A detection method for determining total iron by using potassium thiocyanate as a color developing agent

Abstract

The application belongs to the technical field of analysis and detection, and relates to a detection method for determining total iron (Fe (III)) by using potassium thiocyanate as a chromogenic agent. The application discloses a detection method for determining total iron by using potassium thiocyanate as a chromogenic agent, which comprises wavelength selection, selection of a chromogenic agent, selection of a buffer solution, selection of an oxidizing agent, and treatment of interference. The application can be used to accurately determine the low content of total iron in a nickel smelting intermediate product, and the determination of the iron content in quartz stone, saturated brine and potassium permanganate solution. The method is a basic method, and the application field can be expanded according to actual requirements.

Description

Technical Field

[0001] This invention relates to the field of analytical testing technology, and in particular to a method for determining total iron using potassium thiocyanate as a colorimetric reagent. Background Technology

[0002] Iron is the second most abundant metallic element in the Earth's crust and the fourth most abundant element (5.05%). It is widely found in ores, with iron-bearing minerals including magnetite, hematite, limonite, and siderite. Iron is also widely present in the human body, animals, plants, food, and medicine. Iron is closely related to human life, and qualitative and quantitative analysis of iron plays a crucial role in controlling metallurgical processes, ensuring product quality, and ensuring the healthy growth of humans, animals, and plants, as well as food and drug safety. To date, many methods for determining iron have been developed, including spectrophotometry, atomic absorption spectrometry, titration volumetric method, atomic emission spectrometry, inductively coupled plasma mass spectrometry, electrochemical methods, chemiluminescence methods, gravimetric methods, and fluorescence quenching methods.

[0003] The potassium dichromate titration method is widely recognized as the most accurate method for determining total iron in ores. However, due to the toxicity of HgCl2, meeting national emission standards requires prohibitively high wastewater treatment costs. Improved analytical methods use SnCl2-TiCl3 as a reducing agent, but the TiCl3 solution used in this method has drawbacks such as high cost, easy oxidation, and very poor stability. Titration analysis of total iron in ores is complex, lengthy, and sometimes the endpoint solution color change is not obvious, making it difficult to obtain analytical results promptly and failing to meet the requirements of rapid analysis in modern production. Instrumental analysis methods for determining total iron are simpler and more convenient, but relying solely on instrumental analysis, especially for ores with high iron content, is susceptible to interference, making it difficult to obtain accurate conclusions and conduct in-depth analysis.

[0004] Spectrophotometry is based on Lambert-Beer's law, A=ebc. It utilizes the reaction of the analyte in solution with a colorimetric reagent to form a colored compound. The intensity of the color is directly proportional to the concentration of the analyte, thus determining its concentration. Currently, many colorimetric systems exist for iron determination, with reagents including o-phenanthroline, sulfosalicylic acid, potassium thiocyanate, 4-aminoantipyrine, and chromazine. Sulfosalicylic acid is the most commonly used colorimetric reagent for iron determination. Fe ions can form stepwise complexes with sulfosalicylic acid, potentially producing complexes with different colors (1:1, 1:2, and 1:3) under different acidity conditions. This method has the following drawbacks when used for sample analysis: 1. Strict control of solution acidity is crucial; otherwise, significant errors will occur. 2. The method has low sensitivity; errors from a single absorbance reading are substantial. 3. Samples such as high-nickel matte contain a high nickel content (approximately 50%), which interferes with the sulfosalicylic acid spectrophotometric determination of iron and cannot be eliminated. 4. Samples of virgin nickel electrolyte, cobalt-free electrolyte, and copper-free electrolyte contained 1 mg / L of iron and 70 g / L of nickel. When using the sulfosalicylic acid spectrophotometric method, the color of the complex formed by iron and sulfosalicylic acid could not be observed. Therefore, for these samples in nitric acid medium, the iron content of virgin nickel electrolyte, cobalt-free electrolyte, and copper-free electrolyte is determined by titration colorimetry based on the principle of the formation of a red complex between iron(III) and thiocyanate.

[0005] The six ligands formed by the reaction of Fe(III) with potassium thiocyanate are all blood-red, a characteristic reaction of Fe(III). This indicates that the measurement wavelength will not shift and the absorbance is relatively high. However, this characteristic reaction is not the mainstream detection method for Fe(III). In an acidic environment, the color of the ligands formed by the reaction of Fe(III) with potassium thiocyanate deepens with increasing acidity and lightens or even disappears with decreasing acidity. The partition coefficients of various ligands of ferric thiocyanate are unstable and fade immediately after reaching a certain value. The method has poor reproducibility, lacks a clear stable condition region, and does not allow sufficient analytical time for detection, thus limiting its widespread application. Summary of the Invention

[0006] To address the aforementioned technical problems, this invention proposes a method for determining total iron using potassium thiocyanate as a colorimetric reagent, comprising using ferric thiocyanate with an acidity of pH=2.1 as the colorimetric reagent, wherein the colorimetric reagent comprises: a. Potassium hydrogen phthalate-H2SO4 buffer solution: Measure 170 mL of potassium hydrogen phthalate solution (0.2 mol / L) and 40 mL of sulfuric acid solution (3+97) into a 500 mL volumetric flask, make up to volume and mix well, let stand for 24 hours, pH=2.0; b. Potassium thiocyanate solution (300.0 g / L): Weigh 150.0 g of potassium thiocyanate and dissolve it in 500.00 mL of water; c. H2O2 solution (100 g / L): Take 1 unit of H2O2 solution (30%) and dilute it with water to 3 times; d. Sulfuric acid solution (1.5 mol / L): Measure one unit of sulfuric acid solution (1+1) and dilute with water to six times; The specific measurement steps are as follows: (1) Solid sample: Weigh 0.2 g of solid sample and digest it with an acid system; the liquid after digestion contains F - The concentration of ions must not exceed 0.05 mmol / L (1 mg / L). There is no need to introduce fluoride during the digestion of iron in solid samples. Use the supernatant or filtrate as the test solution, and aliquot the sample according to its content to ensure the absorbance of the sample is within the range of the curve. (2) Liquid samples: Liquid samples should be thoroughly mixed; take the supernatant or filtrate. Other requirements are the same as for solid samples; (3) Take Fe 3+ Add the standard solution or the sample solution to a 100 mL volumetric flask, adjust with ammonia (1+1) until a precipitate appears, then adjust with sulfuric acid solution (1.5 mol / L) until the precipitate just disappears, and strain by 1 drop. Add 3 drops of H2O2 solution (100 g / L), add potassium hydrogen phthalate-H2SO4 buffer solution, add 10.00 mL of potassium thiocyanate solution (300.0 g / L), make up to volume and mix well. After developing the color for 10-15 minutes, start the determination. (4) Using the reagent blank as a reference, measure the absorbance at a visible light spectrophotometer using a 1cm cuvette. The measurement should be completed within 30 minutes.

[0007] Furthermore, the suitable wavelength range for visible light spectrophotometers is 475nm to 480nm.

[0008] Furthermore, the optimal wavelength for the visible light spectrophotometer is preferably 478 nm.

[0009] Compared with the prior art, the beneficial effects of the present invention using the above technical solution are as follows: (1) The present invention uses potassium hydrogen phthalate-H2SO4 buffer solution to control the acidity of the colorimetric system, which solves the problem that the partition coefficients of various ligands of ferric thiocyanate are unstable in acidic environment and will fade immediately after reaching a certain value, thus making the colorimetric system stable. The detection method of determining total iron using potassium thiocyanate as a colorimetric agent has been widely used.

[0010] (2) This method has extremely high sensitivity, which meets the requirements for the analysis of iron in various materials with low content. Using this invention, low content total iron in nickel smelting intermediate products can be accurately determined.

[0011] (3) This method is simple to operate and has a wide range of applications. This method describes a basic method, and users can expand the application field according to actual requirements.

[0012] (4) This method studied F - The interference of Co, Ni, Cu, SiO2, NaCl, and KMnO4 on potassium thiocyanate as a colorimetric reagent in the determination of total iron is also applicable to the determination of iron content in quartz, saturated brine, and potassium permanganate solution. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of the standard curve of the present invention; Figure 2 This is a schematic diagram showing the Fe content curves of 1.34% and 0.80% in this invention; Figure 3 This is a schematic diagram showing the Fe content curves of 5.21% and 11.74% in this invention. Detailed Implementation

[0014] The specific embodiments of the present invention will be further described in detail below with reference to the accompanying drawings and examples. These examples are used to illustrate the present invention but are not intended to limit its scope.

[0015] A method for determining total iron using potassium thiocyanate as a colorimetric reagent is disclosed. By adjusting the acidity-oxidant-colorimetric reagent ratio, a stable colorimetric plateau for ferric thiocyanate is achieved at pH 2.1, thus ensuring sufficient measurement time. The method is characterized by including precautions for sample digestion, wavelength selection, preparation and dosage of the colorimetric reagent, preparation and dosage of the buffer solution, preparation and dosage of the oxidant, interference handling, and the order of reagent addition. Specifically, it includes... (1) Preparation of reagents a. Ultrapure water; b. Superior purity; c. Sulfuric acid solution (1+1); d. Sulfuric acid solution (3+97); e. Potassium hydrogen phthalate solution (0.2 mol / L): Weigh 20.4 g of potassium hydrogen phthalate, dissolve it in 6 times the amount of water (about 120-150 mL) by heating, and make up to 500 mL in a volumetric flask. f. Potassium hydrogen phthalate-H₂SO₄ buffer solution (pH=2.0): Measure 170 mL of potassium hydrogen phthalate solution (0.2 mol / L) and 40 mL of sulfuric acid solution (3+97) into a 500 mL volumetric flask, and dilute to volume. Let stand for 24 hours; g. Potassium thiocyanate solution (300.0 g / L): Weigh 150.0 g of potassium thiocyanate and dissolve it in 500.00 mL of water; h. H2O2 solution (100 g / L): Measure one unit of H2O2 solution (30%) and dilute with water to three times the original volume. It is recommended to prepare a 7-day supply each time. i. Ammonia (1+1); j. Sulfuric acid solution (1.5 mol / L): Measure one unit of sulfuric acid solution (1+1) and dilute it with water to six times its original volume; k, Fe 3+ Standard stock solution (0.5 g / L): Accurately weigh 4.3170 g of ferric ammonium sulfate dodecahydrate into a 100 mL beaker. After initial dissolution with a small amount of sulfuric acid solution (3+97), transfer to a 1000 mL volumetric flask, add 30 mL of sulfuric acid solution (3+97), and dilute to volume. Let stand for 72 hours and then mix well. This solution contains 0.5 mg of iron per mL. Or Weigh 0.5000±0.0001g of metallic Fe (99.99%) standard, digest it with 50mL of sulfuric acid solution (3+1) by heating, and dilute to 1000mL in a volumetric flask. 1mL of this solution contains 0.5mg of iron. l, Fe 3+ Standard solution (0.05 g / L): Take 20 mL of Fe 3+ The standard stock solution (0.5 g / L) was placed in a 200 mL volumetric flask, and 10 mL of sulfuric acid solution (3+97) was added. The mixture was then diluted to volume and stirred thoroughly. 1 mL of this solution contains 0.05 mg of iron. m, Visible spectrophotometer.

[0016] Note: All absorbance values ​​are multiplied by 1000.

[0017] (2) Optimal absorption wavelength The suitable wavelength range is 475nm to 480nm, and 478nm was selected in this experiment. (3) Solid samples should be reduced by the cone quartering method and processed into sample sieves; take the material passing through an 80-mesh sieve; Liquid samples should be thoroughly mixed; take the supernatant or filtrate.

[0018] (4) Sample digestion: Weigh 0.2 g of the solid sample and digest it with an acid system; the liquid after digestion contains F. - The concentration of ions must not exceed 0.05 mmol / L (1 mg / L). There is no need to introduce fluoride during the digestion of iron in solid samples. Use the supernatant or filtrate as the test solution.

[0019] (5) Fe 3+ Standard curve creation Fe 3+Prepare 1 mL, 2 mL, 3 mL, 4 mL, and 5 mL of a 0.050 g / L standard solution in five 100 mL volumetric flasks pre-filled with 30 mL of water. Adjust the solution with ammonia (1+1) until a precipitate appears, then adjust with 1.5 mol / L sulfuric acid solution until the precipitate just disappears, and strain by one drop. Add 3 drops of 100 g / L H₂O₂ solution, 10 mL of potassium hydrogen phthalate-H₂SO₄ buffer solution (pH=2.0), and 10.00 mL of potassium thiocyanate solution (300.0 g / L). Make up to volume and mix well. Develop the color for 10–15 minutes. Using a reagent blank as a reference, measure the absorbance at 478 nm using a visible spectrophotometer with a 1 cm cuvette. The measurement should be completed within 30 minutes.

[0020] Reagent blank: Add 4 drops of sulfuric acid solution (1.5 mol / L) to a 100 mL volumetric flask containing 30 mL of water. Otherwise, the process is the same as the standard series.

[0021] Five groups of absorbance and Fe 3+ The content value is entered into Excel software, in the form of Fe. 3+ A linear equation is constructed with the content on the x-axis and absorbance A on the y-axis. This curve represents the Fe content. 3+ Standard analysis curve.

[0022] List two sets of standard curves as follows: Figure 1 As shown.

[0023] (6) Sensitivity, detection limit, linear range and estimation error Sensitivity If the sample weight is 200 mg, and the volume is diluted to 200 mL in a volumetric flask, and the directly dispensed volumes are 5 mL and 10 mL respectively, then the corresponding limits of detection can be calculated as follows: Where: 200mg represents the sample weight; 200mL represents the final volume; 5mL and 10mL represent the direct aliquots; 100 represents the coefficient for converting mass to percentage content.

[0024] Linear range The lower limit of the content range is the detection limit: 0.006 mg / 100 mL, and the upper limit of the content range is the highest Fe content in the standard: 0.25 mg / 100 mL. If the sample weight is 200 mg, diluted to a volumetric flask of 200 mL, and the direct aliquot is 5 mL, then the direct detection range of Fe by this method is: [insert range here].

[0025] estimation error In this method, the silicon content of an unknown sample can be calculated using the following formula. The above factors are taken as the average level of significance in the experiment.

[0026] Relative error of sample weighing: fixed at 0.01%; relative error of fading absorbance: taken as the plateau absorbance error when the final reagent volume was determined: 0.3%; statistical error of absorbance: taken as the 100 absorbance corresponding to 1 mL of standard in Chapter 4, 1%; relative error of sample background blank: 3%; relative error of standard curve slope, 1%; relative error of slope conformity: replaced by the curve after Cu interference, 3%.

[0027] (7) Sample determination solid samples Accurately weigh 0.2000±0.0002 g of sample into a 500 mL heat-resistant beaker; digest with a saturated nitric acid-potassium chlorate solution, wash thoroughly, and transfer to a 200 mL volumetric flask; place at the same temperature as water, and dilute to volume and mix well. Use the supernatant or filtrate as the test solution.

[0028] Take 0 mL and 5 mL of sample solution from the test solution and place them into two 100 mL volumetric flasks pre-filled with 30 mL of water. Adjust the solution with ammonia (1+1) until a precipitate appears, then adjust with sulfuric acid solution (1.5 mol / L) until the precipitate just disappears, adding 1 drop in excess. Add 3 drops of H₂O₂ solution (100 g / L), 10 mL of potassium hydrogen phthalate-H₂SO₄ buffer solution (pH=2.0), and 10.00 mL of potassium thiocyanate solution (300.0 g / L). Make up to volume and mix well. Develop the color for 10–15 minutes. Using a reagent blank as a reference, measure the absorbance at 478 nm using a visible spectrophotometer with a 1 cm cuvette. The measurement should be completed within 30 minutes.

[0029] Subtract the blank from the absorbance obtained after measurement, and substitute it into the following formula to calculate the sample content: Where: ω is the percentage Fe content of the sample, %; A S is the net absorbance measured on the sample; k is the slope of the Fe standard analysis curve; m0 is the sample weight, g; v1 is the first aliquot, mL; v0 is the total volume after dissolution and final volume adjustment, mL.

[0030] liquid sample Liquid samples should be thoroughly mixed; collect the supernatant or filtrate. The procedure is the same as for solid samples.

[0031] (8) Matrix matching (accuracy comparison) a. Construct a standard curve and record the slope A1; b. Analyze the sample using the standard curve to obtain the sample content C2; c. Using C2 as the content, construct a sample curve using the same method as the standard curve and record the slope A2; d. Compare A1 and A2 according to the allowable error range. If the error exceeds this tolerance, the sample cannot be analyzed using the standard curve; otherwise, it can.

[0032] (9) Interference In a 100 mL test volume, a, F - Interference occurs when the ion concentration is above 0.005 mmol, and when F - a. This method should not be used in the presence of ions. b. Interference occurs when the Co content is above 8 mg, but is minimal when the Co content in the solid sample is below 10%. c. Interference occurs when the Ni content is above 8 mg, but is not considered interference under the experimental conditions. d. Interference occurs when the Cu content is above 0.6 mg, and is the main interference in high Cu, low Fe samples. When directly aliquoting 5 mL, no correction is required when copper with a Cu content of 50% is present in the solid sample. e. SiO2 content of 100% will not interfere with the determination. f. NaCl saturation will not interfere with the determination. g. KMnO4 content below 0.05 mmol will not interfere with the determination.

[0033] The invention will be further illustrated with examples.

[0034] Example 1: Secondary high-nickel matte Accurately weigh 0.2000±0.0002 g of the secondary high-nickel matte sample into a 500 mL heat-resistant beaker; add 20 mL of saturated nitric acid-potassium chlorate solution, digest on a hot plate at low temperature until nearly dry, remove and cool slightly; add 10 mL of water and boil, let cool for a while, wash and transfer to a 200 mL volumetric flask; place at the same temperature as water, and make up to volume and mix well. Take the supernatant or filtrate as the test solution.

[0035] Take 0 mL and 5 mL of sample solution from the test solution and place them into five 100 mL volumetric flasks pre-filled with 30 mL of water. Adjust the solution with ammonia (1+1) until a precipitate appears, then adjust with sulfuric acid solution (1.5 mol / L) until the precipitate just disappears, adding 1 drop in excess. Add 3 drops of H₂O₂ solution (100 g / L), 10 mL of potassium hydrogen phthalate-H₂SO₄ buffer solution (pH=2.0), and 10.00 mL of potassium thiocyanate solution (300.0 g / L). Make up to volume and mix well. Develop the color for 10–15 minutes. Using a reagent blank as a reference, measure the absorbance at 478 nm using a visible spectrophotometer with a 1 cm cuvette. The measurement should be completed within 30 minutes.

[0036] Subtract the blank from the absorbance obtained after measurement, and substitute it into the following formula to calculate the sample content: Where: ω is the percentage Fe content of the sample, %; A S is the net absorbance measured on the sample; k is the slope of the Fe standard analysis curve; m0 is the sample weight, g; v1 is the first aliquot, mL; v0 is the total volume after dissolution and final volume adjustment, mL.

[0037] matrix matching b. Analyzing the samples using a standard curve, the Fe content of samples 1 and 2 was found to be C1: 1.34%, C2: 1.34%, C3: 1.34%, C4: 1.34%, C5: 1.34%, C6: 1.34%, C7: 1.34%, C8: 1.34%, C9 ... 2： 0.80%; c. Using C1 and C2 as standard contents, construct a sample curve according to the method for constructing a standard curve, and record the two sets of slopes A1 and A2; For example... Figure 2 As shown; d. According to the allowable error range, compare the slope of the sample curve with the slope of the standard curve.

[0038] Where: R1 and R2 represent the difference in slope between the two samples and the standard curve, %; A S A1 represents the slope of the standard curve; A2 represents the slope of sample curves 1 and 2; 100 represents the coefficient for converting mass into percentage content.

[0039] It is evident that the maximum difference between the sample curve and the standard curve is 0.5%, meaning that using the standard curve to analyze iron in secondary high-nickel matte will introduce a maximum deviation of 0.5%. This is far smaller than the estimation error and also less than the minimum permissible error of 2% for visible spectrophotometry. The copper content in secondary high-nickel matte can reach up to 40%, thus interference is inevitable, explaining the lower slopes of both sample curves. However, assuming the sample contains 5% iron, the result from the standard curve analysis would be 5 ± 0.03%, which is not worthwhile to introduce matrix correction for such a small absolute error.

[0040] Precision test Since the study examines the probabilistic error throughout the entire process, rather than determining the stability of a single step, the same method is used repeatedly: the same sample is weighed, digested, and measured multiple times according to the specifications of this method, resulting in the precision shown in the table below. It was observed that none of the samples exceeded the estimated error, but the error was larger for samples with low content.

[0041] Spike experiment The precision experiment results showed that the lower the content, the larger the error, indicating that there might be unidentified contamination or loss during the entire process. Therefore, full-process spiked loading is unsuitable for low content samples. Since the focus was on the recovery rate of the detection method, spiked loading was used at the measurement stage: a certain volume of sample was taken, and a certain volume of Fe was added to it.3+ The content of the standard solution (0.05 g / L) was determined according to the sample determination procedure, and the recovery rate was calculated.

[0042] The original content obtained after spiking refers to the original content obtained by subtracting the standard from the spiked sample. It is calculated using the following formula: Where: ω Ps This indicates the original content obtained after spiking, in %; A a+S A represents the total absorbance measured after the sample has been spiked; S Indicates the absorbance of the standard; A a This indicates the absorbance of the sample without spikes; m S The mass of the standard added is in mg; m0 is the sample weight in mg; v1 is the initial aliquot volume in mL; and v0 is the total volume after dissolution and final dilution in mL.

[0043] The recovery rates are shown in the table below. Example 2: Smelting nickel matte in a side-blown furnace Accurately weigh 0.2000±0.0002 g of nickel matte sample from a side-blown furnace into a 500 mL heat-resistant beaker; add 20 mL of saturated nitric acid-potassium chlorate solution, digest at low temperature on a hot plate until nearly dry, remove and cool slightly; add 10 mL of water and boil, let cool for a while, wash and transfer to a 200 mL volumetric flask; place at the same temperature as water, and make up to volume and mix well. Take the supernatant or filtrate as the test solution.

[0044] Take 0 mL and 5 mL of sample solution from the test solution and place them into two 100 mL volumetric flasks, each pre-filled with 30 mL of water. Adjust the solution with ammonia (1+1) until a precipitate appears, then adjust with sulfuric acid solution (1.5 mol / L) until the precipitate just disappears, adding 1 drop in excess. Add 3 drops of H2O2 solution (100 g / L) and 10 mL of potassium hydrogen phthalate-H2SO4 buffer solution (pH=2.0). 加入10.00m Prepare a 300.0 g / L potassium thiocyanate solution, dilute to volume, mix well, and allow to develop color for 10–15 minutes. Using a reagent blank as a reference, measure the absorbance at 478 nm using a visible spectrophotometer with a 1 cm cuvette. The measurement should be completed within 30 minutes.

[0045] Subtract the blank from the absorbance obtained after measurement, and substitute it into the following formula to calculate the sample content: Where: ω is the percentage Fe content of the sample, %; A Sis the net absorbance measured for the sample; k is the slope of the Fe standard analysis curve; m0 is the sample weight, g; V1 represents the total volume after dissolution and final volume adjustment, mL; v1 represents the volume of the first aliquot, mL; V2 represents the total volume of the second final volume adjustment, mL; v2 represents the volume of the second aliquot, mL.

[0046] If the absorbance obtained according to the detection procedure is greater than 0.5, 20 mL of the diluted test solution should be taken from the 200 mL volumetric flask, diluted to volume again, and mixed thoroughly. This 200 mL solution should be used as the test solution, and the detection should be performed again. Do not use small volumes for detection, as this method has high sensitivity; also, do not perform direct detection if the absorbance exceeds the detection limit.

[0047] matrix matching b. Analyzing the samples using a standard curve, the Fe content of samples 1 and 2 was found to be C1: 5.21% and C2: 5.21%. 2： 11.74%; c. Using C1 and C2 as standard contents, construct a sample curve according to the method for constructing a standard curve, and record two sets of slopes A1 and A2, such as... Figure 3 As shown; d. According to the allowable error range, compare the slope of the sample curve with the slope of the standard curve.

[0048] Where: R1 and R2 represent the difference in slope between the two samples and the standard curve, %; A S A1 represents the slope of the standard curve; A2 represents the slope of sample curves 1 and 2; 100 represents the coefficient for converting mass into percentage content.

[0049] It can be seen that the maximum difference between the sample curve and the standard curve is 0.5%. This means that using the standard curve to analyze iron in side-blown nickel matte will introduce a maximum deviation of 0.5%, which is far smaller than the estimation error and also less than the 2% minimum permissible error for visible spectrophotometry. When the iron content in the side-blown nickel matte increases, the copper content decreases, thus reducing the main interference. This explains why the slope of the second set of curves with higher iron content is closer to the slope of the standard curve. Assuming the sample contains 10% iron, the result analyzed using the standard curve will be 10 ± 0.02%.

[0050] Precision test Since the study examines the probabilistic error throughout the entire process, rather than determining the stability of a single step, the same method is used repeatedly: the same sample is weighed, digested, and measured multiple times according to the specifications of this method, resulting in the precision shown in the table below. It was observed that none of the samples exceeded the estimated error, and the higher the content, the smaller the error.

[0051] Spike experiment Because the precision experiment results exhibit the magnitude of probabilistic error throughout the process, and the reliability of the matrix matching method itself is uncertain, a spiking step is adopted in the measurement process: a certain volume of sample is taken, and a certain volume of Fe is added to it. 3+ The content of the standard solution (0.05 g / L) was determined according to the sample determination procedure, and the recovery rate was calculated.

[0052] The original content obtained after spiking refers to the original content obtained by subtracting the standard from the spiked sample. It is calculated using the following formula: Where: ω Ps This indicates the original content obtained after spiking, in %; A a+S A represents the total absorbance measured after the sample has been spiked; S Indicates the absorbance of the standard; A a This indicates the absorbance of the sample without spikes; m S The values ​​represent the mass of the standard added (mg); m0 represents the sample weight (mg); V1 represents the total volume after dissolution and refilling (mL); v1 represents the volume of the first aliquot (mL); V2 represents the total volume of the second refill (mL); and v2 represents the volume of the second aliquot (mL).

[0053] The recovery rates are shown in the table below. Those skilled in the art should understand that the scope of the invention involved in the embodiments of this disclosure is not limited to the technical solutions formed by a specific combination of the above-mentioned technical features, but should also cover other technical solutions formed by any combination of the above-mentioned technical features or their equivalent features without departing from the above-mentioned inventive concept, such as technical solutions formed by replacing the above-mentioned features with (but not limited to) technical features with similar functions disclosed in the embodiments of this disclosure.

Claims

1. A method for determining total iron using potassium thiocyanate as a colorimetric reagent, characterized in that, This includes using ferric thiocyanate with an acidity of pH=2.1 as a colorimetric reagent, wherein the colorimetric reagent comprises: a. Potassium hydrogen phthalate-H2SO4 buffer solution: Measure 170 mL of potassium hydrogen phthalate solution and 40 mL of sulfuric acid solution into a 500 mL volumetric flask, make up to volume and mix well, let stand for 24 hours, pH=2.0; b. Potassium thiocyanate solution: Weigh 150.0g of potassium thiocyanate and dissolve it in 500.00mL of water; c. H2O2 solution: Measure 1 unit of H2O2 solution and dilute it with water to 3 units; d. Sulfuric acid solution: Measure one unit of sulfuric acid solution and dilute it with water to six units. e. Ammonia water; The specific measurement steps are as follows: (1) Solid sample: Weigh 0.2 g of solid sample and digest it with an acid system; the liquid after digestion contains F - The concentration of ions should not exceed 0.05 mmol / L. There is no need to introduce fluoride during the digestion of iron in solid samples. Take the supernatant or filtrate as the test solution and separate the samples according to the content to ensure that the absorbance of the sample is within the range of the curve. (2) Liquid samples: Liquid samples should be mixed well; take the supernatant or filtrate as the test liquid, and separate the samples according to the content so that the absorbance of the sample is within the range of the curve. (3) Take Fe 3+ Add the standard solution or the sample solution to a 100 mL volumetric flask, adjust with ammonia until a precipitate appears, then adjust with sulfuric acid until the precipitate just disappears, and strain by 1 drop. Add 3 drops of H2O2 solution, add potassium hydrogen phthalate-H2SO4 buffer solution, add 10.00 mL of potassium thiocyanate solution, make up to volume and mix well. After developing the color for 10-15 minutes, start the determination. (4) Using the reagent blank as a reference, measure the absorbance at a visible light spectrophotometer using a 1cm cuvette and complete the measurement within 30 minutes.

2. The method for determining total iron using potassium thiocyanate as a colorimetric reagent according to claim 1, characterized in that, The wavelength range of the visible light spectrophotometer is 475nm to 480nm.

3. The method for determining total iron using potassium thiocyanate as a colorimetric reagent according to claim 2, characterized in that, The wavelength of the visible light spectrophotometer is 478 nm.

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