Method for judging acid value titration end point of electric oil based on color titration method

Abstract

The invention relates to the technical field of chemical analysis of oil products, and particularly discloses a method for judging an acid value titration end point of electric oil based on a color titration method. The method comprises the following steps: acquiring time-varying data of a solution color depth value in a titration process in real time by virtue of a spectrum color sensor, fitting a color titration curve by virtue of a high-order polynomial, calculating the curvature of the fitted curve point by point, and taking a curvature peak point which appears for the first time and has a curvature value of 0.2-2 as a titration end point. The method overcomes the defects that a traditional indicator titration method is strong in subjectivity, an automatic potentiometric titration method is easy to pollute an electrode and has no potential jump, and the like, avoids adverse effects of colors of different oil samples on the RGB value of the titration end point when the end point is judged by the color RGB value in an existing color titration method, and is objective and accurate in titration end point judgment, excellent in repeatability and high in accuracy. The method is suitable for measuring the acid value of electric oil. The method is easy and convenient to operate, economical, environmentally friendly, efficient, accurate and high in application and popularization value.

Description

Technical Field

[0001] This invention relates to the field of petroleum chemical analysis technology, and in particular to a method for determining the endpoint of acid value titration of power generation oil based on color titration. Background Technology

[0002] The acidic components in transformer oil or turbine oil are mainly partial oxidation products generated after oil oxidation, such as naphthenic acids, phenols, and fatty acids. If these acidic substances are not removed promptly, they will corrode equipment and components in contact with the oil, shortening their service life, and in severe cases, leading to major accidents. Acid value is used to determine the content of acidic products in oil and its auto-oxidation tendency. Therefore, the acid value of power generation oils must be measured during the acceptance of new oil and routine monitoring of operating oils.

[0003] Among existing technologies for determining the acid value of oils used in power generation, indicator titration and automatic potentiometric titration are the most widely used. However, both methods have certain shortcomings. Indicator titration has a limited application range, only suitable for determining the acid value of light-colored oils. It cannot measure oils with darker colors or slow titration endpoint changes. Furthermore, color changes during the test rely heavily on manual judgment, leading to significant subjectivity; different testers may give different results, resulting in poor reproducibility. Automatic potentiometric titration uses electrodes that are easily contaminated, has a complex maintenance process, and is affected by numerous factors (solvent rinsing, electrode activation, isopropanol immersion, etc.). Moreover, it is prone to problems such as the absence of a clear titration jump during measurement.

[0004] Currently, colorimetric titration of oil acid value generally uses a defined RGB value as the titration endpoint. However, due to the significant color variations in the original oil samples, it is practically difficult to determine the RGB value of the endpoint color in advance. Therefore, there is an urgent need to develop a new technology for determining the acid value of power-grade oils based on color derivative sensing technology. This technology would overcome the limitations of indicator titration, which requires pre-setting the titration endpoint and results in difficult-to-determine RGB values ​​of the solution color, as well as the lack of obvious endpoint color change, and the absence of potential jumps in automatic potentiometric titration. Summary of the Invention

[0005] To address the aforementioned problems, this invention provides a method for determining the endpoint of acid value titration of power oil based on color titration. This method eliminates the need to predetermine the RGB values ​​of the endpoint color, significantly improving the objectivity and accuracy of endpoint determination and enabling efficient and accurate detection of the acid value of power oil.

[0006] To solve the above-mentioned technical problems, the technical solution provided by the present invention is as follows: A method for determining the endpoint of acid value titration of power-grade oils based on color titration includes the following steps: Weigh the oil sample into a reaction vessel equipped with a spectral color sensor, add alcohol solvent, indicator and titrant, perform acid value titration, record the data of color depth value changing over time, and plot the color titration curve; The color titration curve is fitted using a higher-order polynomial, and the curvature of the fitted curve is calculated point by point. The first curvature peak with a curvature value in the range of 0.2 to 2 is taken as the titration endpoint.

[0007] Compared to existing technologies, the method for determining the endpoint of acid value titration of power-grade oils based on color titration provided by this invention uses a spectral color sensor to collect data on the change of solution color depth over time during the titration process. A high-order polynomial is used to fit the color titration curve, and the curvature of the fitted curve is calculated point by point. The first curvature peak with a curvature value in the range of 0.2 to 2 is taken as the titration endpoint. This invention overcomes the shortcomings of traditional indicator titration methods, such as strong subjectivity, and automatic potentiometric titration methods, such as easy electrode contamination and the potential for missing potential jumps. It also avoids the adverse effects of different oil sample colors on the RGB values ​​of the titration endpoint when using RGB values ​​to determine the endpoint in existing color titration methods. This method has the advantages of objective and accurate endpoint determination and excellent repeatability, and is applicable to the determination of acid value for various power-grade oils. The method for determining the endpoint of acid value titration of power-grade oils based on color titration provided by this invention is simple to operate, economical and environmentally friendly, efficient and accurate, and has high application value.

[0008] Through extensive experimentation, this invention has discovered that only curvature peaks with curvature values ​​between 0.2 and 2 correspond to the inflection point on the solution color depth change curve at the titration endpoint, i.e., the titration endpoint itself. When the indicator color undergoes a sudden change and reaches a stable color change, an inflection point appears on the color depth curve, and a curvature peak appears on the curvature curve. This curvature value can more accurately determine the titration endpoint.

[0009] It should be noted that the method for determining the titration endpoint of acid value for power-grade oil based on color titration provided by this invention can be combined with Python programming to calculate the curvature function of the fitted curve and determine the titration endpoint in real time. Alternatively, Excel or other mathematical calculation software can be used for curve fitting and curvature calculation. When calculating the curvature of the fitted curve point by point, as long as a curvature peak with a curvature value in the range of 0.2 to 2 appears (it must simultaneously satisfy the conditions of "curvature peak" and "curvature peak value of 0.2 to 2"), the curvature peak can be determined as the titration endpoint. This method can achieve full automation and eliminates the need for manual judgment.

[0010] Preferably, the oil sample includes transformer oil or turbine oil.

[0011] Preferably, the alcohol solvent includes ethanol or isopropanol, and more preferably ethanol.

[0012] Preferably, the indicator comprises a bromothymol blue solution.

[0013] For example, the preparation method of bromothymol blue solution includes the following steps: Place 0.50g of bromothymol blue (BTB) into a beaker, add 100mL of anhydrous ethanol, and then neutralize with 0.1mol / L potassium hydroxide ethanol solution to a pH of 5.0.

[0014] Preferably, the mass-to-volume ratio of the oil sample, alcohol solvent, and indicator is (8~10)g: (50~60)mL: (0.15~0.25)mL.

[0015] Preferably, the titrant comprises a potassium hydroxide ethanol standard solution.

[0016] More preferably, the concentration of potassium hydroxide in the titrant is 0.03 mol / L to 0.05 mol / L, and more preferably 0.035 mol / L to 0.04 mol / L.

[0017] Preferably, the temperature for acid value titration is 75℃~85℃.

[0018] For example, the specific steps for acid value titration can be referred to GB / T 28552-2012 "Determination of Acid Value of Transformer Oil and Turbine Oil (BTB Method)". The titrant is added to the reaction vessel at a uniform rate using a peristaltic pump calibrated to the titration flow rate. The specific titration rate and the amount added per drop can be determined using general practices in this field.

[0019] Preferably, the higher-order polynomial includes a sixth-order polynomial, the general formula of which is shown in Equation 1: y = b + a1×x + a2×x 2 +a3×x 3 +a4×x 4 +a5×x 5 +a6×x 6 Formula 1 In Equation 1, x represents time in seconds; y represents color depth value, which is dimensionless; b represents the y-intercept; and a1, a2, a3, a4, a5, and a6 represent fitting coefficients, respectively.

[0020] In this invention, both the color titration curve and the fitted curve are plotted with time on the horizontal axis and color depth on the vertical axis. Extensive experiments have shown that using a sixth-order polynomial for curve fitting yields the best fit. If the order is too low, the fit is poor, the titration endpoint is inaccurately determined, and the final acid value calculation has a large deviation; if the order is too high, the computational burden is excessive. Using a sixth-order polynomial for curve fitting satisfies the required accuracy and ensures that the error at the titration endpoint is within an acceptable range.

[0021] Preferably, fitting begins when the color titration curve reaches 3 data points, and the curvature of the fitted curve is calculated.

[0022] More preferably, the process involves point-by-point fitting starting from the first 7 data points of the color titration curve. The curve is refitted for each additional data point, and the curvature of the fitted curve is calculated. The first curvature peak with a curvature value in the range of 0.2 to 2 is taken as the titration endpoint.

[0023] If there are insufficient data points, such as only 3 data points, fitting a sixth-order polynomial will result in a second-order polynomial. That is, when the number of data points n < 7, only an (n-1)-order polynomial can be fitted; only when the number of data points n ≥ 7 can a sixth-order polynomial be fitted. If the order of the fitted polynomial is < 6, it's equivalent to the coefficients of the missing terms in the sixth-order polynomial being zero, which has no impact on the final curvature calculation or the final result.

[0024] Preferably, after obtaining the titration endpoint, the method further includes: calculating the acid value based on the titrant volume corresponding to the titration endpoint.

[0025] More preferably, the formula for calculating the acid value is shown in Equation 2: Acid value = [(V1-V0)×c×56.1] / M Formula 2 In Equation 2, V1 is the volume of titrant consumed in titrating the oil sample, in mL; V0 is the volume of titrant consumed in the blank test, in mL; c is the concentration of the titrant, in mol / L; 56.1 is the molar mass of potassium hydroxide, in g / mol; M represents the mass of the oil sample, expressed in grams.

[0026] It should be noted that the blank test uses a titrant (potassium hydroxide ethanol standard solution) to directly titrate the solvent (anhydrous ethanol), without any oil sample. When titrating the oil sample, the solvent, indicator, and other reagents will consume a portion of the titrant. To eliminate errors, the volume of titrant consumed by the solvent and other reagents needs to be subtracted from the volume of titrant consumed when titrating the oil sample to obtain the actual volume of titrant consumed by the oil sample.

[0027] The present invention has the following beneficial effects: During acid titration, the color depth change of the solution originates from the neutralization reaction of acidic substances in the oil sample with potassium hydroxide, which causes a change in the pH of the solution. At the same time, it causes a change in the protonation state of the bromothymol blue indicator (pH 4.0~5.4), and the color transitions from yellow to blue-green. Correspondingly, the yellow depth measured by the spectral color sensor gradually decreases, and the blue depth gradually increases. The color depth value change curve can be divided into three stages: (1) rapid reaction period: the color intensity decreases almost linearly, and the slope is significant; (2) transition period: the slope slows down, and the curvature gradually increases; (3) stable period: the color intensity fluctuates within a small range. During the acid titration reaction, due to the change in the proton state of the indicator, the spectral color change of the solution is manifested as a change with the increase of the solution pH (or titrant volume); at the titration endpoint, the solution color will change abruptly, which is reflected in the yellow or blue depth value curve of the solution, where the curve suddenly turns, and the curvature is the largest at this turning point. This invention utilizes this characteristic in the solution reaction process to detect whether the reaction has reached the endpoint, which can significantly improve the objectivity and accuracy of the titration endpoint judgment.

[0028] The present invention provides a method for determining the acid value titration endpoint of power oils based on color titration. This method uses a color titration endpoint based on the maximum curvature method to determine the acid value of the oil sample, avoiding the difficulty of pre-setting the RGB values ​​of the endpoint color before titration. Simultaneously, this method effectively avoids the subjective error of determining the titration endpoint by manual visual observation and does not rely on high-end instruments requiring specialized operating skills, thus effectively avoiding the risk of increased errors caused by non-professional operation. Therefore, the method for determining the acid value titration endpoint of power oils based on color titration provided by the present invention can comprehensively avoid interference from both subjective and objective factors, significantly enhance the consistency of the titration endpoint determination process, effectively reduce the error level of the determination results, and improve the accuracy and reliability of the acid value titration results. Attached Figure Description

[0029] Figure 1 The curvature calculation result is obtained by curve fitting the first 7 data points in Embodiment 1 of the present invention.

[0030] Figure 2 The curvature calculation result is obtained by curve fitting the first 16 data points in Embodiment 1 of the present invention.

[0031] Figure 3 The curvature calculation result is obtained by curve fitting the first 31 data points in Embodiment 1 of the present invention.

[0032] Figure 4 The curvature calculation result is obtained by curve fitting the first 11 data points in Embodiment 2 of the present invention.

[0033] Figure 5This is the curvature calculation result after curve fitting of the first 16 data points in Embodiment 2 of the present invention.

[0034] Figure 6 This is the curvature calculation result after curve fitting of the first 31 data points in Embodiment 2 of the present invention.

[0035] Figure 7 The curvature calculation result is obtained by curve fitting the first 11 data points in Embodiment 3 of the present invention.

[0036] Figure 8 The curvature calculation result is obtained by curve fitting the first 16 data points in Embodiment 3 of the present invention.

[0037] Figure 9 The curvature calculation result is obtained by curve fitting of the first 21 data points in Embodiment 3 of the present invention.

[0038] Figure 10 The curvature calculation result is obtained by curve fitting the first 8 data points in Embodiment 4 of the present invention.

[0039] Figure 11 The curvature calculation result is obtained by curve fitting the first 16 data points in Embodiment 4 of the present invention.

[0040] Figure 12 The curvature calculation result is obtained by curve fitting the first 31 data points in Embodiment 4 of the present invention. Detailed Implementation

[0041] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0042] This invention provides a method for determining the endpoint of acid value titration of power-grade oils based on color titration, comprising the following steps: Weigh the oil sample into a reaction vessel equipped with a spectral color sensor, add alcohol solvent, heat in a constant temperature water bath to 75℃~85℃, add indicator, mix well, then add titrant dropwise at a uniform rate, maintain residual temperature for acid value titration, record the changes in volume of added titrant and color depth value over time, and plot the color titration curve. Starting from the color titration curve reaching 3 data points, the color titration curve is fitted using a sixth-order polynomial with time as the horizontal axis and color depth as the vertical axis. The curvature of the fitted curve is calculated point by point, and the first curvature peak with a curvature value in the range of 0.2 to 2 is taken as the titration endpoint. Blank test: Add the alcohol solvent to the reaction vessel without oil sample, heat it to 75℃~85℃ in a constant temperature water bath, add the indicator, mix well, and then add the titrant dropwise at a uniform rate. Maintain the residual temperature and perform acid value titration. Record the volume V0 of the titrant added when the titration endpoint is reached. The acid value is calculated based on the titrant volumes (V1 and V0) corresponding to the titration endpoint, as shown in Equation 2: Acid value = [(V1-V0)×c×56.1] / M Formula 2 In Equation 2, V1 is the volume of titrant consumed in titrating the oil sample, in mL; V0 is the volume of titrant consumed in the blank test, in mL; c is the concentration of the titrant, in mol / L; 56.1 is the molar mass of potassium hydroxide, in g / mol; M represents the mass of the oil sample, expressed in grams.

[0043] It should be noted that, except for the absence of an oil sample, the operating procedures and parameter settings for the blank test are the same as those for titrating the oil sample. The purpose is to subtract the volume of titrant consumed by solvents, indicators, and other reagents from the volume of titrant consumed when titrating the oil sample. This is common knowledge in the field and will not be elaborated further.

[0044] In this embodiment of the invention, the preparation method of bromothymol blue solution includes the following steps: take 0.50g of bromothymol blue (BTB) and put it into a beaker, add 100mL of anhydrous ethanol, and then neutralize it with 0.1mol / L potassium hydroxide ethanol solution to a pH of 5.0.

[0045] In this embodiment of the invention, the code for calculating the titration endpoint using Python is shown below: import sympy as sp def find_max_curvature_point(poly_coeffs, lower_bound, upper_bound): """ This function is used to calculate the point of maximum curvature of a polynomial within a specified range. :param poly_coeffs: A list of coefficients for the polynomial, from the highest degree term to the constant term. :param lower_bound: The lower bound of the range :param upper_bound: The upper bound of the range :return: x-value of the point of maximum curvature """ # Define symbolic variables x = sp.symbols('x') # Constructing polynomial functions degree = len(poly_coeffs) - 1 poly = sum([coeff * x ** (degree - i) for i, coeff in enumerate(poly_coeffs)]) # Find the first derivative first_derivative = sp.diff(poly, x) # Find the second derivative second_derivative = sp.diff(first_derivative, x) # Find the third derivative third_derivative = sp.diff(second_derivative, x) # Construct an equivalent equation for the equation (k^2)' = 0 satisfied by the point of maximum curvature. equation = third_derivative * (1 + first_derivative ** 2) - 3 *second_derivative * first_derivative # Solve the equation solutions = sp.solve(sp.Eq(equation, 0), x) # Filter out solutions within a specified range valid_solutions = [] for solution in solutions: try: value = float(sp.re(solution)) if lower_bound<= value<= upper_bound: valid_solutions.append(value) except TypeError: continue # Curvature calculation formula k = abs(y'') / (1 + (y')**2)**(3 / 2) def curvature(x_val): yp = first_derivative.subs(x, x_val) ypp = second_derivative.subs(x, x_val) k = abs(ypp) / (1 + yp ** 2) ** (3 / 2) return float(k) # Find the point of maximum curvature max_curvature = 0 max_x = None for x_val in valid_solutions: k = curvature(x_val) if k>max_curvature: max_curvature = k max_x = x_val return max_x The present invention uses the first curvature peak with a curvature value in the range of 0.2 to 2 as the titration endpoint. This is determined based on the titration results of nearly 100 sets of oil samples. It is obtained by comparing, analyzing, and summarizing the curvature of the titration curves of nearly 100 sets of oil samples at the endpoint and at various points during the titration process.

[0046] Optimizing the order of higher-order polynomials involves progressively increasing the polynomial order from second to seventh order for fitting, with the largest squared correlation coefficient value of the fitted equation being considered optimal. This invention fitted titration curves of nearly 100 oil samples and found that the sixth-order polynomial had the largest squared correlation coefficient value.

[0047] Unless otherwise specified in the preparation method, all raw materials used in this invention are commercially available products.

[0048] To better illustrate the present invention, further examples are provided below.

[0049] Example 1 This embodiment provides a method for determining the endpoint of acid value titration of power-grade oil based on color titration, including the following steps: S100, referring to GB / T 28552-2012, weigh 8.0151g of oil sample (fire-resistant oil) into a reaction vessel equipped with a spectral color sensor, add 50mL of anhydrous ethanol, reflux for 5min with constant shaking in an 80℃ constant temperature water bath, then add 0.2mL of bromothymol blue solution, mix well, and then add titrant (0.038mol / L potassium hydroxide ethanol standard solution) dropwise at a uniform rate. Maintain the residual temperature for acid value titration, record the volume of titrant added, and use Origin software to plot the color depth value change over time as a color titration curve.

[0050] S200. Starting from the first 5 data points of the color titration curve, with time as the horizontal axis and color depth as the vertical axis, the color titration curve is fitted point by point using the sixth-order polynomial shown in Equation 1. The curvature of the fitted curve is calculated point by point using Python programming. The first curvature peak with a curvature value in the range of 0.2 to 2 is taken as the titration endpoint.

[0051] The specific data for the color titration curves are shown in Table 1. The results of color titration curves with different point counts, fitted curves, and the curvature at each corresponding point are as follows: Figures 1-3 As shown, the fitted curve for the first 31 data points is y = 4640.73304 - 0.16245x - 11.68899x. 2 +2.12591x 3 -0.17504x 4 +0.0062x 5 -0.000078x 6 R 2 =0.98607.

[0052] Table 1. Specific data for the color titration curve of Example 1

[0053] S300, Blank Test: Referring to GB / T 28552-2012, add 50 mL of anhydrous ethanol to a reaction vessel without oil sample, heat to 80 °C in a constant temperature water bath, add 0.2 mL of bromothymol blue solution, mix well, and then add the titrant (0.038 mol / L potassium hydroxide ethanol standard solution) dropwise at a uniform rate. Maintain the residual temperature and perform acid value titration. Record the volume V0 of the titrant added at the titration endpoint.

[0054] S400, referring to Formula 2, calculate the acid value based on the titrant volume corresponding to the titration endpoint (V0=0.01mL, V1=0.250mL). Acid value = [(0.250mL-0.01mL)×0.038mol / L×56.1g / mol] / 8.0151g ≈ 0.0683mg KOH / g.

[0055] Comparative Example 1 This comparative example provides a method for determining the acid value of power-grade oils based on colorimetric titration, including the following steps: S100. Referring to GB / T 28552-2012, weigh 8.0151g of oil sample (fire-resistant oil) into a reaction vessel equipped with a spectral color sensor, add 50mL of anhydrous ethanol, and reflux for 5min with constant shaking in an 80℃ constant temperature water bath. Then add 0.2mL of bromothymol blue solution, mix well, and then add the titrant (0.038mol / L potassium hydroxide ethanol standard solution) dropwise at a uniform rate. Maintain the residual temperature and perform acid value titration until the solution color changes from yellow to blue-green (i.e., the titration endpoint). Record the volume V1 of titrant added at this time.

[0056] S200, Blank Test: Referring to GB / T 28552-2012, add 50 mL of anhydrous ethanol to a reaction vessel without oil sample, heat to 80°C in a constant temperature water bath, add 0.2 mL of bromothymol blue solution, mix well, and then add the titrant (0.038 mol / L potassium hydroxide ethanol standard solution) dropwise at a uniform rate. Maintain the residual temperature and perform acid value titration. Record the volume V0 of the titrant added at the titration endpoint.

[0057] S300, refer to Formula 1, calculate the acid value based on the titrant volume corresponding to the titration endpoint.

[0058] In this comparative example, the testers had been working in the industry for more than two years and repeated the above operation 10 times. The test results of V0 and V1 were averaged, and the results are shown in Table 2.

[0059] Table 2. Acid value determination results of power oil (Comparative Example 1)

[0060] Example 2 This embodiment provides a method for determining the endpoint of acid value titration of power-grade oil based on color titration, including the following steps: S100, referring to GB / T 28552-2012, weigh 8.0220g of oil sample (fire-resistant oil) into a reaction vessel equipped with a spectral color sensor, add 55mL of anhydrous ethanol, reflux for 5min with constant shaking in an 80℃ constant temperature water bath, then add 0.2mL of bromothymol blue solution, mix well, and then add titrant (0.038mol / L potassium hydroxide ethanol standard solution) dropwise at a uniform rate. Maintain the residual temperature for acid value titration, record the volume of titrant added, and use Origin software to plot the color depth value change over time as a color titration curve.

[0061] S200 is the same as in Example 1, and will not be described again.

[0062] The specific data for the color titration curves are shown in Table 3. The results of color titration curves with different point counts, fitted curves, and the curvature at each corresponding point are as follows: Figures 4-6 As shown, the fitted curve for the first 31 data points is y = 4547.86452 - 163.23056x + 31.17148x. 2 -2.85759x 3 +0.10084x 4 -0.00106x 5 -0.0000046x 6 R 2 =0.98001.

[0063] Table 3. Specific data for the color titration curve in Example 2.

[0064] S300, Blank Test: Referring to GB / T 28552-2012, add 55 mL of anhydrous ethanol to a reaction vessel without oil sample, heat to 80°C in a constant temperature water bath, add 0.2 mL of bromothymol blue solution, mix well, and then add the titrant (0.038 mol / L potassium hydroxide ethanol standard solution) dropwise at a uniform rate. Maintain the residual temperature and perform acid value titration. Record the volume V0 of the titrant added at the titration endpoint.

[0065] S400, referring to Formula 2, calculate the acid value based on the titrant volume corresponding to the titration endpoint (V0=0.01mL, V1=0.240mL). Acid value = [(0.240mL-0.01mL)×0.038mol / L×56.1g / mol] / 8.0220g ≈ 0.0611mg KOH / g.

[0066] Comparative Example 2 This comparative example provides a method for determining the acid value of power-grade oils based on colorimetric titration, including the following steps: S100. Referring to GB / T 28552-2012, weigh 8.0220g of oil sample (fire-resistant oil) into a reaction vessel equipped with a spectral color sensor, add 55mL of anhydrous ethanol, and reflux for 5min with constant shaking in an 80℃ constant temperature water bath. Then add 0.2mL of bromothymol blue solution, mix well, and then add titrant (0.038mol / L potassium hydroxide ethanol standard solution) dropwise at a uniform rate. Maintain the residual temperature and perform acid value titration until the solution color changes from yellow to blue-green (i.e., the titration endpoint). Record the volume V1 of titrant added at this time.

[0067] S200, Blank Test: Referring to GB / T 28552-2012, add 55 mL of anhydrous ethanol to a reaction vessel without oil sample, heat to 80°C in a constant temperature water bath, add 0.2 mL of bromothymol blue solution, mix well, and then add the titrant (0.038 mol / L potassium hydroxide ethanol standard solution) dropwise at a uniform rate. Maintain the residual temperature and perform acid value titration. Record the volume V0 of the titrant added at the titration endpoint.

[0068] S300, refer to Formula 1, calculate the acid value based on the titrant volume corresponding to the titration endpoint.

[0069] In this comparative example, the testers had been working in the industry for more than two years and repeated the above operation 10 times. The test results of V0 and V1 were averaged, and the results are shown in Table 4.

[0070] Table 4. Acid value determination results of power oil in Comparative Example 2

[0071] Example 3 This embodiment provides a method for determining the endpoint of acid value titration of power-grade oil based on color titration, including the following steps: S100, referring to GB / T 28552-2012, weigh 8.0006g of oil sample (transformer oil) into a reaction vessel equipped with a spectral color sensor, add 60mL of anhydrous ethanol, reflux for 5min with constant shaking in an 80℃ constant temperature water bath, then add 0.2mL of bromothymol blue solution, mix well, and then add titrant (0.038mol / L potassium hydroxide ethanol standard solution) dropwise at a uniform rate. Maintain the residual temperature for acid value titration, record the volume of titrant added, and use Origin software to plot the color depth value change over time into a color titration curve.

[0072] S200 is the same as in Example 1, and will not be described again.

[0073] The specific data for the color titration curves are shown in Table 5. The results of color titration curves with different point counts, fitted curves, and the curvature at each corresponding point are as follows: Figures 7-9 As shown, the fitted curve for the first 21 data points is y = 9401.39699 + 1498.78533x - 710.37125x. 2 +80.39243x 3 -3.38639x 4 +0.03087x 5 +0.000760x 6 R 2 =0.92319.

[0074] Table 5. Specific data for the color titration curve in Example 3.

[0075] S300, Blank Test: Referring to GB / T 28552-2012, add 60 mL of anhydrous ethanol to a reaction vessel without oil sample, heat to 80 °C in a constant temperature water bath, add 0.2 mL of bromothymol blue solution, mix well, and then add the titrant (0.038 mol / L potassium hydroxide ethanol standard solution) dropwise at a uniform rate. Maintain the residual temperature and perform acid value titration. Record the volume V0 of the titrant added at the titration endpoint.

[0076] S400, referring to Formula 2, calculate the acid value based on the titrant volume corresponding to the titration endpoint (V0=0.01mL, V1=0.110mL), acid value = [(0.110mL-0.01mL)×0.038mol / L×56.1g / mol] / 8.0006g ≈ 0.0266mg KOH / g.

[0077] Comparative Example 3 This comparative example provides a method for determining the acid value of power-grade oils based on colorimetric titration, including the following steps: S100. Referring to GB / T 28552-2012, weigh 8.0006g of oil sample (transformer oil) into a reaction vessel equipped with a spectral color sensor, add 60mL of anhydrous ethanol, and reflux for 5min with constant shaking in an 80℃ constant temperature water bath. Then add 0.2mL of bromothymol blue solution, mix well, and then add titrant (0.038mol / L potassium hydroxide ethanol standard solution) dropwise at a uniform rate. Maintain the residual temperature and perform acid value titration until the solution color changes from yellow to blue-green (i.e., the titration endpoint). Record the volume V1 of titrant added at this time.

[0078] S200, Blank Test: Referring to GB / T 28552-2012, add 60 mL of anhydrous ethanol to a reaction vessel without oil sample, heat to 80 °C in a constant temperature water bath, add 0.2 mL of bromothymol blue solution, mix well, and then add the titrant (0.038 mol / L potassium hydroxide ethanol standard solution) dropwise at a uniform rate. Maintain the residual temperature and perform acid value titration. Record the volume V0 of the titrant added at the titration endpoint.

[0079] S300, refer to Formula 1, calculate the acid value based on the titrant volume corresponding to the titration endpoint.

[0080] In this comparative example, the testers had been working in the industry for more than two years and repeated the above operation 10 times. The test results of V0 and V1 were averaged, and the results are shown in Table 6.

[0081] Table 6. Acid value determination results of comparative example 3, power oil.

[0082] Example 4 This embodiment provides a method for determining the endpoint of acid value titration of power-grade oil based on color titration, including the following steps: S100, referring to GB / T 28552-2012, weigh 8.0741g of oil sample (transformer oil or turbine oil) into a reaction vessel equipped with a spectral color sensor, add 50mL of anhydrous ethanol, reflux for 5min with constant shaking in an 80℃ constant temperature water bath, then add 0.2mL of bromothymol blue solution, mix well, and then add titrant (0.038mol / L potassium hydroxide ethanol standard solution) dropwise at a uniform rate. Maintain the residual temperature for acid value titration, record the volume of titrant added, and use Origin software to plot the color depth value change over time into a color titration curve.

[0083] S200 is the same as in Example 1, and will not be described again.

[0084] The specific data for the color titration curves are shown in Table 7. The results of color titration curves with different point counts, fitted curves, and the curvature at each corresponding point are as follows: Figures 10-12 As shown, the fitted curve for the first 31 data points is y = 4640.73304 - 0.16245x - 11.68899x. 2 +2.12591x 3 -0.17504x 4 +0.0062x 5 -0.000078x 6 R 2 =0.98607.

[0085] Table 7. Specific data for the color titration curve in Example 4.

[0086] S300, Blank Test: Referring to GB / T 28552-2012, add 50 mL of anhydrous ethanol to a reaction vessel without oil sample, heat to 80 °C in a constant temperature water bath, add 0.2 mL of bromothymol blue solution, mix well, and then add the titrant (0.038 mol / L potassium hydroxide ethanol standard solution) dropwise at a uniform rate. Maintain the residual temperature and perform acid value titration. Record the volume V0 of the titrant added at the titration endpoint.

[0087] S400, referring to Formula 2, calculate the acid value based on the titrant volume corresponding to the titration endpoint (V0=0.01mL, V1=0.250mL). Acid value = [(0.250mL-0.01mL)×0.038mol / L×56.1g / mol] / 8.0741g ≈ 0.0634mg KOH / g.

[0088] Comparative Example 4 This comparative example provides a method for determining the acid value of power-grade oils based on colorimetric titration, including the following steps: S100. Referring to GB / T 28552-2012, weigh 8.0741g of oil sample (transformer oil or turbine oil) into a reaction vessel equipped with a spectral color sensor, add 50mL of anhydrous ethanol, and reflux for 5min with constant shaking in an 80℃ constant temperature water bath. Then add 0.2mL of bromothymol blue solution, mix well, and then add titrant (0.038mol / L potassium hydroxide ethanol standard solution) dropwise at a uniform rate. Maintain the residual temperature and perform acid value titration until the solution color changes from yellow to blue-green (i.e., the titration endpoint). Record the volume V1 of titrant added at this time.

[0089] S200, Blank Test: Referring to GB / T 28552-2012, add 50 mL of anhydrous ethanol to a reaction vessel without oil sample, heat to 80°C in a constant temperature water bath, add 0.2 mL of bromothymol blue solution, mix well, and then add the titrant (0.038 mol / L potassium hydroxide ethanol standard solution) dropwise at a uniform rate. Maintain the residual temperature and perform acid value titration. Record the volume V0 of the titrant added at the titration endpoint.

[0090] S300, refer to Formula 1, calculate the acid value based on the titrant volume corresponding to the titration endpoint.

[0091] In this comparative example, the testers had been working in the industry for more than two years and repeated the above operation 10 times. The test results of V0 and V1 were averaged, and the results are shown in Table 8.

[0092] Table 8. Acid value determination results of comparative example 4, power oil.

[0093] The test results of Examples 1-4 and Comparative Examples 1-4 show that the method for determining the endpoint of acid value titration of power oil based on color titration provided by the present invention is close to the results of the national standard method, and the relative error is within an acceptable range, indicating that the present solution has a certain degree of accuracy and reliability.

[0094] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions or improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for determining the endpoint of acid value titration of power-grade oil based on color titration, characterized in that, Includes the following steps: Weigh the oil sample into a reaction vessel equipped with a spectral color sensor, add alcohol solvent, indicator and titrant, perform acid value titration, record the data of color depth value changing over time, and plot the color titration curve; The color titration curve is fitted using a higher-order polynomial, and the curvature of the fitted curve is calculated point by point. The first curvature peak with a curvature value in the range of 0.2 to 2 is taken as the titration endpoint.

2. The method for determining the endpoint of acid value titration of power-grade oil based on color titration as described in claim 1, characterized in that, The oil sample includes transformer oil or turbine oil.

3. The method for determining the endpoint of acid value titration of power-grade oil based on color titration as described in claim 1, characterized in that, The alcohol solvent includes ethanol or isopropanol, and the indicator includes bromothymol blue solution.

4. The method for determining the endpoint of acid value titration of power-grade oil based on color titration as described in claim 1 or 3, characterized in that, The mass-to-volume ratio of the oil sample, alcohol solvent, and indicator is (8~10)g: (50~60)mL: (0.15~0.25)mL.

5. The method for determining the endpoint of acid value titration of power-grade oil based on color titration as described in claim 1, characterized in that, The titrant includes a potassium hydroxide ethanol standard solution, wherein the concentration of potassium hydroxide in the titrant is 0.03 mol / L to 0.05 mol / L.

6. The method for determining the endpoint of acid value titration of power-grade oil based on color titration as described in claim 1, characterized in that, The temperature for acid value titration is 75℃~85℃.

7. The method for determining the endpoint of acid value titration of power-grade oil based on color titration as described in claim 1, characterized in that, The higher-order polynomials include sixth-order polynomials, the general formula of which is shown in Equation 1: y=b+a1×x+a2×x 2 +a3×x 3 +a4×x 4 +a5×x 5 +a6×x 6 Formula 1 In Equation 1, x represents time in seconds; y represents color depth value, which is dimensionless; b represents the y-intercept; and a1, a2, a3, a4, a5, and a6 represent fitting coefficients, respectively.

8. The method for determining the endpoint of acid value titration of power-grade oil based on color titration as described in claim 1, characterized in that, Fitting begins when the color titration curve reaches 3 data points, and the curvature of the fitted curve is calculated.

9. The method for determining the endpoint of acid value titration of power-grade oil based on color titration as described in claim 1, characterized in that, After obtaining the titration endpoint, the method further includes: calculating the acid value based on the titrant volume corresponding to the titration endpoint.

10. The method for determining the endpoint of acid value titration of power-grade oil based on color titration as described in claim 9, characterized in that, The formula for calculating the acid value is shown in Equation 2: Acid value = [(V1-V0)×c×56.1] / M Formula 2 In Equation 2, V1 is the volume of titrant consumed in titrating the oil sample, in mL; V0 is the volume of titrant consumed in the blank test, in mL; c is the concentration of the titrant, in mol / L; 56.1 is the molar mass of potassium hydroxide, in g / mol; M represents the mass of the oil sample, expressed in grams.

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