A method for measuring water content in glycolide

Abstract

The present invention discloses a method for determining the moisture content in glycolide. The method comprises the following steps: utilizing the Karl Fischer volumetric method to determine the moisture content of a glycolide sample; when the moisture content result is ≤0.1%, utilizing a cassette furnace-Karl Fischer coulometer coupled device to determine the moisture content of another glycolide sample; and setting different cassette furnace temperatures during the determination process to obtain the external moisture content and the total moisture content of the glycolide. Ultimately, the external moisture content and the internal moisture content of the glycolide are separately determined. The method has the advantages of high precision and low analysis cost, and provides accurate process data for the production of polyglycolic acid with a high degree of polymerization.

Description

Technical Field

[0001] The invention relates to the field of moisture determination of degradable materials, and in particular to a method for determining the moisture content in glycolide. Background Art

[0002] Moisture testing is a key indicator for testing the basic properties of glycolide products. In the process of polymerizing glycolide to form polyglycolic acid (PGA), moisture in the glycolide significantly affects the degree of polymerization (DP) of the resulting product, which in turn determines key parameters such as the product's mechanical and thermal properties. Therefore, the presence of moisture in the glycolide-to-PGA process can reduce the DP, molecular weight, and melt index of the polymer, leading to deterioration in the mechanical and thermal properties of the polymer product and failure to meet expected process specifications. Therefore, accurate data on glycolide moisture content is essential.

[0003] Currently, the Karl Fischer method is commonly used to determine the moisture content of conventional petroleum products. This method is further divided into Karl Fischer volumetric analysis (referred to as volumetric analysis) and Karl Fischer coulometric analysis (referred to as coulometric analysis). Both methods determine moisture content based on the quantitative reaction of iodine and sulfur dioxide with water in a pyridine or methanol solution. The iodine in the former is added dropwise via a prepared Karl Fischer reagent, while the latter is generated by electrolysis of an anolyte containing iodide ions, resulting in higher measurement sensitivity. Therefore, the volumetric method is often used to determine samples containing constant moisture (>0.1%), while the coulometric method is commonly used to determine samples containing trace moisture (0.0001%-0.1%, ≤0.1%) and is particularly suitable for determining moisture in chemically inert substances.

[0004] Currently, the industry generally considers glycolide to be a conventional petroleum product, and its moisture content is determined using the Karl Fischer method. In the process of glycolide polymerization to form polyglycolic acid, the internal and external moisture content of glycolide affect the polymerization reaction differently, thereby affecting the properties of the polymer product. Specifically, the internal moisture (water of crystallization) of glycolide is incorporated into the polymer product in the form of side chains during the polymerization reaction. Furthermore, the external moisture of glycolide, in addition to being an impurity, can also hydrolyze with glycolide under certain conditions to produce glycolic acid impurities, which are corrosive to pipelines and affect the degree of polymerization of the product. Therefore, it is necessary to accurately determine both the internal and external moisture content of glycolide simultaneously, which is not possible using conventional volumetric methods or Coulomb's law.

[0005] Furthermore, the coulometric method presents the following difficulties in determining the water content of glycolide: The low solubility of glycolide in the coulometric anolyte results in poor repeatability, and each anolyte replacement incurs higher analytical costs. Furthermore, when injecting glycolide samples into the titration cell using a solid weighing boat, airborne moisture is introduced, and glycolide reacts with the anolyte, significantly interfering with the determination of trace amounts of water. Furthermore, the volumetric method presents the following difficulties: The poor solubility of glycolide in the methanol equilibrium solution results in poor repeatability of water content measurements, and the equilibrium solution needs to be replaced after measuring even a small amount of glycolide, resulting in high analytical costs.

[0006] Therefore, it is crucial to develop a method for determining the moisture content in glycolide that can separately determine the external moisture content and the internal moisture content while reducing the error caused by the measurement environment. Summary of the Invention

[0007] The purpose of the present invention is to provide a method for determining the moisture content in glycolide, so as to accurately analyze the moisture content of glycolide samples and realize the monitoring of process indicators.

[0008] To achieve the above-mentioned purpose of the invention, the method for determining the moisture content in glycolide provided by the present invention adopts the following technical solution:

[0009] A method for measuring the moisture content in a glycolide sample, the method comprising:

[0010] The water content of the glycolide sample is measured using the Karl Fischer volumetric method. When the water content result is ≤ 0.1%, another sample of the glycolide is measured using the Karl Fischer coulometric method to obtain the water content of the glycolide.

[0011] The Karl Fischer coulometric method is carried out by using a cassette furnace-Karl Fischer coulometric instrument combination device, wherein the cassette furnace-Karl Fischer coulometric instrument combination device comprises a cassette furnace sampling unit and a Karl Fischer coulometric moisture meter;

[0012] The cassette furnace sampling unit is used to send the water escaping from the sample into the Karl Fischer coulometric moisture analyzer, which includes a cassette furnace, an air pump, a gas drying device, a sampling bottle carrying the sample, and a gas pipeline; wherein,

[0013] The cassette furnace is used to heat the sample injection bottle to release water from the glycolide sample;

[0014] The air pump is used to deliver carrier gas to the injection bottle;

[0015] The gas drying device is used to dry the carrier gas to be introduced into the injection bottle;

[0016] The gas delivery pipeline is used to deliver the gas flow from the sample injection bottle into the anolyte of the Karl Fischer coulometric titrator;

[0017] The Karl Fischer Coulometry method comprises the following steps:

[0018] (1) injecting the coulometric anolyte and the coulometric catholyte into the anode chamber and the cathode chamber of the electrolytic cell of the Karl Fischer coulometric moisture meter respectively;

[0019] (2) adjusting the iodine electrolysis system in the anode chamber and waiting for the electrolysis system in the electrolytic cell to be balanced and stable;

[0020] (3) Adjust the temperature of the furnace to 120-140°C and perform a blank measurement;

[0021] (4) Then, the sample is placed in the test chamber for measurement, and the measurement result is obtained after the measurement is completed;

[0022] (5) The obtained measurement result is recorded as the sample external moisture content W f ;

[0023] (6) Repeat steps (2)-(4), wherein the temperature of the furnace is adjusted to 180°C-200°C in step (3), and the obtained measurement result is recorded as the total moisture content of the sample W t ;

[0024] (7) According to formula W in =W t -W f Calculate the internal moisture content W of glycolide sample in .

[0025] In the cassette furnace sampling unit, the sampling bottle is used to load the sample, plays a role in sealing and preserving, and also serves as a container for heating the sample. In a preferred embodiment, the sampling bottle can be dried in an oven in advance.

[0026] In some embodiments, the carrier gas is used to purge the moisture that may be present in the injection bottle and the pipeline, and the purge time may be 15 min-30 min, wherein the purge process is before the electrolysis system is balanced and stabilized in step (1);

[0027] The carrier gas is an inert gas that does not react with glycolide and is a conventional choice in this field. It can be nitrogen. The carrier gas flow rate is 30ml / min-60ml / min to ensure that the escaped water can quickly enter the electrolytic cell and will not be carried out of the electrolytic cell because the carrier gas flow rate is too high and it does not have time to react, resulting in analysis errors.

[0028] In some embodiments, the adjustment method in step (1) is to add a certain amount of pure water to the anode chamber to consume the excess iodine added in advance due to water absorption of the anode liquid during storage. At this time, the color of the anode liquid gradually changes from dark brown to light yellow.

[0029] It will be understood by those skilled in the art that if there is no excess iodine in the used anolyte and the newly prepared anolyte, there is no need to add pure water.

[0030] The equilibrium and stability of the electrolytic system is determined by the drift value displayed by the Karl Fischer coulometer. When the drift value is ≤20 μg / min, the electrolytic system is considered to have reached equilibrium and stability. The drift measurement steps are as follows: placing an empty sample bottle in the "drift" position dedicated to the sample position, covering the sample bottle with a sealing film, and clicking the "Start" button of the Karl Fischer coulometer to start the drift measurement.

[0031] In the determination from step (3) to step (4) of the present invention, the time for each determination is not less than 5 minutes, preferably 5 minutes to 8 minutes, and more preferably 6 minutes, so as to reduce the error of the analysis process.

[0032] In step (4) of the present invention, the temperature of the cartridge furnace is set to 120° C.-140° C. to ensure the determination of the external moisture content of the glycolide sample and to prevent the internal moisture from being evaporated under the premise of rapid evaporation of the external moisture.

[0033] In step (6) of the present invention, the temperature of the cartridge furnace is set to 180°C-200°C to ensure the determination of the total moisture content of the glycolide sample, and to minimize the entry of glycolide vapor into the titration cell under the premise that both the external moisture and the internal moisture are rapidly evaporated, thereby extending the service life of the anolyte.

[0034] In the present invention, the blank determination in step (3) is used to eliminate the error caused by the possible presence of trace moisture in the empty sampling bottle or pipeline. The Karl Fischer coulometric moisture meter will automatically deduct the blank determination value when outputting the moisture content of the sample.

[0035] In some embodiments, in step (4), a certain amount of sample is weighed and placed in a clean injection bottle for sample determination, and at least two parallel samples are made. The injection bottles are both covered with a sealing film and placed in the sample position of the injector. After clicking the "Start" button on the Karl Fischer coulometer, the instrument will automatically perform moisture content determination.

[0036] During the measurement process, the water in the sample in the injection bottle escapes due to heat and enters the electrolytic cell along with the carrier gas, and the following reaction occurs:

[0037] I2+SO2+H2O→2HI+SO3,2I- -2e→I2;

[0038] It is understood in this field that the W obtained by the Karl Fischer coulometric method is t The value is the amount of water after the blank value is automatically deducted by the instrument, divided by the sample weight.

[0039] Those skilled in the art will understand that as the temperature of the furnace increases, the crystal water (internal water) in the glycolide sample escapes, and thus the measurement result output by the Karl Fischer coulometric moisture meter is the total moisture content of the glycolide sample.

[0040] In step (7) of the present invention, the internal moisture content W in The determination is based on the different temperatures required for the evaporation of the external moisture and internal moisture of glycolide, that is, the external moisture of the glycolide sample is evaporated at 120-140°C, and the total moisture of the glycolide sample is evaporated at 180-200°C. At this time, the total moisture content minus the external moisture content is the internal moisture content of the glycolide sample.

[0041] In the combined cassette furnace and Karl Fischer coulometer method of the present invention, the cassette furnace is used to heat the sample in the sample injection bottle to release water, thereby preventing the problem of glycolide not being able to completely dissolve in the anolyte and adhering to the electrode, thereby avoiding affecting the electrolytic equilibrium of the Karl Fischer coulometer, thereby greatly increasing the number of times the anolyte can be used. In addition, since the air flow from the sample injection bottle is directly fed into the anode chamber of the Karl Fischer coulometer through the gas transmission pipeline, the measurement process is a closed environment, and there is no need to use a solid injection boat, thereby avoiding the introduction of errors due to the inhalation of atmospheric moisture during the injection process.

[0042] In the present invention, the heating temperature of the cartridge furnace is lower than the boiling point of glycolide, and the nitrogen gas will only carry a small amount of glycolide vapor into the anode chamber of the electrolytic cell, so the anode liquid can be reused.

[0043] In some embodiments, the water content of the glycolide sample is measured using the Karl Fischer volumetric method. When the obtained water content result is greater than 0.1%, the result is directly used as the water content of the glycolide sample.

[0044] In some embodiments of the present invention, when the water content of a glycolide sample is determined using the Karl Fischer volumetric method, a mixture of methanol and ethyl acetate is used as the balancing solution instead of the conventional methanol in the art.

[0045] The Karl Fischer volumetric determination process is well known in the art, for example, see GB6283 "Determination of Water Content in Chemical Products - Karl Fischer Method (General Method)". Specifically, the determination can be performed using a Swiss Metrohm 915 integrated Karl Fischer volumetric moisture meter by the following steps:

[0046] (i) Turning on the power of the moisture meter, adding volumetric Karl Fischer reagent and balancing solution into the titration cup and immersing the electrode therein, and waiting for the instrument system to reach a stable state;

[0047] (ii) Weigh a certain amount of glycolide sample, click "Start," and quickly inject the sample into the titration cup containing the balancing solution and the immersed electrode under magnetic stirring;

[0048] (iii) Click "Continue", enter the sample mass and confirm. The Karl Fischer reagent will be automatically titrated into the mixture containing the glycolide sample in the titration cup.

[0049] (iv) After the titration is completed, record the volume of Karl Fischer reagent consumed as displayed by the moisture meter and calculate the water content of the glycolide sample.

[0050] In a preferred embodiment, the titration cup in step (i) is a closed space connected to a molecular sieve for absorbing moisture mixed in the test environment; the volumetric Karl Fischer reagent can be a conventional commercially available Karl Fischer reagent with a titer of C = 3 mg / ml;

[0051] The balancing liquid is a mixture of methanol and ethyl acetate, which is used to increase the solubility of glycolide in Karl Fischer reagent. In a preferred embodiment, the volume ratio of methanol to ethyl acetate in the balancing liquid is 1-3:1, preferably 2:1.

[0052] The balancing solution can be reused. For example, when the amount of glycolide sample dissolved in every 100 g of the balancing solution does not exceed 15 g, the balancing solution can be reused.

[0053] In traditional Karl Fischer volumetric analysis, the equilibrium solvent for the Karl Fischer reagent is methanol. Due to the limited solubility of glycolide in methanol, as the number of measurements increases, the added solid sample precipitates in the methanol, causing a dull endpoint indication at the electrode and preventing the release of water from the sample. In contrast, in the Karl Fischer volumetric analysis of the present invention, a mixture of methanol and ethyl acetate is used as the equilibrium solvent, increasing the solubility of glycolide and thereby increasing the number of measurements per equilibrium solution.

[0054] The stable state in step (i) means that the drift value displayed on the moisture meter screen is less than 20 μg / min, and the solution in the titration cup is light yellow;

[0055] The sample mass in step (iii) is the actual injection mass after deducting the residual sample in the injection boat; the following reaction occurs in the titration cup:

[0056] CH3OH+SO2+RH→[RNH]SO3CH3

[0057] H2O+I2+[RNH]SO3CH3+2RN→[RNH]SO4CH3+2[RNH]I

[0058] Wherein, RN is an optional organic base.

[0059] The formula for calculating the water content of the glycolide sample in step (4) is:

[0060]

[0061] Where C is the known titer of the Karl Fischer reagent (g / ml), V is the volume consumed by the Karl Fischer reagent (ml), and m is the sample mass (g);

[0062] In a preferred embodiment, the relative deviation of two test results of the volumetric method for determining the moisture content of the sample should not exceed 2%, otherwise the test should be repeated.

[0063] Compared with the prior art, the advantages of the method for determining the moisture content in glycolide provided by the present invention are as follows:

[0064] (1) When measuring glycolide samples with a moisture content of ≤0.1%

[0065] The heating temperature is controlled by a cassette furnace to measure the total moisture content and external moisture content of glycolide separately. The internal moisture content of glycolide is obtained by calculation. The blank measurement also reduces the systematic error of the measurement process and achieves accurate measurement.

[0066] The combined cassette furnace and Karl Fischer coulometer is a closed, anhydrous system. The glycolide sample does not come into direct contact with the electrolyte, thus avoiding the introduction of air moisture when using a solid sampling boat. This also prevents glycolide from adhering to the reaction electrode after multiple measurements due to its limited solubility in the anolyte, which can cause slow or even impossible equilibrium in the electrolytic system.

[0067] (2) When measuring glycolide samples with a water content greater than 0.1%

[0068] In the Karl Fischer volumetric method, a mixture of methanol and ethyl acetate is used instead of methanol as the equilibrium solution, which increases the solubility of glycolide in the equilibrium solution, improves the measurement accuracy, and also reduces the frequency of equilibrium solution replacement and analysis costs. DETAILED DESCRIPTION

[0069] In order to better illustrate the technical solution of the present invention, the following examples will further illustrate the method provided by the present invention, but the present invention is not limited to the listed examples, and should also include any other known changes within the scope of the claims of the present invention.

[0070] Moisture determination by the optimized coulometric method (coulometric method of the present invention):

[0071] Reagent materials: Honeywell Fluka coulometric anolyte, Honeywell Fluka coulometric catholyte, ultrapure water, nitrogen (purity 99.9%), refined glycolide from McLean (purity 99.9%), crude glycolide from Guoneng Chemical Company (purity 99%), and crude glycolide from Guoneng Yulin Chemical Company (purity 99%).

[0072] Instruments and equipment: Swiss Metrohm 831 Karl Fischer Coulometric Titrator, Swiss Metrohm 885 Cardboard Stove Sample Injection Unit. The cardboard stove sample injection unit is used to send the water released from the glycolide sample into the Karl Fischer Coulometric Titrator, which includes

[0073] A heating cassette furnace; a nitrogen pump for delivering carrier gas to the sample injection bottle; a gas drying device for drying the nitrogen carrier gas; a sample injection bottle set on the cassette furnace for carrying the sample; and a gas supply pipeline (for delivering the gas flow from the sample injection bottle to the anode chamber of the Karl Fischer coulometric titrator).

[0074] The specific steps are as follows:

[0075] (A) Set the furnace parameters (autosampler temperature: 140°C, heating time: 6 min, carrier gas: nitrogen, carrier gas flow rate: 50 ml / min, endpoint relative drift: 20 μg / min, transfer tube heating, titration endpoint: delayed time termination mode) and preheat the furnace sampling unit. Connect the nitrogen pump to the injection bottle and purge the injection bottle and tubing with nitrogen for 30 min. Inject 100 ml of anolyte and 5 ml of catholyte into the anode and cathode chambers of the Karl Fischer coulometer electrolytic cell, respectively. Use a syringe to add first-class ultrapure water to adjust the iodine electrolysis system in the anode chamber until the color of the solution in the electrolytic cell changes from dark brown to light yellow. Place an empty injection bottle covered with a sealing film in the "Drift" position and click the instrument start button to measure the instrument drift value. If the drift value is ≤ 20 μg / min, it is considered to have reached equilibrium and stability.

[0076] (B) Then prepare an empty injection bottle without sample and perform blank measurement;

[0077] (C) Weigh a certain amount of sample into a clean injection bottle for sample measurement and cover the injection bottle with a sealing film;

[0078] (D) Place the sample bottle into the corresponding injection position and click "Start" on the moisture meter. The instrument will automatically measure the moisture content. Enter the mass m of the sample and wait for the instrument to complete the automatic test.

[0079] (E) Read the final measurement result of the moisture meter and record it as the external moisture content W in the glycolide sample f .

[0080] Repeat the above steps (A)-(D), wherein the heating temperature of the furnace in step (A) is changed from 140°C to 200°C, and the final measurement result of the moisture meter is read and recorded as the total moisture content W of the glycolide sample. t .

[0081] Internal moisture content W in glycolide sample in It can be calculated according to the following formula: in =W t -W f .

[0082] Moisture determination by the coulometric method before optimization (traditional coulometric method):

[0083] Reagent materials: Honeywell Fluka coulometric anode, Honeywell Fluka coulometric cathode, ultrapure water, refined glycolide (purity 99.9%) from Maclean Company, and crude glycolide (purity 99%) from Guoneng Yulin Chemical Company.

[0084] Instruments and equipment: Swiss Metrohm 831 Fischer coulometric moisture analyzer.

[0085] The specific steps are as follows:

[0086] Inject 100ml of anolyte and 5ml of catholyte into the anode and cathode chambers of the Karl Fischer coulometric titrator, respectively. Use a syringe to add first-grade ultrapure water to adjust the iodine electrolysis system in the anode chamber until the color of the solution in the anode chamber changes from dark brown to light yellow. Click the Start button to measure the drift value of the titrator. Equilibrium is considered stable until the drift value is ≤20μg / min.

[0087] Weigh the sample with a sample boat and transfer it to the anode chamber. Press the "Start" button and enter the sample weight according to the screen prompts. Click "Confirm" to start the measurement. The instrument screen will display the measurement curve. Wait for the titration to reach the end point, send an end signal and display the result, which is recorded as the total water content W of the sample. t .

[0088] Moisture determination by the optimized volumetric method (the volumetric method of the present invention):

[0089] Reagent materials: Comeo Karl Fischer reagent (titer C = 3 mg / ml), anhydrous methanol (chromatographic grade), ethyl acetate (analytical grade), refined glycolide from Maclean (purity 99.9%), crude glycolide from Guoneng Chemical Company (purity 99%), and crude glycolide from Guoneng Yulin Chemical Company (purity 99%).

[0090] Instruments and equipment: Swiss Metrohm 915 integrated calorimetric volumetric moisture analyzer;

[0091] The specific steps are as follows:

[0092] Turn on the moisture meter, add a mixture of 50ml of methanol and 25ml of ethyl acetate to the titration cup as the equilibrium solution, immerse the electrode in the equilibrium solution, and when the drift value displayed on the instrument screen is less than 20μg / min, the instrument is considered to have reached equilibrium. Weigh approximately 0.5g of glycolide sample, click "Start Test" and inject the sample into the titration cup. Press the "Continue" button, enter the sample weight, and press "OK" to start the titration test. After the test is completed, record the volume of Karl Fischer reagent consumed and calculate the total water content of the sample according to the following formula:

[0093]

[0094] Where C is the known titer of the Karl Fischer reagent (g / ml), V is the consumed volume of the Karl Fischer reagent (ml), and m is the sample mass (g).

[0095] Water content determination by volumetric method before optimization (traditional volumetric method):

[0096] The difference between this determination step and the above-mentioned optimized volumetric determination step is that the mixed solution used as the balancing solution is replaced with 75 ml of methanol solution, and the balancing solution needs to be replaced after each determination. The other conditions are the same.

[0097] Example 1

[0098] Several samples of refined glycolide from McLean (about 1.000 g each) and several samples of crude glycolide from Guoneng Chemical (about 1.000 g each) were taken. The optimized Karl Fischer volumetric method was first used to measure the moisture content, which was less than 0.1%. Then, the optimized coulometric method was used to determine the external moisture content and total moisture content, and the internal moisture content was calculated.

[0099] Table 1: Determination results of moisture content of refined glycolide and crude glycolide

[0100]

[0101] Table 1 shows that the total moisture content of Maclean's refined glycolide is low, consistently below 100 ppm, meeting the 99.9% purity standard. The total moisture content of Guoneng Chemical's crude glycolide is within 1000 ppm, meeting the 99% purity standard, consistent with industry standards and expectations. Therefore, the optimized coulometry method is feasible for determining the internal and external moisture content of glycolide samples.

[0102] Example 2

[0103] Several purified glycolide samples (approximately 1.000 g each) from Maclean were measured for total moisture content using both the optimized and pre-optimized coulometry methods. The anolyte in the pre-optimized method needed to be replaced each time, while the anolyte in the optimized method was reused.

[0104] Several crude glycolide samples (approximately 0.500 g each) from Guoneng Yulin Chemical Company were tested for total moisture content using both the optimized and pre-optimized volumetric methods. The balancing solution in the pre-optimized method was replaced after every 1 g of glycolide sample, while the balancing solution was replaced after every 5 g of glycolide sample.

[0105] Table 2: Comparison of experimental results before and after optimization of the coulometric method and the volumetric method

[0106]

[0107]

[0108] The experimental results show that the optimized coulometry method can be reused multiple times because the anolyte does not come into contact with the glycolide sample. The standard deviation of the total moisture content determination results is smaller, indicating that the optimized coulometry method has better experimental repeatability. The overall total moisture content determination results of the optimized coulometry method are smaller. Analysis shows that this is because the error caused by the introduction of moisture from the air during the injection boat is avoided.

[0109] The optimized volumetric method uses a balance solution that needs to be replaced less frequently, but the standard deviation of the results for determining the total water content of the glycolide sample is lower. This shows that the optimized volumetric method reduces the analysis cost while improving the measurement accuracy, making it more economical and applicable.

[0110] Example 3

[0111] To ensure the accuracy of the analytical data and that the analytical method is suitable for the glycolide sample being tested, the external moisture content and the spiked recovery rate of the refined glycolide sample were determined based on Example 1 to verify the controllability of the experimental process.

[0112] Take multiple portions of approximately 100.000g of Maclean's purified glycolide sample (each portion, approximately 100.000g) and spread them flat on different trays (sample thickness after spreading ≤1mm); place these trays in a constant temperature and humidity environment at 23°C and 60% humidity. After 5 minutes, record their weights and take the average value; the purified glycolide sample after water absorption is thoroughly mixed, and take multiple 1.000g samples (each portion, approximately 1.000g) as spiked samples. Use the optimized coulometric method to determine the external moisture content and total moisture content of the spiked samples, and then calculate the spiked recovery using the following formula:

[0113] Spiked recovery rate = (spiked sample measurement value - sample measurement value) ÷ spiked amount × 100%

[0114] The measured value of the spiked sample is the external moisture content of the spiked sample, the measured value of the sample is the average external moisture content of the sample before water absorption, and the spiked amount is the average water absorption; or

[0115] Among them, the measured value of the spiked sample is the total moisture content of the spiked sample, the measured value of the sample is the average total moisture content of the sample before water absorption, and the spiked amount is the average water absorption.

[0116] Table 3: Water absorption test results of refined glycolide samples

[0117]

[0118] Table 4: Recovery results of spiked water per gram of purified glycolide sample

[0119]

[0120] Table 5: Total water content of purified glycolide per gram of sample spiked recovery test results

[0121]

[0122] The results in Tables 4 and 5 show that the average recoveries of the external water content and total water content of glycolide determined by the optimized coulometric method are both close to 100%, that is, the Karl Fischer coulometric method of the present invention is suitable for determining glycolide with a water content ≤ 0.1%.

[0123] Unless otherwise defined, the terms used in the present invention have the same meanings as those commonly understood by those skilled in the art.

Claims

1. A method for measuring the moisture content in glycolide, the method comprising: The water content of the glycolide sample is measured using the Karl Fischer volumetric method. When the water content result is ≤ 0.1%, another sample of the glycolide is measured using the Karl Fischer coulometric method to obtain the water content of the glycolide. When the moisture content result obtained is > 0.1%, the result is directly used as the moisture content of the glycolide sample; The Karl Fischer coulometric method is carried out by using a cassette furnace-Karl Fischer coulometric instrument combination device, wherein the cassette furnace-Karl Fischer coulometric instrument combination device comprises a cassette furnace sampling unit and a Karl Fischer coulometric moisture meter; The cassette furnace sampling unit is used to send the water escaping from the sample into the Karl Fischer coulometric moisture analyzer, which includes a cassette furnace, an air pump, a gas drying device, a sampling bottle carrying the sample, and a gas pipeline; wherein The cassette furnace is used to heat the sample injection bottle to release water from the glycolide sample; The air pump is used to deliver carrier gas to the injection bottle; The gas drying device is used to dry the carrier gas to be introduced into the injection bottle; The gas delivery pipeline is used to deliver the gas flow from the sample injection bottle into the anolyte of the Karl Fischer coulometric titrator; The Karl Fischer Coulometry method comprises the following steps: (1) Injecting the coulometric anolyte and the coulometric catholyte into the anode chamber and cathode chamber of the electrolytic cell of the Karl Fischer coulometric titrator respectively; (2) Adjust the iodine electrolysis system in the anode chamber and wait for the electrolysis system in the electrolytic cell to be balanced and stable; (3) Adjust the temperature of the furnace to 120-140°C and perform a blank measurement; (4) Then put the sample in for measurement, and obtain the measurement result after the measurement is completed; (5) The obtained measurement result is recorded as the sample external moisture content W f ; (6) Repeat steps (2) to (4), wherein in step (3), adjust the temperature of the card furnace to 180℃-200℃, and record the obtained measurement result as the total moisture content of the sample W t ; (7) According to formula W in = W t -W f Calculate the internal moisture content W of glycolide sample in .

2. The method for measuring the moisture content in glycolide according to claim 1, wherein When the Karl Fischer volumetric method is used for determination, a mixture of methanol and ethyl acetate is used as the equilibrium solution of the Karl Fischer reagent in the Karl Fischer volumetric method.

3. The method for measuring the moisture content in glycolide according to claim 2, wherein: The volume ratio of methanol to ethyl acetate in the equilibrium solution is 1-3:

1.

4. The method for measuring the moisture content in glycolide according to claim 2, wherein: The volume ratio of methanol to ethyl acetate in the equilibrium solution is 2:

1.

5. The method for measuring the moisture content in glycolide according to claim 3, wherein: When the amount of glycolide sample dissolved in every 100 g of the balancing solution does not exceed 15 g on average, the balancing solution is reused.

6. The method for measuring the moisture content in glycolide according to any one of claims 1 to 5, characterized in that: In the determination from step (3) to step (4), the time for each determination is not less than 5 minutes.

7. The method for measuring the moisture content in glycolide according to claim 6, characterized in that: In the determination from step (3) to step (4), the time for each determination is 5 min to 8 min.

8. The method for measuring the moisture content in glycolide according to claim 7, characterized in that: In the determination from step (3) to step (4), the time for each determination is 6 minutes.

9. The method for measuring the moisture content in glycolide according to any one of claims 1 to 5 and 7 to 8, characterized in that: The carrier gas is an inert carrier gas that can carry water vapor into the Karl Fischer titrator.

10. The method for measuring the moisture content in glycolide according to claim 9, characterized in that: The carrier gas is nitrogen.

11. The method for measuring the moisture content in glycolide according to claim 9, characterized in that: The flow rate of the carrier gas is 30 ml / min-60 ml / min.

12. The method for measuring the moisture content in glycolide according to claim 10, characterized in that: The flow rate of the carrier gas is 50 ml / min.

13. The method for measuring the moisture content in glycolide according to any one of claims 1-5, 7-8 and 10-12, characterized in that: Before the electrolysis system in step (2) is balanced and stabilized, the sample injection bottle and gas pipeline of the cassette furnace sampling unit are purged with carrier gas for 15 min to 30 min.

14. The method for measuring the moisture content in glycolide according to any one of claims 1-5, 7-8 and 10-12, characterized in that: In the step (2), when adjusting the iodine electrolysis system in the anode chamber, pure water is added to the anode chamber to consume excess iodine in the anode solution.

15. The method for measuring the moisture content in glycolide according to claim 13, characterized in that: In the step (2), when adjusting the iodine electrolysis system in the anode chamber, pure water is added to the anode chamber to consume excess iodine in the anode solution.

16. The method for measuring the moisture content in glycolide according to any one of claims 1-5, 7-8, 10-12 and 15, characterized in that: When the drift value displayed by the Karl Fischer coulometric moisture analyzer is ≤20 μg / min, the electrolysis system reaches equilibrium and stability.

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