Method for determining total iron content in manganese-containing positive electrode material

Abstract

The present application relates to the technical field of chemical analysis, in particular to a method for determining total iron content in manganese-containing positive electrode material, comprising the following steps: solution preparation, sample pretreatment, sample determination, blank correction and result calculation. The present application can accurately measure the total iron content in manganese-containing positive electrode material by measuring the volume of potassium dichromate standard solution consumed when different potential jump points appear in the titration process, effectively eliminates the interference of manganese element, simplifies the operation steps, shortens the analysis time, reduces the detection cost, and improves the accuracy of the total iron content in manganese-containing positive electrode material, and has important application value.

Description

Technical Field

[0001] This invention relates to the field of chemical analysis technology, specifically to a method for determining the total iron content in manganese-containing cathode materials. Background Technology

[0002] Current methods for determining total iron content, such as the national standard GB / T6730.65-2009, use the titration method of reducing potassium dichromate with titanium trichloride. This method requires the consumption of various chemical reagents, such as hydrochloric acid, sulfuric acid, phosphoric acid, tin dichloride, titanium trichloride, sodium tungstate, and potassium dichromate, as well as a large amount of distilled water. Moreover, the operation steps are cumbersome, the test time is long, the specific operation is difficult to master, and it is easy to cause systematic deviations.

[0003] In production, most titration methods for determining total iron are manual, which is labor-intensive and inevitably introduces human error. Furthermore, the specialized nature and high operational difficulty of manual titration lead to high training costs for testing personnel. Compared to manual titration, automated potentiometric titration using platinum electrodes to indicate potential jumps has significant advantages. However, traditional potentiometric titration for total iron determination uses mercuric chloride, a highly toxic substance that can easily cause experimental accidents, increasing the experimental hazard and causing environmental pollution. Inductively coupled plasma atomic emission spectrometry (ICP-AES) is also used for detection; however, ICP-AES is susceptible to instrument variations, environmental factors, and other random influences, resulting in significantly lower precision compared to titration.

[0004] CN117741041A discloses an analytical method for determining the content of manganese and iron by potassium dichromate titration. The manual titration method used in this patent uses sodium tungstate and sodium diphenylamine sulfonate as indicators. The endpoint of the titration is determined by the color change of the indicators. Not only are the detection sensitivity, indication accuracy and analytical efficiency low, but the reduction process also requires the operator to judge the endpoint based on the color, which is highly subjective and causes the results to be biased.

[0005] CN116858987A discloses a method for detecting total iron content, but the sample tested by this method does not contain manganese. That is, it does not provide a method for determining iron content when manganese is present.

[0006] Therefore, there is an urgent need to find a safer and more environmentally friendly method for determining total iron content that can significantly reduce reagent usage, simplify the operation process, shorten the detection time, effectively eliminate manganese interference, and improve the accuracy of the determination. Summary of the Invention

[0007] The purpose of this invention is to at least partially solve or improve one of the following technical problems:

[0008] This application proposes a method for determining the total iron content in manganese-containing cathode materials. Compared with existing technologies, this application solves the problems of high indicator consumption (requiring sodium tungstate, sodium diphenylamine sulfonate, etc.), cumbersome steps, long cycle, difficulty in endpoint determination, and interference from manganese in manual titration methods. Potentiometric titration has advantages such as low reagent cost, good repeatability and accuracy, and fast detection speed, and is especially suitable for manganese-containing cathode material systems. It has strong anti-interference ability, and the endpoint is determined by the electrode potential jump, avoiding high concentrations of Mn²⁺. + Solution color or turbidity can interfere with the indicator method; the potential jump characteristic can effectively distinguish Fe²⁺. + / Fe³ + With Mn 2+ / Mn 3+ The redox reaction steps ensure the accuracy of total iron content determination and effectively eliminate interference from manganese. Specifically, the detection method provided in this application includes the following steps: (1) Solution preparation: Prepare potassium dichromate standard solution, stannous chloride solution, sulfuric acid-phosphoric acid mixed acid solution and ferrous ammonium sulfate solution; (2) Sample pretreatment: Place a manganese-containing cathode material sample with a mass of m0 in a reaction vessel, add 0.1 mL of water to wet the manganese-containing cathode material sample, then add concentrated hydrochloric acid, cover and digest, remove the acid and cool, transfer the carbon powder and the solution after removing the acid to a sample bottle, and make up the volume with ultrapure water to a mass of m1 of the contents of the sample bottle; filter out the carbon powder to obtain the sample digest solution; (3) Sample determination; Take the sample digestion solution m2 obtained in step (2), place it in the reaction vessel, add concentrated hydrochloric acid, and then add the stannous chloride solution prepared in step (1) drop by drop until the yellow color disappears, then add 1-2 drops in excess, and then add the sulfuric acid-phosphoric acid mixed solution prepared in step (1). Titrate with the potassium dichromate standard solution prepared in step (1) in a potentiometric titrator until the third potential jump point appears, and stop titrating. Record the volume of potassium dichromate standard solution consumed when each potential jump point appears. (4) Blank correction: Take 1g-12g of blank sample solution and place it in a reaction vessel. Before titration, add 1mL of the ferrous ammonium sulfate solution prepared in step (1). Titrate with the potassium dichromate standard solution prepared in step (1) to the endpoint. The volume of potassium dichromate standard solution consumed is V1. Then add 1mL of the ferrous ammonium sulfate solution prepared in step (1) and continue titrating with the potassium dichromate standard solution prepared in step (1) to the endpoint. The volume of potassium dichromate standard solution consumed in step (1) is V2. The volume of potassium dichromate standard solution consumed by the blank sample solution is V0 = V1 - V2. Perform two parallel determinations and take the average value. The preparation of blank sample solution is consistent with the sample pretreatment steps, the only difference being that no manganese-containing cathode material sample is added during the preparation of blank sample solution. (5) Calculation of results: The total iron content is expressed as the mass fraction of iron (%) and is calculated according to the following formula:

[0009] In the formula: c(1 / 6 K2Cr2O7) is the oxidation concentration of the potassium dichromate standard solution, in mol / L; V represents the volume of potassium dichromate standard solution consumed when the second potential jump point occurs during sample determination, minus the volume of potassium dichromate standard solution consumed when the first potential jump point occurs, in mL. The average volume of potassium dichromate standard solution consumed in two parallel determinations for blank correction, in mL; M is the molar mass of iron, 55.845 g / mol; m0 is the sample amount of the manganese-containing cathode material sample taken in step (2), in g; m1 is the final mass of the contents of the sample bottle after mass adjustment in step (2), in g; m2 is the amount of sample digestion solution taken in step (3), in g.

[0010] 1 / 6 represents the reaction ratio of potassium dichromate and ferrous ions, because the equation for the reaction of potassium dichromate and ferrous ions is: Cr2O7 2－ +6Fe 2+ +14H + →2Cr 3+ +6Fe 3+ +7H2O, meaning that 1 part of potassium dichromate consumes 6 parts of ferrous ions.

[0011] In this application, there are three potential jump points (equivalence points). The first potential jump point indicates that the reaction between potassium dichromate and stannous chloride is complete. The second potential jump point indicates that the reaction between ferrous ions and potassium dichromate is complete. The third potential jump point indicates that Mn²⁺… + It reacts with potassium dichromate. The endpoint is determined by the abrupt change in electrode potential, avoiding high concentrations of Mn²⁺. + Solution color or turbidity can interfere with the indicator method; the potential jump characteristic can effectively distinguish Fe²⁺. + / Fe³ + With Mn 2+ / Mn 3+ The oxidation-reduction reaction steps ensure the accuracy of total iron content determination.

[0012] In this application, the acid removal is performed by heating at 150℃-200℃ for 70min-150min. The purpose of acid removal is to make the mass volume determination more accurate.

[0013] In some embodiments, in step (1), the solvents for the potassium dichromate standard solution, stannous chloride solution, sulfuric acid-phosphoric acid mixed solution, and ferrous ammonium sulfate solution are each water.

[0014] In some embodiments, in step (2), the carbon powder is contained in the manganese-containing cathode material sample, and after digestion, it is suspended in the solution after acid removal, and its content is extremely small.

[0015] In some embodiments, the mass fraction of HCl in the concentrated hydrochloric acid in step (2) is 36%-37%. Specifically, the mass fraction of HCl in the concentrated hydrochloric acid in step (2) is 36%, 36.5%, 37%, or any value within the range of any two of these values.

[0016] In some embodiments, in step (2), the volume ratio of the concentrated hydrochloric acid to the mass ratio of the manganese-containing cathode material sample is (20mL-40mL):3g. Specifically, the volume ratio of the concentrated hydrochloric acid to the mass ratio of the manganese-containing cathode material sample is 20mL:3g, 25mL:3g, 30mL:3g, 35mL:3g, 40mL:3g, or any value within any two of these ranges.

[0017] In some embodiments, the digestion temperature is 90℃-200℃. Specifically, the digestion temperature is 90℃, 100℃, 110℃, 120℃, 130℃, 140℃, 150℃, 160℃, 170℃, 180℃, 190℃, 200℃, or any value within a range of any two of these values.

[0018] In some embodiments, the digestion time is 30 min to 60 min. Specifically, the digestion time is 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, or any value within the range of any two of these values.

[0019] In some embodiments, the preparation method of the stannous chloride solution in step (1) includes: adding concentrated hydrochloric acid to stannous chloride, heating to dissolve, cooling after the system becomes clear, adding water to make up to the volume, adding 2-3 high-purity tin particles to the prepared stannous chloride solution, storing it in a brown dropper bottle, sealing it and storing it away from light; wherein, the concentration of stannous chloride in the stannous chloride solution is 50 g / L-200 g / L, and the mass fraction of HCl in the concentrated hydrochloric acid is 36%-37%. Specifically, the concentration of stannous chloride in the stannous chloride solution is 50 g / L, 60 g / L, 70 g / L, 80 g / L, 90 g / L, 100 g / L, 110 g / L, 120 g / L, 130 g / L, 140 g / L, 150 g / L, 160 g / L, 170 g / L, 180 g / L, 190 g / L, 200 g / L, or any value within any two of these ranges; the mass fraction of HCl in the concentrated hydrochloric acid in step (1) is 36%, 36.5%, 37%, or any value within any two of these ranges. In this application, the purpose of adding high-purity tin particles is to prevent the oxidation of the stannous chloride solution. In this application, stannous chloride, as a selective reducing agent, can reduce Fe³⁺… + Reduced to Fe² + .

[0020] In some embodiments, in step (1), the volume ratio of phosphoric acid aqueous solution: concentrated sulfuric acid: water in the sulfuric acid-phosphoric acid mixed acid solution is (2.5-3.5):(2.5-3.5):(3-5); wherein, before mixing, the mass fraction of phosphoric acid in the phosphoric acid aqueous solution is 80%-85%; and the mass fraction of H2SO4 in the concentrated sulfuric acid is 95%-98%. Specifically, the volume ratio of phosphoric acid aqueous solution: concentrated sulfuric acid: water in the sulfuric acid-phosphoric acid mixed acid solution is 2.5:2.5:3, 3:2.5:3, 3.5:2.5:3, 2.5:3:3, 3:3.5:3, 2.5:2.5:4, 3:2.5:4, 3.5:2.5:4, 2.5:3:4, 3:3.5:4, 2.5:2.5:5, 3:2.5:5, 3.5:2.5:5, 2.5:3:5, 3:3.5:5, or any value within the range of any two of these values. Specifically, before mixing, the mass fraction of phosphoric acid in the phosphoric acid aqueous solution is 80%, 81%, 82%, 83%, 84%, 85%, or any value within the range of any two of these values; specifically, before mixing, the mass fraction of H2SO4 in the concentrated sulfuric acid is 95%, 96%, 97%, 98%, or any value within the range of any two of these values.

[0021] In some embodiments, in step (3), the mass-to-volume ratio of m2 to the added sulfuric acid-phosphoric acid mixed solution is (7.5g-8.5g):(15 mL-30 mL). Specifically, the volume of the added sulfuric acid-phosphoric acid mixed solution is 7.5g:15mL, 8g:20mL, 8.5g:25mL, 8.5g:30mL, or any value within the range of any two of these values.

[0022] In some embodiments, before pretreatment, the manganese-containing cathode material sample further includes: drying the manganese-containing cathode material sample at 100℃-120℃ for 2 h-4 h; or drying the manganese-containing cathode material sample at 100℃-120℃ to constant weight. Specifically, the manganese-containing cathode material sample is dried at 100℃, 105℃, 110℃, 115℃, and 120℃ for 2 h, 3 h, and 4 h, respectively, or dried at 100℃, 105℃, 110℃, 115℃, and 120℃ to constant weight.

[0023] In some embodiments, in step (1), the oxidation concentration c(1 / 6K2Cr2O7) of the potassium dichromate standard solution is 0.1 mol / L-0.4 mol / L. Specifically, the oxidation concentration c(1 / 6K2Cr2O7) of the potassium dichromate standard solution is 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, or any value within the range of any two of these values.

[0024] In some embodiments, in step (1), the concentration of ferrous ammonium sulfate in the ferrous ammonium sulfate solution is 0.5 mol / L to 2 mol / L. Specifically, the concentration of ferrous ammonium sulfate in the ferrous ammonium sulfate solution is 0.5 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L, or any value within the range of any two of these values.

[0025] In some embodiments, the manganese-containing cathode material sample is at least one of manganese iron phosphate and lithium manganese iron phosphate.

[0026] In some embodiments, in step (3), the mass fraction of HCl in the concentrated hydrochloric acid is 36%-37%. Specifically, the mass fraction of HCl in the concentrated hydrochloric acid is 36%, 37%, or any value within the range of any two of these values.

[0027] In some embodiments, in step (3), the mass-to-volume ratio of m2 to concentrated hydrochloric acid is (7.5g-8.5g):(10mL-30mL). Specifically, the mass-to-volume ratio of m2 to concentrated hydrochloric acid is 7.5g:10mL, 7.8g:15mL, 8g:20mL, 8.5g:25mL, 8.5g:30mL, or any value within the range of any two of these values.

[0028] In some embodiments, in step (3), the potentiometric titrator selects a redox electrode as the indicator electrode.

[0029] In some embodiments, in step (4), the titration endpoint is reached when the first potential jump point occurs.

[0030] Beneficial effects The beneficial effects of this invention are as follows: This application solves the problems of high indicator consumption (requiring sodium tungstate, sodium diphenylamine sulfonate, etc.), cumbersome steps, long cycle, difficulty in endpoint determination, and interference from manganese in manual titration methods. The potentiometric titration method adopted in this invention has advantages such as low reagent cost, good repeatability and accuracy, and fast detection speed, and is especially suitable for manganese-containing cathode material systems; it has strong anti-interference ability, and the endpoint is determined by the electrode potential jump, avoiding high concentrations of Mn². + Solution color or turbidity can interfere with the indicator method; the potential jump characteristic can effectively distinguish Fe²⁺. + / Fe³ + With Mn 2+ / Mn 3+ The oxidation-reduction reaction steps ensure the accuracy of total iron content determination.

[0031] Terminology Definition Unless explicitly stated otherwise, all scopes referenced in this invention include end values.

[0032] The term "at least one" is used in this invention to describe the elements and components described herein. This is done merely for convenience and to provide a general meaning regarding the scope of the invention. This description should be understood to include one or at least one, and the singular includes the plural, unless clearly otherwise indicated.

[0033] All numbers in this invention are approximate values, regardless of whether words such as "approximately" or "about" are used. The numerical values ​​may vary by 1%, 2%, 5%, 7%, 8%, 10%, etc. Whenever a number with a value of N is disclosed, any number having a value of N+ / -1%, N+ / -2%, N+ / -3%, N+ / -5%, N+ / -7%, N+ / -8%, or N+ / -10% will be explicitly disclosed, where "+ / -" indicates addition or subtraction, and the range between N-10% and N+10% is also disclosed.

[0034] Unless otherwise stated, the following definitions as used in this invention shall apply. For the purposes of this invention, chemical elements are consistent with the CAS version of the periodic table and the 75th edition of the *Handbook of Chemistry and Physics*, 1994. Furthermore, general principles of organic chemistry can be found in *Organic Chemistry*, Thomas Sorrell, University Science Books, Sausalito: 1999, and *March's Advanced Organic Chemistry* by Michael B. Smith and Jerry March, John Wiley & Sons, New York: 2007, the entire contents of which are incorporated herein by reference.

[0035] Unless otherwise defined, all technical terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Although similar or equivalent methods and materials may be used in the implementation or testing of embodiments of this invention, suitable methods and materials are described below. All publications, patent applications, patents, and other references mentioned in this invention are incorporated herein by reference in their entirety, except where specific paragraphs are cited. In case of any conflict, this specification and its included definitions shall prevail. Furthermore, materials, methods, and examples are illustrative only and are not intended to be limiting.

[0036] “Constant weight”: Unless otherwise specified, refers to the weight of the test sample after two consecutive drying or ignitions with a difference of less than 0.3 mg; the second and subsequent weighings after drying to constant weight shall be carried out after drying for another hour under the specified conditions; the second weighing after ignition to constant weight shall be carried out after ignition for another 30 minutes. Detailed Implementation

[0037] The following description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

[0038] The potentiometric titrators used in the following examples are all Model 916 Metrohm potentiometric titrators from Switzerland.

[0039] Preparation of potassium dichromate standard solution: Take 9.8085 g of potassium dichromate (standard grade), dried to constant weight at 120℃, place it in a beaker, add a small amount of ultrapure water to dissolve it, transfer it to a 1000 mL volumetric flask, dilute to the mark with ultrapure water, shake to mix, and then transfer to a brown glass reagent bottle. Seal and store in a cool, dark place. The concentration of 1 L of solution prepared from 9.8085 g of potassium dichromate is calculated to be c(K₂Cr₂O₇) ≈ 0.033342 mol / L; converted to a catalytic oxidation concentration, it is c(1 / 6 K₂Cr₂O₇) = 0.033342 × 6 = 0.20005 mol / L.

[0040] Prepare stannous chloride solution: Weigh 10 g of stannous chloride dihydrate into a beaker, add 20 mL of concentrated hydrochloric acid (HCl mass fraction of 36%), and heat on an electric furnace to dissolve. After the system becomes clear, allow it to cool naturally to room temperature, transfer it to a 100 mL volumetric flask, and dilute to volume with ultrapure water. Add 2-3 high-purity tin granules to the stannous chloride solution, store in a brown dropper bottle, seal, and protect from light. The concentration of stannous chloride dihydrate in the prepared stannous chloride solution is 100 g / L.

[0041] Prepare a sulfuric acid-phosphoric acid mixed solution: In a beaker containing 400 mL of ultrapure water, 300 mL of concentrated sulfuric acid is slowly poured in while stirring with a glass rod. After the system cools to room temperature, 300 mL of phosphoric acid aqueous solution is added, and the mixture is stirred until homogeneous before being stored in a 1000 mL glass bottle. Before mixing, the concentrated sulfuric acid contains 98% H₂SO₄ by mass, and the phosphoric acid aqueous solution contains 85% phosphoric acid by mass.

[0042] Prepare ferrous ammonium sulfate solution: Take 9.8 g of ferrous ammonium sulfate hexahydrate in a beaker, add 25 mL of 5% sulfuric acid aqueous solution, sonicate until completely dissolved, cool, and store in a brown bottle protected from light. The concentration of ferrous sulfate hexahydrate in the prepared ferrous ammonium sulfate solution is 1 mol / L.

[0043] Preparation of iron standard solution: Take 1.5650 g of GBW07827(GFe-6) iron ore standard sample and place it in a PFA beaker. Add a small amount of ultrapure water to wet the sample and prevent powder from flying out. Then add 30 mL of concentrated hydrochloric acid (mass fraction of 36%), cover, and place on an electric heating plate to digest at 160℃ for 90 min. Remove the acid (180℃, 120 min) until the solution is nearly dry. After cooling, transfer the carbon powder and solution mixture to a 100 mL PET plastic sample bottle. Rinse directly with ultrapure water 2-3 times to ensure complete sample transfer. Make up to 99.8786 g with precise mass adjustment. Filter out the carbon powder using a 0.45 μm aqueous filter membrane syringe filter. Shake the filtrate well and set aside for later use to obtain the iron ore sample digest solution. Take 5.325 g of iron ore sample digestion solution and place it in a 100 mL beaker. Add 20 mL of 36% hydrochloric acid and add stannous chloride solution dropwise until the yellow color disappears. Then add 1-2 drops in excess and add 20 mL of sulfuric acid-phosphoric acid mixed acid solution prepared by the above method. Select the redox electrode as the indicator electrode and titrate with potassium dichromate standard solution with an oxidation concentration of 0.20005 mol / L until the second potential jump point appears (the iron standard solution only has 2 potential jump points). Stop the titration and record the volume of potassium dichromate standard solution consumed when each potential jump point appears. Calculate the total iron content based on the amount of potassium dichromate standard solution consumed.

[0044] Example 1 Sample pretreatment: Take 3.0065 g (m0) of manganese ferric phosphate sample dried to constant weight at 110℃ and place it in a PFA beaker. Add 0.1 mL of ultrapure water to wet the sample and prevent powder from flying out. Then add 30 mL of 36% concentrated hydrochloric acid. Cover and place on an electric heating plate to digest at 160℃ for 90 min. Remove the acid (180℃, 120 min) until the solution is nearly dry. After cooling, transfer the carbon powder and solution mixture to a 100 mL PET plastic sample bottle. Rinse directly with ultrapure water 2-3 times to ensure complete sample transfer. Make up to a final volume of 101.8969 g (m1). Filter out the carbon powder using a 0.45 μm aqueous filter membrane syringe filter. The filtrate is the sample digestion solution.

[0045] Sample determination: Take sample digestion solution m2 and place it in a beaker. Add 20 mL of concentrated hydrochloric acid with a mass fraction of 36%. Add 100 g / L stannous chloride solution dropwise until the yellow color disappears. Add 1-2 drops in excess and then add 20 mL of sulfuric acid-phosphoric acid mixed solution. Select the redox electrode as the indicator electrode and titrate with potassium dichromate standard solution with an oxidation concentration c (1 / 6 K2Cr2O7) of 0.20005 mol / L until the third potential jump point appears. Stop the titration when the third potential jump point appears. Record the volume of potassium dichromate standard solution consumed when each potential jump point appears. V is the volume of potassium dichromate standard solution consumed when the second potential jump point appears during sample determination minus the volume of potassium dichromate standard solution consumed when the first potential jump point appears, in mL. Perform blank correction on the titration results.

[0046] Blank correction: The blank sample preparation steps are the same as the sample preparation steps, the only difference being that no manganese-containing cathode material sample is added to the blank sample to prepare a blank sample solution. Weigh 10.2867 g of the blank sample solution and place it in a beaker. Before titration, add 1 mL of 1 mol / L ferrous ammonium sulfate solution. Titrate with a potassium dichromate standard solution with an oxidation concentration c (1 / 6 K2Cr2O7) of 0.20005 mol / L until the first potential jump point appears, then stop the titration. The volume of standard solution consumed is V1. Then add another 1 mL of 1 mol / L ferrous ammonium sulfate solution and continue titrating until the first potential jump point appears, then stop the titration. The volume of standard solution consumed is V2. Therefore, the volume of potassium dichromate standard solution consumed by the blank sample solution is V0 = V1 - V2, in mL. Perform two parallel determinations and take the average value. .

[0047] Result calculation: Total iron content is expressed as a percentage of iron by mass (%) and is calculated using the following formula:

[0048] In the formula: c(1 / 6 K2Cr2O7) is the oxidation concentration of the potassium dichromate standard solution, mol / L; V represents the volume of potassium dichromate standard solution consumed when the second potential jump point occurs during sample determination, minus the volume of potassium dichromate standard solution consumed when the first potential jump point occurs, in mL. The average volume of potassium dichromate standard solution consumed in two parallel determinations for blank correction is given in mL; M is the molar mass of iron, 55.845 g / mol; m0 is the sample volume of the manganese-containing cathode material sample, g; m1 is the final mass of the contents of the sample vial after volume adjustment, g; and m2 is the sample volume of the digestion solution, g.

[0049] The total iron content and RSD were calculated by performing three parallel determinations. The results are shown in Table 1.

[0050] Table 1 Results of three parallel determinations of ferric manganese phosphate

[0051] The method has a relative standard deviation (RSD) of 0.34% (≤0.5%), indicating good precision.

[0052] Example 2 Six sets of lithium iron phosphate digestion solutions were prepared in parallel: the digestion method was the same as in Example 1, three of which were sample digestion solutions, and the other three were used to prepare sample digestion solution + manganese tetroxide digestion solution. The sample volume m0 of the solid lithium iron phosphate sample and the final volume m1 of the digestion solution are shown in Table 2.

[0053] Preparation of manganese tetroxide digestion solution: The digestion method is the same as the sample pretreatment. The amount of solid manganese tetroxide sample weighed m0 is 3.0012 g, and the final volume of the digestion solution m1 is 100.8364 g.

[0054] Prepare two sets of beakers. In the first set, weigh out the prepared lithium iron phosphate sample digestion solution m2 into each of the three beakers. In the second set, weigh out the combined mass of the prepared lithium iron phosphate sample digestion solution m2 and manganese tetroxide digestion solution into each of the three beakers (the mass of the manganese tetroxide digestion solution is shown in Table 2). The determination method is the same as in Example 1, with each set measured in triplicate. Calculate the total iron content of the sample, investigate whether the reducing element manganese interferes with the total iron content determination results, and evaluate the applicability of this method to cathode material samples.

[0055] Table 2. Results of iron content determination for each group of samples.

[0056]

[0057]

[0058] The addition of manganese source solution to the test sample did not interfere with the determination of the total iron content of the manganese-containing cathode material or the determination of the titration endpoint.

[0059] Specificity test Preparation of lithium iron phosphate digestion solution: The digestion method is the same as that for sample pretreatment.

[0060] Three sets of beakers were prepared. In the first set of beakers, 5.0089 g of lithium iron phosphate digestion solution was accurately weighed. In the second set of beakers, 5.0177 g of lithium iron phosphate digestion solution and 2.5328 g of iron standard solution were accurately weighed. In the third set of beakers, 3.5663 g of lithium iron phosphate digestion solution and 3.5325 g of manganese tetroxide digestion solution prepared in Example 2 were accurately weighed. The determination method was the same as in Example 1, with each set measured in triplicate. The number of potential jump points and the potential information at the potential jumps were observed to evaluate the specificity of the method. The measurement results are shown in Table 3.

[0061] Table 3

[0062] According to Table 3, the first equivalence point is the reaction endpoint of potassium dichromate and stannous chloride, and the second equivalence point is the reaction endpoint of potassium dichromate and Fe. 2+ The reaction endpoint, the third equivalence point is the reaction between potassium dichromate and Mn. 2+ The reaction endpoint was determined. The equivalence point potential of the sample was basically the same before and after the addition of manganese tetroxide digestion solution, and there was a clear and single titration jump for iron during the titration process, indicating that the method has good specificity.

[0063] Example 3 The difference from Example 1 is that the sample is lithium manganese iron phosphate. Three sets of lithium manganese iron phosphate digestion solutions were prepared in parallel: the samples of lithium manganese iron phosphate, m0 and m1, were weighed as shown in Table 4. The remaining steps were the same as in Example 1. The test results are detailed in Table 4.

[0064] Comparative Example 1 Prepare a manganese ferric phosphate digestion solution, and the digestion method is the same as in Example 1, as shown in Table 4.

[0065] The preparation of the lithium manganese iron phosphate digestion solution differs from that in Example 1 in that the sample weighing amount m0 ​​is 3.0110 g and m1 is 134.7641 g. The remaining steps are the same as in Example 1. The test results are detailed in Table 4.

[0066] Prepare sodium diphenylamine sulfonate indicator solution: Dissolve 0.5 g of sodium diphenylamine sulfonate in a small amount of water, and bring the volume to 100 mL. Store the solution in a brown glass bottle. The concentration of the prepared sodium diphenylamine sulfonate solution is 5 g / L.

[0067] Prepare sodium tungstate indicator: Weigh 25g of sodium tungstate and dissolve it in 30mL of ultrapure water (filter if turbid). Add 5mL of 85% phosphoric acid, transfer to a volumetric flask, wash three times with ultrapure water, and dilute to 100mL. Mix well. The prepared sodium tungstate solution has a sodium tungstate mass fraction of 25wt%.

[0068] Prepare a titanium trichloride solution (freshly prepared): 5 mL of 36% hydrochloric acid was added to 20 mL of ultrapure water, followed by 5 mL of 15% titanium trichloride solution. The resulting titanium trichloride solution had a mass fraction of 2.5%.

[0069] Referring to CN117741041A, the determination method is the same as in Example 1, except that a manual titration method is used. Weigh an appropriate amount of the sample digestion solution prepared in Example 1 or Example 3 and place it in an Erlenmeyer flask. Add hydrochloric acid and shake well. Heat to boiling, and while hot, reduce with tin dichloride solution until pale yellow. After cooling, add sodium tungstate indicator and titrate with titanium trichloride solution until a stable blue color is achieved. Add sulfuric acid-phosphoric acid mixed solution, and add potassium dichromate standard solution until the blue color disappears (prepare a potassium dichromate standard solution with an oxidation concentration of c(1 / 6 K2Cr2O7) = 0.1005 mol / L). The volume of potassium dichromate standard solution consumed is V3. Immediately add sodium diphenylamine sulfonate indicator solution and titrate with potassium dichromate standard solution until the solution is a stable purple color. V4, V5 = V4 - V3.

[0070] Table 4 Results of determination of ferromanganese phosphate and lithium manganese phosphate by different methods

[0071] As shown in Table 4, Comparative Example 1 used manual titration with stannous chloride and titanium trichloride solution for redox reaction, and sodium tungstate and sodium diphenylamine sulfonate as indicators. The titration endpoint was determined by the color change of the indicators. Although the test results were close to those of this application, the test process was complicated. Furthermore, the method in Comparative Example 1 could not eliminate the interference of manganese. The method for determining the total iron content in manganese-containing cathode materials provided in this application has good accuracy and parallelism, and the operation process is simple, requires fewer reagents, reduces human error and labor costs. Moreover, the recovery rate of total iron content spiked in the method for determining the total iron content in manganese-containing cathode materials provided in this application is within the range of 98.0-102.0%, which meets the requirements.

[0072] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for determining the total iron content in a manganese-containing cathode material, characterized in that, Includes the following steps: (1) Solution preparation: Prepare potassium dichromate standard solution, stannous chloride solution, sulfuric acid-phosphoric acid mixed acid solution and ferrous ammonium sulfate solution; (2) Sample pretreatment: Place a manganese-containing cathode material sample with a mass of m0 in a reaction vessel, add 0.1 mL of water to wet the manganese-containing cathode material sample, then add concentrated hydrochloric acid, cover and digest, remove the acid and cool, transfer the carbon powder and the solution after removing the acid to a sample bottle, and make up the volume with ultrapure water to a mass of m1 of the contents of the sample bottle; filter out the carbon powder to obtain the sample digest solution; (3) Sample determination; Take the sample digestion solution m2 obtained in step (2), place it in the reaction vessel, add concentrated hydrochloric acid, and then add the stannous chloride solution prepared in step (1) drop by drop until the yellow color disappears, then add 1-2 drops in excess, and then add the sulfuric acid-phosphoric acid mixed solution prepared in step (1). Titrate with the potassium dichromate standard solution prepared in step (1) in a potentiometric titrator until the third potential jump point appears, and stop titrating. Record the volume of potassium dichromate standard solution consumed when each potential jump point appears. (4) Blank correction: Take 1g-12g of blank sample solution and place it in a reaction vessel. Before titration, add 1mL of the ferrous ammonium sulfate solution prepared in step (1). Titrate with the potassium dichromate standard solution prepared in step (1) to the endpoint. The volume of potassium dichromate standard solution consumed is V1. Then add 1mL of the ferrous ammonium sulfate solution prepared in step (1) and continue titrating with the potassium dichromate standard solution prepared in step (1) to the endpoint. The volume of potassium dichromate standard solution consumed is V2. The volume of potassium dichromate standard solution consumed by the blank sample solution is V0 = V1 - V2. Perform two parallel determinations and take the average value. The preparation of blank sample solution is consistent with the sample pretreatment steps, the only difference being that no manganese-containing cathode material sample is added during the preparation of blank sample solution. (5) Calculation of results: The total iron content is expressed as the mass fraction of iron (%) and is calculated according to the following formula: In the formula: c(1 / 6 K2Cr2O7) is the oxidation concentration of the potassium dichromate standard solution, in mol / L; V represents the volume of potassium dichromate standard solution consumed when the second potential jump point occurs during sample determination, minus the volume of potassium dichromate standard solution consumed when the first potential jump point occurs, in mL. The average volume of potassium dichromate standard solution consumed in two parallel determinations for blank correction, in mL; M is the molar mass of iron, 55.845 g / mol; m0 is the sample amount of the manganese-containing cathode material sample taken in step (2), in g; m1 is the final mass of the contents of the sample bottle after mass adjustment in step (2), in g; m2 is the amount of sample digestion solution taken in step (3), in g.

2. The determination method according to claim 1, characterized in that, The concentrated hydrochloric acid in step (2) has a HCl mass fraction of 36%-37%; and / or, The volume ratio of the concentrated hydrochloric acid to the mass ratio of the manganese-containing cathode material sample is (20 mL-40 mL): 3 g; and / or, The digestion temperature is 90℃-200℃; and / or, The digestion time is 30 min-60 min.

3. The determination method according to any one of claims 1-2, characterized in that, In step (1), the method for preparing the stannous chloride solution includes: adding concentrated hydrochloric acid to stannous chloride, heating to dissolve, cooling after the system becomes clear, adding water to make up to the volume, adding 2-3 high-purity tin particles to the prepared stannous chloride solution, storing it in a brown dropper bottle, sealing it, and protecting it from light; wherein, the concentration of stannous chloride in the stannous chloride solution is 50 g / L-200 g / L, and the mass fraction of HCl in the concentrated hydrochloric acid is 36%-37%.

4. The determination method according to any one of claims 1-3, characterized in that, In step (1), the volume ratio of phosphoric acid aqueous solution: concentrated sulfuric acid: water in the sulfuric acid-phosphoric acid mixed acid solution is (2.5-3.5):(2.5-3.5):(3-5); wherein, before mixing, the mass fraction of phosphoric acid in the phosphoric acid aqueous solution is 80%-85%; the mass fraction of H2SO4 in the concentrated sulfuric acid is 95%-98%; and / or, In step (3), the mass-volume ratio of m2 to the added sulfuric acid-phosphoric acid mixed solution is (7.5g-8.5g):(15 mL-30 mL).

5. The determination method according to any one of claims 1-4, characterized in that, Before pretreatment, the manganese-containing cathode material sample further includes: drying the manganese-containing cathode material sample at 100℃-120℃ for 2 h-4 h; or drying the manganese-containing cathode material sample at 100℃-120℃ to constant weight.

6. The determination method according to any one of claims 1-5, characterized in that, In step (1), the oxidation concentration c(1 / 6 K2Cr2O7) of the potassium dichromate standard solution is 0.1 mol / L-0.4 mol / L; and / or, The concentration of ferrous ammonium sulfate in the ferrous ammonium sulfate solution is 0.5 mol / L-2 mol / L.

7. The determination method according to any one of claims 1-6, characterized in that, The manganese-containing cathode material sample is at least one of manganese iron phosphate and lithium manganese iron phosphate.

8. The determination method according to any one of claims 1-7, characterized in that, In step (3), the mass fraction of HCl in the concentrated hydrochloric acid is 36%-37%; and or, The mass-to-volume ratio of m2 to concentrated hydrochloric acid is (7.5g-8.5g):(10mL-30mL).

9. The determination method according to any one of claims 1-8, characterized in that, In step (3), the potentiometric titrator selects the redox electrode as the indicator electrode.

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