Method for analyzing content of sodium acetate in alkaline water in process for producing benzyl alcohol by toluene oxidation method

Abstract

The invention provides a method for analyzing the content of sodium acetate in alkaline water in a process for producing benzyl alcohol through a toluene oxidation method, and belongs to the technical field of chemical substance detection. According to the method, a sodium acetate solution is separated from alkaline water obtained by a process for producing benzyl alcohol through a toluene oxidation method, and a method for carrying out gradient elution by taking a mixed solution of a monopotassium phosphate aqueous solution and acetonitrile as a mobile phase and carrying out liquid chromatography quantitative analysis on the content of sodium acetate by adopting an external standard method is developed. The method is simple in treatment operation of the sample to be detected, simple in detection process, short in detection time, high in analysis result accuracy and good in reproducibility, and compared with isocratic elution, fewer solvents are used, and the analysis cost is lower.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical substance detection, and particularly relates to an analytical method for the content of sodium acetate in the alkaline water of the process for producing benzyl alcohol by toluene oxidation method. Background Art

[0002] In the process for producing benzyl alcohol by toluene oxidation method, toluene reacts with air. In addition to the main product benzyl alcohol, a series of by-products such as methylphenol, benzoic acid, and acetic acid are also formed due to changes in reaction conditions and impurities introduced in raw materials and auxiliaries. To remove these impurities, an alkali washing method is usually adopted to turn organic acids into salts and enter the aqueous phase to obtain alkaline water. When the concentration of sodium acetate in the alkaline water is too high, it will increase the corrosiveness of the alkaline water, causing corrosion of metal equipment and pipelines. At the same time, it will also affect the efficiency of subsequent chemical reactions, thus affecting the quality and production efficiency of the finished benzyl alcohol. In addition, sodium acetate is easy to crystallize, and too high a concentration will cause blockage of pipelines and equipment, increasing the equipment maintenance cost. Therefore, it is particularly important to control the content of sodium acetate in the alkaline water phase during the benzyl alcohol process.

[0003] At present, the determination of sodium acetate content mainly focuses on food, sewage treatment, and environmental monitoring, and its application in benzyl alcohol production has not been reported yet. Moreover, among the current methods for determining the content of sodium acetate in solution, the common one is acid-base titration method, which is easily interfered by other alkaline substances and the titration end point is difficult to grasp. In addition, potentiometric titration method and ion chromatography method are also common methods, but the pre-preparation work and operation of these methods are cumbersome and complex, time-consuming, and the costs of drugs, reagents, and instruments are relatively high. Although gas chromatography can be used for the analysis of acetic acid, since sodium acetate is easy to crystallize and belongs to salts, it has no response in gas chromatography and will cause column residue, affecting the analysis results. Summary of the Invention

[0004] Based on the above technical problems, the present invention provides an analytical method for the content of sodium acetate in the alkaline water of the process for producing benzyl alcohol by toluene oxidation method. This analytical method is simple, fast, accurate, and has good reproducibility.

[0005] Specifically, in order to achieve the above object, the present invention adopts the following technical scheme: An analytical method for the content of sodium acetate in the alkaline water of the process for producing benzyl alcohol by toluene oxidation method, comprising the following steps: S1. Weigh the sample to be tested accurately and record its mass. Extract the sample to be tested with toluene, let it stand for layering to obtain alkaline aqueous phase A. Adjust the pH value of the alkaline aqueous phase A to 7 - 8, let it stand for layering to obtain alkaline aqueous phase B, and record the mass of the alkaline aqueous phase B. Weigh a certain mass of the alkaline aqueous phase B accurately, dilute it with the mixed solution C composed of potassium dihydrogen phosphate aqueous solution and acetonitrile, adjust the pH to 2.5 - 3.0, record the dilution factor, place it at 0 - 2 °C until benzoic acid completely precipitates, and filter it through a microporous membrane. The obtained filtrate is the sample solution. Weigh the sodium acetate reference substance accurately, use the mixed solution C as the solvent, and prepare standard solutions with different concentrations of the sodium acetate reference substance. S2. After the baseline of the liquid chromatography instrument is stable, measure the peak areas of the standard solutions with different concentrations and draw a standard curve. Measure the sample solution under the same conditions as those for measuring the standard solutions to obtain the peak area of the sample solution. The detection conditions of liquid chromatography are as follows: Mobile phase: It includes mobile phase I and mobile phase II. The mobile phase I is a 0.03 mol / L potassium dihydrogen phosphate aqueous solution with a pH of 2 - 3, and the mobile phase II is acetonitrile. Gradient elution is carried out, and the elution program is as follows:

[0006] Flow rate: 1 mL / min; Detection wavelength: 210 nm; Column temperature: 30 °C; Injection volume: 20 μL; Chromatographic column: WondaCract ODS - 2, with a specification of 4.6×250 mm, 5 - Micron; S3. According to the peak area of the sample solution and the standard curve, calculate the content of sodium acetate in the sample solution, and then calculate the content of sodium acetate in the sample to be tested according to the dilution factor and the mass relationship between the sample to be tested and the alkaline aqueous phase B.

[0007] In the preferred scheme, the mass ratio of the sample to be tested to toluene in step S1 is 1:(1 - 3).

[0008] In a further preferred scheme, the mass ratio of the sample to be tested to toluene in step S1 is 1:1.

[0009] In the preferred scheme, when adjusting the pH of the alkaline aqueous phase A to 7 - 8 in step S1, an 80 wt% sulfuric acid solution is used for adjustment.

[0010] In the preferred scheme, the concentration of the potassium dihydrogen phosphate aqueous solution in step S1 is 0.03 mol / L, and the pH value is 2 - 3.

[0011] In a further preferred embodiment, the volume ratio of the potassium dihydrogen phosphate aqueous solution to the acetonitrile in step S1 is 99:1.

[0012] In a preferred embodiment, the concentration range of sodium acetate in the standard solution in step S1 is 0.2 mg / g to 4.0 mg / g.

[0013] In a preferred embodiment, the dilution factor in step S1 is 100 to 200.

[0014] In a preferred embodiment, the pore size of the microporous filter membrane in step S1 is 0.22 μm.

[0015] Compared with the prior art, the technical solution of the present invention has the following beneficial effects: (1) In the present invention, the alkaline water obtained by the toluene oxidation method for producing benzyl alcohol is pretreated to reduce the contents of benzyl alcohol, sodium methylphenoxide and sodium benzoate in the alkaline water, avoiding the interference of these substances on ultraviolet absorption and reducing the interference on the target peak of liquid chromatography.

[0016] (2) In the present invention, by adjusting the pH of the sample solution to 2.5 - 3.0, on the one hand, the peak shape of liquid chromatography can be improved, and on the other hand, sodium benzoate is completely converted into benzoic acid crystals and precipitated, and then filtered to avoid clogging of the equipment pipeline.

[0017] (3) In the present invention, a mixed solution of 0.03 mol / L potassium dihydrogen phosphate aqueous solution with pH = 2 - 3 and acetonitrile is used as the mobile phase, and the separation degree between the main peak and other peaks is better, the baseline fluctuation is small, the peak shape is sharp, and the retention time is stable. This mobile phase can reduce the dissolution of organic substances and reduce the interference of other substances on the analysis results.

[0018] (4) In the present invention, gradient elution can make the impurity peaks other than the target peak flow out faster, the analysis time is shorter, sodium acetate can elute in 5 minutes, and the analysis can be completed in 20 minutes. At the same time, it uses less solvent than isocratic elution, reducing the analysis cost.

[0019] (5) The present invention solves the problem of the determination of the sodium acetate content in the alkaline water generated by the toluene oxidation method for producing benzyl alcohol. The determination operation of the sodium acetate content is simple and fast, the detection result is highly accurate and reliable. When the standard deviation of the instrument baseline noise is 0.05 mV, the detection limit concentration of this method is 0.0003 mg / g, and the quantitative limit concentration is 0.0011 mg / g. The error of detection by different operators on different instruments does not exceed 0.0002% (that is, consistent detection results can be obtained), and the reproducibility is good. Description of the Drawings

[0020] Figure 1The standard curve graph of the standard solution in the specific embodiment provided by the present invention; Figure 2 The liquid chromatography graph of the standard solution in the specific embodiment provided by the present invention; Figure 3 The liquid chromatography graph of the sample solution in the specific embodiment provided by the present invention. Specific embodiment

[0021] The following content describes the technical solution of the present application clearly and completely in combination with the embodiments, so that those skilled in the art can fully understand the present application. Obviously, the described embodiments are only some preferred embodiments of the present application, rather than all embodiments. Any equivalent transformation or substitution made to the following embodiments by those of ordinary skill in the art without creative work shall fall within the protection scope of the present application.

[0022] The inventors of the present application found in the experiment of detecting sodium acetate by liquid chromatography that when the contents of benzyl alcohol, sodium methylphenoxide and sodium benzoate in the process alkali water obtained by the toluene oxidation method for producing benzyl alcohol are relatively high, it will affect the resolution of sodium acetate, and when the dilution multiple of the alkali water is relatively large, it will affect the accuracy of the detection result. Therefore, the inventors studied the detection conditions of the sodium acetate content in the process alkali water of the toluene oxidation method for producing benzyl alcohol. Specifically, it includes various conditions such as the selection of the mobile phase, the selection of the detection wavelength, the sample pretreatment method, and the study of the liquid chromatography conditions.

[0023] The process for producing benzyl alcohol by the toluene oxidation method described in the present application includes the following steps: P1. Toluene oxidation reaction P1-1. Prepare the catalyst mixture M: Add the catalyst, initiator and auxiliary agent to toluene, mix evenly, and preheat to 60°C - 80°C to obtain the catalyst mixture M. Among them, the catalyst is one or a combination of cobalt naphthenate (CAS No.: 61789-51-3), cobalt benzoate, cobalt acetate, cobalt acetylacetonate. The initiator is one or a combination of benzaldehyde, benzyl alcohol, methanol, acetic acid. The auxiliary agent is diphosphonate (for example, 1-hydroxyethylidene-1,1-diphosphonic acid dioctyl ester, tetraethyl propylene-1,3-diphosphonate).

[0024] P1-2. Air pretreatment: Pass the air through a multi-stage dust removal and filtration device to remove mechanical impurities in the air, and then increase the pressure of the air to 0.8 - 1.5 MPa and heat it to 150°C - 200°C through an air feed preheater.

[0025] P1-3. Toluene pretreatment: Increase the pressure of toluene with a content ≥99.5% to 0.8 - 1.5 MPa and heat it to 153°C - 203°C.

[0026] P1-4, Oxidation of Toluene by Air: The air pretreated in step P1-2 is fully dispersed to form micron-sized bubbles. The micron-sized bubbles are mixed with the pretreated toluene in step P1-3 and the catalyst mixture M obtained in step P1-1 and then transported to the oxidation reactor. Control the mass percentage of the catalyst in toluene in the oxidation reactor to be 0.5% - 1%, the mass percentage of the initiator in toluene to be 0.1% - 0.5%, and the mass percentage of the promoter in toluene to be 0.01% - 0.05%. Control the gas-liquid volume ratio to be (1:8) - (1:12) (the volume of the gas is calculated under standard conditions), the reaction temperature to be 155 - 205 °C, and the reaction pressure to be 0.8 - 1.5 MPa. Under the synergistic action of the catalyst, the initiator, and the promoter, toluene undergoes an oxidation reaction with the oxygen in the micron-sized air bubbles. Control the residence time of toluene in the oxidation reactor to be 30 - 60 min to obtain the oxidation reaction liquid K containing benzyl peroxide. The tail gas after the reaction is cooled by a tail gas condenser, and the organic substances (mainly benzene, toluene, formic acid, acetic acid, etc.) in the tail gas are condensed and then transported to the oxidation reaction liquid K and mixed with the oxidation reaction liquid K as the reaction raw material for step P2.

[0027] P2, Directed Decomposition of Benzyl Peroxide P2-1, Preparation of Catalyst Mixture N: Mix the basic inorganic compound, sodium vanadate or / and triphenylphosphine, and diphosphonate evenly, preheat to 35 °C - 50 °C, and pressurize to 0.4 - 0.6 MPa to prepare the catalyst mixture N. Among them, the basic inorganic compound is one or a combination of several of sodium carbonate, potassium carbonate, sodium hydroxide, and potassium hydroxide.

[0028] P2-2, Directed Decomposition of Benzyl Peroxide into Benzyl Alcohol: Transport the oxidation reaction liquid K from the bottom of the directed decomposition tower into the tower, and transport the catalyst mixture N from the middle-upper part of the directed decomposition tower into the tower. Control the pressure to be 0.4 - 0.6 MPa and the reaction temperature to be 50 °C - 70 °C. The mass percentage of the basic inorganic compound in the catalyst mixture N in the oxidation reaction liquid K is 5% - 15%, the mass percentage of sodium vanadate or triphenylphosphine or a mixture of the two in any mass ratio in the oxidation reaction liquid K is 3% - 5%, and the mass percentage of diphosphonate in the oxidation reaction liquid K is 1% - 3%. The oxidation reaction liquid K and the catalyst mixture N are in reverse contact and react fully to direct the decomposition of benzyl peroxide into benzyl alcohol. The acidic organic impurities react with the basic inorganic compound to form organic acid salts and dissolve in the aqueous phase. The acidic organic impurities mainly include acetic acid, formic acid, propionic acid, benzoic acid, and phenolic impurities, etc. After the reaction, the aqueous phase (this aqueous phase is the alkaline water obtained in the process of producing benzyl alcohol by the toluene oxidation method in this application) is discharged from the bottom of the tower, and the organic phase overflows from the top of the tower to obtain the reaction liquid L containing benzyl alcohol.

[0029] In the above-mentioned process steps for producing benzyl alcohol by toluene oxidation, in the aqueous phase obtained in step P2-2, the total content of organic substances such as benzaldehyde and benzyl alcohol is 2% - 4%, the content of sodium formate is 5% - 7%, the content of sodium acetate is 8% - 11%, the content of sodium benzoate is 11% - 20%, and the total content of methylphenol is 8% - 10%. The contents of the foregoing components vary within their respective concentration ranges due to factors such as the degree of oxidation during each reaction process and the dosage of the added basic inorganic compound.

[0030] In the above-mentioned process steps for producing benzyl alcohol by toluene oxidation, taking sodium hydroxide as the basic compound, the reaction equations for generating acetic acid and sodium acetate are as follows: 。

[0031] 1. Selection of mobile phase In this application, a mixed solution of potassium dihydrogen phosphate aqueous solution and acetonitrile is selected as the mobile phase. Compared with methanol, acetonitrile as the organic phase can effectively adjust the polarity of the mobile phase, optimize the separation degree between sodium acetate and impurities, and has a lower viscosity, which can also improve the column efficiency. Sodium acetate is a weak acid salt. Adding potassium dihydrogen phosphate as a buffer salt can maintain the pH of the mobile phase stable at 2.5 - 3.0, inhibit the ionization of sodium acetate, and make it exist in molecular form, so as to achieve better retention and separation on the chromatographic column.

[0032] It is found during the experiment that generally, the higher the content of the organic phase (acetonitrile), the faster the peak emergence time of the standard solution. However, when the acetonitrile content exceeds 2% (v / v), during sample analysis, the separation degree between the target peak and impurities is poor, and salt precipitation will also occur at a relatively low room temperature; as the content of the buffer salt (potassium dihydrogen phosphate) increases, the peak emergence time of the standard solution is slow and the peak shape is poor. When the buffer salt concentration is too high, crystals are easily precipitated, causing blockage of the chromatographic column or the instrument system, and also affecting the stability of the pH value of the mobile phase, resulting in unstable retention time of the target peak. Therefore, in order to balance the separation effect and analysis time, a mixed solution composed of 0.03mol / L potassium dihydrogen phosphate aqueous solution with pH = 2 - 3 and acetonitrile is selected as the mobile phase for target peak analysis. To further improve the analysis efficiency, gradient elution is adopted to increase the proportion of the organic phase to accelerate the outflow of impurities other than the target peak and shorten the total analysis time.

[0033] 2. Selection of detection wavelength The sodium acetate sample is continuously scanned with a UV-visible spectrophotometer at wavelengths from 190nm to 800nm, and it is found that sodium acetate has the maximum absorption at 200nm - 210nm. Since the maximum absorption of acetic acid is at the ultraviolet end and is easily interfered by solvents and impurities, and the response value is good at 210nm, the detection wavelength is determined to be 210nm.

[0034] 3. Sample pretreatment Take an appropriate amount of alkaline water obtained from the process of producing benzyl alcohol by toluene oxidation. Add toluene to the alkaline water according to the mass ratio of alkaline water to toluene of 3:1, 2:1, 1:1, 1:2, and 1:3, and perform shaking extraction, then let it stand for phase separation. Among them, the phase separation is slow and an intermediate phase appears at the ratios of 3:1 and 2:1, while the phase separation time is fast at the ratios of 1:1, 1:2, and 1:3, without an intermediate phase and all can achieve the same extraction effect. Considering reducing the usage amount of toluene to lower costs, the ratio of alkaline water:toluene = 1:1 (mass ratio) is selected for shaking extraction.

[0035] Accurately weigh multiple portions of the lower-layer alkaline water phase (alkaline water phase A) obtained by toluene extraction. Drop sulfuric acid solutions with mass concentrations of 50%, 60%, 70%, 80%, and 98% into each portion of alkaline water phase A in turn, adjust the pH to 7 - 8, let it stand for phase separation, and record the mass of the lower-layer water phase (alkaline water phase B). Among them, the reaction is fast when dropping 80% sulfuric acid solution, the end-point phenomenon is easy to observe, the oil phase is clearly layered, and there is no intermediate phase. Under the condition of pH = 7 - 8, without affecting sodium acetate, sodium methylphenolate can be completely converted into methylphenol with less acid consumption. Therefore, the reaction condition of dropping 80wt% sulfuric acid to adjust the pH to 7 - 8 is selected.

[0036] 4. Sample solution preparation (1)Screening of sample solution treatment conditions Accurately weigh an appropriate amount of alkaline water phase B, add mixed solution C, shake well, add phosphoric acid to adjust the pH to 2.5 - 3.0, and mix evenly. Through experiments, it is found that when the dilution factor of alkaline water phase B is 50 - 100 times, there are many impurities in the sample, and the separation degree between sodium acetate and the impurities in the sample is poor. When the dilution factor exceeds 200 times, the concentration of sodium acetate in the sample is too low, and the analysis error is large. Whether the dilution factor is too high or too low will affect the accuracy of the detection result. Therefore, the dilution factor of alkaline water phase B is controlled at 100 - 200 times.

[0037] Divide the evenly mixed sample into multiple portions, and place them at four temperatures of 0 - 2°C, 2 - 5°C, 5 - 10°C, and room temperature for 10 minutes of cooling respectively. Among them, the most precipitation occurs at 0 - 2°C, and benzoic acid is completely precipitated. Therefore, 0 - 2°C is selected as the optimal cooling temperature.

[0038] (2)Preparation of sample solution Accurately weigh 50.1500 g of the alkaline water obtained from the process of producing benzyl alcohol by toluene oxidation. Perform shaking extraction according to the mass ratio of alkaline water to toluene of 1:1. After standing for layering, 48.0256 g of alkaline water phase A is obtained. Add sulfuric acid solution with a mass concentration of 80% dropwise to alkaline water phase A, adjust the pH to 7 - 8, stand for layering, and obtain 47.8949 g of alkaline water phase B. Weigh 0.1025 g of alkaline water phase B, add 20.0228 g of mixed solution C, shake well, add 0.4425 g of phosphoric acid to adjust the pH to 2.8, and mix evenly. Place the prepared solution in the refrigerator and store it at 0 - 2 °C for ten minutes to completely precipitate benzoic acid. Then filter it through a 0.22 - μm microporous filter membrane to obtain the sample solution.

[0039] 5. Liquid Chromatograph Condition Setting Instrument: Agilent 1260 Liquid Chromatograph; Mobile phase: It includes mobile phase I and mobile phase II. Mobile phase I is an aqueous solution of potassium dihydrogen phosphate with a concentration of 0.03 mol / L and a pH of 2 - 3, and mobile phase II is acetonitrile. Gradient elution is performed, and the elution program is shown in Table 1.

[0040] Table 1 Liquid Phase Detection Gradient Elution Program

[0041] Adjust the pH of the sample solution to 2.5 - 3.0. Within this pH range, sodium benzoate can be completely converted into benzoic acid and precipitated, and the peak shape is the best. Flow rate: 1 mL / min; Detection wavelength: 210 nm; Detector: Ultraviolet - Visible Light Detector; Column temperature: 30 °C; Injection volume: 20 μL; Chromatographic column: WondaCract ODS - 2 (4.6 × 250 mm, 5 - Micron).

[0042] 6. Preparation of Standard Solution Accurately weigh 1.4948 g of sodium acetate trihydrate standard (with a content of 99%), place it in a 100 - mL solvent bottle, add mixed solution C (an aqueous solution of potassium dihydrogen phosphate with a concentration of 0.03 mol / L and a pH of 2 - 3 and acetonitrile in a volume ratio of 99:1) until the total mass of the liquid is 96.2380 g, and shake well to obtain a standard stock solution with a concentration of 9.2750 mg / g.

[0043] Take 1.0303 g of the standard stock solution, dilute it to 47.5528 g with mixed solution C, and adjust the pH to 2.5 - 3.0 with 0.1275 g of phosphoric acid (with a content of 99.5%) to obtain standard solution 1# with a concentration of 0.2004 mg / g.

[0044] Take 2.6699 g of the standard stock solution, dilute it to 50.2268 g with mixed solution C, and adjust the pH to 2.5 - 3.0 with 0.1587 g of phosphoric acid (content 99.5%) to obtain standard solution 2# with a concentration of 0.4915 mg / g.

[0045] Take 5.3908 g of the standard stock solution, dilute it to 50.2629 g with mixed solution C, and adjust the pH to 2.5 - 3.0 with 0.1805 g of phosphoric acid (content 99.5%) to obtain standard solution 3# with a concentration of 0.9912 mg / g.

[0046] Take 10.3551 g of the standard stock solution, dilute it to 46.3365 g with mixed solution C, and adjust the pH to 2.5 - 3.0 with 0.3344 g of phosphoric acid (content 99.5%) to obtain standard solution 4# with a concentration of 2.0579 mg / g.

[0047] Take 20.1120 g of the standard stock solution, dilute it to 45.6996 g with mixed solution C, and adjust the pH to 2.5 - 3.0 with 0.4949 g of phosphoric acid (content 99.5%) to obtain standard solution 5# with a concentration of 4.0381 mg / g.

[0048] 7. Establish a standard curve Use mixed solution C as the sample blank solution to calibrate the zero point of the liquid chromatograph. Under the same liquid chromatographic conditions as those for measuring the sample solution (the chromatographic conditions in item 5), measure the peak areas of standard solutions 1#, 2#, 3#, 4#, and 5#. Take the peak area A as the ordinate (y) and the corresponding concentration C (mg / g) as the abscissa (x), perform linear regression, and obtain the standard curve equation y = 462.31x - 2.0473 (as Figure 1 shown), and the correlation coefficient R 2 = 1. The results show that sodium acetate has a good linear relationship in the range of 0.2004 mg / g - 4.0381 mg / g.

[0049] Use mixed solution C as the sample blank solution, continuously run for 20 min under the condition of mobile phase I:mobile phase II = 99:1 (volume ratio), record the baseline signal during this period, and continuously test 3 times. The standard deviation of the instrument baseline noise is 0.05 mV. Through calculation, the detection limit concentration of sodium acetate detected by the detection method of the present invention is 0.0003 mg / g, and the quantification limit concentration is 0.0011 mg / g.

[0050] 8. Determination of the sample solution After the baseline of the instrument is stable, select any one of the standard solutions 1#, 2#, 3#, 4#, and 5# for two determinations. After the area change of the two injections of the standard solution is less than 1%, perform the determination of the sample solution. The liquid chromatogram of the standard solution is shown in Figure 2 , and the liquid chromatogram of the sample solution is shown in Figure 3 . The retention time of sodium acetate is 5.078 min ± 0.010 min, the retention time is stable, and the peak shape is sharp. According to the external standard method, the chromatographic result is 0.5148 mg / g. According to the dilution factor, the content of sodium acetate in the test solution is 10.3300%, and then according to the conversion relationship, the content of sodium acetate in the sample is calculated to be 9.8655%.

[0051] In summary, the analysis method for the content of sodium acetate in the alkaline water of the process for producing benzyl alcohol by toluene oxidation provided in the present invention includes the following steps: S1. Accurately weigh the test sample (i.e., the alkaline water obtained from the process for producing benzyl alcohol by toluene oxidation), and record the mass of the test sample. Extract the test sample with toluene, let it stand for layering to obtain the alkaline water phase A. Adjust the pH of the alkaline water phase A to 7 - 8, let it stand for layering to obtain the alkaline water phase B, and record the mass of the alkaline water phase B. Accurately weigh a certain mass of the alkaline water phase B, dilute it with the mixed solution C composed of potassium dihydrogen phosphate aqueous solution and acetonitrile, and adjust the pH to 2.5 - 3.0. Record the dilution factor, place it at 0 - 2 °C until benzoic acid completely precipitates, and filter it through a microporous filter membrane. The obtained filtrate is the sample solution.

[0052] Accurately weigh the sodium acetate reference substance, and use the mixed solution C as the solvent to prepare standard solutions with different concentrations of the sodium acetate reference substance.

[0053] S2. After the baseline of the liquid chromatograph is stable, measure the peak areas of the standard solutions with different concentrations, and draw a standard curve; measure the sample solution under the same conditions as those for measuring the standard solutions to obtain the peak area of the sample solution.

[0054] S3. According to the peak area of the sample solution and the standard curve, calculate the content of sodium acetate in the sample solution, and then calculate the content of sodium acetate in the test sample according to the dilution factor and the mass relationship between the test sample and the alkaline water phase B.

[0055] Among them, the mixed solution C is a mixed solution of 0.03 mol / L potassium dihydrogen phosphate aqueous solution with pH = 2 - 3 and acetonitrile. Further specifically, the mixed solution C is a mixed solution of 0.03 mol / L potassium dihydrogen phosphate aqueous solution with pH = 2 - 3 and acetonitrile in a volume ratio of 99:1.

[0056] Further, when adjusting the pH of the alkaline water phase A to 7 - 8, use an 80 wt% sulfuric acid solution for adjustment.

[0057] Further, the concentration range of sodium acetate in the standard solution in step S1 is 0.2 mg / (g solution) to 4.0 mg / (g solution). When preparing the standard solution in step S1, first prepare the standard stock solution, and then dilute the standard stock solution into standard solutions with different concentrations. For example, first prepare a standard stock solution with a sodium acetate concentration of 10 mg / (g solution), and then dilute this standard stock solution into solutions with sodium acetate concentrations of 0.2 mg / g, 0.5 mg / g, 1.0 mg / g, 2.0 mg / g, and 4.0 mg / g respectively.

[0058] 9. Standard recovery test According to the above detection method, determine the content of sodium acetate in the sample solution and determine the standard recovery rate. The results are shown in Table 2.

[0059] Table 2 Detection results of standard recovery rate

[0060] It can be seen from Table 2 that the standard recovery rate of the method of the present invention for detecting the content of sodium acetate in the alkaline water of the benzyl alcohol production process by toluene oxidation is 100% to 101%, indicating that the method of the present invention has high accuracy and good reliability.

[0061] 10. Reproducibility test Using the method provided by the present invention, consistent results can be obtained when analyzing the same sample by different operators, on different instrument and equipment, at different or the same times, and the error does not exceed 0.0002%, indicating good reproducibility.

[0062] 11. Precision test The same sample was continuously measured 10 times under the optimized conditions (the sample treatment conditions and preparation conditions in items 3 and 4 above and the detection conditions in item 5), and the relative standard deviation was 2.04%.

[0063] The above-described embodiments are only the preferred embodiments of the present application and are not used to limit the protection scope of the present application. For any person skilled in the art, the present application can have various changes and modifications. Any simple equivalent changes and modifications made according to the protection scope of the present application and the content of the specification shall be included in the protection scope of the present application.

Claims

1. A method for analyzing the sodium acetate content in the alkali water of the process for producing benzyl alcohol by toluene oxidation, characterized in that, It includes the following steps: S1. Accurately weigh the sample to be tested and record its mass. The sample to be tested is extracted with toluene, allowed to stand for layer separation to obtain an alkaline aqueous phase A. Adjust the pH value of the alkaline aqueous phase A to 7 - 8, allow it to stand for layer separation to obtain an alkaline aqueous phase B, and record the mass of the alkaline aqueous phase B. Accurately weigh a certain mass of the alkaline aqueous phase B, dilute it with a mixed solution C composed of a potassium dihydrogen phosphate aqueous solution and acetonitrile, adjust the pH to 2.5 - 3.0, record the dilution factor, place it at 0 - 2 °C until benzoic acid completely precipitates, and filter it through a microporous membrane. The obtained filtrate is the sample solution. Accurately weigh a sodium acetate reference substance, use the mixed solution C as a solvent, and prepare standard solutions with different concentrations of the sodium acetate reference substance. S2. After the baseline of the liquid chromatography instrument is stable, measure the peak areas of the standard solutions with different concentrations and draw a standard curve. Measure the sample solution under the same conditions as those for measuring the standard solutions to obtain the peak area of the sample solution. The detection conditions of the liquid chromatography are as follows: Mobile phase: It includes mobile phase I and mobile phase II. Mobile phase I is a 0.03 mol / L potassium dihydrogen phosphate aqueous solution with a pH of 2 - 3, and mobile phase II is acetonitrile. Gradient elution is performed, and the elution program is as follows: Flow rate: 1 mL / min; Detection wavelength: 210 nm; Column temperature: 30 °C; Injection volume: 20 μL; Chromatographic column: WondaCract ODS - 2, with a specification of 4.6×250 mm, 5 - Micron; S3. According to the peak area of the sample solution and the standard curve, calculate the content of sodium acetate in the sample solution, and then calculate the content of sodium acetate in the sample to be tested based on the dilution factor and the mass relationship between the sample to be tested and the alkaline aqueous phase B.

2. The analysis method according to claim 1, characterized in that, In step S1, the mass ratio of the sample to be tested to the toluene is 1:(1 - 3).

3. The analysis method according to claim 1, characterized in that In step S1, when adjusting the pH of the alkaline aqueous phase A to 7 - 8, an 80 wt% sulfuric acid solution is used for adjustment.

4. The analysis method according to claim 1, characterized in that In step S1, the concentration of the potassium dihydrogen phosphate aqueous solution is 0.03 mol / L, and the pH value is 2 - 3.

5. The analysis method according to claim 4, characterized in that In step S1, the volume ratio of the potassium dihydrogen phosphate aqueous solution to the acetonitrile is 99:

1.

6. The analysis method according to claim 1, characterized in that In step S1, the concentration range of sodium acetate in the standard solution is 0.2 mg / g - 4.0 mg / g.

7. The analysis method according to claim 1, characterized in that In step S1, the dilution factor is 100 - 200.

8. The analysis method according to claim 1, characterized in that In step S1, the pore size of the microporous membrane is 0.22 μm.

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