A method for detecting chloride ion content in ferric phosphate

Abstract

The invention provides a method for detecting the chloride ion content in ferric phosphate. The method comprises: firstly dissolving a ferric phosphate sample in a mixed acid solution of sulfuric acid and phosphoric acid to dissolve all the chloride ions in the phosphoric acid sample, then adding a silver chloride turbid solution formed by excessive silver nitrate into the chloride ion solution, adding a stabilizer containing gelatin and polyhydroxy alcohol to stabilize the silver chloride system, and testing the chloride ion content in the ferric phosphate sample by spectrophotometry. The method has the characteristics of high accuracy and good stability of the test result.

Description

Technical Field

[0001] The invention relates to a method for detecting the chloride ion content in ferric phosphate, and belongs to the technical field of chloride ion content analysis and testing. Background Art

[0002] Lithium iron phosphate, as a positive electrode material for lithium-ion batteries, has attracted much attention at home and abroad, and iron phosphate, as the current mainstream raw material for synthesizing lithium iron phosphate, has also become a research hotspot. Synthesizing high-purity, high-quality iron phosphate products plays an important role in the better application of lithium iron phosphate positive electrode materials in lithium-ion batteries. Chloride ions, as common impurity ions in iron phosphate, will affect the cycle life and safety performance of lithium-ion batteries. The presence of chloride ions in production will corrode the metal parts of the equipment and affect the service life of the equipment.

[0003] At present, the main methods for detecting chloride ions include ion chromatography, turbidimetry, spectrophotometry, etc. However, there are few reports on the detection methods of chloride ion content in iron phosphate. The existing technical solutions are subject to the following problems due to the iron phosphate system.

[0004] (1) There is no method for directly detecting chloride ions in iron phosphate in the prior art. Generally, an indirect method is used, such as using deionized water ultrasound to dissolve chloride ions in iron phosphate during pretreatment, filtering out the powder, and indirectly testing the content of chloride ions dissolved in water. However, the pretreatment process cannot guarantee the complete dissolution of chloride ions, and the filtration process will cause the loss of chloride ions.

[0005] (2) The sample pretreatment process for the determination of chloride ions by ion chromatography is complex. Ion chromatography has high requirements for samples. The content of transition metal ions in the sample needs to be controlled at a low level. Fe is the main component in the iron phosphate sample. Therefore, the sample needs to be processed by column before testing. The process is complicated and will cause a certain degree of loss of chloride ions.

[0006] (3) The turbidimetric method for determining chloride ions has high requirements for the test solution. The test solution must be a colorless solution, otherwise it will interfere with the judgment of the turbidity of the precipitation. The turbidimetric method is generally judged by the naked eye, which is highly subjective and has large errors.

[0007] (4) When the spectrophotometric method is used to determine the absorbance of chloride ions, the chloride ion test solution is first converted into a suspension containing silver chloride precipitate, and the absorbance of the suspension is determined by spectrophotometry. At this time, it is necessary to ensure that the suspension is stably and evenly distributed. If the precipitate is easy to settle, the test error will increase.

[0008] Therefore, it is necessary to develop an improved detection method for the chloride ion content in ferric phosphate. Summary of the invention

[0009] The technical solution provided by the present invention is made by the inventor based on the following cognition:

[0010] For the field of iron phosphate, the test method in the prior art is generally to dissolve the chloride ions with water first, and then filter out the iron phosphate powder, which has the disadvantage of incomplete chloride ion dissolution. Iron phosphate materials are very soluble in hydrochloric acid, but the addition of hydrochloric acid will interfere with the determination of chloride ions, and other acids, such as nitric acid, sulfuric acid, phosphoric acid and other medium and strong acids, cannot completely dissolve iron phosphate samples. Oxalic acid can dissolve iron phosphate samples as a complexing acid, but the dissolved solution is grass green, and the color will interfere with the determination of absorbance when tested by spectrophotometry. Therefore, the inventors have conducted in-depth and detailed research on the acid that dissolves iron phosphate, and finally selected a mixed solution of phosphoric acid and sulfuric acid of a specific composition to dissolve iron phosphate, and the solution does not develop color and there is no test interference. Although chloride ions are easy to form volatile hydrochloric acid in a strong acid environment, in a closed autoclave, chloride ions will not volatilize in the form of hydrochloric acid, and after cooling to room temperature, chloride ions can be ensured to be stably present in the solution. This method can ensure that all chloride ions present in the iron phosphate material are directly detected, and the accuracy of pre-treatment is improved. On the other hand, the present invention also uses a stabilizer of a specific composition to stabilize the solution, ensuring that the formed AgCl suspension will not settle and can be evenly dispersed in the solution, thereby finally completing the present invention.

[0011] Specifically, the present invention provides a method for detecting the chloride ion content in ferric phosphate, comprising:

[0012] a: Configuration of matrix solution

[0013] Concentrated sulfuric acid, concentrated phosphoric acid and water are mixed in a volume ratio of 30-50%:30-50%:0-40% to obtain a mixed acid solution;

[0014] The chlorine-free ferric phosphate sample is placed in a reaction kettle, the mixed acid solution is added, heated to dissolve, cooled, and constant volume is obtained to obtain a matrix solution;

[0015] b: Drawing of standard working curve

[0016] Add the above matrix solution to the chlorine standard solution, make up to the volume, and prepare standard solutions with different concentrations;

[0017] Adding an excess of silver nitrate solution to the above standard solution, adding a stabilizer containing gelatin and polyhydric alcohol, testing the absorbance of the standard solution using a spectrophotometer, and drawing a standard working curve;

[0018] c: Preparation of test solution

[0019] Dissolve the ferric phosphate sample to be tested according to the operation in step a, cool it, and make it to volume. Take the solution after making it to volume, and add

[0020] The stabilizer of gelatin and polyhydric alcohol is shaken and allowed to stand to obtain the test solution;

[0021] d: Determination of chloride ion content

[0022] The absorbance of the test solution was tested by a spectrophotometer, and the chloride ion content in the test solution was calculated according to the drawn standard working curve. The chloride ion content in the iron phosphate sample to be tested was obtained according to the following calculation formula:

[0023] W(Cl - )=C*m2 / m1

[0024] in:

[0025] W(Cl - )—Chloride ion content in iron phosphate (ppm);

[0026] C—chloride ion content in the test solution (ppm);

[0027] m1—the mass (g) of the iron phosphate sample to be tested in step c;

[0028] m2—the mass (g) of the iron phosphate sample to be tested after dissolution and constant volume in step c.

[0029] According to the embodiment provided by the present invention, the method for detecting the chloride ion content in iron phosphate may further include the following additional technical features.

[0030] According to some embodiments provided by the present invention, the concentration of the concentrated sulfuric acid is 98%, and the concentration of the concentrated phosphoric acid is 85%.

[0031] According to some embodiments provided by the present invention, the temperature for heating and dissolving the chlorine-free ferric phosphate or the ferric phosphate to be tested in the mixed acid solution is 150-200° C., and the dissolution time is 1-3 hours.

[0032] According to some embodiments provided by the present invention, the temperature for heating and dissolving can be listed as: 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, and the like.

[0033] According to some embodiments provided by the present invention, the time for heating and dissolving can be listed as: 1h, 1.5h, 2h, 2.5h, 3h, and the like.

[0034] In some embodiments, the chloride-free ferric phosphate or the ferric phosphate to be tested is dissolved in the mixed acid solution at 180° C. for 1-1.5 hours.

[0035] According to some embodiments provided by the present invention, the stabilizer containing gelatin and polyhydric alcohol is composed of gelatin, polyhydric alcohol and water, and the mass of the gelatin accounts for 0.1%-0.2% of the total mass of the stabilizer, for example: 0.1%, 0.11%, 0.12%, 0.13%, 0.14%, 0.15%, 0.16%, 0.17%, 0.18%, 0.19%, 0.2%, etc.

[0036] According to some embodiments provided by the present invention, in the stabilizer containing gelatin and polyhydric alcohol, the volume of the polyhydric alcohol accounts for 30%-50% of the total volume of the polyhydric alcohol and water, for example: 30%, 35%, 40%, 45%, 50%, and the like.

[0037] According to some embodiments provided by the present invention, the polyhydric alcohol is at least one of glycerol, propylene glycol, ethylene glycol, polyethylene glycol, and polyvinyl alcohol.

[0038] In some embodiments, the polyhydric alcohol is glycerol.

[0039] According to some embodiments provided by the present invention, in the stabilizer containing gelatin and glycerol, the volume of glycerol accounts for 30%-50% of the total volume of glycerol and water, for example: 30%, 35%, 40%, 45%, 50%, and the like.

[0040] According to some embodiments provided by the present invention, the preparation process of the above stabilizer includes: adding gelatin into water to dissolve it, and then adding polyhydric alcohol to obtain the stabilizer.

[0041] According to some embodiments provided by the present invention, the mass of the gelatin accounts for 0.1%-0.2% of the total mass of the stabilizer.

[0042] According to some embodiments provided by the present invention, the volume of the polyhydric alcohol accounts for 30%-50% of the total volume of the polyhydric alcohol and water, for example: 30%, 35%, 40%, 45%, 50%, and so on.

[0043] According to some embodiments provided by the present invention, the polyhydric alcohol is at least one of glycerol, propylene glycol, ethylene glycol, polyethylene glycol, and polyvinyl alcohol.

[0044] In some embodiments, the polyhydric alcohol is glycerol.

[0045] According to some embodiments provided by the present invention, the volume of the glycerol accounts for 30%-50% of the total volume of glycerol and water, for example: 30%, 35%, 40%, 45%, 50%, and the like.

[0046] The dissolution temperature is not particularly limited, and may be, for example, 60-80°C.

[0047] According to some embodiments provided by the present invention, the concentration of the silver nitrate solution is 0.05 mol / L.

[0048] According to some embodiments provided by the present invention, the method for detecting the chloride ion content in ferric phosphate comprises:

[0049] a: Configuration of matrix solution

[0050] Concentrated sulfuric acid, concentrated phosphoric acid and water are mixed in a volume ratio of 30-50%:30-50%:0-40% to obtain a mixed acid solution;

[0051] The chlorine-free ferric phosphate sample is placed in a reaction kettle, the mixed acid solution is added, heated to dissolve, cooled, and constant volume is obtained to obtain a matrix solution;

[0052] b: Preparation of stabilizer

[0053] Add gelatin into water and heat to dissolve, then add glycerol to obtain a stabilizer, wherein the mass of the gelatin accounts for 0.1%-0.2% of the total mass of the stabilizer, and the volume of the glycerol accounts for 30%-50% of the total volume of the glycerol and water;

[0054] c: Drawing of standard working curve

[0055] Add the above matrix solution to the chlorine standard solution, make up to the volume, and prepare standard solutions with different concentrations;

[0056] Add an excess of silver nitrate solution to the above standard solution, add a stabilizer, test the absorbance of the standard solution using a spectrophotometer, and draw a standard working curve;

[0057] d: Preparation of test solution

[0058] Dissolve the ferric phosphate sample to be tested according to the operation in step a, cool it, and make it to volume. Take the solution after making it to volume, and add

[0059] The stabilizer of gelatin and polyhydric alcohol is shaken and allowed to stand to obtain the test solution;

[0060] e: Determination of chloride ion content

[0061] The absorbance of the test solution was tested by a spectrophotometer, and the chloride ion content in the test solution was calculated according to the drawn standard working curve. The chloride ion content in the iron phosphate sample to be tested was obtained according to the following calculation formula:

[0062] W(Cl - )=C*m2 / m1

[0063] in:

[0064] W(Cl -)—the content of chloride ions in the iron phosphate sample to be tested (ppm);

[0065] C—chloride ion content in the test solution (ppm);

[0066] m1—the mass (g) of the iron phosphate sample to be tested in step d;

[0067] m2—the mass (g) of the iron phosphate sample to be tested after dissolution and constant volume in step d.

[0068] Compared with the prior art, the present invention has the following technical effects:

[0069] (1) In the detection method provided by the present invention, a mixed acid solution of sulfuric acid and phosphoric acid in a specific ratio can ensure that the iron phosphate sample is completely dissolved, and the solution does not show color and there is no test interference. Although chloride ions are easy to form volatile hydrochloric acid in a strong acid environment, in a closed autoclave, chloride ions will not volatilize in the form of hydrochloric acid. After cooling to room temperature, it can be ensured that chloride ions are stably present in the solution. Therefore, this method can ensure that all chloride ions present in the iron phosphate material are directly detected, thereby improving the accuracy of pre-treatment.

[0070] (2) In the detection method provided by the present invention, for the iron phosphate system, a chloride-free iron phosphate sample is used as a matrix, and a chloride ion standard solution is prepared by the standard addition method to improve the accuracy of the standard solution. A mixed stabilizer of gelatin and polyhydroxy alcohol is added to the silver chloride turbid solution formed by adding excess silver nitrate to the chloride ion solution, which solves the problem that the suspension is easy to settle during spectrophotometric testing and can improve the stability of the test.

[0071] (3) The detection method provided by the present invention can directly test the total chloride ion content in the ferric phosphate sample. The improved spectrophotometric method has high sensitivity, good stability and high precision. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 A flow chart showing a method for detecting chloride ion content in ferric phosphate provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0073] Unless otherwise specified, all scientific and technological terms used in the present invention have the same meaning as those commonly understood by those skilled in the art to which the present invention belongs. All patents and publications related to the present invention are incorporated herein by reference in their entirety. The term "comprising" or "including" is an open expression, that is, including the contents specified in the present invention, but does not exclude contents in other aspects.

[0074] The embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings and examples, but it will be appreciated by those skilled in the art that the following examples are only used to illustrate the present invention and should not be considered to limit the scope of the present invention. If specific conditions are not specified in the examples, they are carried out according to normal conditions or conditions recommended by the manufacturer. If the manufacturer is not specified for the reagents or instruments used, they are all conventional products that can be purchased commercially.

[0075] Example

[0076] 1. Operation process

[0077] like Figure 1 As shown, the method for detecting the chloride ion content in iron phosphate provided in this embodiment includes:

[0078] a. Preparation of matrix solution

[0079] (1) Concentrated sulfuric acid: concentrated phosphoric acid: water are mixed in a volume ratio of 40%:40%:20% to obtain a mixed acid solution, and a sealed high-pressure reactor is placed in a high-temperature oven and heated at 180° C. for 1 hour to dissolve.

[0080] (2) Weigh 2 g of chlorine-free ferric phosphate sample and place it in a reactor. Add 10 mL of the above mixed acid solution and heat to dissolve at 180°C for 1 hour. After the autoclave cools to room temperature, dilute and fix the volume to 20 g. Transfer the completely dissolved chlorine-free ferric phosphate solution to a clean PET bottle. This solution is the matrix solution.

[0081] b. Configuration of stabilizer

[0082] (3) Weigh 0.1 g of gelatin and dissolve it in 30 mL of hot water, then add 20 mL of glycerol to the dissolved gelatin solution to prepare a stabilizer, wherein the mass percentage of gelatin is 0.2% and the volume of glycerol accounts for 40% of the total volume of glycerol and water.

[0083] c. Drawing of standard working curve

[0084] (4) Take 0.00mL, 1.00mL, 2.00mL, 3.00mL, 4.00mL, and 5.00mL of a 20ug / mL chlorine standard solution, add 2g of the matrix solution prepared in step (2), dilute with distilled water, and make up to 20g to prepare 0, 1, 2, 3, 4, and 5ppm standard solutions, respectively.

[0085] (5) Take 10mL of the above standard solution and place it in a 25mL colorimetric tube, add 5mL of excess 0.05mol / L silver nitrate solution, and add 10mL of the prepared stabilizer. After the solution is shaken evenly and left to stand for a period of time, take out 3mL of the test solution and place it in a 1cm colorimetric dish. Use a spectrophotometer to test the absorbance of a series of standard solutions at a wavelength of 420nm. The instrument automatically draws a standard working curve with the concentration of chloride ions as the horizontal axis and the absorbance as the vertical axis. The linear correlation coefficient is between 99.9% and 100%.

[0086] d. Preparation of test solution

[0087] (6) Weigh 1 g (denoted as m1) of the ferric phosphate sample to be tested and place it in a reactor. Add 5 mL of the mixed acid solution prepared in step (1) and heat to dissolve at 180° C. for 1 h. After the autoclave cools to room temperature, dilute and fix the volume to 100 g (denoted as m2). Transfer the completely dissolved ferric phosphate solution to a clean PET bottle.

[0088] (7) Take 10 mL of the above solution and place it in a 25 mL colorimetric tube. Add 5 mL of excess 0.05 mol / L silver nitrate solution and 10 mL of the prepared mixed stabilizer of gelatin and glycerol. Shake the solution evenly and let it stand. This solution is the test solution. Prepare six samples in parallel.

[0089] e. Determination of chloride ion content

[0090] (8) Take 3 ml of the above test solution and place it in a 1 cm cuvette. Use a spectrophotometer to test the absorbance of the test solution at a wavelength of 420 nm. Automatically calculate the chloride ion content (denoted as C) in the test solution according to the drawn chloride ion standard working curve. According to the following calculation formula, the chloride ion content in the iron phosphate sample to be tested is obtained:

[0091] W(Cl - )=C*m2 / m1

[0092] in:

[0093] W(Cl - )—the content of chloride ions in the iron phosphate sample to be tested (ppm);

[0094] C—chloride ion content in the test solution (ppm);

[0095] m1—mass of the iron phosphate sample to be tested in step d (g);

[0096] m2—the mass (g) of the iron phosphate sample to be tested after dissolution in step d.

[0097] 2. Data

[0098] The test results are shown in Table 1.

[0099] Table 1

[0100]

[0101] Comparative Example 1

[0102] 1. Operation process

[0103] The difference from the embodiment is that the composition of the mixed acid solution is: concentrated sulfuric acid: concentrated phosphoric acid: water are mixed in a volume ratio of 2:5:3 to obtain a mixed acid solution.

[0104] 2. Data

[0105] Table 2

[0106]

[0107] Comparative Example 2

[0108] 1. Operation process

[0109] The difference from the embodiment is that the composition of the mixed acid solution is: concentrated sulfuric acid: concentrated phosphoric acid: water are mixed in a volume ratio of 5:2:3 to obtain a mixed acid solution.

[0110] 2. Data

[0111] Table 3

[0112]

[0113] The results of Comparative Examples 1 and 2 show that due to the low content of sulfuric acid or phosphoric acid, the iron phosphate samples are not completely dissolved, and the chloride ion content in the detected iron phosphate samples is lower than the result of Example 1. And the relative deviation of the test results is large. The credibility of the results is low. It shows that when sulfuric acid and phosphoric acid are mixed, the acidity of each component needs to be within a suitable range to ensure that the sample is completely dissolved.

[0114] Comparative Example 3

[0115] 1. Operation process

[0116] The difference from the embodiment is that the composition of the stabilizer is: 0.1 g of gelatin is weighed and dissolved in 50 mL of hot water to prepare the stabilizer.

[0117] 2. Data

[0118] Table 4

[0119]

[0120]

[0121] Comparative Example 4

[0122] 1. Operation process

[0123] The difference from the embodiment is that the composition of the stabilizer is: 20 mL of glycerol and 30 mL of distilled water are mixed to prepare the stabilizer.

[0124] 2. Data

[0125] Table 5

[0126]

[0127] The results of Comparative Examples 3 and 4 show that when using a spectrophotometer to test, only a single-component stabilizer is added, such as only gelatin or only glycerol. The linearity of the standard curve obtained by the test is poor, and the correlation coefficient is less than 99%. When the test liquid is tested, the results of the six parallel samples gradually decrease, indicating that the AgCl suspension formed after adding the stabilizer is unstable. As the placement time increases, AgCl settles, resulting in a decrease in absorbance, affecting the accuracy of the test results.

[0128] In the description of this specification, the description with reference to the terms "some embodiments", "other embodiments", "embodiment", "example", etc. means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials or characteristics described may be combined in any one or more embodiments or examples in a suitable manner. In addition, those skilled in the art may combine and combine the different embodiments or examples described in this specification and the features of the different embodiments or examples, unless they are contradictory.

[0129] Although the embodiments and examples of the present invention have been shown and described above, it is to be understood that the above embodiments and examples are exemplary and are not to be construed as limitations of the present invention. A person skilled in the art may change, modify, replace and modify the above embodiments and examples within the scope of the present invention.

Claims

1. A method for detecting the chloride ion content in ferric phosphate, characterized in that: include: a: Configuration of matrix solution Concentrated sulfuric acid, concentrated phosphoric acid and water are mixed in a volume ratio of 30-50%:30-50%:0-40% to obtain a mixed acid solution; The chlorine-free ferric phosphate sample is placed in a sealed high-pressure reactor, the mixed acid solution is added, heated to dissolve, cooled, and constant volume is obtained to obtain a matrix solution; b: Drawing of standard working curve Add the above matrix solution to the chlorine standard solution, make up to the volume, and prepare standard solutions with different concentrations; Add an excess of silver nitrate solution to the above standard solution, add a stabilizer containing gelatin and polyhydric alcohol, test the absorbance of the standard solution using a spectrophotometer, and draw a standard working curve; c: Preparation of test solution Dissolve the ferric phosphate sample to be tested according to the operation in step a, cool it, and adjust the volume. Take the adjusted solution, add an excess of silver nitrate solution, add a stabilizer containing gelatin and polyhydroxy alcohol, shake well and let it stand to obtain a solution to be tested; d: Determination of chloride ion content The absorbance of the test solution was tested by a spectrophotometer, and the chloride ion content in the test solution was calculated according to the drawn standard working curve. The chloride ion content in the iron phosphate sample to be tested was obtained according to the following calculation formula: W(Cl - )=C*m2 / m1 in: W(Cl - )—the content of chloride ions in the iron phosphate sample to be tested; C—the content of chloride ions in the test solution; m1—the mass of the iron phosphate sample to be tested in step c; m2—the mass of the iron phosphate sample to be tested after dissolution and constant volume in step c; The concentration of the concentrated sulfuric acid is 98%, and the concentration of the concentrated phosphoric acid is 85%; The temperature for heating and dissolving the iron phosphate in the mixed acid solution is 150-200° C., and the dissolution time is 1-3 hours; The stabilizer containing gelatin and polyhydric alcohol consists of gelatin, polyhydric alcohol and water, and the mass of the gelatin accounts for 0.1%-0.2% of the total mass of the stabilizer; The volume of the polyhydric alcohol accounts for 30%-50% of the total volume of the polyhydric alcohol and water; The preparation process of the stabilizer containing gelatin and polyhydric alcohol comprises: adding gelatin into water to dissolve it, and then adding polyhydric alcohol to obtain the stabilizer.

2. The method for detecting chloride ion content in ferric phosphate according to claim 1, characterized in that: The polyhydric alcohol is at least one of glycerol, propylene glycol, ethylene glycol, polyethylene glycol and polyvinyl alcohol.

3. The method for detecting chloride ion content in ferric phosphate according to claim 2, characterized in that: The polyhydric alcohol is glycerol.

4. The method for detecting chloride ion content in ferric phosphate according to claim 1, characterized in that: The concentration of the silver nitrate solution is 0.05 mol / L.

5. The method for detecting chloride ion content in ferric phosphate according to claim 1, characterized in that: The volume in steps a and b is adjusted to 20 g with water.

6. The method for detecting chloride ion content in ferric phosphate according to claim 1, characterized in that: The volume in step c is adjusted to 100 g with water.

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