A method for determining the vanillin oxidation end point based on thin layer chromatography

Abstract

The present application relates to the technical field of chemical analysis, in particular to a method for determining vanillin oxidation end point based on thin layer chromatography, comprising the following steps: S1: taking a control product to prepare a control solution; S2: preparing a to-be-tested solution for standby; S3: taking the control solution obtained in S1 and the to-be-tested solution obtained in S2, and developing in a developing agent according to thin layer chromatography conditions, and after the development is completed, confirming the corresponding spots of vanillyl mandelic acid in the to-be-tested solution, the control solution and 3-methoxy-4-hydroxy-5-ethanol acid benzaldehyde on a silica gel plate; S4: judging the oxidation reaction process and the required reaction time according to the diameter and color of the vanillyl mandelic acid spot in the to-be-tested solution. The present application solves the technical problem of long analysis time in the existing method for determining vanillin reaction end point.

Description

Technical Field

[0001] This invention relates to the field of chemical analysis technology, specifically to a method for determining the oxidation endpoint of vanillin based on thin-layer chromatography. Background Technology

[0002] Vanillin, also known as vanillin, scientifically named 3-methoxy-4-hydroxybenzaldehyde, has the characteristic aroma of vanilla and is an indispensable raw material in the food additive industry. It is also the world's largest producer of synthetic flavorings and an important intermediate for many drugs.

[0003] Currently, vanillin is mainly synthesized in China using the glyoxylic acid method. The specific steps are as follows: using guaiacol and glyoxylic acid as raw materials, 3-methoxy-4-hydroxyphenylethanolic acid is synthesized; 3-methoxy-4-hydroxyphenylethanolic acid is then oxidized to 3-methoxy-4-hydroxyvanillin; and finally, vanillin is obtained through decarboxylation. Most domestic vanillin production companies choose to oxidize 3-methoxy-4-hydroxyphenylethanolic acid by introducing air into the oxidation stage. However, if the aeration time is too long, more impurities and tar will be generated, reducing the quality of the produced vanillin and increasing environmental protection costs. If the aeration time is insufficient, the conversion of 3-methoxy-4-hydroxyphenylethanolic acid will be incomplete, reducing yield and production efficiency. Therefore, effective determination of the oxidation endpoint in vanillin production is of great significance for improving oxidation yield and production efficiency, and controlling the quality indicators of vanillin products.

[0004] Existing technologies mostly employ traditional analytical detection methods for determination, such as spectrophotometry, high-performance liquid chromatography (HPLC), liquid chromatography-mass spectrometry (LC-MS), gas chromatography (GC), and gas chromatography-mass spectrometry (GC-MS). These methods determine the extent of the oxidation reaction by detecting the content of various substances in the reaction solution. However, these methods require too much detection time and are not suitable for rapid and accurate determination in such industrial production processes.

[0005] Therefore, developing an accurate and rapid thin-layer chromatography method for determining the oxidation endpoint of vanillin can not only improve the production efficiency and oxidation yield of the oxidation reaction section, but also avoid problems such as poor product quality caused by excessive oxidation time and large amounts of tar in wastewater from environmental protection treatment, which is of great significance for subsequent production. Summary of the Invention

[0006] The present invention aims to provide a method for determining the oxidation endpoint of vanillin based on thin-layer chromatography, so as to solve the technical problem that the analysis time is too long in the existing methods for determining the reaction endpoint of vanillin.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: a method for determining the oxidation endpoint of vanillin based on thin-layer chromatography, characterized by comprising the following steps:

[0008] S1: Prepare a control solution from the reference standard;

[0009] S2: Take the vanillin oxidation solution whose reaction progress needs to be determined, add acidification solution to acidify it, shake well and prepare the test solution for later use;

[0010] S3: Take the control solution obtained in S1 and the test solution obtained in S2, and develop them in the developing solvent according to the thin layer chromatography conditions. After development, confirm the corresponding spots of vanillylmandelic acid, control solution and 3-methoxy-4-hydroxy-5-ethanolic benzaldehyde in the test solution on the silica gel plate.

[0011] S4: Measure the diameter (d) of the vanillic mandelic acid spot in the test solution to obtain the required oxidation reaction time T1. At the same time, take out the colorimetric card and compare it with the color of the vanillic mandelic acid spot in the test solution to obtain the required oxidation reaction time T2. If T1 and T2 are in the same interval, substitute the diameter of the vanillic mandelic acid spot in the test solution into the formula to calculate the specific reaction time.

[0012] The principles and advantages of this scheme are:

[0013] This method uses thin-layer chromatography to determine the vanillin oxidation endpoint. The spot corresponding to vanillylmandelic acid in the test solution can be identified on the silica gel plate of the thin-layer chromatography through the color reaction. The diameter and color of the spot corresponding to vanillylmandelic acid in the test solution can be confirmed in a simple way, and the degree of reaction can be judged by its diameter and color.

[0014] Compared to existing technologies that typically require more than 30 minutes to determine the vanillin oxidation endpoint using methods such as liquid chromatography, this method can yield a conclusion within 4 minutes, making it very quick and convenient.

[0015] In addition, this application can also calculate the remaining reaction time based on the diameter of the vanillic mandelic acid spot in the test solution, which improves the accuracy of the judgment of the reaction endpoint compared to the previous empirical judgment.

[0016] Preferably, as an improvement, the control solution in S1 is a 0.01-0.2% (wt) vanillylmandelic acid solution.

[0017] In this application, the control solution can be used as a basis for judging the content of vanillylmandelic acid in the test solution. When the color of the spot corresponding to vanillylmandelic acid in the test solution is close to the color of the spot corresponding to the control solution, it indicates that the concentration of vanillylmandelic acid in the test solution is equivalent to the concentration of the control solution, which can be used as one of the standards for judging that the oxidation reaction has been completed.

[0018] Preferably, as an improvement, in step S2, 2-10 ml of vanillin oxidation solution is taken, the acidification solution is a sulfuric acid solution with a concentration of 20%-60%, and the amount of acidification solution added is 0.25 ml-1.5 ml.

[0019] In this application, acidifying the vanillin oxidation solution allows for the display of the corresponding components during testing. Specifically, it displays spots of 3-methoxy-4-hydroxy-5-ethanol benzaldehyde, serving as one of the verification conditions for comparison with the vanillylmandelic acid spots in the test solution. When the 3-methoxy-4-hydroxy-5-ethanol benzaldehyde spot and the vanillylmandelic acid spot in the test solution are similar in color and size, it indicates that the oxidation reaction has not yet reached its endpoint. When the vanillylmandelic acid spot in the test solution is significantly lighter in color and much smaller in area compared to the 3-methoxy-4-hydroxy-5-ethanol benzaldehyde spot, it indicates that the oxidation reaction has been completed.

[0020] Preferably, as an improvement, the vanillin oxidation solution in S2 has undergone an oxidation reaction for 110-120 minutes.

[0021] In this application, the vanillin oxidation reaction requires 120–160 minutes. Therefore, testing with an oxidation solution that has undergone 110–120 minutes of oxidation avoids the problem of the oxidation reaction being completed due to excessive time. If an oxidation solution with a shorter reaction time is used for testing, the accuracy will be relatively low due to incomplete oxidation.

[0022] Preferably, as an improvement, the developing agent in S3 is a mixture of dichloromethane, isopropanol and glacial acetic acid, wherein the volume ratio of dichloromethane, isopropanol and glacial acetic acid in the mixture is 8-20:2-7:1.

[0023] In this application, a mixture of dichloromethane, isopropanol and glacial acetic acid is used as the developing agent, which has a shorter development time and a higher relative displacement value compared to other developing agents.

[0024] Preferably, as an improvement, the specific relationship between the diameter d of the vanillic mandelic acid spot in the test solution and the required oxidation reaction time T1 in step S4 is as follows:

[0025] When 0mm≦d<1mm, then 0≦T1<3min;

[0026] When 1mm≦d<2mm, then 3min≦T1<15min;

[0027] When 2mm≦d<3mm, then 15min≦T1<25min;

[0028] When 3mm≦d<4mm, then 25min≦T1<35min;

[0029] When 4mm≦d, then 35min≦T1.

[0030] In this application, the diameter of the spot corresponding to vanillic mandelic acid in the test solution is directly proportional to the required reaction time. The larger the diameter of the spot corresponding to vanillic mandelic acid in the test solution, the more substances in the oxidation solution have not yet undergone oxidation reaction, and the longer the reaction time is required.

[0031] Preferably, as an improvement, the colorimetric card in step S4 includes a blank background and several colored stripes, with the number of colored stripes being 4 to 8.

[0032] In this application, the colorimetric card is used as a reference when comparing the color of vanillic mandelic acid in the test solution. The more colored bands there are, the more accurately the color of the vanillic mandelic acid spots in the test solution can be compared, reducing errors.

[0033] Preferably, as an improvement, the colorimetric card has five colored bands, including colored band one, colored band two, colored band three, colored band four, and colored band five; the RGB color of colored band one is (230, 230, 255); the RGB color of colored band two is (203, 195, 227); the RGB color of colored band three is (170, 152, 169); the RGB color of colored band four is (159, 43, 104); and the RGB color of colored band five is (128, 0, 128).

[0034] In this application, the endpoint of the vanillin oxidation reaction can be accurately determined when there are 5 colored bands on the colorimetric card. Each of the 5 colored bands has a standard color, allowing for a more accurate determination of the color of the vanillic mandelic acid spots in the test solution during comparison.

[0035] Preferably, as an improvement, the specific relationship between the color of the vanillic mandelic acid spot in the test solution and the colorimetric card comparison result in step S4, and the remaining oxidation reaction time T2, is as follows:

[0036] When the color of the vanillic mandelic acid spot in the test solution is closest to that of the colored band, then 0 ≦ T2 < 3 min;

[0037] When the color of the vanillic mandelic acid spot in the test solution is closest to that of the second colored band, then 3min ≦ T2 < 15min;

[0038] When the color of the vanillic mandelic acid spot in the test solution is closest to that of the colored band three, then 15min ≦ T2 < 25min;

[0039] When the color of the vanillic mandelic acid spot in the test solution is closest to that of colored band four, then 25min≦T2<35min;

[0040] When the color of the vanillic mandelic acid spot in the test solution is closest to that of colored band five, then 35min ≦ T2.

[0041] In this application, the required oxidation reaction time can be obtained by comparing the color of the vanillic mandelic acid spots in the test solution with the colored bands.

[0042] Preferably, as an improvement, the formula in step S4 is T = 9.55d - 3.725, where d is the diameter of the vanillic mandelic acid spot in the test solution in millimeters; and T is the time required for further oxidation reaction in minutes.

[0043] In this application, based on the relationship between the diameter of vanillic mandelic acid spots in the test solution and the time required for further oxidation reaction, the formula in this application can accurately calculate the specific time required for further reaction. Attached Figure Description

[0044] Figure 1 This is a schematic diagram showing thin-layer chromatography silica gel plates at different oxidation reaction stages in this invention under a 254nm ultraviolet light device.

[0045] Figure 2 This is a schematic diagram of the colorimetric card of the present invention. Detailed Implementation

[0046] The following detailed description illustrates the specific implementation method:

[0047] A method for determining the vanillin oxidation endpoint based on thin-layer chromatography includes the following steps:

[0048] S1: Prepare a reference solution from the reference standard;

[0049] Specifically, take 1g of vanillin-mandelic acid with a content (area) greater than 98.5% as a reference standard, dilute it with methanol, shake well, and prepare a 0.05% (wt) vanillin-mandelic acid reference solution. Seal the solution and store it in a refrigerator for later use. Besides methanol, water, toluene, acetonitrile, ethanol, chloroform, ethyl acetate, cyclohexane, butanone, acetone, petroleum ether, etc., can also be used as solvents to dilute vanillin-mandelic acid.

[0050] S2: Take the vanillin oxidation solution to be judged, add acidification solution to acidify it, shake well and prepare vanillylmandelic acid in the test solution for later use.

[0051] Specifically, when the vanillin oxidation reaction reaches 120 min, the gas supply to the oxidation tower is stopped. Take 5 ml of the vanillin oxidation solution to be tested from the middle of the oxidation tower into a centrifuge tube, add 10 drops (about 0.5 ml) of 30% sulfuric acid solution to acidify the sample, shake well, and then proceed with thin-layer chromatography analysis.

[0052] S3: Take the reference solution obtained in S1 and the vanillic mandelic acid solution obtained in S2, and develop them in the developing solvent according to the thin layer chromatography conditions. After development, confirm the corresponding spots of vanillic mandelic acid in the test solution on the silica gel plate.

[0053] Specifically, using a capillary tube with a pore size of 0.3 mm, spot the vanillic mandelic acid solution and the control solution from the test solution onto the same thin-layer chromatography silica gel plate (model: GF254; 2.5 cm long, 2.5 cm wide), with a spot diameter of 4 mm. The spotting procedure is as follows: Place the thin-layer chromatography plate face up, and use a pencil to draw a straight line on a relatively flat side of the plate, about 3-4 mm from the top. Mark a point at the midpoint of the line and another point about 6-8 mm to the left and right of the midpoint. Use the capillary tube to take 1-5 μL of the test solution and spot it on the left and right points respectively. Then, take the control solution and spot it on the midpoint. After spotting, dry the plate, then use tweezers to hold the unspotted wide edge of the thin-layer chromatography silica gel plate and gently place it at a horizontal angle of 60-80° into a developing tank containing the developing solvent for development. When the solvent front on the plate approaches the edge of the thin-layer chromatography silica gel plate (running time approximately 50-60 seconds), use tweezers to remove the thin-layer chromatography silica gel plate and develop it under a 254nm UV device to confirm the corresponding spots of vanillylmandelic acid and other components in the test solution. The thin-layer chromatography silica gel plate after development will look approximately as shown in the attached image under UV light. Figure 1 As shown. (Attached) Figure 1 The image shows thin-layer chromatography silica gel plates with different degrees of oxidation reaction.

[0054] The developing solvent is a mixture of dichloromethane, isopropanol, and glacial acetic acid in a volume ratio of 20:7:1. The developing tank measures 35mm in diameter and 60mm in height.

[0055] S4: Measure the diameter d (mm) of the vanillic mandelic acid spot in the test solution to obtain the remaining oxidation reaction time T1min. At the same time, take out the colorimetric card and compare it with the color of the vanillic mandelic acid spot in the test solution to obtain the remaining oxidation reaction time T2min. If T1 and T2 are in the same interval, substitute the diameter of the vanillic mandelic acid spot in the test solution into the formula to calculate the specific reaction time.

[0056] Specifically, after identifying the spot corresponding to vanillylmandelic acid in the test solution, measure its diameter (d) using a ruler and record it. Determine the required reaction time T1 according to the following criteria:

[0057] When 0mm≦d<1mm, then 0≦T1<3min;

[0058] When 1mm≦d<2mm, then 3min≦T1<15min;

[0059] When 2mm≦d<3mm, then 15min≦T1<25min;

[0060] When 3mm≦d<4mm, then 25min≦T1<35min;

[0061] When 4mm≦d, then 35min≦T1.

[0062] Simultaneously, the self-made colorimetric card was taken out to confirm the color range of the spot corresponding to vanillic mandelic acid in the test solution on the colorimetric card. The colorimetric card, as shown in the attached figure, includes a blank background and five colored bands, namely colored band one, colored band two, colored band three, colored band four, and colored band five. The RGB color corresponding to colored band one is (230, 230, 255); the RGB color corresponding to colored band two is (203, 195, 227); the RGB color corresponding to colored band three is (170, 152, 169); the RGB color corresponding to colored band four is (159, 43, 104); and the RGB color corresponding to colored band five is (128, 0, 128).

[0063] Different colored bands correspond to different required further reaction times, T2, as follows:

[0064] The colored band one corresponds to a reaction time of 0 ≤ T2 < 3 min;

[0065] The second colored band corresponds to a reaction time of 3 min ≤ T2 < 15 min;

[0066] The colored band three corresponds to a reaction time of 15 min ≤ T2 < 25 min;

[0067] The colored band four corresponds to a reaction time of 25 min ≤ T2 < 35 min;

[0068] The colored band five corresponds to a reaction time of 35 min ≤ T2.

[0069] Compare the times obtained from T1 and T2. If they are within the same range, substitute the diameter of the vanillic mandelic acid spot in the test solution into the formula T = 9.55d - 3.725 to calculate the accurate reaction time T, in minutes, where d is in millimeters. If they are not within the same range, repeat the experiment.

[0070] In this embodiment, the diameter of the spot corresponding to vanillic mandelic acid in the test solution was measured to be 2.1 mm, and its color was closest to that of colored band three. Therefore, according to the above judgment criteria, the reaction needed to proceed for approximately 16 minutes. The reaction was then continued with ventilation for another 16 minutes, ventilation was stopped, and a sample was taken for retesting. The diameter of the spot corresponding to vanillic mandelic acid in the test solution measured in the second test was approximately 0.5 mm, and its color was closest to that of colored band one. It was determined that the oxidation reaction had met the acceptable standard, and the oxidized solution could be proceeded to subsequent processes. The total time spent on the two thin-layer chromatography analyses, as well as the determination of the remaining reaction time and the oxidation endpoint, was approximately 4 minutes.

[0071] A simpler method, especially when the oxidation reaction is nearing its endpoint, is to directly compare the vanillic mandelic acid spot in the test solution with the spots in the control solution and the 3-methoxy-4-hydroxy-5-ethanolic acid benzaldehyde spot, without requiring complex calculations. (See attached image.) Figure 1 As shown in the two diagrams on the right, when the color of the vanillic mandelic acid spot in the test solution is close to the color of the spot in the control solution, and the area of ​​the vanillic mandelic acid spot in the test solution is much smaller than that of the 3-methoxy-4-hydroxy-5-ethanolic benzaldehyde spot, it can be directly determined that the oxidation reaction is nearing its end.

[0072] Example 2

[0073] The difference between this embodiment and Example 1 is that the vanillin oxidation solution taken in S2 comes from a different production batch than that in Example 1. The diameter of the spot corresponding to vanillic mandelic acid in the test solution was measured to be 1.2 mm, and its color was closest to the second colored band. Therefore, according to the above judgment criteria, the reaction needed to proceed for approximately 8 minutes. The reaction was then continued with aeration for 16 minutes, aeration was stopped, and a sample was taken for retesting. In the second test, the diameter of the spot corresponding to vanillic mandelic acid in the test solution was approximately 0.4 mm, and its color was closest to the first colored band. It was determined that the oxidation reaction had met the acceptable standard, and the oxidation solution could be proceeded to subsequent processes. The two thin-layer chromatography analyses and the determination of the remaining reaction time and oxidation endpoint took approximately 4 minutes in total.

[0074] Example 3

[0075] The difference between this embodiment and Example 1 is that the vanillin oxidation solution taken in S2 comes from a different production batch than that in Example 1. The diameter of the spot corresponding to vanillylmandelic acid in the test solution was measured to be 0.5 mm, and its color was closest to that of the colored band. Therefore, according to the above judgment criteria, the reaction needs to continue for about 1 minute to reach the reaction endpoint. The total time spent on thin-layer chromatography analysis and determining the remaining reaction time and oxidation endpoint was about 2 minutes.

[0076] Comparative Example 1

[0077] The difference between this comparative example and Example 1 lies in the measurement method. This comparative example uses HPLC (high performance liquid chromatography) to determine whether the oxidation reaction has reached its endpoint.

[0078] The vanillin oxidation solution tested after 120 min of aeration had a vanillic mandelic acid area of ​​0.91% and an impurity area of ​​0.40%, with an oxidation yield of 94.8%. This indicates that the reaction did not reach the endpoint.

[0079] The vanillin oxidation solution, after reacting for 120 minutes, showed a vanillylmandelic acid area of ​​0.07% and an impurity area of ​​0.78%, with an oxidation yield of 97%, indicating that the reaction had reached its endpoint.

[0080] The two reaction tests took a total of about 60 minutes.

[0081] Comparative Example 2

[0082] The difference between this comparative example and Example 2 lies in the measurement method. This comparative example uses HPLC (High Performance Liquid Chromatography) to determine whether the oxidation reaction has reached its endpoint. The experimental method is as follows:

[0083] The vanillin oxidation solution tested after 120 min of aeration had a vanillic mandelic acid area of ​​0.58% and an impurity area of ​​0.60%, with an oxidation yield of 95.6%. This indicates that the reaction did not reach its endpoint.

[0084] The vanillin oxidation solution, which reacted simultaneously for 128 minutes, had a vanillic mandelic acid area of ​​0.13% and an impurity area of ​​0.88%, with an oxidation yield of 96.8%, indicating that the reaction had reached its endpoint. The two tests took approximately 65 minutes in total.

[0085] Comparative Example 3

[0086] The determination method in this comparative example differs from that in Example 3. This comparative example uses HPLC (High Performance Liquid Chromatography) to determine whether the oxidation reaction has reached its endpoint. The vanillin oxidation solution after 120 min of aeration showed a vanillylmandelic acid area of ​​0.16% and an impurity area of ​​0.63% after 6.5 min, with an oxidation yield of 96.7%, thus verifying successful oxidation. The detection time was 32 min.

[0087] The experimental results of the above embodiments and comparative examples are recorded in Table 1 below:

[0088] Table 1: Data Record Table for Examples and Comparative Examples

[0089]

[0090] Analysis of experimental results: As can be seen from the comparison of the results of Comparative Examples 1-3 and Examples 1-3, the results of determining the vanillin reaction endpoint and the required reaction time using the present invention are very accurate.

[0091] The above descriptions are merely embodiments of the present invention, and common knowledge such as specific technical solutions and / or characteristics are not described in detail here. It should be noted that those skilled in the art can make various modifications and improvements without departing from the technical solutions of the present invention, and these should also be considered within the scope of protection of the present invention. These modifications and improvements will not affect the effectiveness of the implementation of the present invention or the practicality of the patent. The scope of protection claimed in this application should be determined by the content of its claims, and the specific embodiments described in the specification can be used to interpret the content of the claims.

Claims

1. A method for determining the oxidation endpoint of vanillin based on thin-layer chromatography, characterized in that: Includes the following steps: S1: Prepare a control solution from the reference standard; S2: Take the vanillin oxidation solution whose reaction progress needs to be determined, add acidification solution to acidify it, shake well and prepare the test solution for later use; S3: Take the control solution obtained in S1 and the test solution obtained in S2, and develop them in the developing solvent according to the thin-layer chromatography conditions. After development, confirm the corresponding spots of vanillylmandelic acid, control solution, and 3-methoxy-4-hydroxy-5-ethanol benzaldehyde in the test solution on a silica gel plate. The developing solvent is a mixture of dichloromethane, isopropanol and glacial acetic acid, and the volume ratio of dichloromethane, isopropanol and glacial acetic acid in the mixture is 8-20:2-7:

1. S4: Measure the diameter (d) of the vanillic mandelic acid spot in the test solution to obtain the required oxidation reaction time T1. At the same time, take out the colorimetric card and compare it with the color of the vanillic mandelic acid spot in the test solution to obtain the required oxidation reaction time T2. If T1 and T2 are in the same interval, substitute the diameter of the vanillic mandelic acid spot in the test solution into the formula to calculate the specific reaction time. The formula is T = 9.55d - 3.725, where d is the diameter of the vanillic mandelic acid spot in the test solution in millimeters; T is the required reaction time in minutes.

2. The method for determining the vanillin oxidation endpoint based on thin-layer chromatography according to claim 1, characterized in that: The control solution in S1 is a 0.01-0.2% (wt) vanillylmandelic acid solution.

3. The method for determining the vanillin oxidation endpoint based on thin-layer chromatography according to claim 1, characterized in that: In step S2, 2-10 ml of vanillin oxidation solution is taken, and the acidification solution is a sulfuric acid solution with a concentration of 20%-60%, and the amount of acidification solution added is 0.25 ml-1.5 ml.

4. The method for determining the vanillin oxidation endpoint based on thin-layer chromatography according to claim 3, characterized in that: The vanillin oxidation solution in S2 has undergone oxidation for 110-120 minutes.

5. The method for determining the vanillin oxidation endpoint based on thin-layer chromatography according to claim 1, characterized in that: The specific relationship between the diameter d of the vanillic mandelic acid spot in the test solution and the required oxidation reaction time T1 in step S4 is as follows: When 0mm≦d<1mm, then 0≦T1<3min; When 1mm≦d<2mm, then 3min≦T1<15min; When 2mm≦d<3mm, then 15min≦T1<25min; When 3mm≦d<4mm, then 25min≦T1<35min; When 4mm≦d, then 35min≦T1.

6. The method for determining the vanillin oxidation endpoint based on thin-layer chromatography according to claim 1, characterized in that: In step S4, the colorimetric card includes a blank background and several colored stripes, with the number of colored stripes being 4 to 8.

7. The method for determining the vanillin oxidation endpoint based on thin-layer chromatography according to claim 1, characterized in that: In step S4, the color chart includes a blank background and five colored bands. The colored bands include colored band one, colored band two, colored band three, colored band four, and colored band five. The RGB color of colored band one is (230, 230, 255); the RGB color of colored band two is (203, 195, 227); the RGB color of colored band three is (170, 152, 169); the RGB color of colored band four is (159, 43, 104); and the RGB color of colored band five is (128, 0, 128).

8. The method for determining the vanillin oxidation endpoint based on thin-layer chromatography according to claim 7, characterized in that: The specific relationship between the color of the vanillic mandelic acid spot in the test solution in step S4 and the remaining oxidation reaction time T2 is as follows: When the color of the vanillic mandelic acid spot in the test solution is closest to that of the colored band, then 0≦T2<3 min; When the color of the vanillic mandelic acid spot in the test solution is closest to that of the second colored band, then 3min ≦ T2 < 15min; When the color of the vanillic mandelic acid spot in the test solution is closest to that of the colored band three, then 15min ≦ T2 < 25min; When the color of the vanillic mandelic acid spot in the test solution is closest to that of colored band four, then 25min≦T2<35min; When the color of the vanillic mandelic acid spot in the test solution is closest to that of colored band five, then 35min ≦ T2.

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