Method for determining trace calcium and magnesium in high-concentration lithium chloride electrolyte

Abstract

The invention discloses a method for measuring trace calcium and magnesium in a high-concentration lithium chloride electrolyte, which comprises the following steps: weighing analytically pure lithium chloride, stirring and dissolving with pure water to a constant volume, precipitating the solution by lithium hydroxide, adjusting the pH value, standing and filtering; and adsorbing through chelating resin to obtain a lithium chloride electrolyte, and then adding a proportionally standard solution. Diluting to the scale, and uniformly shaking; and finally, measuring the content of calcium and magnesium ions in the obtained constant-volume solution on an inductively coupled plasma emission spectrum. The invention provides the method for determining trace calcium and magnesium in the high-concentration lithium chloride electrolyte, the lower detection limit of the method is lower than that of the prior art, the test result repeatability is good, the accuracy is high, a high-purity reagent does not need to be additionally purchased as a matrix, the detection cost is reduced, a standard addition method is adopted, the sample pretreatment time is shortened, and the detection efficiency is improved.

Description

Technical Field

[0001] The invention relates to the technical field of electrolyte component determination, and in particular to a method for determining trace amounts of calcium and magnesium in a high-concentration lithium chloride electrolyte. Background Art

[0002] With the rapid development of emerging industries such as renewable resources and new energy vehicles, the demand for energy storage batteries has increased significantly. As one of the important raw materials for battery positive electrode materials, the importance of lithium hydroxide is self-evident. The method of preparing battery-grade lithium hydroxide by lithium chloride electrolysis can simplify the process and greatly reduce environmental pollution, thereby improving production efficiency. Since the electrolysis process of lithium chloride has very high requirements for the impurities of lithium chloride electrolyte, calcium and magnesium ions must be less than 20μg / L. The lower limit of calcium and magnesium detection in the existing national standard method GB / T11064.16-2023 is 0.0002%, which is converted to a solution concentration of calcium and magnesium. The lower limit is 440μg / L (based on 220g / L lithium chloride). This method can no longer meet the detection requirements of calcium and magnesium in the electrolyte entering the tank, and it is necessary to purchase high-purity matrix reagents and use the matrix matching method to determine, which increases the reagent cost, and the high-purity reagents are easily contaminated during the configuration and weighing process.

[0003] In view of the above, it is necessary to propose a method for determining trace amounts of calcium and magnesium in high-concentration lithium chloride electrolyte to solve the above problems. Summary of the invention

[0004] The purpose of the present invention is to overcome the defects in the prior art and provide a method for determining trace amounts of calcium and magnesium in a high-concentration lithium chloride electrolyte. The method of the present invention solves the defects of the prior art, such as high detection limit, high detection cost and complicated operation.

[0005] To achieve the above object, the technical solution of the present invention is as follows: A method for determining trace amounts of calcium and magnesium in a high-concentration lithium chloride electrolyte, the specific steps are as follows: 1. Sample processing 1. Weigh 220g lithium chloride (AR) and dissolve it in pure water until the volume reaches 1L.

[0006] 2. Add 0.3g / L lithium hydroxide to solution 1, adjust the pH to between 10-12, let it stand and filter, then enter the chelating resin to adsorb calcium and magnesium ions.

[0007] 3. The solution obtained after adsorption by chelating resin is lithium chloride electrolyte with a concentration of 220g / L.

[0008] 2. Test conditions 1. Instrument: SPECTROGREEN inductively coupled plasma optical emission spectrometer (ICP-OES) was selected. The optimal parameters for using the instrument were: generator power: 1400W, cooling gas flow: 14L / min, auxiliary gas flow: 1.8L / min, nebulizing gas flow: 0.9L / min, additional gas flow: 0.2L / min, sample lifting volume: 2.0L / min.

[0009] 2. Selection of analysis lines: Select spectral lines with high sensitivity and less interference.

[0010] 3. National standard solution: The mixed standard concentration of calcium and magnesium is 100 mg / L.

[0011] 4. Intermediate stock solution: Accurately transfer 1 mL of calcium and magnesium mixed standard (100 mg / L) into a 100 mL volumetric flask, dilute to scale with ultrapure water and shake well. This stock solution contains calcium and magnesium at a concentration of 1 mg / L.

[0012] 5. Test water: ultrapure water.

[0013] 3. Preparation of test solution and working curve Take 5 portions of 1 mL of 220 g / L lithium chloride sample solution and place them in 5 10 mL polypropylene centrifuge tubes, then add 0, 0.2, 0.4, 0.6, and 0.8 mL of calcium and magnesium intermediate reserve solution (1 mg / L) respectively, and dilute to 10 mL with ultrapure water. The concentrations of calcium and magnesium spiked in the 5 centrifuge tubes are 0, 20, 40, 60, and 80 μg / L. Shake well to obtain the solution to be tested by the standard addition method. The first portion without the standard solution is the blank sample.

[0014] 4. Testing Place the above 5 portions of standard addition test solution in an inductively coupled plasma emission spectrometer, select the working parameters and use the standard addition mode, measure the emission intensity of the specified wavelength of calcium and magnesium from low to high, and obtain the regression curve after regression. The correlation coefficient of the curve should be above 0.999. Measure the blank sample again to obtain the calcium and magnesium content in the test solution. The spectral line wavelengths of calcium and magnesium are 396.847nm and 279.553nm respectively.

[0015] Furthermore, a certain amount of internal standard elements, such as scandium (Sc), are added to the sample and standard solution. The internal standard element should be similar in nature to the target element (calcium, magnesium) but will not exist naturally in the sample. The internal standard method can correct errors caused by factors such as instrument drift and sample matrix effect, and improve the accuracy and reproducibility of the measurement. The same concentration of internal standard element solution is added to each sample and standard solution, and then the emission intensity of the target element and the internal standard is simultaneously measured in the inductively coupled plasma emission spectrometer. The signal of the target element is corrected by the signal of the internal standard element.

[0016] Scandium (Sc) was selected as the internal standard element.

[0017] Prepare the internal standard solution: weigh 0.1 g of scandium standard substance, dissolve it in 1% nitric acid and make up to 1 L to obtain a 100 mg / L scandium standard solution.

[0018] Sample treatment: Take 5 portions of 1 mL of 220 g / L lithium chloride sample solution and place them in 5 10 mL polypropylene centrifuge tubes. Add 0, 0.2, 0.4, 0.6, and 0.8 mL of calcium and magnesium intermediate stock solution (1 mg / L) respectively, and dilute to 10 mL with ultrapure water. Add 0.1 mL of 100 mg / L scandium standard solution to each centrifuge tube and shake well.

[0019] Determination: The emission intensity of calcium, magnesium and scandium in each standard solution and sample was determined using ICP-OES. The emission intensities of calcium (396.847 nm), magnesium (279.553 nm) and scandium (399.092 nm) were recorded.

[0020] Data processing: Calculate the internal standard-corrected emission intensity of calcium and magnesium in each standard solution: Draw a standard curve with the internal standard corrected emission intensity as the ordinate and the concentration of calcium or magnesium as the abscissa to obtain a regression equation. Substitute the internal standard corrected emission intensity of the sample into the regression equation to calculate the concentration of calcium and magnesium in the sample.

[0021] The advantages and beneficial effects of the present invention are: (1) The present invention provides a method for determining trace amounts of calcium and magnesium in a high-concentration lithium chloride electrolyte. The detection limit of the method is lower than that of the prior art, and the test results have good repeatability and high accuracy.

[0022] (2) The present invention provides a method for determining trace amounts of calcium and magnesium in a high-concentration lithium chloride electrolyte. The method does not require the purchase of additional high-purity reagents as a matrix, thereby reducing the detection cost.

[0023] (3) The present invention provides a method for determining trace amounts of calcium and magnesium in a high-concentration lithium chloride electrolyte. The method adopts a standard addition method, which shortens the sample pretreatment time and improves the detection efficiency.

[0024] (4) The internal standard method is used in the present invention to correct errors caused by factors such as instrument drift and sample matrix effect, thereby improving the accuracy and reproducibility of the measurement. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 is a calibration curve diagram of calcium element in one embodiment of the present invention; Figure 2is a calibration curve diagram of magnesium element in one embodiment of the present invention; Figure 3 is a spectrum of calcium in a sample solution in one embodiment of the present invention; Figure 4 is a spectrum of a sample solution magnesium in one embodiment of the present invention; DETAILED DESCRIPTION

[0026] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and embodiments. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present invention.

[0027] Example: This example provides a method for determining trace amounts of calcium and magnesium in a high-concentration lithium chloride electrolyte, comprising the following steps: 1. Sample processing 1. Weigh 220g lithium chloride (AR) and dissolve it in pure water until the volume reaches 1L.

[0028] 2. Add 0.3g / L lithium hydroxide to solution 1, adjust the pH to between 10-12, let it stand and filter, then enter the chelating resin to adsorb calcium and magnesium ions.

[0029] 3. The solution obtained after adsorption by chelating resin is lithium chloride electrolyte with a concentration of 220g / L.

[0030] 2. Sample determination 1. Working curve configuration: Take 5 portions of 1 mL of 220 g / L lithium chloride sample solution and place them in 5 10 mL polypropylene centrifuge tubes, then add 0, 0.2, 0.4, 0.6, and 0.8 mL of calcium and magnesium intermediate reserve solution, respectively, and dilute to 10 mL with ultrapure water. The calcium and magnesium spike concentrations in the 5 centrifuge tubes are 0, 20, 40, 60, and 80 μg / L. Shake well to obtain the standard addition method test solution. The first portion without the standard solution is the blank sample.

[0031] 2. Place the above 5 portions of standard addition method test solution in an inductively coupled plasma emission spectrometer, select the working parameters and use the standard addition mode, measure the emission intensity of the specified wavelength of calcium and magnesium from low to high, and obtain the regression curve after regression. The correlation coefficient of the curve should be above 0.999. Measure the blank sample again to obtain the calcium and magnesium content in the sample. The spectral line wavelengths of calcium and magnesium are 396.847nm and 279.553nm respectively. The calcium and magnesium content in the sample obtained by computer processing is:

[0032] element Calcium (μg / L) Magnesium (μg / L) Sample 39.0 2.48 This experiment also uses the spike recovery method to verify the accuracy of the above method: ① Verification of spike recovery rate The calcium and magnesium standard substances were added to the sample and subjected to the same treatment before determination. The ratio of the result after removing the calcium and magnesium content in the sample to the amount of calcium and magnesium added was the spike recovery rate. As shown in Table 1, the calcium and magnesium recovery rates were 96.2% and 99.8% respectively, which met the recovery rate requirement of 90-120%, indicating that the method has high test accuracy.

[0033] Table 1 Sample spike recovery experiment element Original value (μg / L) Standard addition value (μg / L) Measured value (μg / L) Recovery rate (%) calcium 37.8 100 134 96.2 magnesium 2.22 100 102 99.8 In order to examine the precision of this method, this example also provides a precision test: ②Precision test The example sample was measured repeatedly 11 times, and the relative standard deviation (RSD) of the measurement results was calculated. See Table 2. The RSDs of calcium and magnesium were 2.66% and 4.13%, respectively, which met the analysis requirement of RSD ≤ 10%, indicating that the method has high precision and good stability.

[0034] Table 2 Precision test of samples 11 times element 1 2 3 4 5 6 7 8 9 10 11 SD average value RSD(%) Calcium (μg / L) 37.8 38.5 36.3 38.1 36.5 36.3 37.0 36.5 36.0 38.0 38.7 0.99 37.2 2.66 Magnesium (μg / L) 2.22 2.54 2.35 2.31 2.36 2.28 2.29 2.22 2.28 2.27 2.19 0.095 2.30 4.13 ③ Method detection limit and lower detection limit The sample results were measured 11 times, and the standard deviation (SD) was calculated. Three times the standard deviation was used as the detection limit (MDL) of the method, and 10 times the standard deviation was used as the lower limit of detection of the method. As shown in Table 3, the detection limits of calcium and magnesium were 2.97μg / L and 0.285μg / L, respectively, and the lower limits of calcium and magnesium were 9.9μg / L and 0.95μg / L, respectively. This shows that the lower limit of detection of this method can meet the requirements of lithium chloride electrolyte (calcium and magnesium are less than 20μg / L).

[0035] Table 3 Detection limit and lower limit of determination element 11 measurement results SD MDL Determination limit Calcium (μg / L) 37.8,38.5,36.3,38.1,36.5,36.3,37.0,36.5,36.0,38.0,38.7 0.99 2.97 9.9 Magnesium (μg / L) 2.22,2.54,2.35,2.31,2.36,2.28,2.29,2.22,2.28,2.27,2.19 0.095 0.285 0.95 The above results show that the method of determining trace calcium and magnesium in high-concentration lithium chloride electrolyte by standard addition method is simple, the results are stable and reliable, and the detection limit is as low as 9.9μg / L, which can meet the detection needs of lithium chloride electrolyte for calcium and magnesium.

[0036] Furthermore, scandium was used as an internal standard element.

[0037] To prepare the internal standard solution, weigh 0.1 g of scandium standard substance, dissolve it in 1% nitric acid and make up to 1 L to obtain a 100 mg / L scandium standard solution.

[0038] Prepare the standard solution by accurately transferring 1 mL of calcium and magnesium mixed standard (100 mg / L) into a 100 mL volumetric flask, dilute to the mark with ultrapure water and shake well to obtain a 1 mg / L intermediate stock solution. Take 5 portions of 1 mL of 220 g / L lithium chloride sample solution and place them in 5 10 mL polypropylene centrifuge tubes. Add 0, 0.2, 0.4, 0.6, and 0.8 mL of calcium and magnesium intermediate stock solution (1 mg / L) respectively and make up to 10 mL with ultrapure water. Add 0.1 mL of 100 mg / L scandium standard solution to each centrifuge tube and shake well.

[0039] Determination, use ICP-OES to measure the emission intensity of calcium, magnesium and scandium in each standard solution and sample. Record the emission intensity of calcium (396.847 nm), magnesium (279.553 nm) and scandium (399.092 nm).

[0040] Data processing, calculate the internal standard corrected emission intensity of calcium and magnesium in each standard solution: A standard curve was drawn with the internal standard-corrected emission intensity as the ordinate and the concentration of calcium or magnesium as the abscissa to obtain a regression equation.

[0041] The internal standard-corrected emission intensity of the sample was substituted into the regression equation to calculate the concentrations of calcium and magnesium in the sample.

[0042] Table 4 Preparation of standard solutions Sample No. Ca / Mg spike amount (mL) Ca / Mg concentration (μg / L) Sc spike amount (mL) Sc concentration (μg / L) 1 0 0 0.1 10 2 0.2 20 0.1 10 3 0.4 40 0.1 10 4 0.6 60 0.1 10 5 0.8 80 0.1 10 Table 5 Measurement results Sample No. Ca emission intensity (I_Ca) Mg emission intensity (I_Mg) Sc emission intensity (I_Sc) 1 100 150 500 2 200 300 500 3 300 450 500 4 400 600 500 5 500 750 500 Table 6 Internal standard correction emission intensity Sample No. Ca internal standard corrected emission intensity (I_Ca / Sc) Mg internal standard corrected emission intensity (I_Mg / Sc) 1 0.2 0.3 2 0.4 0.6 3 0.6 0.9 4 0.8 1.2 5 1.0 1.5 Standard curve for calcium: I Ca / Sc =0.0125×Ca concentration+0.1 Correlation coefficient R 2 =0.999 Standard curve for magnesium: I Mg / Sc =0.01875×Mg concentration+0.15 Correlation coefficient R 2 =0.999 Assume that there is an unknown sample, after the same processing steps, the measured emission intensity is as follows: Ca emission intensity (I_Ca) = 250 Mg emission intensity (I_Mg) = 400 Sc emission intensity (I_Sc) = 500 Calcium concentration: 0.5=0.0125×Ca concentration+0.1 0.4=0.0125×Ca concentration Ca concentration=0.4 / 0.0125=32 μg / L Magnesium concentration: 0.8=0.01875×Mg concentration+0.15 0.65=0.01875×Mg concentration Mg concentration=0.65 / 0.01875=34.67 μg / L In summary, the internal standard method can correct the determination results of calcium and magnesium in the sample, improving the accuracy and reliability of the determination. The final measured calcium concentration in the above unknown sample was 32μg / L, and the magnesium concentration was 34.67μg / L.

[0043] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A method for determining trace amounts of calcium and magnesium in a high concentration lithium chloride electrolyte, characterized in that: The following steps are involved: Step 1, weigh analytical pure lithium chloride, dissolve it in pure water with stirring to make up the volume, precipitate the solution with lithium hydroxide, adjust the pH, and filter it after standing; Step 2, adsorbing the solution of step 1 through a chelating resin to obtain a lithium chloride electrolyte; Step 3, take several portions of the solution in step 2 of the same volume into a centrifuge tube, add a proportional amount of intermediate reserve solution, dilute to the scale, and shake well; Step 4: The calcium and magnesium ion contents of the fixed volume solution obtained in step 3 are measured by inductively coupled plasma emission spectroscopy.

2. The method for determining trace amounts of calcium and magnesium in a high concentration lithium chloride electrolyte according to claim 1, wherein: In step 1, 0.3 g / L of lithium hydroxide is added, the volume is fixed to 1 L, and the pH is adjusted to between 10 and 12.

3. The method for determining trace amounts of calcium and magnesium in a high concentration lithium chloride electrolyte according to claim 1, wherein: The electrolyte concentration in step 2 is 220 g / L.

4. The method for determining trace amounts of calcium and magnesium in a high concentration lithium chloride electrolyte according to claim 1, wherein: The intermediate reserve solution is prepared by adding 1 ml of the national standard solution to a 100 ml volumetric flask and diluting to the scale with ultrapure water. The calcium and magnesium concentrations in the intermediate reserve solution are 1 mg / L. The mixed standard concentration of calcium and magnesium in the national standard solution is 100 mg / L.

5. The method for determining trace amounts of calcium and magnesium in a high concentration lithium chloride electrolyte according to claim 1 or 4, characterized in that: In the step 3, five 1 ml portions of the lithium chloride electrolyte in step 2 are respectively placed in 10 ml centrifuge tubes, and the matrix is ​​calibrated by the standard addition method, and the concentrations of 0, 20, 40, 60, and 80 μg / L calcium and magnesium intermediate reserve solutions are added, and the volume is fixed to 10 ml to make the concentration of the curve matrix and the sample matrix consistent.

6. The method for determining trace amounts of calcium and magnesium in a high concentration lithium chloride electrolyte according to claim 1, wherein: In step 4, a SPECTROGREEN FMD46 inductively coupled plasma emission spectrometer is used to measure the calcium and magnesium contents at a selected wavelength.

7. The method for determining trace amounts of calcium and magnesium in a high concentration lithium chloride electrolyte according to claim 5, wherein: Select an internal standard element, prepare a standard solution of the internal standard element of a certain concentration, add the internal standard solution of the same concentration to each sample and standard solution, correct the signal of the target element by the signal of the internal standard element, and calculate the concentration of calcium and magnesium in the sample.

8. The method for determining trace amounts of calcium and magnesium in a high concentration lithium chloride electrolyte according to claim 6, wherein: The internal standard element is scandium (Sc); the internal standard element standard solution is: weigh 0.1 g of scandium standard substance, dissolve it in 1% nitric acid and make it dilute to 1 L, to obtain a 100 mg / L scandium standard solution.

9. The method for determining trace amounts of calcium and magnesium in a high concentration lithium chloride electrolyte according to claim 6, wherein: Add 0.1 mL of 100 mg / L scandium standard solution to each of the five centrifuge tube samples and shake well; The emission intensity of calcium, magnesium and scandium in each standard solution and sample was determined using an inductively coupled plasma emission spectrometer; the internal standard-corrected emission intensity of the sample was substituted into the regression equation to calculate the concentration of calcium and magnesium in the sample.

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