Method for drawing gas-liquid equilibrium phase diagram of completely mutually soluble two-liquid system

Abstract

The invention relates to the technical field of chemical detection, in particular to a method for drawing a gas-liquid equilibrium phase diagram of a completely mutually soluble two-liquid system. The drawing method comprises the following steps: establishing an absorbance-ethanol volume fraction standard curve; preparing a target double-liquid system and realizing gas-liquid balance; measuring the absorbance and converting the composition; and drawing a phase diagram. The invention provides a bromophenol blue-based spectrophotometric method to replace a refractive index method, the method is suitable for a double-liquid system with one liquid being ethanol, and by utilizing the light absorption specificity of bromophenol blue to ethanol, when the ethanol content is changed, the absorbance of the double-liquid system at a specific wavelength is changed along with the increase of the ethanol content, and the ethanol content can be converted from the absorbance. The testing method is large in absorbance change range and high in sensitivity, and manual adjustment and manual reading are not needed. According to the method, the drawing precision and accuracy of the two-liquid system gas-liquid equilibrium phase diagram can be improved, and the selection range of the two-liquid system for drawing the phase diagram is widened.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of chemical detection, in particular to a method for drawing a gas-liquid equilibrium phase diagram of a completely miscible binary liquid system. BACKGROUND

[0002] The gas-liquid equilibrium phase diagram of a completely miscible binary liquid system (hereinafter referred to as a binary liquid system) is a core thermodynamic diagram that describes the corresponding relationship between the gas phase composition and the liquid phase composition of a binary liquid system at a specific temperature. The accuracy of the diagram directly determines the design efficiency and operational stability of separation processes such as chemical distillation and extraction, and has irreplaceable application value in the fields of petroleum and chemical industry, pharmaceutical synthesis, and fine chemical industry.

[0003] Currently, the key to drawing a gas-liquid equilibrium phase diagram of a binary liquid system lies in accurately determining the component content of the gas phase (distillate) and the liquid phase (residual liquid) under gas-liquid equilibrium conditions. The most widely used method in existing technology is the refractive index method: by establishing a standard curve of "refractive index-component content" in advance, the refractive index of the distillate and the residual liquid is measured, and the component content is converted by combining the standard curve. However, this method has significant defects: first, refractive index measurement requires manual adjustment of light path focusing, which is tedious and prone to errors due to differences in personnel operation; second, refractive index is sensitive to temperature, and fluctuations in environmental temperature will directly lead to measurement errors, requiring additional constant temperature devices, increasing experimental cost and operational complexity; third, for some solvents with small differences in refractive index compared to ethanol (such as acetone, ethyl acetate), the refractive index does not change significantly over the entire concentration range, which can lead to insufficient accuracy in component content conversion and affect the accuracy of the phase diagram data points.

[0004] To solve the above problems, there is an urgent need in the field for a binary liquid system component content determination method that is simple to operate, has strong anti-interference ability, and has high measurement accuracy, to support efficient and accurate drawing of a binary liquid system gas-liquid equilibrium phase diagram. SUMMARY

[0005] To address the problems of existing refractive index method for drawing a binary liquid system gas-liquid equilibrium phase diagram, such as tedious operation, susceptibility to temperature interference, and insufficient accuracy, the present application provides a method for drawing a completely miscible binary liquid system gas-liquid equilibrium phase diagram based on bromophenol blue photometry, aiming to simplify and accurately determine component content, reduce experimental operation difficulty and environmental dependence, and improve the efficiency and reliability of phase diagram drawing.

[0006] To solve the above technical problems, the technical solution provided by the present application is as follows: A method for drawing a completely miscible binary liquid system gas-liquid equilibrium phase diagram, comprising the following steps: A bromophenol blue is used as a chromogenic agent to prepare a double-liquid standard solution containing different ethanol volume fractions, the double-liquid standard solution being composed of ethanol and a second liquid; the absorbance of the double-liquid standard solution containing different ethanol volume fractions is measured at a set wavelength to establish an absorbance-ethanol volume fraction standard curve; A target double-liquid solution is added into a boiling point apparatus and heated to gas-liquid equilibrium, the boiling point is recorded, and the distillate and the remaining liquid are collected; the target double-liquid solution is composed of ethanol and a second liquid; The distillate and the remaining liquid are mixed with a bromophenol blue solution respectively, the absorbance of each mixed solution is measured at the set wavelength, and the ethanol volume fraction in the distillate and the remaining liquid is converted into a mole fraction based on the absorbance-ethanol volume fraction standard curve; The gas phase line point and the liquid phase line point are obtained with the boiling point as the ordinate and the ethanol mole fraction of the distillate and the remaining liquid as the abscissa, the ratio of ethanol to the second liquid of the target double-liquid system is replaced, the above-mentioned gas-liquid equilibrium to line point obtaining step is repeated, and the gas-liquid equilibrium phase diagram of the target double-liquid system is obtained by connecting the line points.

[0007] In an embodiment of the present application, the second liquid is selected from at least one of water, cyclohexane, acetone, ethyl acetate, and benzene.

[0008] In an embodiment of the present application, the concentration of the bromophenol blue in the double-liquid standard solution is 25-50 μmol / L.

[0009] In an embodiment of the present application, the second liquid is water, and the set wavelength is 590 nm.

[0010] In an embodiment of the present application, the second liquid is one of cyclohexane, acetone, ethyl acetate, and benzene, and the set wavelength is 425 nm.

[0011] In an embodiment of the present application, before heating to gas-liquid equilibrium, a zeolite is added into the boiling point apparatus, and condensed water is continuously introduced during the heating process.

[0012] In an embodiment of the present application, the volume ratio of the distillate or the remaining liquid to the bromophenol blue solution is (1:1)-(1:2).

[0013] In an embodiment of the present application, the total volume of the mixed solution is 1-15 mL.

[0014] In an embodiment of the present application, when the absorbance of the double-liquid standard solution containing different ethanol volume fractions is measured, each double-liquid standard solution is subjected to at least three parallel determinations, and the average value is used to establish the absorbance-ethanol volume fraction standard curve.

[0015] In an embodiment of the present application, the goodness of fit R2 of the absorbance-ethanol volume fraction standard curve is ≥0.99.

[0016] In an embodiment of the present application, when the bromophenol blue solution is prepared, the solvent used is the second liquid in the binary system standard solution or ethanol.

[0017] The present application proposes a bromophenol blue-based photometric method to replace the refractive index method, which is suitable for a binary system in which one liquid is ethanol. The method utilizes the light absorption specificity of bromophenol blue to ethanol. When the ethanol content changes, the absorbance of the binary system at a specific wavelength changes with the increase of the ethanol content, and the ethanol content can be converted from the absorbance. The test method has a large range of absorbance change, high sensitivity, and does not require manual adjustment and manual reading. The implementation of the present application can improve the drawing accuracy and accuracy of the binary system gas-liquid equilibrium phase diagram, and expand the selection range of the binary system for drawing the phase diagram.

[0018] Other features and advantages of the present application will be set forth in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or can be learned by practice of the present application. The objects and other advantages of the present application can be realized and attained by the structure particularly pointed out in the description and claims and the appended drawings. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description. Obviously, the drawings described in the following description are some embodiments of the present application, and for those skilled in the art, other drawings can also be obtained without creative labor on the basis of these drawings. In the following description, the positional relationship described in the drawings is the direction of the components drawn in the drawings as the reference unless otherwise specified.

[0020] Figure 1 Structure diagram of the boiling point apparatus used in the embodiments of the present application; Figure 2 Binary phase diagram with the lowest constant boiling point (left) and binary phase diagram with the highest constant boiling point (right); Figure 3 Refractive index apparatus used in the embodiments of the present application; Figure 4 Refractive index apparatus used in the embodiments of the present application; Figure 3 Refractive index apparatus before and after adjustment and theoretical and actual appearance; Figure 5 Absorption spectrum of the ethanol-water binary system with different composition ratios; Figure 6 Appearance of the ethanol-water binary system with different composition ratios; Figure 7Absorption spectrum of ethanol-cyclohexane binary system with different composition ratios; Figure 8 Appearance of ethanol-cyclohexane binary system with different composition ratios; Figure 9 Absorption spectrum of ethanol-acetone binary system with different composition ratios; Figure 10 Appearance of ethanol-acetone binary system with different composition ratios; Figure 11 Absorption spectrum of ethanol-ethyl acetate binary system with different composition ratios; Figure 12 Appearance of ethanol-ethyl acetate binary system with different composition ratios; Figure 13 Absorption spectrum of ethanol-benzene binary system with different composition ratios; Figure 14 Appearance of ethanol-benzene binary system with different composition ratios; Figure 15 Standard curve of Example 1; Figure 16 Standard curve of Example 2; Figure 17 Binary system gas-liquid equilibrium phase diagram of Example 1; Figure 18 Binary system gas-liquid equilibrium phase diagram of Example 2; Figure 19 Standard curve of Example 3; Figure 20 Standard curve of Example 4; Figure 21 Binary system gas-liquid equilibrium phase diagram of Example 3; Figure 22 Binary system gas-liquid equilibrium phase diagram of Example 4. DETAILED DESCRIPTION

[0021] In order to make the objects, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some but not all of the embodiments of the present application; and the technical features in the different embodiments of the present application described below can be combined with each other as long as there is no conflict. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the protection scope of the present application.

[0022] In the description of this invention, it should be noted that all terms used in this invention (including technical and scientific terms) have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains, and should not be construed as limiting the invention; it should be further understood that the terms used in this invention should be understood to have the same meaning as those in the context of this specification and in the relevant field, and should not be understood in an idealized or overly formal sense, except as expressly defined in this invention.

[0023] A binary liquid system is a solution system consisting of two completely miscible liquids, such as water or an organic solvent, let's say one is A and the other is B. Common binary liquid systems include water-ethanol, ethanol-butanone, and ethanol-cyclohexane. Drawing the vapor-liquid equilibrium phase diagram of a binary liquid system is a common undergraduate physical chemistry teaching experiment. The experimental steps are as follows (please refer to the instructions). Figure 1 A binary liquid system of a certain composition is added to a boiling point apparatus along with a small amount of boiling chips. Cooling water is then passed through the system, and the mixture is heated until vapor-liquid equilibrium is reached (the solution boils and the thermometer reading remains constant). The temperature at this point is the boiling point of the solution. A portion of the liquid vaporizes, is cooled by cooling water, and flows back into the groove of the boiling point apparatus, forming the distillate. The remaining liquid in the apparatus is the residue. Heating is stopped, and the mixture is cooled to room temperature. Stoppers 1 and 2 are opened, and the distillate and residue are removed and their refractive indices are measured.

[0024] The refractive index-composition curves of the standard solutions are converted into the distillate composition yA and the residual composition xA. Plotting the boiling point as the ordinate and yA as the abscissa, a point (T-yA) on the vapor phase line of the phase diagram can be determined. Plotting the boiling point as the ordinate and xA as the abscissa, a point (T-xA) on the liquid phase line of the phase diagram can be determined. The two liquid compositions are changed, and this process is repeated several times. Each time, boiling is required until vapor-liquid equilibrium is reached. The boiling points are recorded, the solutions are cooled, and the refractive indices of the distillate and residual liquids are measured. The T-yA values ​​recorded in all steps form a series of points. A curve is plotted according to the variation pattern of these points, representing the vapor phase line of the phase diagram. Similarly, the T-xA values ​​recorded in all steps form a series of points. A curve is plotted according to the variation pattern of these points, representing the liquid phase line of the phase diagram. This completes the two-liquid phase diagram (reference). Figure 2 Zuohe Figure 2 As shown on the right, the liquidus line and the vapor line intersect at a single point, which is called the azeotropic point. If the temperature of the azeotropic point is higher than the boiling points of A and B (TA* and TB*), it is the highest azeotropic point; otherwise, it is the lowest azeotropic point.

[0025] The composition of the residual liquid and distillate was determined by refractive index analysis, using an Abbe refractometer (reference). Figure 3 As shown in the diagram, during measurement, open the light shield, loosen the lock, lift the prism mount, drop several drops of sample solution onto the sample plate to spread the solution evenly, close the prism mount, and tighten the lock. At this point, the field of view observed through the eyepiece will be... Figure 4The scene in the middle is unadjusted, the prism rotation hand wheel and the dispersion adjustment hand wheel are adjusted to adjust the position of the light and dark interface, eliminate the dispersion, and the field of view should present at this time Figure 4 The scene in the middle is adjusted (theoretical), but due to the accuracy of the instrument itself, the dispersion cannot be completely eliminated, and the actual presentation is Figure 4 The scene in the middle is adjusted (actual). After adjustment, the scale below the field of view is read, that is, the refractive index. When the sample is replaced, if the boiling point of the sample is lower than the boiling point of water, use an ear bulb to blow dry the remaining sample on the sample plate, otherwise use a lens paper to wipe dry. After replacement, the prism rotation hand wheel and the dispersion adjustment hand wheel need to be adjusted again, and the reading is re-read.

[0026] From the above refractive index measurement process, it can be seen that the measurement of the refractive index relies on manual adjustment and manual reading, and the subjectivity is high. Especially in the case where the dispersion cannot be completely eliminated, the accuracy of multiple measurements is low. In addition, the change range of the refractive index is relatively small, resulting in low sensitivity and large measurement error. As shown in Table 1, taking the binary system with the largest change range of ethanol-cyclohexane as an example, the volume fraction of ethanol is from 0% to 100%, and the volume fraction of cyclohexane is from 100% to 0%, and the change range of the refractive index is only from about 1.43 to 1.36. And other such as ethanol-acetone, ethanol-ethyl acetate, etc. The change range is smaller, so the binary system for the phase diagram drawing experiment project in the course teaching can be selected, and the binary system of ethanol-cyclohexane is generally selected.

[0027] Table 1: Change range of refractive index of common binary system (20℃)

[0028] Therefore, the present application provides a photometric method based on bromophenol blue to replace the refractive index method, which is suitable for binary systems in which one liquid is ethanol. The method uses the specific absorption of bromophenol blue to ethanol. When the ethanol content changes, the absorbance of the binary system at a specific wavelength changes with the increase of the ethanol content, and the ethanol content can be calculated from the absorbance.

[0029] Specifically, an embodiment of the present application provides a method for drawing a complete miscible binary system gas-liquid equilibrium phase diagram, comprising the following steps: Step 1, prepare a double liquid system standard solution containing different ethanol volume fractions with bromophenol blue as a chromogenic agent, the double liquid system standard solution is composed of ethanol and at least one second liquid selected from water, cyclohexane, acetone, ethyl acetate, benzene, the concentration of bromophenol blue in the double liquid system standard solution is 25-50 μmol / L, for example 25 μmol / L, 30 μmol / L, 35 μmol / L, 40 μmol / L, 45 μmol / L, 50 μmol / L, etc., but not only limited to the listed values, other values not listed in this range are also applicable, increasing the concentration will increase the absorbance and sensitivity, but too high concentration will make the absorbance too large, leading to increased measurement error; determine the absorbance of the double liquid system standard solution containing different ethanol volume fractions at the set wavelength, and establish the absorbance-ethanol volume fraction standard curve; Step 2, add the target double liquid system solution to the boiling point apparatus and heat to gas-liquid equilibrium, record the boiling point and collect the distillate and the remaining liquid, the distillate is the liquid that is vaporized from the target double liquid system solution and cooled by condensate water backflow into the groove of the boiling point apparatus; the target double liquid system solution is composed of ethanol and at least one second liquid selected from water, cyclohexane, acetone, ethyl acetate, benzene; preferably, before heating to gas-liquid equilibrium, add zeolite to the boiling point apparatus, and continuously introduce condensate water during heating.

[0030] Step 3, mix the distillate and the remaining liquid with bromophenol blue solution respectively; Preferably, the volume ratio of the distillate or the remaining liquid to the bromophenol blue solution is (1:1)-(1:2), for example (1:1), (1:1.2), (1:1.5), (1:1.8), (1:2), etc., but not only limited to the listed values, other values not listed in this range are also applicable, the purpose of controlling this volume ratio is that although increasing the proportion of bromophenol blue standard solution will increase the overall absorbance and sensitivity, but too much will make the absorbance too large, and the measurement error is also relatively large. The total volume of the mixed liquid is 1-15 mL, for example 1 mL, 2 mL, 3 mL, 4 mL, 5 mL, 6 mL, 7 mL, 8 mL, 9 mL, 10 mL, 11 mL, 12 mL, 13 mL, 14 mL, 15 mL, etc., but not only limited to the listed values, other values not listed in this range are also applicable; it should be noted that since the remaining liquid sample is generally 1-5 mL, but the distillate can only be taken from the groove of the boiling point apparatus, and since the capacity of the groove of the boiling point apparatus varies, it is generally 0.5-1 mL in the laboratory scene, therefore, the total volume of their mixed liquid depends on the previous volume ratio, for example, if the volume of the distillate is 1 mL, then if the volume ratio of the distillate to the bromophenol blue solution is 1:2, then the total volume of the mixed liquid is 3 mL.

[0031] The absorbance of each mixed solution at a set wavelength is determined, and the ethanol volume fraction in the distillate and the remaining liquid is converted into molar fraction based on the absorbance-ethanol volume fraction standard curve and converted into molar fraction; the set wavelength is determined according to the highest point of the standard solution absorption spectrum, so that the absorbance is the largest and the sensitivity is the highest; for example, if the second liquid is water, the set wavelength is 590 nm; and if the second liquid is one of cyclohexane, acetone, ethyl acetate, and benzene, the set wavelength is 425 nm. This is because the ethanol-water binary liquid system has two maximum absorption wavelengths, one at 425 nm and the other at 590 nm, and the absorbance at 425 nm is less related to the ethanol concentration, so 590 nm is used.

[0032] Step 4: Obtain the gas-liquid line points with the boiling point as the abscissa and the ethanol molar fraction of the distillate and the remaining liquid as the ordinate, replace the ethanol and the second liquid ratio of the target binary liquid system, repeat the above steps of heating to gas-liquid equilibrium→ recording boiling point→ collecting distillate / remaining liquid→ measuring composition→ obtaining line points, and connect the line points to obtain the gas-liquid equilibrium phase diagram of the target binary liquid system.

[0033] In this embodiment, the target binary liquid system solution and the determined binary liquid system standard solution are the same type of binary liquid system, such as an ethanol-water binary liquid system solution, an ethanol-cyclohexane binary liquid system solution, an ethanol-acetone binary liquid system solution, an ethanol-ethyl acetate binary liquid system solution, or an ethanol-benzene binary liquid system solution.

[0034] In this embodiment, to improve the accuracy, when determining the absorbance of the binary liquid system standard solution containing different ethanol volume fractions, at least 3 parallel determinations are performed for each binary liquid system standard solution, and the average value is used to establish the absorbance-ethanol volume fraction standard curve.

[0035] In this embodiment, when preparing the bromophenol blue solution, the solvent used is the second liquid or ethanol in the binary liquid system standard solution.

[0036] Those skilled in the art can understand that the order of writing each step in the method of each embodiment or example does not mean a strict execution order and does not constitute any limitation on the implementation process. The detailed execution order of each step should be determined by its function and possible internal logic. If not specifically stated, all steps of the present application can be performed in sequence or randomly, preferably in sequence. For example, the method comprises step 1 and step 2, which means that the method can comprise step 1 and step 2 in sequence, or step 2 and step 1 in sequence. For example, the method also comprises step 3, which means that step 3 can be added to the method in any order, for example, the method can comprise step 1, step 2 and step 3, or step 1, step 3 and step 2, or step 3, step 1 and step 2, etc.

[0037] Further, the present application tests the absorption spectrum of bromophenol blue in ethanol-water, ethanol-cyclohexane, ethanol-acetone, ethanol-ethyl acetate, ethanol-benzene binary system, wherein the concentration of bromophenol blue is 50 umol / L, and the volume fraction of ethanol is 0%, 5%, 25%, 50%, 75%, 95% and 100%, respectively. The results are shown in Figures 5-14 The absorption spectra of these binary systems change significantly with the change of the volume fraction of ethanol, and the color also changes significantly, indicating that the bromophenol blue photometric method can be used for the determination of the ethanol content in the binary system, thereby completing the phase diagram.

[0038] Hereinafter, examples of the present application will be described. The examples described below are exemplary and are only used to explain the present application and cannot be understood as a limitation of the present application. If the technology or conditions are not specified in the examples, the technology or conditions described in the literature in the art or according to the product manual are used. If the reagents or instruments are not specified by the manufacturer, they are all conventional products that can be obtained by purchase.

[0039] Example 1 I. Reagents and Instruments Reagents: anhydrous ethanol (analytical pure), cyclohexane (analytical pure), bromophenol blue (analytical pure); Instruments: UV-visible spectrophotometer (Shanghai Yuanzhi Company, model UV5500), artificial custom boiling point instrument, thermometer (accuracy 0.1℃), pipette (range 0.1-1mL), 2mL cuvette (quartz material).

[0040] II. Experimental Steps Step 1, establishment of standard curve (refractive index method): prepare ethanol-cyclohexane standard solutions with ethanol volume fractions of 0%, 20%, 40%, 60%, 80% and 100%, respectively. Use a pipette to take 0.5 mL of standard solutions of different compositions on the sample plate of the refractometer, close the prism seat, adjust the dispersion, and then read the refractive index of the standard solution to establish the standard curve of refractive index-volume fraction. Repeat the measurement three times.

[0041] Step 2, drawing phase diagram (refractive index method): add a certain amount of ethanol and cyclohexane to the boiling point instrument, add a small amount of zeolite, pass condensed water, heat, and read the temperature when the gas-liquid balance is reached (the solution boils and the thermometer reading is constant). The boiling point of the solution. After a portion of the liquid vaporizes, it is cooled by the condenser water and then returns to the groove of the boiling point instrument to form a distillate. The remaining liquid in the boiling point instrument is the remaining liquid. Stop heating and cool to room temperature. Open the ground plug and use a pipette to take 0.5 mL of distillate and remaining liquid, respectively, to test the refractive index. Convert the volume fraction of the remaining liquid and the distillate to mole fraction by the refractive index-volume fraction curve of the standard solution. The mole fraction of the remaining liquid is xA, and the mole fraction of the distillate is yA. With the boiling point T as the vertical coordinate and yA as the horizontal coordinate, a point (T-yA) on the gas phase line of the phase diagram can be determined. With the boiling point T as the vertical coordinate and xA as the horizontal coordinate, a point (T-xA) on the liquid phase line of the phase diagram can be determined. Empty the boiling point instrument and add different amounts of ethanol and cyclohexane, repeat the experiment, and obtain two new points. Repeat this cycle each time the experiment is conducted with 2+38, 4+36, 8+32, 16+24, 24+16, 30+10, and 34+6 mL of ethanol and cyclohexane, respectively. In addition, add anhydrous ethanol and cyclohexane (both pure components) to the boiling point instrument, heat to boiling, and record the boiling point. However, the refractive indices of the remaining liquid and the distillate are not measured because the refractive indices of the pure components are equal and equal to the refractive indices of the standard solutions with ethanol volume fractions of 100% and 0%, respectively.

[0042] Example 2 I. Reagents and Instruments Reagents: anhydrous ethanol (analytical pure), cyclohexane (analytical pure), bromophenol blue (analytical pure); Instruments: UV-visible spectrophotometer (Shanghai Yuanzhi Company, model UV5500), custom boiling point instrument, thermometer (precision 0.1°C), pipette (range 0.1-1 mL), 2 mL cuvette (quartz material).

[0043] II. Experimental Steps Step 1, establishment of standard curve (spectrophotometry): prepare bromophenol blue solution with ethanol and cyclohexane as solvent, concentration is 50 μmol / L, mix the two solutions to form standard solutions with ethanol volume fraction of 0%, 20%, 40%, 60%, 80% and 100%, test the absorbance of standard solutions at 425 nm, establish standard curve of absorbance-ethanol volume fraction, test 3 times in parallel.

[0044] Step 2, phase diagram drawing (spectrophotometry): add a certain amount of ethanol and cyclohexane to the boiling point apparatus, add a small amount of zeolite, pass cold water, heat, read the temperature when gas-liquid equilibrium is reached (solution boils and thermometer reading is constant), that is, the boiling point of the solution. After a part of the liquid is vaporized, it is cooled by condenser water and then flows back into the groove of the boiling point apparatus to form distillate. The remaining liquid in the boiling point apparatus is the remaining liquid. Stop heating, cool to room temperature, open the ground plug, accurately transfer 0.5 mL to 2 mL of bromophenol blue cyclohexane solution (100 μmol / L) to a cuvette with a pipette, accurately transfer 0.5 mL of distillate or remaining liquid to the same cuvette with a pipette, close the cuvette, mix thoroughly, then test the absorbance at 425 nm with a spectrophotometer, then convert the ethanol volume fraction in the cuvette to molar fraction according to the standard curve, and multiply the value by 2 to obtain the molar fraction of ethanol in the distillate or remaining liquid. The molar fraction of the remaining liquid is xA, and the molar fraction of the distillate is yA. With the boiling point T as the vertical coordinate and yA as the horizontal coordinate, one point (T-yA) on the gas phase line of the phase diagram can be determined. With the boiling point T as the vertical coordinate and xA as the horizontal coordinate, one point (T-xA) on the liquid phase line of the phase diagram can be determined. Empty the boiling point apparatus, add different amounts of ethanol and cyclohexane, and repeat the experiment to obtain new two points. Cycle each time with 2+38, 4+36, 8+32, 16+24, 24+16, 30+10, 34+6 (units are mL). In addition, add anhydrous ethanol and cyclohexane (both pure components) to the boiling point apparatus, heat to boiling, and record the boiling point, but do not test the absorbance of the remaining liquid and distillate, because the absorbance of the pure component remaining liquid and distillate is equal, and is equal to the absorbance of the standard solution with ethanol volume fraction of 100% and 0%, respectively.

[0045] Determination of detection limit (refractive index method of Example 1): test the refractive index of cyclohexane standard solution 10 times in parallel, and calculate the detection limit according to 3σ / k, where σ is the deviation of 10 test data, and k is the slope of the standard curve.

[0046] Determination of detection limit (spectrophotometry of Example 2): test the absorbance of bromophenol blue cyclohexane standard solution at 425 nm 10 times in parallel, and calculate the detection limit according to 3σ / k, where σ is the deviation of 10 test data, and k is the slope of the standard curve.

[0047] Please refer toFigure 15 and Figure 16 , respectively, the results of the determination of the detection limit of Example 1 and Example 2, wherein, Figure 15 is the standard curve of the refractive index method, and the volume fraction of the detection limit thereof is 0.029; Figure 16 is the standard curve of the photometry method, and the volume fraction of the detection limit thereof is 0.022, and the difference between the two is small.

[0048] Please refer to Figure 17 and Figure 18 , respectively, the phase diagrams drawn by the method of Example 1 and Example 2; both methods have the lowest constant boiling point, and the constant boiling point liquid phase molar fraction, gas phase molar fraction and boiling point of Example 1 (refractive index method) are 0.55, 0.48, 64.7℃, respectively, and those of Example 2 (photometry method) are 0.59, 0.51, 64.8℃, respectively, which are relatively close to the results (0.55, 0.55, 64.9℃) in the literature (Cheng Nenglin, Solvent Handbook Fifth Edition, Chemical Industry Press, 2007.11), indicating that both the refractive index method and the photometry method can be applied to the drawing of the ethanol-cyclohexane phase diagram.

[0049] Example 3 Example 3 of the present embodiment adopts the refractive index method.

[0050] I. Reagents and instruments Reagents: anhydrous ethanol (analytical pure), ethyl acetate (analytical pure), bromophenol blue (analytical pure); Instruments: ultraviolet visible spectrophotometer (Shanghai Yuanzhi Company, model UV5500), artificially customized boiling point instrument, thermometer (accuracy 0.1℃), pipette (range 0.1-1mL), 2mL cuvette (quartz material).

[0051] II. Experimental steps Step 1, establishment of standard curve (refractive index method): except that ethyl acetate is used instead of cyclohexane, the remaining steps are the same as those of Example 1.

[0052] Step 2, drawing of phase diagram (refractive index method): except that ethyl acetate is used instead of cyclohexane, the remaining steps are the same as those of Example 1.

[0053] Step 3, determination of detection limit (refractive index method): except that ethyl acetate is used instead of cyclohexane, the remaining steps are the same as those of Example 1.

[0054] Example 4 Example 4 of the present embodiment adopts the photometry method.

[0055] I. Reagents and instruments Reagents: anhydrous ethanol (analytical pure), ethyl acetate (analytical pure), bromophenol blue (analytical pure); Instrument: UV-Vis spectrophotometer (Shanghai Yuanzhi Company, Model UV5500), artificial boiling point instrument, thermometer (accuracy 0.1 °C), pipette (range 0.1-1 mL), 2 mL cuvette (quartz material).

[0056] II. Experimental steps Step 1, establishment of standard curve (spectrophotometry): except that ethyl acetate is used instead of cyclohexane, the rest of the steps are the same as Example 2.

[0057] Step 2, phase diagram drawing (spectrophotometry): except that ethyl acetate is used instead of cyclohexane, the rest of the steps are the same as Example 2.

[0058] Step 3, determination of detection limit (spectrophotometry): except that ethyl acetate is used instead of cyclohexane, the rest of the steps are the same as Example 2.

[0059] The experimental results of Example 3 and Example 4 are as follows: Please refer to Figure 19 and Figure 20 , respectively, the determination of the detection limit of Example 3 and Example 4, wherein, Figure 19 is the standard curve of refractive index method, and the volume fraction of its detection limit is 0.09; Figure 20 is the standard curve of spectrophotometry, and the volume fraction of its detection limit is 0.023; It can be seen that the detection limit of the refractive index method is significantly higher than that of the spectrophotometry, and the main reason is that the refractive index of ethanol and ethyl acetate is very small, and the slope of the standard curve is very small.

[0060] Please refer to Figure 21 and Figure 22, respectively, the phase diagrams were plotted according to the methods of Example 3 and Example 4; the constant boiling point of the photometric method is clear, the boiling point is 72.2℃, the liquid phase mole fraction is 0.48, and the gas phase mole fraction is 0.54, which is close to the literature (Cheng Nenglin, Solvent Handbook Fifth Edition, Chemical Industry Press, 2007.11) result (0.46, 0.46, 71.8oC). The constant boiling point of the refractive index method is not clear, the boiling point is between 72.1-72.3℃, the liquid phase mole fraction is between 0.38-0.54, and the gas phase mole fraction is between 0.4-0.59, the change range is large, and the difference is large compared with the literature, and even the abnormal situation of two lines crossing occurs. This shows that the photometric method can be applied to the drawing of the ethanol-ethyl acetate phase diagram, while the refractive index method is not suitable, the main reasons of which include: the difference between the refractive index of ethanol and ethyl acetate is small, the test error of the composition of the distillate and the remaining liquid is large; before testing the refractive index, sample liquid needs to be added dropwise on the sample plate, so that the sample liquid is flat on the sample plate, and the sample liquid has a low boiling point, and volatilizes very quickly after being flat, and the volatilization degree of the two components of the sample liquid is inconsistent, resulting in that the test result deviates from the actual value; the measurement of the refractive index relies on manual adjustment and manual reading, which is highly subjective. In the drawing experiment of the ethanol-cyclohexane two-liquid phase diagram, the problems of sample liquid volatilization and manual reading also exist, but since the refractive index of ethanol and cyclohexane is quite different, these problems are not obvious.

[0061] In addition, those skilled in the art should understand that although there are many problems in the prior art, each embodiment or technical solution of the present application can only improve in one or several aspects, and it is not necessary to solve all the technical problems listed in the prior art or background art at the same time. Those skilled in the art should understand that what is not mentioned in a claim should not be regarded as a limitation to the claim.

[0062] Although terms such as two-liquid system, color developer, standard solution, boiling point instrument, ethanol, second liquid, absorbance, gas phase line point, liquid phase line point, etc. are used more in this paper, but the possibility of using other terms is not excluded. The use of these terms is only to facilitate the description and explanation of the essence of the present application; any additional limitation is contrary to the spirit of the present application; the terms "first", "second", etc. in the specification and claims of the embodiments of the present application and the above-mentioned drawings are used to distinguish similar objects, and do not necessarily describe a specific order or sequence.

[0063] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present application, and are not intended to limit the present application; although the present application has been described in detail with reference to the above embodiments, those skilled in the art should understand that the technical solutions recorded in the above embodiments can be modified, or some or all of the technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for drawing a gas-liquid equilibrium phase diagram of a completely miscible binary liquid system, characterized in that, Includes the following steps: Using bromophenol blue as a colorimetric reagent, two-component standard solutions containing different volume fractions of ethanol were prepared. The two-component standard solutions consisted of ethanol and a second liquid. The absorbance of the two-component standard solutions containing different volume fractions of ethanol was measured at a set wavelength, and an absorbance-ethanol volume fraction standard curve was established. The target binary liquid system solution was added to a boiling point apparatus and heated to vapor-liquid equilibrium. The boiling point was recorded, and the distillate and the remaining liquid were collected. The target binary liquid system solution consisted of ethanol and a second liquid. The distillate and the residue were respectively mixed with bromophenol blue solution, and the absorbance of each mixture was measured at the set wavelength. Based on the absorbance-ethanol volume fraction standard curve, the ethanol volume fraction in the distillate and the residue was calculated and converted into mole fraction. Using the boiling point as the ordinate and the ethanol mole fraction of the distillate and the remaining liquid as the abscissa, obtain the gas phase line point and the liquid phase line point. Change the ratio of ethanol to the second liquid in the target binary liquid system, repeat the above steps from gas-liquid equilibrium to line point acquisition, and connect the line points to obtain the gas-liquid equilibrium phase diagram of the target binary liquid system.

2. The method for drawing a gas-liquid equilibrium phase diagram of a completely miscible binary liquid system according to claim 1, characterized in that, The second liquid is selected from one of water, cyclohexane, acetone, ethyl acetate, and benzene.

3. The method for drawing a gas-liquid equilibrium phase diagram of a completely miscible binary liquid system according to claim 1, characterized in that, The concentration of bromophenol blue in the two-component standard solution is 25-50 μmol / L.

4. The method for drawing a gas-liquid equilibrium phase diagram of a completely miscible binary liquid system according to claim 1, characterized in that, The second liquid is water, and the set wavelength is 590nm.

5. The method for drawing a gas-liquid equilibrium phase diagram of a completely miscible binary liquid system according to claim 1, characterized in that, The second liquid is one of cyclohexane, acetone, ethyl acetate, and benzene, and the set wavelength is 425 nm.

6. The method for drawing a gas-liquid equilibrium phase diagram of a completely miscible binary liquid system according to claim 1, characterized in that, The volume ratio of the distillate or the residue to the bromophenol blue solution is (1:1) to (1:2).

7. The method for drawing a gas-liquid equilibrium phase diagram of a completely miscible binary liquid system according to claim 1, characterized in that, The total volume of the mixture is 1-15 mL.

8. The method for drawing a gas-liquid equilibrium phase diagram of a completely miscible binary liquid system according to claim 1, characterized in that, When determining the absorbance of the two-component standard solutions containing different volume fractions of ethanol, at least three parallel measurements are performed for each two-component standard solution, and the average value is used to establish the absorbance-ethanol volume fraction standard curve.

9. The method for drawing a gas-liquid equilibrium phase diagram of a completely miscible binary liquid system according to claim 1, characterized in that, The goodness of fit R of the absorbance-ethanol volume fraction standard curve 2 ≥0.

99.

10. The method for drawing a gas-liquid equilibrium phase diagram of a completely miscible binary liquid system according to claim 1, characterized in that, When preparing the bromophenol blue solution, the solvent used is the second liquid in the two-liquid standard solution or ethanol.

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