Selective determination method for calcium content under coexistence of zinc and tin

Abstract

The invention discloses a selective determination method for calcium content in the presence of zinc and tin, and belongs to the technical field of analytical chemistry. The method comprises the following steps: precisely weighing a sample, adding hydrochloric acid and nitric acid, heating, dissolving, and steaming to be nearly dry; adding 3-8mL of hydrobromic acid, evaporating to dryness at low temperature, and repeating for at least one time to remove tin; adding hydrochloric acid and water into residues, heating and dissolving, cooling and fixing the volume to obtain a sample stock solution; taking 10.00-50.00 mL of the stock solution, adding water, a triethanolamine solution, a magnesium sulfate solution and a potassium hydroxide solution, stirring, and standing for 1-2 minutes; adding a calcein-thymol phenolphthalein mixed indicator, titrating with an EGTA standard titrating solution, and observing under a black background until green fluorescence disappears and is in stable rose red as an end point; and performing a blank test, and calculating the calcium content according to a formula. The method can accurately measure the calcium content in the zinc-tin-calcium composite system, is simple and convenient to operate, low in cost and suitable for quality control of related materials.

Description

Technical Field

[0001] This invention relates to the field of analytical chemistry, specifically to a method for the selective determination of calcium content in the presence of zinc and tin. Background Technology

[0002] In recent years, zinc-tin-calcium composite systems have become a research hotspot in inorganic functional materials due to their excellent synergistic effects in flame retardancy and catalysis, such as the composite application of zinc hydroxystannate and calcium carbonate. In such multi-component materials, calcium can be a key component for achieving specific functions, or it can be an impurity element affecting the purity and performance of the material. This characteristic makes the accurate determination of calcium content particularly important. However, a key bottleneck for in-depth research and future industrial applications of such systems lies in the lack of a rapid, accurate, and low-cost method for determining the calcium content.

[0003] Currently, the determination of calcium content mainly relies on ethylenediaminetetraacetic acid (EDTA) titration, inductively coupled plasma optical emission spectrometry (ICP-OES), and atomic absorption spectrometry (AAS). However, these existing technologies all have significant limitations when applied to zinc-tin-calcium composite systems:

[0004] (1) Although instrumental analysis methods (such as ICP-OES and AAS) have high detection accuracy, the equipment is expensive, the operating cost is high, the professional skills of the operators are required to be strict, and the analysis cycle is long, making it difficult to promote them as a routine quality control method in industrial sites and small and medium-sized enterprises.

[0005] (2) The traditional EDTA titration method is inexpensive and easy to operate, but in this composite system, the coexisting zinc (Zn) 2+ ), Tin (Sn) 4+ Metal ions such as calcium ions, due to their higher complexation stability constant with EDTA than calcium ions, will preferentially react with EDTA and seriously interfere with the endpoint determination of commonly used calcium indicators, resulting in significantly higher measurement results and compromising accuracy and reliability.

[0006] Zinc and tin ions severely interfere with traditional calcium titration methods, rendering existing technologies inadequate for the research and quality control requirements of this novel material system. The purpose of this invention is to develop a method capable of interference-resistant and selective determination of calcium content in a zinc-tin-calcium composite system, thereby facilitating the transition of this new material from the laboratory to practical applications. Summary of the Invention

[0007] To address the aforementioned shortcomings, this invention provides a selective method for determining calcium content in the presence of zinc and tin. By removing tin with hydrobromic acid, masking zinc with triethanolamine-magnesium sulfate, and combining this with EGTA titration, the method solves the problems of interference in calcium determination under zinc-tin coexistence and the high cost of instrumental methods.

[0008] To achieve the above technical objectives, the present invention adopts the following technical solution:

[0009] A selective method for determining calcium content in the presence of zinc and tin, comprising the following steps:

[0010] S1. Sample decomposition: Accurately weigh the sample, add hydrochloric acid and nitric acid, heat to dissolve and then evaporate to near dryness;

[0011] S2, Removal of tin interference: Add hydrobromic acid to the product obtained in S1, heat at low temperature to evaporate to dryness until no bromine vapor escapes, and repeat at least once;

[0012] S3. Preparation and volume adjustment of the test solution: Add hydrochloric acid and water to the residue obtained in S2, heat to dissolve, cool and then adjust to volume to obtain the sample stock solution.

[0013] S4. Interference masking: Take the sample stock solution obtained in S3, add water, triethanolamine solution, magnesium sulfate solution and potassium hydroxide solution in sequence, stir evenly and let stand;

[0014] S5. Titration determination: Add calcein-thymolphthalein mixed indicator to the solution treated in S4, and titrate with EGTA standard titrant until the green fluorescence disappears and a stable rose red color appears.

[0015] S6. Blank correction and result calculation: Perform a blank test and calculate the calcium content in the sample based on the net consumption of EGTA standard titrant.

[0016] Preferably, in step S1, the sample weight is 0.1g~0.5g, the amount of hydrochloric acid added is 8mL~15mL, and the amount of nitric acid added is 3mL~8mL.

[0017] Preferably, in step S2, the amount of hydrobromic acid added is 3 mL to 8 mL.

[0018] Preferably, in step S3, hydrochloric acid and water are added to the residue, heated to dissolve, cooled, and then transferred to a 250mL~500mL volumetric flask and diluted to volume.

[0019] Preferably, in step S4, the volume of the sample stock solution is 10.00 mL to 50.00 mL; 120 mL to 150 mL of water, 5 mL to 15 mL of triethanolamine solution, 1 mL to 3 mL of magnesium sulfate solution and 15 mL to 25 mL of potassium hydroxide solution are added sequentially.

[0020] Preferably, in step S4, the triethanolamine solution is prepared by mixing triethanolamine and water in a volume ratio of 1:3; the magnesium sulfate solution is prepared by dissolving 1.0 g of MgSO4·7H2O in water and diluting it to 100 mL; and the potassium hydroxide solution is prepared by dissolving 20.0 g of potassium hydroxide in water and diluting it to 100 mL.

[0021] Preferably, in step S5, the calcein-thymolphthalein mixed indicator is prepared by grinding and mixing calcein and thymolphthalein in a mass ratio of 0.10:0.14 with dried potassium chloride in a mass ratio of 1:100.

[0022] Preferably, in step S6, the formula for calculating the calcium content is:

[0023] In the formula:

[0024] C represents the accurate concentration of the EGTA standard titration solution, in mol / L;

[0025] V1 represents the volume of EGTA standard titration solution consumed in titrating the sample solution, in mL;

[0026] V0 represents the volume of EGTA standard titration solution consumed in titrating the blank solution, in mL;

[0027] 40.08 represents the molar mass of calcium, in g / mol;

[0028] D represents the separation ratio, which is the ratio of the total volume of the sample stock solution to the volume of the sample stock solution transferred during titration;

[0029] m represents the sample mass, expressed in grams.

[0030] Preferably, the sample is a zinc-tin-calcium composite material.

[0031] Preferably, the zinc-tin-calcium composite system material is a composite material of zinc hydroxystannate and calcium carbonate.

[0032] Compared with the prior art, the present invention has the following advantages and technical effects:

[0033] 1. Achieved selective determination in complex systems: This invention effectively overcomes the severe interference from multiple coexisting metal ions such as zinc and tin by combining the evaporation of hydrobromic acid to remove tin with the synergistic masking of triethanolamine-magnesium sulfate. It solves the technical problem that traditional EDTA titration cannot accurately determine the tin in this specific complex system, and fills the gap in chemical titration methods in this field.

[0034] 2. Sensitive and reliable endpoint determination: By optimizing the indicator system and solution environment, the endpoint change of "disappearance of green fluorescence to rose red" in EGTA titration is used. The phenomenon is sensitive and intuitive, which greatly reduces the subjective error caused by inaccurate endpoint determination and ensures the repeatability and reliability of the analytical results.

[0035] 3. Simple and quick to operate, easy to promote: The entire analysis process is based on classic titration operation, requiring no special equipment or superb operating skills. The analysis cycle is short, the steps are clear, and it is easy to master and apply in conventional analytical laboratories and production lines. It has good universality and promotion value. Detailed Implementation

[0036] The present invention will now be described in detail with reference to specific embodiments. These embodiments will help those skilled in the art to further understand the present invention, but do not limit the invention in any way. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention. These all fall within the scope of protection of the present invention.

[0037] In this invention, a method for selectively determining calcium content in the presence of zinc and tin includes the following steps:

[0038] S1. Sample decomposition: Accurately weigh the sample, add hydrochloric acid and nitric acid, heat to dissolve and then evaporate to near dryness;

[0039] S2, Removal of tin interference: Add hydrobromic acid to the product obtained in S1, heat at low temperature to evaporate to dryness until no bromine vapor escapes, and repeat at least once;

[0040] S3. Preparation and volume adjustment of the test solution: Add hydrochloric acid and water to the residue obtained in S2, heat to dissolve, cool and then adjust to volume to obtain the sample stock solution.

[0041] S4. Interference masking: Take the sample stock solution obtained in S3, add water, triethanolamine solution, magnesium sulfate solution and potassium hydroxide solution in sequence, stir evenly and let stand;

[0042] S5. Titration determination: Add calcein-thymolphthalein mixed indicator to the solution treated in S4, and titrate with EGTA standard titrant until the green fluorescence disappears and a stable rose red color appears.

[0043] S6. Blank correction and result calculation: Perform a blank test and calculate the calcium content in the sample based on the net consumption of EGTA standard titrant.

[0044] The working mechanism and principle of each step in this invention:

[0045] I. Sample Decomposition (S1)

[0046] Using a hydrochloric acid-nitric acid mixed acid digestion system, the hydrochloric acid is passed through Cl... - Coordination disrupts the bond between metal ions and the matrix, forming [ZnCl4]. 2- [SnCl6] 2- Nitric acid has both oxidizing and acidic properties, and it oxidizes low-valent tin to Sn. 4+ It also decomposes organic impurities. Weigh 0.1g~0.5g of the sample to balance the detection limit and masking capacity. Mix hydrochloric acid (8mL~15mL) and nitric acid (3mL~8mL) to ensure complete dissolution of the sample, while avoiding excessive acid from affecting subsequent steps.

[0047] II. Removal of Tin Interference (S2)

[0048] Using Sn 4+ With Br - The volatile SnBr4 is formed; low-temperature heating causes SnBr4 to escape with bromine vapor, thus separating tin from calcium and zinc. 3-8 mL of hydrobromic acid is used to ensure the release of SnBr4. 4+ Ensure thorough complexation, avoiding excessive amounts that could trigger side reactions. Repeat the procedure at least twice for Sn. 4+ Stepwise complexation characteristics ensure residual Sn 4+ Complete volatilization eliminates measurement errors caused by its preferential reaction with the titrant due to its high complexation stability constant.

[0049] III. Preparation and Volume Adjustment of Test Solution (S3)

[0050] Add hydrochloric acid to redissolve calcium and zinc chloride, and use water to control the acidity; boil to accelerate dissolution and remove residual Br2. Adjust the calcium concentration to the appropriate titration range by making up to 250mL~500mL, ensuring that the EGTA consumption is within the optimal reading range of the burette, and at the same time providing a benchmark for the calculation of the separation ratio, thus improving the repeatability of the method.

[0051] IV. Interference Masking (S4)

[0052] The synergistic construction of a calcium-selective environment by triethanolamine-magnesium sulfate-potassium hydroxide: triethanolamine reacts with Zn under alkaline conditions. 2+ Fe 3+ Al 3+ To form a stable chelate, a dosage of 5-15 mL is used to cover the zinc ion concentration; magnesium sulfate provides Mg 2+ It complexes with excess triethanolamine, and because its complexing ability with EGTA is weaker than that of calcium, it helps to improve endpoint sensitivity. A dosage of 1-3 mL balances the masking and indicating effects. Potassium hydroxide is used to adjust the pH to 12-13, enhancing the complexing ability of triethanolamine and preventing calcium and zinc precipitation. Take 10.00 mL-50.00 mL of stock solution to match the masking agent volume, dilute with water to reduce ionic strength, and let stand for 1-2 minutes to ensure complexation equilibrium.

[0053] V. Titration Determination (S5)

[0054] EGTA is more selective for calcium than EDTA, and its affinity for Ca is higher. 2+ The complexation stability constant is significantly higher than that of Zn. 2+ Ensure that the titrant only reacts with Ca 2+ Reaction. In the calcein-thyme phenolphthalein mixed indicator, calcein reacts with Ca... 2+ A green fluorescent complex is formed, and thymolphthalein masks the background fluorescence; at the titration endpoint, EGTA reacts with Ca... 2+ The reaction was complete, the fluorescence disappeared, and the thymolphthalein turned rose-red. Observation against a black background eliminates light interference, and a top-down viewing angle avoids judgment bias.

[0055] VI. Blank Correction and Result Calculation (S6)

[0056] The blank test eliminates systematic errors caused by trace amounts of calcium in the reagent and contamination introduced during the operation process, and improves accuracy by deducting the volume consumed in the blank test. In the calculation formula, the separation ratio (D) converts the calcium content of the separated test solution into the total content, and 40.08 (calcium molar mass) and 1000 (unit conversion factor) ensure that the result is presented as a mass fraction, which conforms to the specifications for the determination of constant components.

[0057] Pre-experimental preparation: reagent preparation and standardization

[0058] (a) Experimental reagents

[0059] All reagents used in the embodiments and comparative examples of this invention meet analytical purity standards, specifically as follows: hydrochloric acid, nitric acid, hydrobromic acid; triethanolamine, magnesium sulfate (MgSO4·7H2O), potassium hydroxide, ethylene glycol bis(2-aminoethyl ether)tetraacetic acid (EGTA), standard calcium carbonate, calcein, thymolphthalein, potassium chloride (which needs to be dried at 105°C); the experimental water is deionized water.

[0060] (II) Reagent Preparation

[0061] Triethanolamine solution (1+3): Measure 100 mL of analytical grade triethanolamine, mix it with 300 mL of deionized water, stir well and set aside.

[0062] Magnesium sulfate solution (10g / L): Accurately weigh 1.0g of analytical grade magnesium sulfate (MgSO4·7H2O), place it in a 100mL volumetric flask, add an appropriate amount of deionized water to dissolve it, dilute to the mark, shake well and set aside.

[0063] Potassium hydroxide solution (200g / L): Accurately weigh 20.0g of analytical grade potassium hydroxide, place it in a 100mL volumetric flask, add an appropriate amount of deionized water to dissolve it (note that the dissolution is exothermic, and the solution needs to be cooled before diluting to the mark), and shake well before use.

[0064] Calcein-thymolphthalein mixed indicator: Accurately weigh 0.10g calcein and 0.14g thymolphthalein, and place them together with 10.0g of analytical grade potassium chloride that has been dried to constant weight at 105℃ in an agate mortar. Grind thoroughly until evenly mixed, and store in a brown ground glass bottle to protect from light and seal for later use.

[0065] (III) Preparation and Standardization of EGTA Standard Solution

[0066] Preparation of EGTA standard solution: Weigh 3.8g of ethylene glycol bis(2-aminoethyl ether)tetraacetic acid (EGTA) and place it in a 300mL beaker. Add 150mL of deionized water and heat in a constant temperature water bath at 60~80℃. While stirring continuously, slowly add about 15mL of 200g / L potassium hydroxide solution until the EGTA is completely dissolved. Stop heating and let the solution cool to room temperature. Transfer it to a 1000mL volumetric flask, dilute to the mark with deionized water, and shake well to obtain the EGTA standard solution to be standardized.

[0067] EGTA standard solution calibration: Accurately weigh 0.1000 g (accurate to 0.0001 g) of calcium carbonate dried to constant weight at 110℃, place it in a 250 mL Erlenmeyer flask, and moisten it with a small amount of deionized water; add hydrochloric acid solution (1+1) dropwise until the calcium carbonate is completely dissolved, and then titrate according to the steps S4~S5 in the following embodiments of the present invention, record the volume of EGTA standard solution consumed, and calculate its accurate concentration.

[0068] To make the present invention more fully disclosed, more specific embodiments are described below.

[0069] Example 1

[0070] A selective method for determining calcium content in the presence of zinc and tin, comprising the following steps:

[0071] S1. Sample pretreatment: Accurately weigh 0.1500 g (accurate to 0.0001 g) of the zinc-tin-calcium composite sample and place it in a 250 mL beaker. Add 10 mL of hydrochloric acid and 5 mL of nitric acid, heat on a hot plate until the sample is completely dissolved, and evaporate to near dryness.

[0072] S2. Removing Tin Interference: Remove the beaker, let it cool slightly, add 5 mL of hydrobromic acid, and evaporate to dryness at low temperature until no red bromine vapor is emitted. Add another 5 mL of hydrobromic acid and repeat the above operation once to ensure that the tin is completely removed.

[0073] S3. Preparation and dilution of the solution: Add 5 mL of hydrochloric acid and 20 mL of water to the residue after tin removal, and heat to boiling to completely dissolve the soluble salts. After cooling, transfer the entire solution to a 500 mL volumetric flask, dilute to the mark with water, and shake well to obtain the sample stock solution.

[0074] S4. Interference Masking: Accurately transfer 25.00 mL of the above sample stock solution into a 250 mL Erlenmeyer flask. Add 120 mL of water, 10 mL of triethanolamine solution (1+3), 2 mL of magnesium sulfate solution (10 g / L), and 20 mL of potassium hydroxide solution (200 g / L) in sequence, and stir with a magnetic stirrer for 1-2 minutes.

[0075] S5. Titration determination: Add 0.10g of calcein-thymolphthalein indicator and titrate with pre-calibrated EGTA standard solution (0.01 mol / L). Observe against a black background until the green fluorescence of the solution completely disappears and turns a stable rose red color. Record the volume V1 consumed.

[0076] S6. Blank correction and result calculation: Perform a blank test along with the sample. Except for not adding the sample, the other steps are exactly the same as S1 to S5. Record the volume of EGTA standard titrant consumed, V0 (mL).

[0077] Example 2

[0078] A selective method for determining calcium content in the presence of zinc and tin, comprising the following steps:

[0079] S1. Sample pretreatment: Accurately weigh 0.2000 g (accurate to 0.0001 g) of the zinc-tin-calcium composite sample and place it in a 250 mL beaker. Add 10 mL of hydrochloric acid and 5 mL of nitric acid, heat on a hot plate until the sample is completely dissolved, and evaporate to near dryness.

[0080] S2. Removing Tin Interference: Remove the beaker, let it cool slightly, add 5 mL of hydrobromic acid, and evaporate to dryness at low temperature until no red bromine vapor is emitted. Add another 5 mL of hydrobromic acid and repeat the above operation once to ensure that the tin is completely removed.

[0081] S3. Preparation and dilution of the solution: Add 5 mL of hydrochloric acid and 20 mL of water to the residue after tin removal, and heat to boiling to completely dissolve the soluble salts. After cooling, transfer the entire solution to a 500 mL volumetric flask, dilute to the mark with water, and shake well to obtain the sample stock solution.

[0082] S4. Interference Masking: Accurately transfer 25.00 mL of the above sample stock solution into a 250 mL Erlenmeyer flask. Add 120 mL of water, 10 mL of triethanolamine solution (1+3), 2 mL of magnesium sulfate solution (10 g / L), and 20 mL of potassium hydroxide solution (200 g / L) in sequence, and stir with a magnetic stirrer for 1-2 minutes.

[0083] S5. Titration determination: Add 0.10g of calcein-thymolphthalein indicator and titrate with pre-calibrated EGTA standard solution (0.01 mol / L). Observe against a black background until the green fluorescence of the solution completely disappears and turns a stable rose red color. Record the volume V1 consumed.

[0084] S6. Blank correction and result calculation: Blank correction and result calculation: Perform a blank test along with the sample. That is, except that no sample is added, the other steps are exactly the same as S1 to S5. Record the volume of EGTA standard titrant consumed, V0 (mL).

[0085] Example 3

[0086] A selective method for determining calcium content in the presence of zinc and tin, comprising the following steps:

[0087] S1. Sample pretreatment: Accurately weigh 0.3000 g (accurate to 0.0001 g) of the zinc-tin-calcium composite sample and place it in a 250 mL beaker. Add 10 mL of hydrochloric acid and 5 mL of nitric acid, heat on a hot plate until the sample is completely dissolved, and evaporate to near dryness.

[0088] S2. Removing Tin Interference: Remove the beaker, let it cool slightly, add 5 mL of hydrobromic acid, and evaporate to dryness at low temperature until no red bromine vapor is emitted. Add another 5 mL of hydrobromic acid and repeat the above operation once to ensure that the tin is completely removed.

[0089] S3. Preparation and dilution of the solution: Add 5 mL of hydrochloric acid and 20 mL of water to the residue after tin removal, and heat to boiling to completely dissolve the soluble salts. After cooling, transfer the entire solution to a 500 mL volumetric flask, dilute to the mark with water, and shake well to obtain the sample stock solution.

[0090] S4. Interference Masking: Accurately transfer 25.00 mL of the above sample stock solution into a 250 mL Erlenmeyer flask. Add 120 mL of water, 10 mL of triethanolamine solution (1+3), 2 mL of magnesium sulfate solution (10 g / L), and 20 mL of potassium hydroxide solution (200 g / L) in sequence, and stir with a magnetic stirrer for 1-2 minutes.

[0091] S5. Titration determination: Add 0.10g of calcein-thymolphthalein indicator and titrate with pre-calibrated EGTA standard solution (0.01 mol / L) against a black background until the green fluorescence of the solution completely disappears and turns a stable rose red color. Record the volume V1 consumed.

[0092] S6. Blank correction and result calculation: Blank correction and result calculation: Perform a blank test along with the sample. That is, except that no sample is added, the other steps are exactly the same as S1 to S5. Record the volume of EGTA standard titrant consumed, V0 (mL).

[0093] The mass fraction of calcium, ω(Ca), is calculated using the following formula:

[0094] In the formula:

[0095] C: The accurate concentration of the EGTA standard titration solution, in moles per liter (mol / L).

[0096] V1: The volume of EGTA standard titration solution consumed in titrating the sample solution, in milliliters (mL).

[0097] V0: The volume of EGTA standard titration solution consumed in titrating the blank solution, in milliliters (mL);

[0098] 40.08: Molar mass of calcium, in grams per mole (g / mol);

[0099] D: Separation ratio (total volume of sample stock solution / volume of sample stock solution transferred during titration);

[0100] m: The mass of the sample, in grams (g).

[0101] Comparative Example 1

[0102] No hydrobromic acid tin removal treatment is performed; the remaining steps are the same as in Example 2.

[0103] Comparative Example 2

[0104] Without adding triethanolamine and magnesium sulfate solution for masking, the remaining steps are the same as in Example 2.

[0105] Comparative Example 3

[0106] Titration was performed using a conventional EDTA standard solution, and the remaining steps were the same as in Example 2.

[0107] Effect verification and data analysis

[0108] To verify the reliability of the method of the present invention, a systematic methodological verification was conducted.

[0109] (1) Accuracy test (recovery rate)

[0110] Nine samples with known calcium content were accurately weighed, and calcium standard solution was accurately added to them at three levels: approximately 80%, 100%, and 120% of the calcium content in the samples. The calcium content was determined according to the method of this invention, and the recovery rate was calculated. The results are shown in Table 1.

[0111]

[0112] (2) Precision test (repeatability)

[0113] Six zinc-tin-calcium composite samples from the same batch were accurately weighed and their calcium content was determined in parallel according to the method of this invention. The results are shown in Table 2.

[0114]

[0115] (3) Durability test

[0116] The stability of the method of this invention under different conditions was investigated. A sample was selected, and the effects of the amount of hydrobromic acid, the amount of masking agent, and the pH condition after fine-tuning on the results were examined. The RSD of the results were all less than 3%, indicating good robustness. The results are shown in Table 3.

[0117]

[0118] Summary of experimental results:

[0119] (a) Test Results

[0120] The test results of Examples 1-3 and Comparative Examples 1-3 of the present invention are shown in Table 4.

[0121] (II) Summary of Comparison Results between Examples and Comparative Examples

[0122] The measurement results, endpoint color change response time, and endpoint evaluation of Examples 1-3 and Comparative Examples 1-3 were statistically analyzed, as follows:

[0123]

[0124] 1. Accuracy of measurement results

[0125] As shown in Table 4, the calcium content measured in Examples 1-3 was 8.38%-8.44%, which was stable. The measured values ​​for Comparative Example 1 were 9.38%, Comparative Example 2 were 9.48%, and Comparative Example 3 were 9.07%, all of which were higher than those in Examples 1-3. This indicates that failure to remove tin interference, failure to mask zinc ions, or the use of traditional EDTA titration would all lead to higher measured results. However, the present invention effectively ensures the accuracy of the measurement through a combination strategy of removing tin with hydrobromic acid, masking with triethanolamine-magnesium sulfate, and titrating with EGTA.

[0126] 2. Endpoint Judgment Effect

[0127] The endpoint color change response time of Examples 1-3 is ≤1 second, and the endpoint evaluation is "sharp", which facilitates quick and accurate endpoint judgment; the response time of Comparative Example 1 is 5-8 seconds, and the endpoint evaluation is "sluggish"; the response time of Comparative Example 2 is >30 seconds and the endpoint is difficult to stabilize, and the evaluation is "unable to judge"; the response time of Comparative Example 3 is 3-5 seconds, and the evaluation is "sluggish change". This shows that the indicator system and solution environment optimized by the present invention can improve the sensitivity and reliability of endpoint judgment.

[0128] 3. Overall performance of the method

[0129] Based on the methodological validation results, the average recovery rate of the method of this invention is 100.7%, the repeatability RSD is 0.7%, and the robustness RSD is <0.3%. Moreover, the operation is based on classical titration, which does not require expensive instruments. It solves the defects of high cost of existing instrumental analysis methods and large interference of traditional EDTA titration method, and can meet the needs of zinc-tin-calcium composite material research and development and industrial on-site quality control.

[0130] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for selectively determining calcium content in the presence of zinc and tin, characterized in that, Includes the following steps: S1. Sample decomposition: Accurately weigh the sample, add hydrochloric acid and nitric acid, heat to dissolve and then evaporate to near dryness; S2, Removal of tin interference: Add hydrobromic acid to the product obtained in S1, heat at low temperature to evaporate to dryness until no bromine vapor escapes, and repeat at least once; S3. Preparation and volume adjustment of the test solution: Add hydrochloric acid and water to the residue obtained in S2, heat to dissolve, cool and then adjust to volume to obtain the sample stock solution. S4. Interference masking: Take the sample stock solution obtained in S3, add water, triethanolamine solution, magnesium sulfate solution and potassium hydroxide solution in sequence, stir evenly and let stand; S5. Titration determination: Add calcein-thymolphthalein mixed indicator to the solution treated in S4, and titrate with EGTA standard titrant until the green fluorescence disappears and a stable rose red color appears. S6. Blank correction and result calculation: Perform a blank test and calculate the calcium content in the sample based on the net consumption of EGTA standard titrant.

2. The selective determination method for calcium content in the presence of zinc and tin according to claim 1, characterized in that, In step S1, the sample weight is 0.1g~0.5g, the amount of hydrochloric acid added is 8mL~15mL, and the amount of nitric acid added is 3mL~8mL.

3. The selective determination method for calcium content in the presence of zinc and tin according to claim 1, characterized in that, In step S2, the amount of hydrobromic acid added is 3 mL to 8 mL.

4. The selective determination method for calcium content in the presence of zinc and tin according to claim 1, characterized in that, In step S3, hydrochloric acid and water are added to the residue, heated to dissolve, cooled, and then transferred to a 250mL~500mL volumetric flask and diluted to volume.

5. The selective determination method for calcium content in the presence of zinc and tin according to claim 1, characterized in that, In step S4, the volume of the sample stock solution is 10.00 mL to 50.00 mL; 120 mL to 150 mL of water, 5 mL to 15 mL of triethanolamine solution, 1 mL to 3 mL of magnesium sulfate solution and 15 mL to 25 mL of potassium hydroxide solution are added sequentially.

6. The selective determination method for calcium content in the presence of zinc and tin according to claim 1, characterized in that, In step S4, the triethanolamine solution is prepared by mixing triethanolamine and water in a volume ratio of 1:3; the magnesium sulfate solution is prepared by dissolving 1.0 g of MgSO4·7H2O in water and diluting it to 100 mL; and the potassium hydroxide solution is prepared by dissolving 20.0 g of potassium hydroxide in water and diluting it to 100 mL.

7. The selective determination method for calcium content in the presence of zinc and tin according to claim 1, characterized in that, In step S5, the calcein-thymolphthalein mixed indicator is prepared by grinding and mixing calcein and thymolphthalein in a mass ratio of 0.10:0.14 with dried potassium chloride in a mass ratio of 1:

100.

8. The selective determination method for calcium content in the presence of zinc and tin according to claim 1, characterized in that, In step S6, the formula for calculating the calcium content is: In the formula: C represents the accurate concentration of the EGTA standard titration solution, in mol / L; V1 represents the volume of EGTA standard titration solution consumed in titrating the sample solution, in mL; V0 represents the volume of EGTA standard titration solution consumed in titrating the blank solution, in mL; 40.08 represents the molar mass of calcium, in g / mol; D represents the separation ratio, which is the ratio of the total volume of the sample stock solution to the volume of the sample stock solution transferred during titration; m represents the sample mass, expressed in grams.

9. A method for selectively determining calcium content in the presence of zinc and tin according to any one of claims 1-8, characterized in that, The sample is a zinc-tin-calcium composite material.

10. The method for selectively determining calcium content in the presence of zinc and tin according to claim 9, characterized in that, The zinc-tin-calcium composite system is a composite material of zinc hydroxystannate and calcium carbonate.

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