for detecting hexavalent chromium in cement, characterized in that, Includes the following steps: (1) Preparation of standard solution: Prepare a hexavalent chromium standard stock solution with a concentration of 50 mg / L, and dilute it stepwise to obtain a hexavalent chromium standard working solution with a concentration of 5 mg / L; (2) Preparation of colorimetric reagent: Weigh diphenylcarbazide and add solvent to prepare a colorimetric reagent containing 0.1 to 0.2 g per 50 mL; (3) Plotting the standard curve: Transfer different volumes of hexavalent chromium standard working solution into a 50 mL volumetric flask, dilute with water to about 20 mL, add 5 mL of color reagent and 5 mL of 0.04 mol / L hydrochloric acid solution, dilute to the mark of the volumetric flask, shake well, let stand for 8 to 20 min for color development, and measure the absorbance at a wavelength of 540 nm. Plot the standard curve with hexavalent chromium content as the abscissa and absorbance as the ordinate. (4) Cement sample preparation: Cement sample, water and standard sand are mixed in the mass ratio to make mortar, the preliminary filtrate is collected by vacuum filtration, and then a second filtration is performed to obtain clear filtrate to be tested; (5) Sample determination: Take the filtrate to be tested, dilute it, add color reagent and masking agent, adjust the pH of the solution to 2.1 to 2.5 with 1 mol / L hydrochloric acid, make up the volume and let it stand for 8 to 20 min to develop color, measure the absorbance at 540 nm wavelength, and calculate the hexavalent chromium content in cement according to the standard curve; (6) Waste liquid treatment: Collect the waste liquid containing color reagent and hexavalent chromium after the test is completed, and discharge it after harmless treatment. Specifically, first adjust the pH value of the waste liquid to 2.0-3.0 with sulfuric acid, add sodium bisulfite to fully reduce hexavalent chromium to trivalent chromium; after standing for 15-30 minutes, add sodium hydroxide or calcium hydroxide to adjust the pH value to 10.0-10.5, so that chromium ions precipitate in the form of hydroxide; after filtration, the filter residue is disposed of harmlessly; after the supernatant is tested and meets the standards, it is discharged.

2. The method according to claim 1, characterized in that, In the preparation of the standard solution, the hexavalent chromium standard stock solution is prepared by the following steps: 0.1414 g of analytical grade potassium dichromate is weighed, dissolved, and then diluted to 1000 mL. The analytical grade potassium dichromate is then dried at 140 °C for 2 hours.

3. The method according to claim 1, characterized in that, In the cement sample processing, the mass ratio of cement sample, water, and standard sand is 2:1:6, and the mortar degassing and filtration time is 10 minutes.

4. The method according to claim 1, characterized in that, The colorimetric agent described in step (2) is prepared by step a or step b: Step a: Weigh 0.125g of diphenylcarbazide, dissolve it in 25mL of anhydrous ethanol, transfer it to a 50mL volumetric flask, dilute it to the mark with anhydrous ethanol, shake well, and store it in the dark to obtain anhydrous ethanol-based colorimetric reagent. Step b: Weigh 0.125g of diphenylcarbazide, dissolve it in a compound solvent, add stabilizer, color-developing synergist and anti-interference modifier, stir until completely dissolved, transfer to a 50mL volumetric flask, dilute to the mark with anhydrous ethanol, shake well and store in the dark to obtain the modified color-developing agent.

5. The method according to claim 4, characterized in that, The compound solvent is anhydrous ethanol, glycerol, and glacial acetic acid. Glacial acetic acid accounts for 6% of the total volume of the color developer, glycerol accounts for 2% to 4% of the same volume of the color developer, and the remainder is anhydrous ethanol.

6. The method according to claim 4, characterized in that, The stabilizer is maltitol, and the amount added is 1% to 3% of the mass of diphenylcarbazide.

7. The method according to claim 4, characterized in that, The color-enhancing agent is a compound of o-phenanthroline and bipyridine with a mass ratio of 1:(0.5-1), and the amount added is 2%-3% of the mass of diphenylcarbazide.

8. The method according to claim 4, characterized in that, The anti-interference modifier is a diethylenetriaminepentaacetic acid-sodium gluconate composite system, with a mass ratio of 1:(1-2), and the amount added is 1% to 1.5% of the mass of diphenylcarbazide.

9. The method according to claim 1, characterized in that, The masking agent mentioned in step (5) is aminotrimethylenephosphonic acid, and its volume is 0.2% to 0.5% of the volume of the colorimetric agent.

10. The method according to claim 1, characterized in that, In the waste liquid treatment process, the waste liquid collection tank is made of corrosion-resistant material, and the amount of sodium bisulfite added is 3 to 5 times the mass of hexavalent chromium in the waste liquid. After confirming that the hexavalent chromium content of the supernatant is ≤0.005mg / L and the pH value is between 6.5 and 8.5, it is discharged into the sewage treatment system.

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