Method for testing total base number of lubricating oil

Abstract

The application provides a test method for total base number of lubricating oil. First, the peak area of a voltammetric peak of copper ions obtained by the reaction of a mixed test solution with excessive copper oxide is detected, and then the peak area of a voltammetric peak of copper ions in a standard sample obtained by the reaction of the mixed test solution with excessive copper oxide is detected. Then, the peak areas obtained by the two detections are fitted to obtain a base number-peak area standard curve. Finally, the peak area of a voltammetric peak obtained by detecting a lubricating oil sample to be tested is substituted into the base number-peak area standard curve to obtain the base number of the lubricating oil sample to be tested. The test method of the application can make the basic substances in the lubricating oil fully dissolved in the solvent by selecting a more suitable mixed reagent, and does not need to use chlorobenzene and other highly toxic and irritating reagents, thereby improving the safety factor, reducing the reagent usage, and simplifying the operation. The test only needs 2 minutes. The base number result obtained by the test method of the application has good consistency with the standard method and good repeatability.

Description

Technical Field

[0001] This invention belongs to the field of testing technology, and in particular relates to a method for testing the total base number of lubricating oil. Background Technology

[0002] Total base number (TBC) is a crucial performance indicator for lubricating oils, representing the amount of alkaline substances they contain. The national standard defines TBC as the amount of acid required to neutralize all alkaline components in 1g of lubricating oil sample under specified conditions, expressed in milligrams of KOH (mgKOH / g). These alkaline substances are primarily amino compounds, weak acid salts such as soaps, basic salts of polybasic acids, and salts of heavy metals. Internal combustion engine oils commonly contain super-base-number detergents and dispersants to neutralize acids generated from the combustion products of sulfur-containing diesel fuel, preventing equipment corrosion. In metalworking fluids, a certain base number ensures the activity of additives in the lubricating oil, such as corrosion inhibitors and stabilizers for non-ferrous metals. For new oils, measuring the base number can determine whether the amount of additives added is sufficient. For oils in use, regular sampling and analysis of TBC changes can monitor the lubricating oil's condition and determine the approximate consumption of additives, providing a reference for rational oil changes.

[0003] In existing technologies, non-electrochemically active substances are converted into electrochemically active substances through chemical conversion reactions. Specifically, an oil sample is mixed with a slight excess of acid, causing the acid to react with alkaline substances in the oil sample. The remaining acid then reacts chemically with added CuO. Finally, cyclic voltammetry is used to determine the Cu content in the mixture. 2+ The total base number of the oil sample is indirectly determined by the content of ions. However, the selected system solution cannot fully dissolve the lubricating oil, and the alkaline substances in the oil sample cannot completely react with the acid, resulting in a large deviation in the measured value.

[0004] In another existing technique, an excess of oxalic acid is mixed with an oil sample and heated to allow the reacting acid (oxalic acid) to fully react with the alkaline substances in the oil sample. The resulting acidic solution is then added to a highly alkaline sodium phenolate solution. The remaining acid value is measured using linear voltammetry in electrochemistry, and the alkalinity of the oil sample is then indirectly calculated. However, the sensitivity of linear sweep voltammetry is severely affected by the capacitive current generated during the measurement process. Differential pulse voltammetry (DPV) is developed based on classical voltammetric analysis. A linearly varying DC voltage is applied to the working electrode, and a rectangular pulse voltage with an amplitude of 5-100 mV and a duration of 40-80 ms is synchronously superimposed at certain time intervals using a time controller. The current difference is recorded twice, 20 ms before the pulse begins and 20 ms before the pulse ends. This difference is maximized at the half-wave potential of the DC polarographic wave. The longer pulse duration allows for sufficient attenuation of the charging current, reducing background current and thus improving the sensitivity and resolution of the measurement. Summary of the Invention

[0005] In view of this, and in response to the aforementioned problems in the prior art, the present invention proposes a method for testing the total base number of lubricating oil.

[0006] This invention proposes a method for testing the total base number of lubricating oil, comprising the following steps:

[0007] S1: Prepare the mixed test solution;

[0008] S2: React the mixed solution prepared in step S1 with an excess of copper oxide, centrifuge and take the supernatant, and detect the peak area of ​​the copper ion voltammetric peak.

[0009] S3: Mix the standard sample with the mixed solution prepared in step S1 to obtain the first reaction product;

[0010] S4: The first reaction product obtained in step S3 is reacted with excess copper oxide to obtain the second reaction product;

[0011] S5: Centrifuge the second reaction product obtained in step S4 and take the supernatant to detect the peak area of ​​the copper ion voltammetric peak in the standard sample.

[0012] S6: Fit the peak area obtained in steps S2 and S5 to the plot to obtain the base number-peak area standard curve;

[0013] S7: Use the lubricating oil sample to be tested instead of the standard sample, and perform the determination according to steps S3-S5. The peak area of ​​the obtained voltammetric peak is used to obtain the base value of the oil sample based on the base value-peak area standard curve obtained in step S6.

[0014] As a specific embodiment of the present invention, in step S1, the mixed test solution includes a solvent and hydrochloric acid and lithium perchlorate.

[0015] In this invention, the volume-to-mass ratio of the solvent to lithium perchlorate is 100 ml : (1-1.2) g. Lithium perchlorate is an electrolyte, and as an ion transport carrier, dissolving 1 g of lithium perchlorate in 100 ml of solvent is sufficient to fulfill its function as a transport carrier. In this invention, the amount of lithium perchlorate used is selected as 1-1.2 g dissolved in 100 ml of reagent.

[0016] In the test method of this invention, the hydrochloric acid reacts completely with the alkaline substances in the sample, and the remaining hydrochloric acid then reacts with copper oxide. Therefore, it is essential to ensure that the hydrochloric acid concentration for each sample is consistent and known. To reduce the amount of lubricating oil used in the reagent test, the hydrochloric acid concentration in the mixed test solution is 0.1-0.3 mol / L, preferably 0.2 mol / L, thereby ensuring that the amount of reagent used to react with the hydrochloric acid is small and effective.

[0017] As a specific embodiment of the present invention, the solvent includes acetone, butanone, isopropanol and carbon dichloride, and the volume ratio of acetone, butanone, isopropanol and dichloromethane is (1-2):(3-4):(2-3):(1-3).

[0018] The inventors of this application have discovered that using such a solvent can fully dissolve alkaline substances in oil samples. It is believed that the alkaline substances in oil samples are mainly amino compounds, soaps, basic salts of polybasic acids, and salts of heavy metals, mostly polar organic substances. According to the principle of "like dissolves like," a mixed organic solvent of suitable polarity is necessary for complete dissolution. Acetone, butanone, and dichloromethane are all used as organic solvents to dissolve oil samples, but their polarities differ: acetone is 5.4, and butanone is 4.5. By combining two of these, the solvent's polarity is adjusted to a moderate level, thus improving the dissolution of the sample. Isopropanol, with a polarity of 4.3, primarily functions to dissolve inorganic acid salts in organic reagents.

[0019] In a specific embodiment of the present invention, the water content in the mixed test solution does not exceed 10% of the total volume of the mixed test solution. Water is only miscible with organic reagents containing hydrophilic groups, such as aldehydes and alcohols. Isopropanol in this mixed reagent is miscible with water, but excessive water will cause water and organic reagents to separate into layers. After adding lubricating oil, water-oil-organic reagents separate into layers, thus failing to dissolve the oil sample well.

[0020] In a specific embodiment of the present invention, in step S3, the number of standard samples is at least two, preferably standard oils with alkalinity values ​​of 6-30 mg KOH / g. Such standard oils can be standard samples provided by a qualified manufacturer of standard substances, and these standard samples are accompanied by a verification certificate and statements regarding their uncertainty and metrological traceability. For example, in this embodiment of the present invention, the standard samples selected are standard oils produced by VHG Labs in the United States, with verification certificates and alkalinity values ​​of 6 and 30 mg KOH / g.

[0021] In a specific embodiment of the present invention, in step S3, the mass-to-volume ratio of the standard sample and the mixed test solution is (0.1-0.3) g: (2-6) ml.

[0022] In a specific embodiment of the present invention, in step S4, the copper oxide includes one of copper hydroxide powder and copper oxide powder. Exemplarily, in 100 ml of solvent, the preferred mass of copper oxide added is 15-30 mg.

[0023] In a specific embodiment of the present invention, in steps S2, S5 and S7, the detection of the copper ion voltammetric peak is independently performed by pulse voltammetry; the detection parameters of the pulse voltammetry are independently: voltage amplification 0.005-0.01V, pulse height 0.10-0.20V, pulse width 0.02-0.05s and pulse period 0.2-0.5s.

[0024] In a specific embodiment of the present invention, in step S7, the mass-to-volume ratio of the lubricating oil sample to be tested and the mixed test solution is (0.1-0.3) g : (2-6) ml.

[0025] All of the above-mentioned raw materials used in this invention can be prepared in-house or purchased commercially; this invention does not impose any particular limitations on them.

[0026] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0027] 1. The method for testing the total base number of lubricating oil of the present invention improves the safety factor by selecting a more suitable mixed reagent to fully dissolve the alkaline substances in the lubricating oil in the solvent, without the need to use highly toxic and irritating reagents such as chlorobenzene; the amount of reagent used is about 4 mL, which greatly reduces the amount of reagent used compared with the standard method.

[0028] 2. The method for testing the total base number of lubricating oil of the present invention is simple to operate, requiring only mixing and shaking, and the test only takes 2 minutes.

[0029] 3. The total base number test method of the present invention was used to test six kinds of lubricating oils. The test results were compared with the test results of the petrochemical industry standard SH / T 0251, which showed that the base number results had good consistency with the standard method and good repeatability. Attached Figure Description

[0030] Figure 1 This is the standard curve of copper ion peak area versus alkalinity in Example 2 of the present invention;

[0031] Figure 2 The current-voltage characteristic curve of sample 2 in Example 3 of this invention;

[0032] Figure 3 The current-voltage characteristic curve of sample 3 in Example 4 of this invention;

[0033] Figure 4 This is a repeatability test of sample 4 in Example 5 of the present invention. Detailed Implementation

[0034] The present invention will be further described below with reference to specific embodiments, but this does not constitute any limitation on the present invention.

[0035] The reagents used in the various embodiments of this invention are as follows:

[0036] Acetone, analytical grade, ≥99.5%, Sinopharm Reagent;

[0037] Butanone, analytical grade, ≥99.0%, Sinopharm Reagent;

[0038] Isopropanol, analytical grade, ≥99.7%, Sinopharm Reagent;

[0039] Dichloromethane, analytical grade, ≥99.5%, National Pharmaceutical Reagent;

[0040] Concentrated hydrochloric acid, analytical grade, 36.0–38.0%, National Pharmaceutical Reagent;

[0041] Lithium perchlorate (trihydrate), analytical grade, ≥99.0%, Sinopharm Reagent;

[0042] Copper oxide powder, analytical grade, ≥99.0%, National Pharmaceutical Reagent.

[0043] In each embodiment of the present invention, the testing instrument used is:

[0044] The SCRC-M3 mini vortex mixer (maximum stirring speed 4000 r / min) is manufactured by Sinopharm Chemical Reagent Co., Ltd.

[0045] The Autolab electrochemical workstation (model PGSTAT 302N) is manufactured by Metrohm Autolab in the Netherlands.

[0046] The first sample of the lubricating oil to be tested was a new oil of grade 4030.

[0047] Samples 2-6 are all lubricating oil samples to be tested.

[0048] Example 1

[0049] This embodiment provides a screening solution with three functions: dissolving the oil sample, reacting with alkaline substances in the oil sample, and providing electrolytes. Specific details are as follows:

[0050] Prepare 100 mL of 9 different mixed test solutions, with specific proportions shown in Table 1:

[0051] Table 1. Nine different mixed solutions

[0052]

[0053] Note: The concentration of hydrochloric acid is 0.2 mol / L.

[0054] The experimental steps are as follows:

[0055] S1: Take the reagents specified in 1-9 of the table into 9 reagent bottles, shake and mix them to fully dissolve the lithium perchlorate, and prepare 9 mixed test solutions.

[0056] S2: Take 9 test tubes, labeled 1#-9#, and weigh 0.2g of lubricating oil No. 1 (brand name 4030 new oil) into each. Shake the tubes in a mini vortex apparatus to ensure that the oil sample and reagents are thoroughly mixed. Observe the state of the lubricating oil No. 1 sample after adding the 9 mixed reagents. Among them, the No. 3 test solution showed a layering phenomenon, while the others were uniformly pale yellow, indicating that the other 8 reagents except No. 3 could dissolve the oil sample well.

[0057] S3: Add 0.02g of copper oxide powder to each test tube, continue to centrifuge in a mini vortex apparatus, let stand for 2 minutes, take the supernatant and observe the color phenomenon: the test solution in test tubes 1# and 2# is dark brown, the test solution in test tube 3# is transparent white and separated into layers, the test solutions in test tubes 4# and 5# are yellow, and the test solutions in test tubes 6#-9# are light yellow.

[0058] S4: Perform DPV detection on the samples in each test tube. The peak area values ​​are shown in Table 2.

[0059] Table 2. Peak areas of copper ions obtained from the tests of 9 mixed solutions.

[0060] 1 2.95 2 2.53 3 5.62 4 3.59 5 3.87 6 6.52 7 6.79 8 0 9 9.68

[0061] As shown in Table 2, the copper ion peak area is smallest in reagent #2, while no copper ion peak area is detected in reagent #8. This is because isopropanol, an organic reagent, was not added as a carrier to dissolve the inorganic acid salt into the organic reagent, thus the copper oxide powder failed to convert into the active substance copper ions. If the mixed reagent is appropriate, it can dissolve the oil sample, allowing all the alkaline substances in the oil sample to react with hydrochloric acid. Since the hydrochloric acid concentration is consistent in all eight test solutions, the more alkaline substances react with the oil sample, the less remains to react with copper oxide, resulting in a lower copper ion concentration in the test solution, which is reflected in a smaller peak area. Therefore, reagent #2 is selected as the optimal mixed reagent for this method, where the volume ratio of acetone, butanone, isopropanol, and dichloromethane is 1:2:1:1, and each 100 ml of the mixed reagent contains 1 g of lithium perchlorate.

[0062] Example 2

[0063] This embodiment provides a method for quantitatively determining the total base number of lubricating oil, including the plotting of a standard curve. Specific details are as follows:

[0064] S1: Prepare the mixed solution according to the mixing ratio of reagent 2 in Example 1;

[0065] S2: React the mixed test solution with an excess of copper oxide, centrifuge and take the supernatant to detect the peak area of ​​the copper ion voltammetric peak;

[0066] S3: Weigh 0.2g of standard samples with alkalinity values ​​of 6 and 30mgKOH / g respectively, and then shake them thoroughly with 4mL of mixed test solution to obtain the first reaction product respectively;

[0067] S4: The first reaction product obtained in step S3 is reacted with excess copper oxide powder to obtain the second reaction product;

[0068] S5: Centrifuge the second reaction product obtained in step S4 and take the supernatant to detect the peak area of ​​copper ions in the two standard samples.

[0069] S6: Fit the peak areas obtained in steps S2 and S5 to the plot to obtain the base number-peak area standard curve, as shown. Figure 1 As shown, within a certain range, the peak area of ​​copper ions exhibits good linearity with the base value.

[0070] Example 3

[0071] This embodiment provides a method for testing the total base number of lubricating oil, used to test a new sample of a certain No. 2 lubricating oil. Specific details are as follows:

[0072] S1: Mix 20ml acetone, 40ml butanone, 20ml isopropanol and 20ml dichloromethane evenly, then add 1.7ml hydrochloric acid and 1g lithium perchlorate (trihydrate) to form a mixed test solution;

[0073] S2: After thoroughly shaking and mixing 0.2g of lubricating oil sample and 4ml of mixed test solution, the first reaction product is obtained;

[0074] S3: React the first reaction product obtained in step S2 with 0.02g of copper hydroxide to obtain the second reaction product;

[0075] S4: Centrifuge the product of the second reaction and collect the supernatant. Detect the voltammetric peak of copper ions using differential pulse voltammetry. Figure 2 As shown;

[0076] S5: The peak area of ​​the voltammetric peak detected in step S4 was compared with that of the standard sample for quantitative analysis, and the alkalinity of sample No. 2 was found to be 23.1 mg KOH / g.

[0077] Example 4

[0078] This embodiment provides a method for testing the total base number of lubricating oil, used to test a new sample of a certain No. 3 lubricating oil. Specific details are as follows:

[0079] S1: Mix 20ml acetone, 40ml butanone, 20ml isopropanol and 20ml dichloromethane evenly, then add 1.7ml hydrochloric acid and 1g lithium perchlorate (trihydrate) to form a mixed test solution;

[0080] S2: After thoroughly shaking and mixing 0.2g of lubricating oil sample and 4ml of mixed test solution, the first reaction product is obtained;

[0081] S3: React the first reaction product obtained in step S2 with 0.02g of copper hydroxide to obtain the second reaction product;

[0082] S4: Centrifuge the product of the second reaction and collect the supernatant. Detect the voltammetric peak of copper ions using differential pulse voltammetry. Figure 3 As shown;

[0083] S5: The peak area of ​​the voltammetric peak detected in step S4 was compared with that of the standard sample for quantitative analysis, and the alkalinity of sample No. 3 was found to be 25.9 mg KOH / g.

[0084] Example 5

[0085] This embodiment provides a repeatability test for the total base number of a lubricating oil, used to test the repeatability of a certain lubricating oil No. 4. Specific details are as follows:

[0086] S1: Weigh 0.2g of the lubricating oil sample No. 4, add 4ml of mixed test solution, shake and mix thoroughly to obtain the first reaction product;

[0087] S3: React the first reaction product obtained in step S2 with 0.02g of copper hydroxide to obtain the second reaction product;

[0088] S4: Centrifuge the product of the second reaction and collect the supernatant. Use differential pulse voltammetry to continuously detect the voltammetric peak of copper ions for 10 seconds. Measure the peak area corresponding to the measured voltammetric peak, such as... Figure 4 As shown in Table 3, the standard deviation of the peak area was 0.133%, and the alkalinity of lubricating oil sample No. 4 was 13.3 mg KOH / g.

[0089] Table 3 shows the peak areas measured in 10 measurements.

[0090]

[0091] Excluding systematic errors in the experimental instruments, the determination of total alkalinity using the optimized DPV parameters can be considered to have good repeatability.

[0092] Examples 6-8

[0093] The alkalinity values ​​of lubricating oils No. 1, No. 5 and No. 6 were tested using the same method as in Example 3, and were 7.87 mg KOH / g, 6.71 mg KOH / g and 5.80 mg KOH / g, respectively.

[0094] Comparative Example 1

[0095] This comparative example provides the existing technical method SH / T0251 "Petroleum Products Alkali Number Titration Method (Perchloric Acid Potentiometric Titration Method)" for the determination of total alkali number of six oil products, and then compares the results with those obtained by this method. The comparison results are shown in Table 4.

[0096] Table 4 Comparison of alkalinity determination results from the two methods

[0097] 1 7.95 7.87 1.02 2 24.6 23.1 6.49 3 27.2 25.9 5.02 4 13.6 13.3 2.26 5 6.74 6.71 0.45 6 5.82 5.80 0.34

[0098] Comparing Examples 3-8 with Comparative Example 1 shows that differential pulse voltammetry is feasible for determining total alkalinity and can meet the requirements for feasibility, repeatability and accuracy in the determination.

[0099] In summary, the method for testing the total base number of lubricating oil of the present invention uses a more suitable mixed reagent to fully dissolve the alkaline substances in the lubricating oil in the solvent, thereby allowing them to react completely with the hydrochloric acid in the reagent. By adding excess copper oxide powder to react with the remaining hydrochloric acid, electrochemically active copper ions are obtained, and then detected by differential pulse voltammetry. If the expected base number of the sample is 6 mg KOH / g, the relative error of the result can be reduced to 0.34% using this test method.

[0100] Any numerical value mentioned in this invention, if there is only a two-unit interval between any minimum and any maximum value, includes all values ​​that increase by one unit each time from the minimum to the maximum value. For example, if the amount of a component, or the value of a process variable such as temperature, pressure, or time, is stated as 50-90, in this specification it means specifically listing values ​​such as 51-89, 52-88… and 69-71 and 70-71, etc. For non-integer values, it may be appropriately considered that a unit is 0.1, 0.01, 0.001, or 0.0001. These are merely some specifically specified examples. In this application, in a similar manner, all possible combinations of numerical values ​​between the listed minimum and maximum values ​​are considered to have been disclosed.

[0101] It should be noted that the embodiments described above are only for explaining the present invention and do not constitute any limitation on the present invention. The present invention has been described with reference to typical embodiments, but it should be understood that the words used therein are descriptive and explanatory terms, not limiting terms. Modifications can be made to the present invention within the scope of the claims, and revisions can be made to the present invention without departing from the scope and spirit of the present invention. Although the present invention described herein relates to specific methods, materials, and embodiments, it does not mean that the present invention is limited to the specific examples disclosed herein; on the contrary, the present invention can be extended to all other methods and applications with the same function.

Claims

1. A method for testing the total base number of lubricating oil, characterized in that, Includes the following steps: S1: Prepare a mixed test solution, wherein the mixed test solution includes a solvent and hydrochloric acid and lithium perchlorate, wherein the solvent includes acetone, butanone, isopropanol and dichloromethane, and the volume ratio of acetone, butanone, isopropanol and dichloromethane is (1~2):(3~4):(2~3):(1~3), and the water content in the mixed test solution does not exceed 10% of the total volume of the mixed test solution; S2: React the mixed solution prepared in step S1 with an excess of copper oxide, centrifuge and take the supernatant, and detect the peak area of ​​the copper ion voltammetric peak. S3: Mix the standard sample with the mixed solution prepared in step S1 to obtain the first reaction product; the number of the standard sample is at least 2, and the standard oils with different amounts of alkalinity are 6-30 mgKOH / g respectively; S4: The first reaction product obtained in step S3 is reacted with excess copper oxide to obtain a second reaction product; the copper oxide includes at least one of copper hydroxide powder and copper oxide powder. S5: Centrifuge the second reaction product obtained in step S4 and take the supernatant to detect the peak area of ​​the copper ion voltammetric peak in the standard sample. S6: Fit the peak area obtained in steps S2 and S5 to the plot to obtain the base number-peak area standard curve; S7: Use the lubricating oil sample to be tested instead of the standard sample, and perform the determination according to steps S3-S5 to obtain the peak area of ​​the copper ion voltammetry peak. According to the base number-peak area standard curve obtained in step S6, obtain the base number of the oil sample.

2. The test method according to claim 1, characterized in that, The volume-to-mass ratio of the solvent to lithium perchlorate is 100 ml : (1~1.2) g; And / or, the concentration of hydrochloric acid in the mixed test solution is 0.1-0.3 mol / L.

3. The test method according to claim 2, characterized in that, The concentration of hydrochloric acid in the mixed test solution is 0.2 mol / L.

4. The test method according to any one of claims 1-3, characterized in that, In step S3, the mass-to-volume ratio of the standard sample and the mixed test solution is (0.1~0.3) g : (2~6) ml.

5. The test method according to any one of claims 1-3, characterized in that, In steps S2 and S5, and in step S7, the voltammetric peaks of the copper ions are detected independently using pulse voltammetry.

6. The test method according to claim 5, characterized in that, The detection parameters of the pulse voltammetry method are each independent of the following: voltage amplification 0.005-0.01V, pulse height 0.10-0.20V, pulse width 0.02-0.05s, and pulse period 0.2-0.5s.

7. The test method according to any one of claims 1-3, characterized in that, In step S7, the mass-to-volume ratio of the lubricating oil sample to be tested and the mixed test solution is (0.1~0.3) g : (2~6) ml.

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