WO2023206109A1 - 草酸二甲酯中草酸含量的测定方法 - Google Patents

草酸二甲酯中草酸含量的测定方法 Download PDF

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WO2023206109A1
WO2023206109A1 PCT/CN2022/089413 CN2022089413W WO2023206109A1 WO 2023206109 A1 WO2023206109 A1 WO 2023206109A1 CN 2022089413 W CN2022089413 W CN 2022089413W WO 2023206109 A1 WO2023206109 A1 WO 2023206109A1
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dimethyl oxalate
acid content
sample
nitric acid
oxalate sample
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PCT/CN2022/089413
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French (fr)
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张亚春
白文娟
李春雷
黄起中
葛喜慧
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中国神华煤制油化工有限公司
国能榆林化工有限公司
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Priority to PCT/CN2022/089413 priority Critical patent/WO2023206109A1/zh
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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
    • G01N30/02—Column chromatography
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
    • G01N30/02—Column chromatography
    • G01N30/04—Preparation or injection of sample to be analysed
    • G01N30/06—Preparation
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N30/00—Investigating or analysing materials by separation into components using adsorption, absorption or similar phenomena or using ion-exchange, e.g. chromatography or field flow fractionation
    • G01N30/02—Column chromatography
    • G01N30/62—Detectors specially adapted therefor
    • G01N30/64—Electrical detectors
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N31/00—Investigating or analysing non-biological materials by the use of the chemical methods specified in the subgroup; Apparatus specially adapted for such methods
    • G01N31/16—Investigating or analysing non-biological materials by the use of the chemical methods specified in the subgroup; Apparatus specially adapted for such methods using titration

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  • the present invention relates to the field of detection technology, and specifically to a method for detecting the content of oxalic acid impurities in dimethyl oxalate.
  • the polyglycolic acid (PGA) device adopts a non-petroleum process technology route.
  • the product has excellent full degradation characteristics, excellent mechanical properties and high barrier properties. It is mainly used in medical sutures, medical stents, sanitary products, food packaging, tableware, etc.
  • dimethyl oxalate is an important intermediate raw material for the production of polyglycolic acid, so the composition of dimethyl oxalate has an extremely important impact on the polyglycolic acid process and polyglycolic acid products.
  • dimethyl oxalate will undergo a hydrolysis reaction when it meets water to generate the corresponding oxalic acid
  • oxalic acid has the following hazards to production: 1) It has a corrosive effect on equipment pipelines, affecting the service life of the equipment; 2) It generates salt substances after corroding the equipment, Salt will scale in the equipment and cause blockage of equipment and pipelines; 3) Oxalic acid corrodes equipment and causes iron to enter PGA products in the form of ions, affecting the quality of PGA products. Therefore, strictly controlling the oxalic acid content in dimethyl oxalate has become an important process indicator.
  • the existing analysis method for oxalic acid content in dimethyl oxalate mainly uses the traditional acid-base titration method, which has problems such as large errors, poor repeatability, and inaccurate results.
  • the purpose of the present invention is to provide a method for measuring the oxalic acid content in dimethyl oxalate, aiming to eliminate the influence of other impurities in dimethyl oxalate on the oxalic acid measurement results, so as to obtain a more accurate oxalic acid content.
  • the present invention provides a method for determining the oxalic acid content in dimethyl oxalate, which includes the following steps:
  • the total acid content is measured by titrating the dimethyl oxalate sample, and the total acid content is calculated based on the amount of titrant consumed when the first potential jump point occurs.
  • the total acid content is measured by titrating the dimethyl oxalate sample, and the total acid content is calculated based on the amount of titrant consumed when the second potential jump point occurs.
  • the total acid content is determined by potentiometric titrator.
  • the titrant is KOH in methanol.
  • the nitric acid content is determined by acid-base titration.
  • the nitric acid content is determined by ion chromatography.
  • the method for determining nitric acid content using ion chromatography includes:
  • the content of nitric acid in the dimethyl oxalate sample is calculated.
  • the concentrations of the nitrate ion standard solution are 6.25 mg/L, 12.50 mg/L, 25.00 mg/L, 50.00 mg/L, and 100.00 mg/L respectively.
  • the ion exchange column is an anion exchange column.
  • the method for determining the oxalic acid content in dimethyl oxalate includes the following steps:
  • the oxalic acid content in the dimethyl oxalate sample is calculated according to Formula I;
  • ⁇ (%) represents the mass content percentage of oxalic acid
  • C KOH represents the concentration of KOH methanol standard solution, in mol/L
  • V KOH represents the volume V KOH of the KOH methanol standard solution consumed when the first potential jump point occurs, in mL;
  • m is the mass of the dimethyl oxalate sample, in g;
  • ⁇ HNO3 represents the nitric acid content in the dimethyl oxalate sample, in %
  • 63 is the relative molecular mass of nitric acid
  • 90.03 is the relative molecular mass of oxalic acid.
  • the present invention has the following advantages:
  • the method for measuring the oxalic acid content in dimethyl oxalate overcomes the problem of measuring two mixed acids separately, and can avoid the problem of high oxalic acid measurement results caused by the participation of nitric acid impurities, which are also acidic substances, in the titration reaction. Furthermore, the measurement method provided by the present invention also solves the problem of large differences in the measurement data of oxalic acid content caused by the insignificant second potential jump when measuring oxalic acid in dimethyl oxalate. Using the measurement method provided by the invention to measure the oxalic acid content in dimethyl oxalate can eliminate the interference of nitric acid and the solution base, and the test results have better parallelism and higher accuracy.
  • Figure 1 is the E&dE/dV diagram of the oxalic acid standard solution (1mol/L) measured using a potentiometric titrator.
  • the pH of the first potential jump point is 4.17, and the pH of the second potential jump point is 8.14;
  • Figure 2 shows the E&dE/dV diagram of oxalic acid in dimethyl oxalate sample measured using a potentiometric titrator.
  • the pH of the first potential jump point is 4.74, and the pH of the second potential jump point is 7.54;
  • Figure 3 is the E&dE/dV diagram of the nitric acid standard solution measured using a potentiometric titrator, in which the pH of the potential jump point is 5.08;
  • Figure 4 is the standard curve of different concentrations of nitrate and corresponding conductivity
  • the E&dE/dV diagram refers to: plotting the potential of the indicating electrode against the titration volume and plotting the first derivative of the potential of the indicating electrode against the titration volume against the titration volume.
  • E is the potential of the indicating electrode
  • V is the titration volume
  • dE/dV is the first derivative of the potential of the indicating electrode to the titration volume).
  • Oxalic acid is a binary weak acid with an ionization constant Ka1 of 5.9 ⁇ 10 -2 and a Ka2 of 6.4 ⁇ 10 -5 .
  • an alkaline standard solution is used for titration, and the oxalic acid content is obtained by the classic acid-base titration method.
  • the first sudden jump in pH occurs, the first hydrogen ion is dissociated from oxalic acid, and when the second sudden jump in pH occurs, the second hydrogen ion is dissociated from oxalic acid.
  • the mass percentage content of oxalic acid in the sample is calculated according to Formula II:
  • V KOH represents the volume of KOH methanol standard solution consumed by titrating the sample, in mL;
  • C KOH represents the concentration of KOH methanol standard solution, in mol/L
  • ⁇ represents the mass content percentage of oxalic acid
  • m represents the sample mass, unit is g;
  • 90.03 is the relative molecular mass of oxalic acid.
  • the inventor of the present invention discovered during the research process that the pH of the first potential jump point of the oxalic acid standard solution (ie, the first pH jump) is 4.17, and the pH of the second potential jump point (ie, the second pH jump) is 4.17. jump) is 8.14 (as shown in Figure 1); and when measuring oxalic acid in the dimethyl oxalate sample, the pH of the first potential jump point is 4.74, and the pH of the second potential jump point is 7.54 (Figure 2 shown). It can be seen that when measuring oxalic acid in the dimethyl oxalate sample, both the first potential jump point and the second potential jump point shifted.
  • the inventor of the present invention continued research and found that when using a potentiometric titrator to measure the E&dE/dV of a nitric acid standard solution, the pH of the potential jump point was 5.08 (as shown in Figure 3). Therefore, it is believed that due to the influence of nitric acid in the dimethyl oxalate sample, both the first potential jump point and the second potential jump point of oxalic acid will shift toward the potential jump point (5.08) of nitric acid. Research has confirmed that the main impurities in dimethyl oxalate products include methanol, methyl formate, water, nitric acid, oxalic acid, etc.
  • nitric acid and oxalic acid are acidic substances, so when the oxalic acid content is detected by titration, nitric acid will also Participate in the reaction, resulting in higher measurement results of oxalic acid content.
  • the present invention provides a method for determining the oxalic acid content in dimethyl oxalate, which includes the following steps:
  • the dimethyl oxalate sample includes a finished product of dimethyl oxalate and a process sample of dimethyl oxalate.
  • the dimethyl oxalate finished product refers to a dimethyl oxalate product with a dimethyl oxalate content of more than 99.5%.
  • the process sample of dimethyl oxalate refers to a sample containing dimethyl oxalate obtained during the intermediate process of producing or developing dimethyl oxalate products.
  • the total acid content refers to the sum of the oxalic acid and nitric acid contents in the dimethyl oxalate sample.
  • the total acid content is determined by potentiometric titrator.
  • 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%, and a measurement potential range of ⁇ 1000 mV.
  • Titration of oxalic acid produces two pH jumps.
  • the first pH jump is the first potential jump point
  • the second pH jump is the second potential jump point.
  • the total acid content is measured by titrating the dimethyl oxalate sample, and the total acid content is calculated based on the amount of titrant consumed when the second potential jump point occurs.
  • the total acid content is measured by titrating the dimethyl oxalate sample, and the total acid content is calculated based on the amount of titrant consumed when the first potential jump point occurs.
  • the inventor of the present invention further discovered during research that the first potential jump point and the second potential jump point of the oxalic acid standard solution are both obvious (as shown in Figure 1); and when measuring the dimethyl oxalate sample, When oxalic acid is detected, the second potential jump point becomes less obvious (as shown in Figure 2), which will cause problems such as difficulty in reading and large differences in measurement data when measuring oxalic acid in dimethyl oxalate samples. After research, the reason for this problem is the interference of the solution base.
  • the present invention uses the value calculated from the amount of titrant consumed when the first potential jump point appears as the total acid content result in the dimethyl oxalate sample, and, after repeated verification, it is consistent with the amount of titrant that appears. Compared with the calculated value of the amount of titrant consumed at the second potential jump point as the result of the total acid content in the dimethyl oxalate sample, it has better parallelism and smaller errors, and further improves the oxalic acid content. Accuracy of determination of oxalic acid content in dimethyl ester samples.
  • the titrant is a KOH methanol solution, that is, a solution of KOH dispersed in methanol.
  • the concentration of KOH in the KOH methanol solution is 0.1 mol/L.
  • step (2) when the oxalic acid in the dimethyl oxalate sample is directly measured by titration, the result will be interfered by nitric acid. Therefore, the present invention adopts the method of first measuring the total acid content and then subtracting the nitric acid content to obtain accurate oxalic acid content results, so it is necessary to measure the nitric acid content.
  • the nitric acid content is determined by acid-base titration or ion chromatography, preferably the more accurate ion chromatography method.
  • 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. This method uses the different binding abilities of nitrate ions to the ion exchange column to separate the nitric acid in the dimethyl oxalate sample. The root ions are separated.
  • the separation of nitrate ions in the dimethyl oxalate sample by ion chromatography specifically includes the following steps:
  • the standard curve uses the nitrate ion concentration as the abscissa and the conductivity peak area as the ordinate.
  • the specific concentrations of the 0-100 mg/L nitrate ion standard solution are 6.25 mg/L, 12.50 mg/L, 25.00 mg/L, 50.00 mg/L, and 100.00 mg/L respectively.
  • the ion exchange column is an anion exchange column.
  • the anion exchange column is a Thermo Fisher AS23 anion column, which mainly includes a guard column and an anion separation column.
  • the anion analysis column uses an anion exchange resin as a stationary phase and an eluent as a mobile phase.
  • the method for digesting the dimethyl oxalate sample can adopt conventional methods in the art.
  • the purpose is to convert various elements in the dimethyl oxalate sample into free states so as to perform anion exchange and thereby measure nitric acid.
  • the digestion treatment involves mixing the dimethyl oxalate sample with hydrogen peroxide, heating and drying.
  • the dimethyl oxalate sample and hydrogen peroxide are mixed at a temperature of 70°C, maintained for 1.5 hours, and then heated to 80°C for drying.
  • the material obtained is added back to water to make up the volume to obtain a solution, which is used to pass through the anion exchange column.
  • step (iii) the content of nitric acid can be calculated according to formula III:
  • ⁇ HNO3 represents the nitric acid content in the dimethyl oxalate sample, in %
  • CHNO3 represents the nitrate ion concentration corresponding to the conductivity peak area of nitrate ions detected by ion chromatography on the standard curve, in mg/L;
  • m represents the mass of dimethyl oxalate sample, in g
  • V is the volume of the dimethyl oxalate sample in step (ii) after digestion treatment and constant volume, in mL.
  • the calculation method for obtaining the oxalic acid content in the dimethyl oxalate sample based on the total acid content minus the nitric acid content is: calculated based on the volume of the titrant consumed during the determination of the total acid content. Calculate the molar amount of nitric acid based on the molar amount of titrant consumed, and calculate the molar amount of nitric acid based on the content of nitrate ions.
  • ⁇ (%) is the mass content percentage of oxalic acid
  • C KOH represents the concentration of KOH methanol standard solution, in mol/L
  • V KOH is the volume of the KOH methanol standard solution consumed when the first potential jump point occurs, in mL;
  • m represents the mass of the dimethyl oxalate sample, in g
  • ⁇ HNO3 represents the nitric acid content in the dimethyl oxalate sample, in %
  • 63 is the relative molecular mass of nitric acid
  • 90.03 is the relative molecular mass of oxalic acid.
  • the steps of the method for determining the oxalic acid content in the dimethyl oxalate sample are as follows:
  • ⁇ (%) represents the mass content percentage of oxalic acid
  • C KOH represents the concentration of KOH methanol standard solution, in mol/L
  • V KOH represents the volume of KOH methanol standard solution consumed when the first potential jump point occurs, in mL;
  • m represents the mass of the dimethyl oxalate sample, in g
  • ⁇ HNO3 represents the nitric acid content in the dimethyl oxalate sample, in %
  • 63 is the relative molecular mass of nitric acid
  • 90.03 is the relative molecular mass of oxalic acid.
  • the units of the parameters shown in Formula I, Formula II and Formula III can be converted into international units or secondary units representing the same parameters.
  • mol/L can be converted into mmol/L, mL. It can be converted to L, etc., and Formula I, Formula II and Formula III can be adjusted accordingly according to the specific conversion situation.
  • This example provides an ion chromatographic analysis method for the determination of trace amounts of nitric acid in dimethyl oxalate, an intermediate product of the polyglycolic acid plant of Yulin Company.
  • Nitrate ion standard solution standard solutions with concentrations of 6.25mg/L, 12.50mg/L, 25.00mg/L, 50.00mg/L, and 100.00mg/L respectively;
  • the ion chromatograph consists of an eluent storage bottle, a delivery pump, a sampling valve, a protection and separation column, an anion suppressor, a conductivity detector and a chromatography workstation.
  • guard column and separation column are both resin-filled columns.
  • the models are Thermo Fisher AG23 anion column (4 ⁇ 50mm) and Thermo Fisher AS23 anion column (4 ⁇ 250mm) respectively.
  • nitrate ion standard solutions with contents of 6.25 mg/L, 12.50 mg/L, 25.00 mg/L, 50.00 mg/L, and 100.00 mg/L in five 50 mL volumetric flasks.
  • the conductivity peak area of the corresponding nitrate ion of each nitrate ion standard solution is measured through the above ion chromatography, and a working curve is drawn with the nitrate ion concentration as the abscissa and the conductivity peak area as the ordinate (as shown in Figure 4).
  • Digestion treatment to eliminate matrix interference Take the dimethyl oxalate sample, weigh 2g and place it in a 50mL beaker, add 4mL of water, heat to a homogeneous phase at 70°C, incubate for 0.5h, add 3mL of hydrogen peroxide, and keep at 80°C for 1.5h. Steam to dryness.
  • ⁇ HNO3 The content of nitric acid in the sample, ⁇ HNO3 , is measured in mass percentage and calculated according to Formula III.
  • the dimethyl oxalate sample contains nitric acid impurity.
  • the measured nitric acid content data can be used in the subsequent calculation of oxalic acid content.
  • This embodiment provides calculation of the total acid content in the dimethyl oxalate sample using the first potential jump and the second potential jump respectively.
  • the results of the first potential jump pH, the second potential jump pH, the volume of titrant consumed at the corresponding time points, and the calculated total acid content are shown in Table 2.
  • the calculation formula for the total acid content is as shown in Formula I.
  • V 1 in this embodiment is V KOH in Formula I;
  • V 2 in this embodiment is regarded as V KOH in Formula I.
  • This embodiment provides the total acid content in the dimethyl oxalate sample calculated based on the amount of titrant consumed (V 1 ) when the first potential jump point occurs, and the final oxalic acid content is added as a standard. Recycling experiment.
  • Spiked recovery rate (measured value of spiked sample - measured value of sample) ⁇ amount of spiked ⁇ 100
  • This example provides the determination of the oxalic acid content in dimethyl oxalate, an intermediate product of the polyglycolic acid plant of Yulin Company.
  • Example 2 Weigh the dimethyl oxalate sample used in Example 1 into a clean and dry beaker, add 50 mL of organic solvent for complete dissolution to obtain a dimethyl oxalate solution. Use a potentiometric titrator to titrate the dimethyl oxalate solution with 0.1 mol/L KOH methanol solution (standard solution) as the titrant. When the first potential jump point appears, record the volume of KOH solution consumed at this time. V KOH .

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Abstract

一种草酸二甲酯中草酸含量的测定方法，包括如下步骤：测定草酸二甲酯样品中的总酸含量；测定草酸二甲酯样品中的硝酸含量；以总酸含量减去硝酸含量，所得的值为草酸二甲酯样品中的草酸含量。方法克服了两种混酸分别测量的难题，可以避免滴定时同为酸类物质的硝酸杂质参与反应而导致的草酸测定结果偏高的问题，可以消除硝酸和溶液基底的干扰。

Description

草酸二甲酯中草酸含量的测定方法 技术领域
本发明涉及检测技术领域，具体涉及在草酸二甲酯中检测草酸杂质的含量的方法。
背景技术
聚乙醇酸(PGA)装置采用非石油工艺技术路线，产品具有优异的全降解特性、优异的力学性能和高阻隔性能，主要应用于医疗缝合线、医用支架、卫生用品、食品包装、餐具等。其中，草酸二甲酯是生产聚乙醇酸的重要中间原料，所以草酸二甲酯的成分组成对聚乙醇酸工艺和聚乙醇酸产品都有极其重要的影响。
然而，由于草酸二甲酯遇水会发生水解反应生成相应的草酸，且草酸对生产具有以下危害：1)对设备管道有腐蚀作用，影响设备使用寿命；2)腐蚀设备后生成盐类物质，盐在设备中会结垢导致设备和管道堵塞；3)草酸腐蚀设备使铁以离子形式进入PGA产品，影响PGA产品的质量。因此，严格控制草酸二甲酯中草酸含量成为重要的工艺指标。
现有的草酸二甲酯中草酸含量的分析方法主要是采用传统的酸碱滴定法进行，存在误差大、重复性差、结果不准确等问题。
发明内容
本发明目的在于提供一种草酸二甲酯中草酸含量的测定方法，旨在排除草酸二甲酯中其它杂质对草酸测定结果的影响，以获得更加准确的草酸含量。
为了实现上述发明目的，本发明提供了一种草酸二甲酯中草酸含量的测定方法，其包括如下步骤：
测定草酸二甲酯样品中的总酸含量；
测定所述草酸二甲酯样品中的硝酸含量；
以所述总酸含量减去所述硝酸含量，所得的值为所述草酸二甲酯样品中的草酸含量。
在一些实施方案中，所述总酸含量的测定方法是对所述草酸二甲酯样品进行滴定，以出现第一电位突跃点时所消耗的滴定剂的量计算得到总酸含量。
在一些实施方案中，所述总酸含量的测定方法是对所述草酸二甲酯样品进行滴定，以出现第二电位突跃点时所消耗的滴定剂的量计算得到总酸含量。
在一些实施方案中，所述总酸含量是通过电位滴定仪测定的。
在一些实施方案中，所述滴定剂为KOH甲醇溶液。
在一些实施方案中，所述硝酸含量是通过酸碱滴定法测定的。
在一些实施方案中，所述硝酸含量是通过离子色谱法测定的。
在一些实施方案中，采用离子色谱法测定硝酸含量的方法包括：
建立0-100mg/L的硝酸根离子标准溶液与对应电导率峰面积的标准曲线；
将经过消解处理的草酸二甲酯样品通过离子交换柱，分离所述草酸二甲酯样品中的硝酸根离子，检测所述硝酸根离子的电导率峰面积；
根据所得的电导率峰面积在所述标准曲线上对应的硝酸根离子浓度，计算得出所述草酸二甲酯样品中硝酸的含量。
在一些实施方案中，所述硝酸根离子标准溶液的浓度分别为6.25mg/L、12.50mg/L、25.00mg/L、50.00mg/L、100.00mg/L。
在一些实施方案中，所述离子交换柱为阴离子交换柱。
在一些实施方案中，草酸二甲酯中草酸含量的测定方法包括如下步骤：
将所述草酸二甲酯样品分散在溶剂中得到草酸二甲酯溶液；
以KOH甲醇标准溶液对所述草酸二甲酯溶液进行滴定，当第一电位突跃点出现时，记录所消耗的滴定剂的体积V KOH；
建立0-100mg/L的硝酸根离子标准溶液与对应电导率峰面积的标准曲线；
将经过消解处理的所述草酸二甲酯样品通过离子交换柱，分离所述草酸二甲酯样品中的硝酸根离子，检测所得硝酸根离子的电导率峰面积；
记录所得电导率峰面积在所述标准曲线上对应的硝酸根离子浓度C HNO3，计算所述草酸二甲酯样品中的硝酸含量ω HNO3；
根据式I计算得到所述草酸二甲酯样品中的草酸含量；
Figure PCTCN2022089413-appb-000001
式I中，
ω(％)表示草酸质量含量百分数；
C KOH表示KOH甲醇标准溶液的浓度，单位为mol/L；
V KOH表示当第一电位突跃点出现时所消耗的KOH甲醇标准溶液的体积V KOH，单位为mL；
m为所述草酸二甲酯样品的质量，单位为g；
ω HNO3表示所述草酸二甲酯样品中的硝酸含量，单位为％；
63为硝酸的相对分子质量；
90.03为草酸的相对分子质量。
本发明与现有技术相比，具有以下优点：
本发明提供的草酸二甲酯中草酸含量的测定方法，克服了两种混酸分别测量的难题，可以避免滴定时同为酸类物质的硝酸杂质参与反应而导致的草酸测定结果偏高的问题。进一步地，本发明提供的测定方法还解决了在测定草酸二甲酯中的草酸时存在的第二电位突跃不明显而导致的草酸含量测定数据差异大的问题。采用本发明提供的测定方法测定草酸二甲酯中的草酸含量，可以消除硝酸和溶液基底的干扰，且测试结果平行性更好、准确度更高。
附图说明
图1为采用电位滴定仪测定草酸标准溶液(1mol/L)的E&dE/dV图，其中，第一电位突跃点的pH为4.17，第二电位突跃点的pH为8.14；
图2为采用电位滴定仪测定草酸二甲酯样品中草酸的E&dE/dV图，其中，第一电位突跃点的pH为4.74，第二电位突跃点的pH为7.54；
图3为采用电位滴定仪测定硝酸标准溶液的E&dE/dV图，其中，电位突跃点的pH为5.08；
图4为不同浓度的硝酸根与对应电导率的标准曲线；
图1-3中，E&dE/dV图是指：以指示电极的电位对滴定体积作图和以指示电极的电位对滴定体积的一次微商对滴定体积作图。其中，E为指示电极的电位；V为滴定体积；dE/dV为指示电极的电位对滴定体积的一次微商)。
具体实施方式
草酸是一种二元弱酸，其电离常数Ka1为5.9×10 -2、Ka2为6.4×10 -5。通常在测定样品中的草酸时，采用碱性标准溶液进行滴定，以经典的酸碱滴定法获得草酸含量。其中，当pH出现第一个突跃时为草酸解离出第一个氢离子，当pH出现第二个突跃时为草酸解离出第二个氢离子。以滴定剂为KOH甲醇标准溶液为例，样品中草酸的质量百分数含量按照式II计算：
Figure PCTCN2022089413-appb-000002
式II中：
V KOH表示滴定样品所消耗KOH甲醇标准溶液的体积，单位为mL；
C KOH表示KOH甲醇标准溶液的浓度，单位为mol/L；
ω表示草酸质量含量百分数；
m表示样品质量，单位为g；
90.03为草酸的相对分子质量。
然而，本发明的发明人在研究过程中发现，草酸标准溶液的第一电位突跃点的pH(即第一pH突跃)为4.17，第二电位突跃点的pH(即第二pH突跃)为8.14(如图1所示)；而当测定草酸二甲酯样品中的草酸时，第一电位突跃点的pH为4.74，第二电位突跃点的pH为7.54(如图2所示)。可见，在测定草酸二甲酯样品中的草酸时，第一电位突跃点和第二电位突跃点都发生了偏移。对此，本发明发明人继续研究后发现，采用电位滴定仪测定硝酸标准溶液的E&dE/dV时，电位突跃点的pH为5.08(如图3所示)。因此认为，由于草酸二甲酯样品中硝酸的影响，会导致草酸的第一电位突跃点和第二电位突跃点都发生向硝酸的电位突跃点(5.08)偏移的情况。经研究确认，草酸二甲酯产品中的主要杂质有甲醇、甲酸甲酯、水、硝酸、草酸等；其中，硝酸和草酸都属于酸类物质，因此在滴定法检测草酸含量时，硝酸也会参与反应，进而导致草酸含量的测定结果偏高。为了解决该问题，本发明提供了一种草酸二甲酯中草酸含量的测定方法，其包括如下步骤：
(1)测定草酸二甲酯样品中的总酸含量；
(2)测定所述草酸二甲酯样品中的硝酸含量；
(3)以所述总酸含量减去所述硝酸含量，所得的值为所述草酸二甲酯样品中的草酸含量。
具体地，步骤(1)中，所述草酸二甲酯样品包括草酸二甲酯成品以及草酸二甲酯的过程样品。其中，所述草酸二甲酯成品是指草酸二甲酯含量为99.5％以上的草酸二甲酯产品。所述草酸二甲酯的过程样品是指在生产或开发草酸二甲酯产品的中间过程中获得的含草酸二甲酯的样品。
所述总酸含量，指的是所述草酸二甲酯样品中的草酸和硝酸的含量之和。
在一些实施方案中，所述总酸含量是通过电位滴定仪测定的。优选地，所述电位滴定仪为高精度的电位滴定仪，如最小加液量0.005mL、加液误差≤0.15％、测量电位范围±1000mV的电位滴定仪。
采用滴定法对草酸进行滴定时会产生两个pH突跃。相应地，当使用电位滴定法对草酸进行滴定时(例如使用电位滴定仪)，第一个pH突跃为第一电位突跃点，第二个pH突跃为第二电位突跃点。
在一些实施方案中，所述总酸含量的测定方法是对所述草酸二甲酯样品进行滴定，以出现第二电位突跃点时所消耗的滴定剂的量计算得到总酸含量。
在一些实施方案中，所述总酸含量的测定方法是对所述草酸二甲酯样品进行滴定，以出现第一电位突跃点时所消耗的滴定剂的量计算得到总酸含量。同时，本发明的发明人在研究时进一步发现，草酸标准溶液的第一电位突跃点和第二电位突跃点均明显(如图1所示)；而当测定草酸二甲酯样品中的草酸时，第二电位突跃点变得不明显(如图2所示)，这会造成在测定草酸二甲酯样品中的草酸时出现读数困难、测定数据差异大的问题。经研究，出现该问题的原因是溶液基底的干扰。为了解决该问题，本发明以出现第一电位突跃点时所消耗的滴定剂的量计算得到的数值作为草酸二甲酯样品中的总酸含量结果，并且，经反复验证，其与以出现第二电位突跃点时所消耗的滴定剂的量计算得到的数值作为草酸二甲酯样品中的总酸含量的结果相比，具有更好的平行性和更小的误差，进一步提高的草酸二甲酯样品草酸含量测定的准确度。
在一些实施方案中，所述滴定剂为KOH甲醇溶液，即将KOH分散在甲醇中形成的溶液。在一些具体实施例中，为了便于滴定和计算，KOH甲醇溶液中KOH的浓度为0.1mol/L。
步骤(2)中，由于通过滴定法直接测定草酸二甲酯样品中的草酸时，其结果会受到硝酸的干扰。因此本发明采用先测总酸含量，再减去硝酸含量的方式来获得准确的草酸含量结果，故需要测定硝酸的含量。在一些实施方案中，所述硝酸含量是通过酸碱滴定法或离子色谱法测定的，优选更加准确的离子色 谱法。离子色谱法分离草酸二甲酯样品中的硝酸根离子可通过本领域已知的方法和材料进行，该方法是利用硝酸根离子与离子交换柱结合能力的不同对草酸二甲酯样品中的硝酸根离子进行分离。
在一些实施方案中，离子色谱法分离草酸二甲酯样品中的硝酸根离子具体包括如下步骤：
(i)建立0-100mg/L的硝酸根离子标准溶液与对应电导率峰面积的标准曲线；
(ii)将经过消解处理的草酸二甲酯样品通过离子交换柱，分离所述草酸二甲酯样品中的硝酸根离子，检测所述硝酸根离子的电导率峰面积；
(iii)根据所得的电导率峰面积在所述标准曲线上对应的硝酸根离子浓度，计算得出所述草酸二甲酯样品中硝酸的含量。
在一些实施方案中，步骤(i)中，所述标准曲线以硝酸根离子浓度作为横坐标，以电导率峰面积作为纵坐标。
在一些实施方案中，所述0-100mg/L的硝酸根离子标准溶液的具体浓度分别为6.25mg/L、12.50mg/L、25.00mg/L、50.00mg/L、100.00mg/L。
在一些实施方案中，所述离子交换柱为阴离子交换柱。在一具体实施例中，所述阴离子交换柱为赛默飞AS23型阴离子柱，主要包括保护柱和阴离子分离柱，阴离子分析柱采用阴离子交换树脂作为固定相，以淋洗液作为流动相。
步骤(ii)中，对草酸二甲酯样品进行消解处理的方法可以采用本领域的常规方法，其目的是将草酸二甲酯样品中的各种元素转化为游离态，以便进行阴离子交换从而测定硝酸根离子的含量。在一些实施方案中，所述消解处理是将草酸二甲酯样品与双氧水混合，经加热、干燥处理。优选地，所述草酸二甲酯样品与双氧水混合温度为70℃，保温1.5h，然后加热至80℃干燥处理。消解处理后所得物质重新加入水定容获得溶液，用于通过阴离子交换柱。
步骤(iii)中，硝酸的含量可以按照式III进行计算：
Figure PCTCN2022089413-appb-000003
式III中：
ω HNO3表示草酸二甲酯样品中的硝酸含量，单位为％；
C HNO3表示离子色谱法检测所得硝酸根离子的电导率峰面积在所述标准曲线上对应的硝酸根离子浓度，单位为mg/L；
m表示草酸二甲酯样品的质量，单位为g；
V为步骤(ii)草酸二甲酯样品经过消解处理、定容的体积，单位为mL。
步骤(3)中，在一些实施方案中，根据总酸含量减去硝酸含量得到草酸二甲酯样品中的草酸含量的计算方法是：根据总酸含量测定时所消耗的滴定剂的体积计算出消耗滴定剂的摩尔量，以及根据硝酸根离子的含量计算硝酸的摩尔量，用消耗滴定剂的摩尔量减去硝酸的摩尔量得到草酸的摩尔量，再乘以草酸的相对分子质量，所得的积再除以草酸二甲酯样品的质量，最后乘以100，即为草酸二甲酯样品中的草酸质量含量百分数。以滴定剂为KOH甲醇标准溶液为例，草酸含量的计算如式I所示。
Figure PCTCN2022089413-appb-000004
式I中：
ω(％)为草酸质量含量百分数；
C KOH表示KOH甲醇标准溶液的浓度，单位为mol/L；
V KOH为当第一电位突跃点出现时所消耗的KOH甲醇标准溶液的体积，单位为mL；
m表示所述草酸二甲酯样品的质量，单位为g；
ω HNO3表示所述草酸二甲酯样品中的硝酸含量，单位为％；
63为硝酸的相对分子质量；
90.03为草酸的相对分子质量。
作为本发明的其中一个优选的具体实施方案，草酸二甲酯样品中草酸含量的测定方法的步骤如下：
S1、将所述草酸二甲酯样品分散在溶剂中得到草酸二甲酯溶液；
S2、以KOH甲醇标准溶液对所述草酸二甲酯溶液进行滴定，当第一电位突跃点出现时，记录所消耗的滴定剂的体积V KOH；
S3、建立0-100mg/L的硝酸根离子标准溶液与对应电导率峰面积的标准曲线；
S4、将经过消解处理的所述草酸二甲酯样品通过离子交换柱，分离所述草酸二甲酯样品中的硝酸根离子，检测所得硝酸根离子的电导率峰面积；
S5、记录所得电导率峰面积在所述标准曲线上对应的硝酸根离子浓度C HNO3，计算所述草酸二甲酯样品中的硝酸含量ω HNO3；
S6、根据式I计算得到所述草酸二甲酯样品中的草酸含量；
Figure PCTCN2022089413-appb-000005
式I中：
ω(％)表示草酸质量含量百分数；
C KOH表示KOH甲醇标准溶液的浓度，单位为mol/L；
V KOH表示当第一电位突跃点出现时所消耗的KOH甲醇标准溶液的体积，单位为mL；
m表示所述草酸二甲酯样品的质量，单位为g；
ω HNO3表示所述草酸二甲酯样品中的硝酸含量，单位为％；
63为硝酸的相对分子质量；
90.03为草酸的相对分子质量。
在该优选的具体实施方案中，不仅消除了在测定中硝酸对草酸的干扰，还消除了溶液基底的干扰，使草酸二甲酯样品中草酸含量的测定结果平行性更好，准确度进一步提高。同时，可以理解的是，本发明中，式I、式II和式III所示参数的单位可以换算为表示相同参数的国际单位或次级单位，例如mol/L可以换算为mmol/L，mL可以换算为L等，根据具体换算的情况对式I、式II和式III进行相应调整即可。
为使本发明上述实施细节和操作能清楚地被本领域技术人员理解，以及本发明实施例草酸二甲酯中草酸含量的测定方法的进步性能显著的体现，以下通过多个实施例来举例说明上述技术方案。
下述实施例中所使用的实验方法如无特殊说明，均为常规方法。
下述实施例中所用的材料、试剂等，如无特殊说明，均可从商业途径得到。
实施例1
本实施例提供了离子色谱分析法测定榆林公司聚乙醇酸装置中间产品草酸二甲酯中的微量硝酸。
1、试剂和材料
(1)双氧水：分析纯；
(2)水：超纯水；
(3)硝酸根离子标准溶液：浓度分别为6.25mg/L、12.50mg/L、25.00mg/L、50.00mg/L、100.00mg/L的标准溶液；
(4)淋洗液：配制4.5mmol Na 2CO 3+0.8mmol NaHCO 3的淋洗液2L。
2、仪器和设备(市购)
(1)离子色谱仪由淋洗液储液瓶、输送泵、进样阀、保护和分离柱、阴离子抑制器、电导率检测器和色谱工作站等组成。
Figure PCTCN2022089413-appb-000006
(2)分析天平：感量0.1mg。
(3)保护柱和分离柱：保护柱和分离柱都是树脂填充柱，型号分别为赛默飞AG23型阴离子柱(4×50mm)以及赛默飞AS23型阴离子柱(4×250mm)。
3、分析步骤
(1)标准曲线绘制：
在5个50mL的容量瓶中分别配制含量为6.25mg/L、12.50mg/L、25.00mg/L、50.00mg/L、100.00mg/L的硝酸根离子标准溶液。通过上述离子色谱测定各硝酸根离子标准溶液的对应的硝酸根离子的电导率峰面积，以硝酸根离子浓度为横坐标、电导率峰面积为纵坐标绘制工作曲线(如图4所示)。
(2)样品测定
消解处理以消除基体干扰：取草酸二甲酯样品，称取2g置于50mL烧杯中，加入4mL水，在70℃加热至均一相，保温0.5h后，加入3mL双氧水，保温1.5h，80℃蒸干。
用超纯水洗涤烧杯中蒸干的物质，将其转移到容量瓶中，用水进行稀释至刻度(50mL)，读取结果C HNO3。
样品中硝酸的含量ω HNO3以质量百分数计，按式III计算。
测定结果如表1所示。
表1离子色谱分析法测定草酸二甲酯(DMO)样品中的硝酸含量
样品序号 DMO(g) NO3 -(mg/L) HNO 3(％)
1 2.0034 3.1318 0.0078
2 2.0104 3.2033 0.0080
3 2.0087 3.1969 0.0080
4 2.0004 3.1056 0.0078
平均值 2.0057 3.1594 0.0079
标准偏差(％) - - 0.0001
通过表1可以看出，草酸二甲酯样品中含有硝酸杂质。同时，测定得到的硝酸含量的数据可以在后续草酸含量计算时用到。
实施例2
本实施例提供了分别以第一电位突跃和第二电位突跃计算草酸二甲酯样品中总酸含量。
(11)称取草酸二甲酯样品于洁净干燥的烧杯中，加入有机溶剂50mL进行完全溶解得到草酸二甲酯溶液。采用电位滴定仪，以0.1mol/L的KOH甲醇溶液(标准溶液)作为滴定剂对草酸二甲酯溶液进行滴定，当第一电位突跃点出现时，记录此时所消耗的KOH甲醇溶液的体积V 1。
(12)继续滴定，当第二电位突跃点出现时，记录此时所消耗的KOH甲醇溶液的体积V 2。
第一电位突跃pH、第二电位突跃pH、相应时间点所消耗的滴定剂的体积，以及计算得到的总酸含量结果如表2所示。其中，总酸含量的计算公式如式I所示，当计算以第二电位突跃点出现作为总酸含量的计算基准时，本实施例中的V 1即为式I中的V KOH；当计算以第二电位突跃点出现作为总酸含量的计算基准时，将本实施例中的V 2作为式I中的V KOH。
表2草酸二甲酯样品中的总酸含量结果
Figure PCTCN2022089413-appb-000007
通过表2可以看出，与以第二电位突跃点出现时记录所消耗的滴定剂的量计算得到的草酸二甲酯样品中的总酸含量的结果相比，以第一电位突跃点出现时记录所消耗的滴定剂的量计算得到的草酸二甲酯样品中的总酸含量的标准偏差更小，因此以第一电位突跃点出现时记录所消耗的滴定剂的量计算得到的草酸二甲酯样品中的总酸含量的结果更加准确。
实施例3
本实施例提供了以第一电位突跃点出现时记录所消耗的滴定剂的量(V 1)计算得到的草酸二甲酯样品中的总酸含量为基准，最终得到的草酸含量的加标回收实验。
1、测定过程
取相同重量的已知草酸含量的草酸二甲酯样品两份，向其中分别加入草酸104μg，按照实施例1和实施例2(以第一电位突跃点出现时记录所消耗的滴定剂的量计算草酸二甲酯样品中的总酸含量)的步骤，在相同条件下分别测定两份样品的草酸含量。
2、加标回收率的计算
加标回收率＝(加标试样测定值-试样测定值值)÷加标量×100
加标回收实验结果如表3所示。
表3草酸二甲酯样品中草酸含量以V 1计算加标回收实验结果
Figure PCTCN2022089413-appb-000008
通过表3可以看出，使用本发明方法检测草酸二甲酯样品中草酸含量，其结果的准确性和可靠性可以满足分析专业要求(95％-105％)。
实施例4
本实施例提供了榆林公司聚乙醇酸装置中间产品草酸二甲酯中的草酸含量的测定。
称取实施例1中所用的草酸二甲酯样品于洁净干燥的烧杯中，加入有机溶剂50mL进行完全溶解得到草酸二甲酯溶液。采用电位滴定仪，以0.1mol/L的KOH甲醇溶液(标准溶液)作为滴定剂对草酸二甲酯溶液进行滴定，当第一电位突跃点出现时，记录此时所消耗的KOH溶液的体积V KOH。
结合实施例1测定的硝酸含量的结果的平均值0.0079％，根据式I计算样品中的草酸含量，结果如表4所示。
表4草酸二甲酯样品中草酸含量的测定结果
DMO(g) V KOH(mL) C KOH(mol/L) 硝酸含量(％) 草酸含量(％)
5.0292 0.0594 0.1256 0.0079 0.0021
以上所述实施例仅表达了本发明的几种实施方式，其描述较为具体和详细，但并不能因此而理解为对本发明专利范围的限制。应当指出的是，对于本领域的普通技术人员来说，在不脱离本发明构思的前提下，还可以做出若干变形和改进，这些都属于本发明的保护范围。因此，本发明专利的保护范围应以所附权利要求为准。

Claims (14)

  1. 一种草酸二甲酯中草酸含量的测定方法，其特征在于，包括如下步骤：
    测定草酸二甲酯样品中的总酸含量；
    测定所述草酸二甲酯样品中的硝酸含量；
    以所述总酸含量减去所述硝酸含量，所得的值为所述草酸二甲酯样品中的草酸含量。
  2. 根据权利要求1所述的测定方法，其特征在于，所述总酸含量的测定方法是对所述草酸二甲酯样品进行滴定，以出现第一电位突跃点时所消耗的滴定剂的量计算得到总酸含量。
  3. 根据权利要求1所述的测定方法，其特征在于，所述总酸含量的测定方法是对所述草酸二甲酯样品进行滴定，以出现第二电位突跃点时所消耗的滴定剂的量计算得到总酸含量。
  4. 根据权利要求1-3任一项所述的测定方法，其特征在于，所述总酸含量是通过电位滴定仪测定的。
  5. 根据权利要求1-3任一项所述的测定方法，其特征在于，所述滴定剂为KOH甲醇溶液。
  6. 根据权利要求1-5任一项所述的测定方法，其特征在于，所述硝酸含量是通过酸碱滴定法或离子色谱法测定的。
  7. 根据权利要求6所述的测定方法，其特征在于，采用离子色谱法测定硝酸含量的方法包括：
    建立0-100mg/L的硝酸根离子标准溶液与对应电导率峰面积的标准曲线；
    将经过消解处理的草酸二甲酯样品通过离子交换柱，分离所述草酸二甲酯样品中的硝酸根离子，检测所述硝酸根离子的电导率峰面积；
    根据所得的电导率峰面积在所述标准曲线上对应的硝酸根离子浓度，计算得出所述草酸二甲酯样品中硝酸的含量。
  8. 根据权利要求7所述的测定方法，其特征在于，所述硝酸根离子标准溶液的浓度分别为6.25mg/L、12.50mg/L、25.00mg/L、50.00mg/L、100.00mg/L。
  9. 根据权利要求7所述的测定方法，其特征在于，所述消解处理是将所述草酸二甲酯样品中的各元素转化为游离态。
  10. 根据权利要求9所述的测定方法，其特征在于，所述消解处理包括：将所述草酸二甲酯样品与双氧水混合后，进行加热、干燥处理。
  11. 根据权利要求7所述的测定方法，其特征在于，所述离子交换柱为阴离子交换柱。
  12. 根据权利要求11所述的测定方法，其特征在于，所述阴离子交换柱采用阴离子交换树脂作为固定相，以淋洗液作为流动相；所述淋洗液为碳酸钠与碳酸氢钠的混合溶液。
  13. 根据权利要求7-12任一项所述的测定方法，其特征在于，所述草酸二甲酯样品中硝酸的含量按照式III进行计算：
    Figure PCTCN2022089413-appb-100001
    式III中：
    ω HNO3表示所述草酸二甲酯样品中的硝酸含量，单位为％；
    C HNO3表示离子色谱法检测所得硝酸根离子的电导率峰面积在所述标准曲线上对应的硝酸根离子浓度，单位为mg/L；
    m表示所述草酸二甲酯样品的质量，单位为g；
    V为所述草酸二甲酯样品经过消解处理、定容的体积，单位为mL。
  14. 根据权利要求1-13任一项所述的测定方法，其特征在于，包括如下步骤：
    将所述草酸二甲酯样品分散在溶剂中得到草酸二甲酯溶液；
    以KOH甲醇标准溶液对所述草酸二甲酯溶液进行滴定，当第一电位突跃点出现时，记录所消耗的滴定剂的体积V KOH；
    建立0-100mg/L的硝酸根离子标准溶液与对应电导率峰面积的标准曲线；
    将经过消解处理的所述草酸二甲酯样品通过离子交换柱，分离所述草酸二甲酯样品中的硝酸根离子，检测所得硝酸根离子的电导率峰面积；
    记录所得电导率峰面积在所述标准曲线上对应的硝酸根离子浓度C HNO3，计算所述草酸二甲酯样品中的硝酸含量ω HNO3；
    根据式I计算得到所述草酸二甲酯样品中的草酸含量；
    Figure PCTCN2022089413-appb-100002
    式I中，
    ω(％)表示草酸质量含量百分数；
    C KOH表示KOH甲醇标准溶液的浓度，单位为mol/L；
    V KOH为表示当第一电位突跃点出现时所消耗的KOH甲醇标准溶液的体积，单位为mL；
    m表示所述草酸二甲酯样品的质量，单位为g；
    ω HNO3表示所述草酸二甲酯样品中的硝酸含量，单位为％；
    63为硝酸的相对分子质量；
    90.03为草酸的相对分子质量。
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