Method for determining the oxalic acid content of dimethyl oxalate

Abstract

A method for quantifying the oxalic acid content in dimethyl oxalate, comprising the steps of: quantifying the content of total acid in a dimethyl oxalate sample; quantifying the content of nitric acid in a dimethyl oxalate sample; subtracting the content of nitric acid from the content of total acid, and taking the resulting value as the content of oxalic acid in the dimethyl oxalate sample. This method overcomes the difficult task of separately quantifying two acids to be mixed, avoids the problem of high oxalic acid quantification results due to the reaction of nitric acid impurity, which is also an acidic substance, during titration, and eliminates the interference of nitric acid and solution base.

Description

Detailed Description of the Invention

[0001] [Technical field] The present invention relates to the technical field of detection, in particular to a method for detecting the content of oxalic acid impurity in dimethyl oxalate.

[0002] [Background of the invention] Polyglycolic acid (PGA) equipment adopts non-petroleum process technology route. Polyglycolic acid products have excellent complete decomposition properties, excellent mechanical properties and high barrier properties, and are mainly used in medical sutures, medical stents, sanitary products, food packaging, tableware, etc. Here, dimethyl oxalate is an important intermediate raw material for the production of polyglycolic acid, so the composition of dimethyl oxalate has a vital impact on the production process of polyglycolic acid and polyglycolic acid products.

[0003] However, when dimethyl oxalate comes into contact with water, it will undergo hydrolysis reaction and produce the corresponding oxalic acid, and the production of oxalic acid has the following risks: 1) It will corrode the equipment pipeline and affect the service life of the equipment; 2) Salt substances will be generated after the equipment is corroded, and the salts will form scale in the equipment, causing blockage of the equipment and pipeline; 3) Oxalic acid will corrode the equipment, and iron will be mixed into the PGA product in the form of ions, which will have a negative impact on the quality of the PGA product. Therefore, strictly controlling the oxalic acid content in dimethyl oxalate has become an important technical indicator.

[0004] The existing analytical method for the oxalic acid content in dimethyl oxalate is mainly carried out by traditional acid-base titration, which has problems such as large error, poor reproducibility, and inaccurate results.

[0005] [Summary of the invention] The object of the present invention is to provide a method for quantifying the oxalic acid content in dimethyl oxalate, which aims to eliminate the influence of other impurities in dimethyl oxalate on the quantitative result of oxalic acid and obtain a more accurate oxalic acid content.

[0006] In order to achieve the above object of the invention, the present invention provides a method for quantifying the oxalic acid content in dimethyl oxalate, comprising the steps of: To determine the content of total acid in dimethyl oxalate samples; To determine the nitric acid content in dimethyl oxalate samples; and The nitric acid content is subtracted from the total acid content, and the resulting value is the oxalic acid content in the dimethyl oxalate sample.

[0007] In some embodiments, a method for quantifying the total acid content is to titrate a dimethyl oxalate sample and calculate the total acid content by the amount of titrant consumed when the first potential sudden jump point occurs.

[0008] In some embodiments, a method for quantifying the total acid content is to titrate a dimethyl oxalate sample and calculate the total acid content by the amount of titrant consumed when the second potential spike occurs.

[0009] In some embodiments, the total acid content is determined by a potentiometric titrator.

[0010] In some embodiments, the titrant is a KOH solution in methanol.

[0011] In some embodiments, the nitric acid content is determined by acid-base titration.

[0012] In some embodiments, the nitric acid content is quantified by ion chromatography.

[0013] In some embodiments, a method for quantifying nitric acid content by ion chromatography includes: To prepare a standard curve of 0-100 mg / L standard solutions of nitrate ion and its corresponding electrical conductivity peak area; Passing the digested dimethyl oxalate sample through an ion exchange column to separate nitrate ions in the dimethyl oxalate sample, and detecting the electrical conductivity peak area of ​​the nitrate ions; and Calculate the nitrate content in the dimethyl oxalate sample according to the nitrate ion concentration corresponding to the electrical conductivity peak area on the obtained standard curve.

[0014] In some embodiments, the concentrations of the standard solutions of nitrate ion are 6.25 mg / L, 12.50 mg / L, 25.00 mg / L, 50.00 mg / L, and 100.00 mg / L, respectively.

[0015] In some embodiments, the ion exchange column is an anion exchange column.

[0016] In some embodiments, a method for quantifying the oxalic acid content in dimethyl oxalate comprises the steps of: Dispersing a dimethyl oxalate sample in a solvent to obtain a dimethyl oxalate solution; A solution of dimethyl oxalate was titrated with a standard KOH-methanol solution, and the volume of titrant consumed at the first potential sudden increase point (V KOH ) to record; To prepare a standard curve of 0-100 mg / L standard solutions of nitrate ion and its corresponding electrical conductivity peak area; passing the digested dimethyl oxalate sample through an ion exchange column to separate nitrate ions in the dimethyl oxalate sample, and detecting the electrical conductivity peak area of ​​the resulting nitrate ions; The nitrate ion concentration (C) corresponding to the electrical conductivity peak area obtained on the standard curve HNO3 ) was recorded, and the nitrate content (ω HNO3 ) ; and Calculating the oxalic acid content in the dimethyl oxalate samples according to formula I;

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[0017] Compared with the prior art, the present invention has the following advantages: The method for determining the oxalic acid content in dimethyl oxalate provided by the present invention overcomes the difficulty of measuring two mixed acids separately, and can avoid the problem of high oxalic acid determination results caused by the reaction of nitric acid impurity, which is also an acidic substance, during titration.In addition, the determination method provided by the present invention also solves the problem of large fluctuations in the quantitative data of oxalic acid content when determining oxalic acid in dimethyl oxalate, which is caused by the sudden sudden increase in the second potential that is not obvious.The determination method provided by the present invention is used to determine the oxalic acid content in dimethyl oxalate, which can eliminate the interference of nitric acid and solution base, and the test results have more parallel and higher accuracy.

[0018] [Brief description of the drawings] FIG. 1 shows the E&dE / dV diagram of an oxalic acid standard solution (1 mol / L) measured by a potentiometric titrator. The pH of the first potential sudden increase point is 4.17, and the pH of the second potential sudden increase point is 8.14. FIG. 2 shows the E&dE / dV plot of oxalic acid in a dimethyl oxalate sample as determined by potentiometric titration, with the pH of the first potential spike being 4.74 and the pH of the second potential spike being 7.54. FIG. 3 shows the E&dE / dV diagram of the nitric acid standard solution measured by a potentiometric titrator, with the pH at which the potential suddenly increases being 5.08. FIG. 4 is a standard curve of different concentrations of nitrate and their corresponding electrical conductivity. In Figures 1 to 3, the E&dE / dV diagram refers to a plot of the titer using the potential of the indicator electrode, and a plot of the titer using the first derivative of the potential of the indicator electrode with respect to the titer, where E is the potential of the indicator electrode, V is the titer, and dE / dV is the first derivative of the potential of the indicator electrode with respect to the titer.

[0019] [Detailed Description of the Invention] Oxalic acid has an ionization constant Ka of 5.9x10 -2 , Ka2 is 6.4x10 -5 Oxalic acid is a weak divalent acid. Usually, when quantifying oxalic acid in a sample, an alkaline standard solution is used for titration, and the oxalic acid content is determined by classical acid-base titration, where a first hydrogen ion is dissociated by oxalic acid when the pH shows a first sudden increase, and a second hydrogen ion is dissociated by oxalic acid when the pH shows a second sudden increase. In the example of using KOH-methanol standard solution as the titrant, the mass % content of oxalic acid in the sample is calculated according to Equation II:

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[0020] However, during testing, the inventors of the present invention found that the pH of the first potential point (i.e., the first pH point) of the oxalic acid standard solution was 4.17 and the pH of the second potential point (i.e., the second pH point) was 8.14 (as shown in FIG. 1), while the oxalic acid in the dimethyl oxalate sample was determined to have a pH of 4.74 and a pH of 7.54 (as shown in FIG. 2). It can be seen that both the first potential point and the second potential point shifted during the determination of the oxalic acid in the dimethyl oxalate sample. In this regard, the inventors continued testing and found that the E&dE / dV of the nitric acid standard solution was determined by potentiometric titration to have a pH of 5.08 (as shown in FIG. 3). Therefore, it is believed that the first potential sudden surge point and the second potential sudden surge point of oxalic acid are shifted to the potential sudden surge point of nitric acid (5.08) due to the influence of nitric acid in the dimethyl oxalate sample. The main impurities in the dimethyl oxalate product are methanol, methyl formate, water, nitric acid, oxalic acid, etc., and since nitric acid and oxalic acid are both acid substances, it has been confirmed through tests that when the oxalic acid content is detected by titration, nitric acid also participates in the reaction, resulting in a higher quantitative result of the oxalic acid content. To solve this problem, the present invention provides a method for quantifying the oxalic acid content in dimethyl oxalate, comprising the following steps: (1) To determine the total acid content in dimethyl oxalate samples; (2) quantifying the nitric acid content in the dimethyl oxalate sample; and (3) Subtract the nitric acid content from the total acid content, and use the resulting value as the oxalic acid content in the dimethyl oxalate sample.

[0021] Specifically, in step (1), the dimethyl oxalate sample includes an end product of dimethyl oxalate and a process sample of dimethyl oxalate. Here, the end product of dimethyl oxalate refers to a dimethyl oxalate product having a dimethyl oxalate content of 99.5% or more. The process sample of dimethyl oxalate refers to a sample containing dimethyl oxalate obtained in an intermediate process that produces or generates a dimethyl oxalate product.

[0022] The total acid content refers to the sum of the oxalic acid content and the nitric acid content in a dimethyl oxalate sample.

[0023] In some embodiments, the total acid content is quantified by a potentiometric titrator. Preferably, the potentiometric titrator is a high-precision potentiometric titrator, such as a potentiometric titrator with a minimum liquid addition volume of 0.005 mL, a liquid addition error of 0.15% or less, and a quantitative potential range of ±1000 mV.

[0024] When oxalic acid is titrated by titration, two pH spikes occur: when oxalic acid is titrated potentiometrically (e.g., using a potentiometric titrator), the first pH spike is the first potential spike point, and the second pH spike is the second potential spike point.

[0025] In some embodiments, a method for quantifying the total acid content is to titrate a dimethyl oxalate sample and calculate the total acid content by the amount of titrant consumed when the second potential spike occurs.

[0026] In some embodiments, the method for quantifying the total acid content is to titrate a dimethyl oxalate sample, and calculate the total acid content according to the amount of titrant consumed when the first potential sudden increase point occurs. Meanwhile, the inventors further found that during testing, the first potential sudden increase point and the second potential sudden increase point of the oxalic acid standard solution are both obvious (as shown in FIG. 1), whereas when quantifying oxalic acid in a dimethyl oxalate sample, the second potential sudden increase point becomes unclear (as shown in FIG. 2), and problems such as difficulty in reading and large difference in quantitative data occur when quantifying oxalic acid in a dimethyl oxalate sample. After testing, it was found that the cause of this problem is the interference of the solution base. To solve this problem, the present invention uses the amount of titrant consumed when the first potential sudden surge point occurs as the result of the total acid content in the dimethyl oxalate sample, and after repeated verification, compares it with the amount of titrant consumed when the second potential sudden surge point occurs as the result of the total acid content in the dimethyl oxalate sample, which is more parallel and has smaller error, and further improves the accuracy of the determination of the oxalic acid content in the dimethyl oxalate sample.

[0027] In some embodiments, the titrant is a KOH methanol solution, i.e., a solution formed by dispersing KOH in methanol. In some specific embodiments, the concentration of KOH in the KOH methanol solution is 0.1 mol / L for ease of titration and calculation.

[0028] In step (2), if the oxalic acid in the dimethyl oxalate sample is directly quantified by titration, the result will be interfered with by nitric acid. Therefore, the present invention adopts a method of first quantifying the total acid content, and then subtracting the nitric acid content to obtain an accurate result of the oxalic acid content. Therefore, it is necessary to quantify the nitric acid content. In some embodiments, the nitric acid content is quantified by acid-base titration or ion chromatography, preferably ion chromatography, which is more accurate. The separation of nitrate ions in the dimethyl oxalate sample by ion chromatography can be carried out by methods and materials known in the art. The method uses the different binding capacity of nitrate ions with an ion exchange column to separate nitrate ions in the dimethyl oxalate sample.

[0029] In some embodiments, the separation of nitrate ions from a dimethyl oxalate sample by ion chromatography comprises the following steps: (i) To prepare a standard curve of 0-100 mg / L standard solutions of nitrate ion and its corresponding electrical conductivity peak area; (ii) passing the digested dimethyl oxalate sample through an ion exchange column to separate nitrate ions in the dimethyl oxalate sample, and detecting the electrical conductivity peak area of ​​the nitrate ions; and (iii) Calculating the nitrate content in the dimethyl oxalate sample according to the nitrate ion concentration corresponding to the obtained electrical conductivity peak area on the standard curve.

[0030] In some embodiments, in step (i), the standard curve uses nitrate ion concentration as the horizontal axis and electrical conductivity peak area as the vertical axis.

[0031] In some embodiments, the specific concentrations of the 0-100 mg / L standard solutions of nitrate ion are 6.25 mg / L, 12.50 mg / L, 25.00 mg / L, 50.00 mg / L, and 100.00 mg / L, respectively.

[0032] In some embodiments, the ion exchange column is an anion exchange column. In a specific embodiment, the anion exchange column is a Thermo Fisher AS23 type anion column, mainly including a guard column and an anion separation column, and the anion analysis column uses an anion exchange resin as the stationary phase and an eluent as the mobile phase.

[0033] In step (ii), the digestion method of the dimethyl oxalate sample may be a conventional method in the art, the purpose of which is to convert various elements in the dimethyl oxalate sample into a free state for anion exchange and quantify the content of nitrate ion. In some embodiments, the digestion is to mix the dimethyl oxalate sample with hydrogen peroxide, then heat and dry. Preferably, the dimethyl oxalate sample is mixed with hydrogen peroxide at a temperature of 70°C, and the temperature is kept for 1.5 hours, and then heated to 80°C for drying. The material obtained after digestion is added back to water to make up the volume to obtain a solution for passing through the anion exchange column.

[0034] In step (iii), the nitric acid content can be calculated according to formula III:

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[0035] In step (3), in some embodiments, the method of calculating the oxalic acid content in the dimethyl oxalate sample by subtracting the nitric acid content from the total acid content is as follows: the molar amount of consumed titrant is calculated according to the volume of titrant consumed during the determination of the total acid content, the molar amount of nitric acid is calculated according to the nitrate ion content, the molar amount of oxalic acid is obtained by subtracting the molar amount of nitric acid from the molar amount of consumed titrant, then multiplying by the relative molecular weight of oxalic acid, dividing the resulting product by the mass of the dimethyl oxalate sample, and finally multiplying by 100 to obtain the mass percent content of oxalic acid in the dimethyl oxalate sample. In the example where KOH methanol standard solution is used as titrant, the oxalic acid content is calculated as in Formula I.

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[0036] In one preferred embodiment of the present invention, a method for quantifying the oxalic acid content in a dimethyl oxalate sample comprises the following steps: S1. Dispersing a dimethyl oxalate sample in a solvent to obtain a solution of dimethyl oxalate; S2. A solution of dimethyl oxalate is titrated with a standard KOH-methanol solution. When the first potential sudden increase occurs, what is the volume of titrant consumed (V)? KOH ) to record; S3. Draw a standard curve of 0-100mg / L standard solution of nitrate ion and its corresponding electrical conductivity peak area; S4. passing the digested dimethyl oxalate sample through an ion exchange column to separate nitrate ions in the dimethyl oxalate sample, and detecting the electrical conductivity peak area of ​​the resulting nitrate ions; S5. The nitrate ion concentration (C) corresponding to the obtained electrical conductivity peak area on the standard curve HNO3 ) and calculate the nitric acid content in the dimethyl oxalate sample (ω HNO3 ). S6. Calculating the oxalic acid content in the dimethyl oxalate sample according to formula I;

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[0037] In this preferred embodiment, not only the interference of nitric acid on oxalic acid during quantification, but also the interference of solution base is eliminated, so that the quantitative result of oxalic acid content in dimethyl oxalate sample is more parallel and more accurate.At the same time, it can be understood that in the present invention, the units of parameters shown in formula I, formula II and formula III can be converted into international units or subunits that represent the same parameters, for example, mol / L can be converted into mmol / L, mL can be converted into L, etc., and formula I, formula II and formula III can be appropriately adjusted according to specific conversion conditions.

[0038] In order to make those skilled in the art clearly understand the above implementation details and operations of the present invention, and to prominently reflect the improved performance of the method for determining the oxalic acid content in dimethyl oxalate in the embodiments of the present invention, the above technical solutions are illustrated by the following several examples.

[0039] The experimental methods used in the following examples are conventional unless otherwise specified.

[0040] All materials, reagents and the like used in the following examples are commercially available unless otherwise specified.

[0041] [Example 1] In this example, trace amounts of nitric acid in dimethyl oxalate, an intermediate product of the polyglycolic acid manufacturing plant of Yulin Company, were quantified by ion chromatography.

[0042] 1. Reagents and Materials (1) Hydrogen peroxide: analytical grade; (2) Water: ultrapure water; (3) Nitrate ion standard solutions: 6.25 mg / L, 12.50 mg / L, 25.00 mg / L, 50.00 mg / L, and 100.00 mg / L standards, respectively; (4) Eluent: Prepare 2 L of eluent containing 4.5 mmol Na2CO3 + 0.8 mmol NaHCO3.

[0043] 2. Equipment and devices (commercially available) (1) Ion chromatography consists of an eluent storage bottle, a delivery pump, an injection valve, a guard and separation column, an anion suppressor, an electrical conductivity detector, and a chromatography workstation. [Table 1]

[0044] (2) Analytical balance: sensitivity 0.1 mg. (3) Guard column and separation column: The guard column and separation column were both resin-packed columns, the models being Thermo Fisher AG23 type anion column (4x50mm) and Thermo Fisher AS23 type anion column (4x250mm).

[0045] 3.Analysis process (1) Drawing the standard curve: Nitrate ion standard solutions with concentrations of 6.25 mg / L, 12.50 mg / L, 25.00 mg / L, 50.00 mg / L, and 100.00 mg / L were prepared in five 50 mL flasks. The electric conductivity peak area of ​​the nitrate ion corresponding to each nitrate ion standard solution was quantified by the above-mentioned ion chromatography, and a calibration curve was drawn with the nitrate ion concentration on the horizontal axis and the electric conductivity peak area on the vertical axis (Figure 4).

[0046] (2) Quantification of samples Digestion to eliminate base interference: A sample of dimethyl oxalate was taken, 2 g was weighed into a 50 mL beaker, 4 mL of water was added, the mixture was heated at 70°C to a homogeneous phase, and after keeping for 0.5 hours, 3 mL of hydrogen peroxide was added and kept for 1.5 hours, and the resulting solution was evaporated to dryness at 80°C.

[0047] The evaporated material in the beaker was washed with ultrapure water, transferred to a volumetric flask, and diluted to the mark (50 mL) with water. HNO3 It was read as follows.

[0048] Nitric acid content in the sample (ωHNO3 ) was quantified in mass percent and calculated according to Equation III.

[0049] The quantitative results are shown in Table 1.

[0050] [Table 2]

[0051] From Table 1, it can be seen that the dimethyl oxalate sample contains nitric acid impurity. At the same time, the nitric acid content data obtained by quantification can be used for the subsequent calculation of oxalic acid content.

[0052] [Example 2] In this example, a method is provided for calculating the total acid content in a dimethyl oxalate sample by the first potential burst and the second potential burst, respectively.

[0053] (11) A sample of dimethyl oxalate was weighed into a clean, dry beaker, and 50 mL of an organic solvent was added to completely dissolve the sample, obtaining a solution of dimethyl oxalate. Using a potentiometric titrator, the dimethyl oxalate solution was titrated using a 0.1 mol / L KOH methanol solution (standard solution) as the titrant, and when the first potential sudden increase point occurred, the volume (V1) of the KOH methanol solution consumed at this point was recorded.

[0054] (12) The titration was continued, and the volume of KOH methanol solution consumed (V2) was recorded when the second potential sudden increase occurred.

[0055] The first potential sudden increase pH, the second potential sudden increase pH, the volume of titrant consumed at the corresponding time, and the calculated total acid content are shown in Table 2. Here, the calculation formula for the total acid content is shown in Formula I. When the occurrence of the first potential sudden increase point is used as the calculation basis for the total acid content, V1 in this embodiment is V in Calculation Formula I. KOHWhen the occurrence of the second potential sudden increase point is used as the calculation basis for the total acid content, V2 in this embodiment is V in Calculation Formula I KOH Used as.

[0056] [Table 3]

[0057] From Table 2, compared with the result of the total acid content in the dimethyl oxalate sample calculated by recording the amount of titrant consumed when the second potential sudden increase point occurs, the standard deviation of the total acid content in the dimethyl oxalate sample calculated by recording the amount of titrant consumed when the first potential sudden increase point occurs is smaller, and therefore the result of the total acid content in the dimethyl oxalate sample calculated by recording the amount of titrant consumed when the first potential sudden increase point occurs is more accurate.

[0058] [Example 3] In this example, a spike-and-recovery experiment is provided in which the oxalic acid content was determined based on the total acid content in a dimethyl oxalate sample, calculated by recording the amount of titrant consumed (V1) when the first potential spike occurred.

[0059] 1. Quantitative Process Two samples of dimethyl oxalate with known oxalic acid content were taken at the same weight, 104 μg of oxalic acid was added to each, and the oxalic acid content of the two samples was quantified under the same conditions according to the procedures of Examples 1 and 2 (the total acid content in the dimethyl oxalate sample was calculated by recording the amount of titrant consumed when the first potential spike point occurred).

[0060] 2. Calculation of spike recovery rate Spike recovery rate = (spiked sample quantitative value - sample quantitative value) ÷ spike amount x 100

[0061] The spike-and-recovery experimental results are shown in Table 3.

[0062] [Table 4]

[0063] From Table 3, it can be seen that the accuracy and reliability of the results of the oxalic acid content in dimethyl oxalate samples detected by the method of the present invention can meet the professional requirements of analysis (95%-105%).

[0064] [Example 4] This example provides a quantitative determination of the oxalic acid content in dimethyl oxalate, an intermediate product of Yulin's polyglycolic acid device.

[0065] The dimethyl oxalate sample used in Example 1 was weighed into a clean and dry beaker, and 50 mL of an organic solvent was added to completely dissolve the sample, obtaining a dimethyl oxalate solution. Using a potentiometric titrator, the dimethyl oxalate solution was titrated using a 0.1 mol / L KOH methanol solution (standard solution) as the titrant. When the first potential sudden surge point occurred, the volume of the KOH solution consumed at this point (V KOH ) was recorded.

[0066] Combined with the average result of 0.0079% of the nitric acid content determined in Example 1, the oxalic acid content in the samples was calculated according to Formula I. The results are shown in Table 4.

[0067] [Table 5]

[0068] The above examples merely represent some embodiments of the present invention, and the description is relatively specific and detailed, but should not be understood as limiting the patent scope of the present invention. It should be noted that for a person skilled in the art, some modifications and improvements can be made without departing from the concept of the present invention, and all of these belong to the protection scope of the present invention. Therefore, the patent protection scope of the present invention shall be based on the appended claims. [Brief description of the drawings]

[0069] [Figure 1] FIG. 1 shows the E&dE / dV diagram of an oxalic acid standard solution (1 mol / L) measured by a potentiometric titrator. The pH of the first potential sudden increase point is 4.17, and the pH of the second potential sudden increase point is 8.14. [Diagram 2] FIG. 2 shows the E&dE / dV plot of oxalic acid in a dimethyl oxalate sample as determined by potentiometric titration, with the pH of the first potential spike being 4.74 and the pH of the second potential spike being 7.54. [Diagram 3] FIG. 3 shows the E&dE / dV diagram of the nitric acid standard solution measured by a potentiometric titrator, with the pH at which the potential suddenly increases being 5.08. [Figure 4] Figure 4 is a standard curve of the corresponding electrical conductivity for different concentrations of nitrate. In Figures 1-3, E&dE / dV diagrams refer to plots of titer using the potential of the indicator electrode and plots of titer using the first derivative of the potential of the indicator electrode with respect to the titer. Here, E is the potential of the indicator electrode, V is the titer, and dE / dV is the first derivative of the potential of the indicator electrode with respect to the titer.

Claims

1. A method for quantifying the oxalic acid content in dimethyl oxalate, comprising the steps of: Determining the content of total acid in dimethyl oxalate samples; Determining the nitric acid content in the dimethyl oxalate sample; and Subtracting the nitric acid content from the total acid content to obtain the oxalic acid content in the dimethyl oxalate sample.

2. The method for quantifying the total acid content according to claim 1, wherein the method comprises titrating a dimethyl oxalate sample and calculating the total acid content based on the amount of titrant consumed when the first potential sudden increase point occurs.

3. The method for quantifying the total acid content according to claim 1, wherein the method comprises titrating a dimethyl oxalate sample and calculating the total acid content according to the amount of titrant consumed when the second potential sudden increase point occurs.

4. The method according to any one of claims 1 to 3, wherein the total acid content is quantified by a potentiometric titrator.

5. The method according to any one of claims 1 to 3, wherein the titrant is a KOH methanol solution.

6. The method according to any one of claims 1 to 5, wherein the nitric acid content is quantified by acid-base titration or ion chromatography.

7. 7. The method for quantifying the nitric acid content by ion chromatography according to claim 6, comprising the steps of:

2. Preparing a standard curve of 0-100 mg / L standard solutions of nitrate ion and its corresponding electrical conductivity peak area; Passing the digested dimethyl oxalate sample through an ion exchange column to separate nitrate ions in the dimethyl oxalate sample, and detecting the electrical conductivity peak area of ​​the nitrate ions; and Calculating the nitrate content in the dimethyl oxalate sample according to the nitrate ion concentration corresponding to the obtained electrical conductivity peak area on the standard curve.

8. The quantitative method according to claim 7, wherein the concentrations of the standard solutions of nitrate ions are 6.25 mg / L, 12.50 mg / L, 25.00 mg / L, 50.00 mg / L, and 100.00 mg / L, respectively.

9. The quantitative method according to claim 7, wherein the digestion converts each element in the dimethyl oxalate sample into a free state.

10. 10. The method of claim 9, wherein the digestion comprises mixing the dimethyl oxalate sample with hydrogen peroxide, heating and drying the mixture.

11. The method according to claim 7, wherein the ion exchange column is an anion exchange column.

12. The quantitative method according to claim 11, wherein the anion exchange column uses an anion exchange resin as a stationary phase and an eluent as a mobile phase, and the eluent is a mixed solution of sodium carbonate and sodium bicarbonate.

13. The method according to any one of claims 7 to 12, wherein the nitric acid content in the dimethyl oxalate sample is calculated according to formula III: [0010] In the above formula III, ω ＨＮＯ３ represents the nitric acid content in % in the dimethyl oxalate sample; C ＨＮＯ３ represents the nitrate ion concentration in mg / L corresponding to the electrical conductivity peak area of ​​nitrate ion detected by ion chromatography on the standard curve; m represents the mass of the dimethyl oxalate sample in g; V is the volume in mL of the dimethyl oxalate sample that was digested and then made up to volume.

14. The method according to any one of claims 1 to 13, comprising the steps of: Dispersing the dimethyl oxalate sample in a solvent to obtain a dimethyl oxalate solution; The dimethyl oxalate solution was titrated with a standard KOH methanol solution, and the volume of the titrant consumed when the first potential sudden increase point occurred (V ＫＯＨ ) to record; 2. Preparing a standard curve of 0-100 mg / L standard solutions of nitrate ion and its corresponding electrical conductivity peak area; passing the digested dimethyl oxalate sample through an ion exchange column to separate nitrate ions in the dimethyl oxalate sample, and detecting the electrical conductivity peak area of ​​the obtained nitrate ions; The nitrate ion concentration (C ＨＮＯ３ ) and the nitric acid content ω ＨＮＯ３ Calculating Calculating the oxalic acid content in said dimethyl oxalate sample according to Formula I; [0025] In the above formula I, ω (%) represents the mass percent content of oxalic acid; C ＫＯＨ represents the concentration of the KOH methanol standard solution in mol / L; V ＫＯＨ represents the volume in mL of the KOH methanol standard solution consumed when the first potential spike point occurred; m represents the mass of the dimethyl oxalate sample in g; ω ＨＮＯ３ represents the nitric acid content in % in the dimethyl oxalate sample; 63 is the relative molecular weight of nitric acid; and 90.03 is the relative molecular weight of oxalic acid.

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