A method for determining metal ions in n-methylpyrrolidone

Abstract

The application provides a method for determining metal ions in N-methylpyrrolidone, comprising the following steps: 1) preparation of N-methylpyrrolidone digestion solution: adding an oxidizing agent to a sample; the amount of the oxidizing agent is 150-200% of the mass of the concentrated sample; adding a reducing agent to the sample; the amount of the reducing agent is 200-500% of the mass of the concentrated sample; 2) determination of metal ions in the N-methylpyrrolidone digestion solution obtained in step 1): preferably, the determination is performed by using an inductively coupled plasma atomic emission spectrometer. The application can quickly and accurately determine metal ions with a content of less than 1 mg / kg in N-methylpyrrolidone, and simplifies the determination method and saves the analysis cost.

Description

Technical Field

[0001] This invention belongs to the field of N-methylpyrrolidone detection technology, specifically relating to a method for determining metal ions in N-methylpyrrolidone. Background Technology

[0002] N-Methylpyrrolidone is a C5 cyclic organic compound, miscible with water in any proportion, and soluble in organic solvents such as diethyl ether, acetone, chlorinated hydrocarbons, and aromatic hydrocarbons. N-Methylpyrrolidone is an excellent high-grade solvent with high selectivity and stability, and is widely used in industrial fields such as coatings, inks, electronic chemicals, advanced cleaning agents, and power lithium-ion batteries.

[0003] Especially during the lithium battery manufacturing process, the metal ions in the solvent N-methylpyrrolidone have a lower reduction potential than lithium ions. During charging, metal impurity ions will first intercalate into the carbon anode, reducing the number of lithium ion intercalation sites and thus decreasing the reversible capacity of the lithium-ion battery.

[0004] Excessive impurity ion content not only leads to a decrease in the reversible specific capacity of lithium-ion batteries, but the precipitation of metal ions can also prevent the formation of an effective passivation layer on the graphite electrode surface, thus damaging the entire battery. Lithium ions have a small radius and migrate at a higher rate between graphite layers than other metal ions; therefore, low concentrations of metal ions have little impact on the battery. In the battery industry, the content of various metal impurity ions in organic electrolytes is generally required to be less than 0.007%. Therefore, the metal ion content in N-methylpyrrolidone is crucial in the application of lithium batteries.

[0005] Before determining metal ions in N-methylpyrrolidone, the N-methylpyrrolidone sample needs to be digested. Currently, the most commonly used digestion methods are dry digestion and microwave digestion.

[0006] Dry digestion involves removing a large amount of organic matter through high-temperature carbonization and ashing, followed by dissolution with acid or other solvents to prepare a sample solution. Finally, methods such as solvent extraction, masking, and precipitation are mainly used to eliminate interference from other ions. However, dry digestion is time-consuming, complex, and prone to loss of volatile elements; the crucible also has an adsorption effect on the analyte, reducing the measurement results and recovery rate.

[0007] Microwave digestion is a wet digestion process that uses microwaves to heat the digestion solution (various acids, some alkalis, and salts) and sample in a sealed container, rapidly dissolving the samples under high temperature and pressure. As the pressure in the sealed container increases, the boiling point of the acids also increases, accelerating the digestion process. Furthermore, because each sample container is independently sealed, there is no air contamination, cross-contamination, or acid loss, resulting in relatively thorough digestion. However, microwave digestion systems can only process a limited number of samples (organic samples <0.5g, inorganic samples <3g) and pose significant safety risks.

[0008] Furthermore, the detection of metal ion content in N-methylpyrrolidone digests is still in the exploratory stage in this field, and there are few relevant reports at present.

[0009] Currently, no national or industry standard method has been proposed for the determination of metal ion content in N-methylpyrrolidone. Therefore, in order to improve the accuracy and precision of detection and effectively improve product quality, a method for determining the metal ion content in N-methylpyrrolidone is urgently needed. Summary of the Invention

[0010] To address the shortcomings of existing technologies, this invention provides a method for determining metal ions in N-methylpyrrolidone, enabling rapid and effective detection of metal ion content in N-methylpyrrolidone.

[0011] To achieve the above objectives, the present invention adopts the following technical solution:

[0012] The first aspect of this invention provides a method for determining metal ions in N-methylpyrrolidone, comprising the following steps:

[0013] 1) Preparation of N-methylpyrrolidone digestion solution:

[0014] 1.1) The N-methylpyrrolidone sample was heated and concentrated to obtain a concentrated sample;

[0015] 1.2) An oxidant is added to the concentrated sample, and an oxidation reaction is carried out under heating conditions; the amount of the oxidant is 150-200% of the mass of the concentrated sample;

[0016] 1.3) Add a reducing agent to the sample obtained in step 1.2) and carry out a reduction reaction under heating conditions to obtain N-methylpyrrolidone digest; the amount of the reducing agent is 200-500% of the mass of the concentrated sample;

[0017] 2) Determination of metal ions in the N-methylpyrrolidone digest obtained in step 1):

[0018] Preferably, the determination is performed using an inductively coupled plasma atomic emission spectrometer.

[0019] This invention employs a wet digestion process to prepare N-methylpyrrolidone digestion solution. The N-methylpyrrolidone sample is heated and concentrated to obtain a concentrated sample. A strong oxidant is added to initiate an oxidation reaction; the amount of oxidant is approximately 150-200% of the concentrated sample mass. A reducing agent is then added to initiate a reduction reaction; the amount of reducing agent is approximately 200-500% of the concentrated sample mass. This process completely decomposes and oxidizes the N-methylpyrrolidone, causing it to escape in a gaseous state. The analyte metal is converted into an inorganic (ionic) state and exists in the digestion solution.

[0020] In some preferred embodiments, in step 1.2), the oxidant is concentrated sulfuric acid, concentrated nitric acid, or concentrated hydrochloric acid. Preferably, the concentrated sulfuric acid has a mass concentration of 96-98%, the concentrated nitric acid has a mass concentration of 98-99%, and the concentrated hydrochloric acid has a mass concentration of 35-37%.

[0021] In some preferred embodiments, in step 1.3), the reducing agent is hydrogen peroxide. Preferably, the reducing agent is hydrogen peroxide with a mass concentration of 29-30%.

[0022] In some preferred embodiments, in step 1), the amount of the N-methylpyrrolidone sample used is 15-20g.

[0023] Because the content of metal ions in N-methylpyrrolidone is low, and the sample amount for N-methylpyrrolidone digestion is too low, the metal content cannot reach the instrument detection limit and therefore cannot be detected. Alternatively, due to various reasons, the test data fluctuates greatly, making accurate metal ion determination impossible. The inventors have discovered that using the above-mentioned preferred sample amount allows for rapid and accurate quantification of metal ions in N-methylpyrrolidone. Excessive sample amount leads to excessively long sample digestion time, making rapid testing of the target metal ion content impossible.

[0024] In some preferred embodiments, in step 1.1), the heating concentration temperature is 450-550°C; the heating concentration is preferably carried out until the evaporation reaches 90%-98% of the mass of the N-methylpyrrolidone sample.

[0025] In some preferred embodiments, in step 1.2), the temperature of the oxidation reaction is 450-550°C;

[0026] In step 1.3), the temperature of the reduction reaction is 450-550℃.

[0027] The oxidation and reduction reactions are carried out at temperatures above 450℃. The oxidation reaction can proceed fully, N-methylpyrrolidone is completely decomposed, and the reduction reaction can proceed fully, ensuring that residual carbon is completely removed.

[0028] In some preferred embodiments, in step 1.2), N-methylpyrrolidone decomposes to produce a black liquid, and the oxidation reaction yields a thick black liquid to confirm that the oxidation reaction has occurred; in step 1.3), the reduction reaction yields a clear and transparent liquid to ensure that the reduction reaction proceeds fully and that residual carbon is completely removed.

[0029] In some preferred embodiments, in step 1.1), the purity of the N-methylpyrrolidone sample is ≥99.0%.

[0030] In some preferred embodiments, in step 2), an inductively coupled plasma atomic emission spectrometer is used to detect the target metal ions in the N-methylpyrrolidone digestion solution. Based on the detection results and the pre-established standard working curve of the target metal ions, the content of the target metal ions in the N-methylpyrrolidone sample is calculated and determined.

[0031] Preferably, the test conditions of the inductively coupled plasma atomic emission spectrometer include: pressure 0.6-0.7 MPa, plasma flow rate 14.0-16.0 L / min; auxiliary gas flow rate 0.9-1.1 L / min; nebulizer flow rate 0.6-0.8 L / min.

[0032] In some preferred embodiments, the metal ion is one or more of the following metal element ions: K, Na, Ca, Mg, Zn, Fe, Cu, Ni.

[0033] This invention provides a method for determining the metal ion content in N-methylpyrrolidone, effectively overcoming the drawbacks of traditional dry digestion methods, such as long processing time, complex digestion procedures, and easy loss of metal elements. Since the content of various metal elements in N-methylpyrrolidone samples is at a low level, traditional microwave digestion methods require small sample volumes, making it difficult to reach the detection limits of each metal and thus difficult to determine their content. The method of this invention employs a simpler wet digestion pretreatment method and uses inductively coupled plasma atomic emission spectrometry (ICP-AES) to determine the metal ions in the N-methylpyrrolidone digestion solution. This method enables rapid and accurate quantitative detection of metal ions with a content of less than 1 mg / kg in N-methylpyrrolidone, while simplifying the determination method and saving analytical costs. Attached Figure Description

[0034] Figure 1 This is the standard curve for Na.

[0035] Figure 2 This is the standard curve for Fe.

[0036] Figure 3 This is the standard curve for the K element.

[0037] Figure 4 This is the standard curve for Ni.

[0038] Figure 5 This is the standard curve graph for Zn.

[0039] Figure 6 This is the standard curve graph for Cu.

[0040] Figure 7 This is the standard curve for Mg.

[0041] Figure 8 This is the standard curve graph for Ca. Detailed Implementation

[0042] The present invention will be further illustrated below with specific embodiments. These embodiments are merely illustrative and do not imply that the scope of the invention is limited thereto.

[0043] In the following examples, the determination of metal ions in N-methylpyrrolidone digest specifically includes the following steps:

[0044] a) Preparation of working solutions: Take the multi-element mixed standard solution and use 1% HNO3 solution as a diluent to prepare working solutions of 0.1 mg / kg, 0.5 mg / kg, and 1 mg / kg respectively; use inductively coupled plasma atomic emission spectrometry to detect the working curves of each standard solution.

[0045] b) Preparation of the test solution: Transfer the N-methylpyrrolidone digest to a 10 mL LFA volumetric flask, rinse the container 2-3 times with rinsing solution, transfer the rinsing solution to the volumetric flask, and make up to volume to obtain the N-methylpyrrolidone test solution.

[0046] c) Determination of metal ions: Measure the emission intensity of the element to be tested, and calculate the content of each element in N-methylpyrrolidone based on the working curve.

[0047] The working solution is prepared using a multi-element mixed standard solution, specifically as follows:

[0048] Accurately pipette 1.00 mL of a 1000 mg / L multi-element mixed standard solution into a 10 mL PFA volumetric flask, and dilute to volume with 1% HNO3 solution to obtain an intermediate solution of 100 mg / L. Accurately pipette 1.00 mL of the intermediate solution into a 10 mL PFA volumetric flask, and dilute to volume with 1% HNO3 solution to obtain a standard solution of 10 mg / L. Pipe 0.1 mL, 0.5 mL, and 1.0 mL of the standard solution into 10 mL PFA volumetric flasks, and dilute to volume with 1% HNO3 solution to obtain working solutions of 0.1 mg / kg, 0.5 mg / kg, and 1 mg / kg, respectively.

[0049] The rinsing solution was a 1% (w / w) HNO3 solution.

[0050] The content of the metal element to be tested is calculated using the following formula: C = C1 * V * D / m, where C is the content of the metal element to be tested, mg / kg; C1 is the concentration of the sample calculated from the standard curve, mg / L; V is the sample dilution volume, mL; D is the sample dilution factor; and m is the mass of the sample to be tested, g.

[0051] Reagents and instruments:

[0052] N-methylpyrrolidone sample 1: analytical grade reagent, 99% by mass, purchased from Aladdin;

[0053] N-Methylpyrrolidone Sample 2: This is a sample of the product prepared by our company in March 2023, batch number 2023032309, with a mass concentration of 99.99%;

[0054] N-Methylpyrrolidone Sample 3: This is a sample of the product prepared by our company in February 2023, batch number 2023020901, with a mass concentration of 99.99%.

[0055] Nitric acid: analytical grade reagent, concentration 65%-68%, purchased from General-Reagent;

[0056] Concentrated sulfuric acid: analytical grade reagent, concentration 95%-98%, purchased from General-Reagent;

[0057] Hydrogen peroxide: analytical grade reagent, 30% by mass, purchased from Sinopharm Chemical Reagent Co., Ltd.

[0058] Multi-element mixed standard solution: A multi-element standard solution containing K, Na, Ca, Mg, Zn, Fe, Cu, and Ni, with a standard value of 1000 μg / mL, in a medium of 5% HNO3, 3% HCl, and trace HF, purchased from the National Nonferrous Metals and Electronic Materials Analysis and Testing Center;

[0059] Inductively coupled plasma atomic emission spectrometer: PerkinElmerAvio 550Max.

[0060] The parameters of the inductively coupled plasma atomic emission spectrometer are as follows:

[0061] Test parameters Parameter value plasma flow rate 15.0L / min Auxiliary air flow 1.00L / min Atomizer Flow 0.70L / min One reading time 5.00s Instrument stabilization time 15s Pump speed 12rpm Cleaning time 30s Number of readings 3 Observation direction Axial

[0062] Turn on the inductively coupled plasma atomic emission spectrometer and its auxiliary equipment. Open the main argon valve and the pressure reducing valve, and adjust the pressure to 0.65 MPa. Connect the peristaltic pump tubing and the autosampler tubing. Check that the nebulizer, nebulization chamber, and rectangular tube are normal. Turn on the instrument software and check the optical path temperature. After the argon purging is complete, turn on the cooling water, ignite the plasma, and start the test after the plasma stabilizes.

[0063] Example 1

[0064] 1) Preparation of N-methylpyrrolidone digestion solution: Weigh 15.0000±0.0001g of N-methylpyrrolidone sample 1 into a 100mL platinum crucible, place it on a hot plate, and heat at 500℃ to evaporate until 0.5g of black liquid remains in the crucible. Remove and cool to room temperature to obtain a concentrated sample. Add 1g of concentrated sulfuric acid and heat at 500℃. When a thick black liquid appears in the crucible, remove the crucible and cool to room temperature. Add 2.5g of hydrogen peroxide, place the crucible on the hot plate again, and heat at 500℃ until the solution becomes clear and transparent. Remove and cool to room temperature to obtain the N-methylpyrrolidone digestion solution.

[0065] 2) Determination of metal ions:

[0066] a) Preparation of working solutions: Take the multi-element mixed standard solution and use 1% HNO3 solution as a diluent to prepare working solutions of 0.1 mg / kg, 0.5 mg / kg, and 1 mg / kg respectively; use inductively coupled plasma atomic emission spectrometry to detect the working curves of each standard solution.

[0067] Working solutions of 0.1 mg / kg, 0.5 mg / kg, and 1 mg / kg were analyzed in ascending order of concentration. The emission intensity of each element in the standard series solutions was measured from low to high. Working curves for each element were plotted with concentration on the x-axis and analytical line intensity on the y-axis. Spectra with a correlation coefficient > 0.999 were retained. The working curves for each element are shown below. Figure 1-8 As shown.

[0068] Background correction sequentially selected the analytical spectral lines of K, Na, Ca, Mg, Zn, Fe, Cu, and Ni elements to eliminate interference and obtain the standard curve equations for each element, as shown in Table 1.

[0069] Table 1. Equations of the standard curves for each element

[0070]

[0071]

[0072] b) Preparation of the test solution: Transfer the N-methylpyrrolidone digest obtained in step 1) into a 10 mL LFA volumetric flask, rinse the crucible three times with 1% dilute nitric acid, transfer the washing solution to the volumetric flask, dilute to the mark, and shake well to obtain the N-methylpyrrolidone test solution; prepare an equal volume blank sample without adding N-methylpyrrolidone digest using the same method.

[0073] c) Determination of metal ions: Measure the emission intensity of the element to be tested, and calculate the content of each element in N-methylpyrrolidone based on the working curve.

[0074] The solution to be tested was measured, and the blank solution was measured at the same time. The concentration C1 of the metal element to be tested was obtained from the working curve, and the metal element content C was calculated according to the calculation formula: C=C1*V*D / m. The results are shown in Table 2.

[0075] Table 2 shows the metal element concentrations measured in N-methylpyrrolidone sample 1.

[0076]

[0077] Example 2

[0078] The metal ion concentration C1 of N-methylpyrrolidone sample 2 was determined using the same method and parameters as in Example 1, and the metal element content C was calculated according to the formula: C=C1*V*D / m. The test results are shown in Table 3 below.

[0079] Table 3. Metal element concentrations measured in N-methylpyrrolidone sample 2.

[0080]

[0081]

[0082] Example 3

[0083] The metal ion concentration C1 of N-methylpyrrolidone sample 3 was determined using the same method and parameters as in Example 1, and the metal element content C was calculated according to the formula: C=C1*V*D / m. The test results are shown in Table 4 below.

[0084] Table 4 shows the metal element concentrations measured in N-methylpyrrolidone sample 3.

[0085]

[0086] Example 4 Recovery and Precision Analysis

[0087] The metal ion concentration of N-methylpyrrolidone sample 3 was determined using the same method and parameters as in Example 1. Five samples were set for each concentration, and each sample was measured in triplicate. The average recovery rate was calculated to verify the recovery rate and precision of the method. The total target content in the sample minus the target content in the blank sample can effectively eliminate the influence of other factors on the target content. The ratio of the difference to the theoretical content of the standard substance added to the sample is the spiked recovery rate of the sample.

[0088] Recovery rate is calculated using the following formula: Recovery rate (%) = (Total target analyte content in the sample - Target analyte content in the blank sample) / Amount of standard added to the sample × 100

[0089] N-methylpyrrolidone sample 3: This is a sample of the product prepared by our company in February 2023, with batch number 2023020901. Spiking experiments were conducted to determine the recovery rate RSD (%) of the analyte. The results are shown in Table 5. Each sample was measured three times, and the average value was calculated.

[0090] Table 5. N-Methylpyrrolidone Spiking Test

[0091]

[0092] The above recovery rates indicate that, at low, medium, and high concentrations (where low, medium, and high concentrations refer to spiking at 50%, 100%, and 120% of the content of a certain element in the unknown sample, respectively), the recovery rates of several target analytes are between 80% and 120%, and the RSD% is less than 10%, indicating that the method of the present invention meets the quantitative requirements.

[0093] Comparative Example 1

[0094] The heating concentration, oxidation, and reduction temperatures of Comparative Example 1-1 were all 150°C, and those of Comparative Example 1-2 were all 650°C. The metal ion concentration C1 of N-methylpyrrolidone sample 1 was determined using the same method and parameters as in Example 1. The test results are shown in Table 6 below.

[0095] Table 6. Metal element content of N-methylpyrrolidone sample 1 determined at different concentration, oxidation, and reduction temperatures.

[0096]

[0097] In Comparative Example 1-1, if the wet digestion temperature is too low (e.g., 100℃-200℃), the sample digestion time will be relatively long, about 4 hours, and the analysis and testing time will be too long, which cannot meet the production requirements. In Comparative Example 1-2, if the digestion temperature is too high (e.g., >600℃), a large amount of organic vapor will volatilize, posing a risk of sample flash fire, resulting in lower test data.

[0098] Comparative Example 2

[0099] Comparative Example 2-1 was performed without hydrogen peroxide (reducing agent), while Comparative Example 2-1 was performed with only a small amount of hydrogen peroxide (reducing agent, amount 0.8g). The metal ion concentration C1 of N-methylpyrrolidone sample 1 was determined using the same method and parameters as in Example 1. The test results are shown in Table 7 below.

[0100] Table 7. Metal element content of N-methylpyrrolidone sample 1 under different reducing agent addition conditions.

[0101]

[0102]

[0103] In Comparative Example 2-1, without the addition of a reducing agent, carbon black appeared, making carbon removal impossible, and the entire sample did not meet the test requirements. In Comparative Example 2-2, the amount of reducing agent added was too low, causing some metal ions to adsorb onto the carbon black, resulting in distorted test data. In Example 1, the amount of reducing agent added was 2.5g. An excess of reducing agent was added during the digestion process to ensure complete carbon removal. Once the digestion solution was clear and transparent, it was concentrated by heating, and the excess reducing agent would volatilize and not affect subsequent test results.

[0104] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for determining metal ions in N-methylpyrrolidone, characterized in that, Includes the following steps: 1) Preparation of N-methylpyrrolidone digestion solution: 1.1) The N-methylpyrrolidone sample was concentrated by heating at 450-550℃ to obtain a concentrated sample; 1.2) Add an oxidant to the concentrated sample and carry out an oxidation reaction under heating conditions of 450-550℃; the amount of the oxidant is 150-200% of the mass of the concentrated sample; the oxidant is concentrated sulfuric acid with a mass concentration of 96-98%, concentrated nitric acid with a mass concentration of 98-99%, or concentrated hydrochloric acid with a mass concentration of 35-37%. 1.3) Add a reducing agent to the sample obtained in step 1.2), and carry out a reduction reaction under heating conditions of 450-550℃ to obtain N-methylpyrrolidone digest; the amount of the reducing agent is 200-500% of the mass of the concentrated sample; 2) The N-methylpyrrolidone digest obtained in step 1) was analyzed for metal ions using inductively coupled plasma atomic emission spectrometry.

2. The method for determining metal ions in N-methylpyrrolidone according to claim 1, characterized in that: In step 1.3), the reducing agent is hydrogen peroxide.

3. The method for determining metal ions in N-methylpyrrolidone according to claim 2, characterized in that: The reducing agent is hydrogen peroxide with a mass concentration of 29-30%.

4. The method for determining metal ions in N-methylpyrrolidone according to claim 1, characterized in that: In step 1), the amount of the N-methylpyrrolidone sample used is 15-20g.

5. The method for determining metal ions in N-methylpyrrolidone according to claim 1, characterized in that: In step 1.1), the heating and concentration are carried out until the evaporation amount reaches 90%-98% of the mass of the N-methylpyrrolidone sample.

6. The method for determining metal ions in N-methylpyrrolidone according to any one of claims 1-5, characterized in that: In step 1.2), a thick black liquid is obtained through the oxidation reaction; In step 1.3), a clear and transparent liquid is obtained through the reduction reaction.

7. The method for determining metal ions in N-methylpyrrolidone according to any one of claims 1-5, characterized in that: In step 1.1), the purity of the N-methylpyrrolidone sample is ≥99.0%.

8. The method for determining metal ions in N-methylpyrrolidone according to any one of claims 1-5, characterized in that: In step 2), the target metal ions in the N-methylpyrrolidone digest are detected using an inductively coupled plasma atomic emission spectrometer. Based on the detection results and the pre-established standard working curve of the target metal ions, the content of the target metal ions in the N-methylpyrrolidone sample is calculated and determined.

9. The method for determining metal ions in N-methylpyrrolidone according to claim 8, characterized in that: The test conditions for the inductively coupled plasma atomic emission spectrometer include: pressure 0.6-0.7 MPa, plasma flow rate 14.0-16.0 L / min; auxiliary gas flow rate 0.9-1.1 L / min; and nebulizer flow rate 0.6-0.8 L / min.

10. The method for determining metal ions in N-methylpyrrolidone according to any one of claims 1-5, characterized in that: The metal ion is one or more of the following metal element ions: K, Na, Ca, Mg, Zn, Fe, Cu, Ni.

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