Method for testing iodine value of ester oil

Abstract

The invention discloses a method for testing the iodine value of ester oil, and belongs to the technical field of iodine value testing. Respectively titrating the sample and the blank sample, recording the volume V of the standard solution consumed at the titration end point of the sample, and recording the volume V0 of the standard solution consumed at the titration end point of the blank sample; calculating an iodine value according to the amount of consumed standard solution during titration; according to the present invention, the # imgabs0 # uses the method for determining the iodine value of the ester oil by using the potentiometric titration method to test a variety of ester oil, the repeatability and the reproducibility test result are good, the detection result has no significant difference with the national standard GB / T 5532 determination result, the determination of the iodine value of various ester oil can be met, and the result is accurate and reliable.

Description

Technical Field

[0001] The invention belongs to the technical field of iodine value testing, and in particular relates to a method for measuring the iodine value of ester oil by potentiometric titration. Background Art

[0002] Iodine value is an important characteristic index of ester oil, which is used to indicate the unsaturation of ester oil. It is defined as the number of grams of iodine that can be absorbed (added) in 100g of substance, and the unit is g / 100g. Ester oil includes natural oils and synthetic esters, which have good lubrication performance and load-bearing capacity. They are mainly used in products such as gear oil, compressor oil, metalworking fluid and machine tool guide oil. Iodine value can be used as an important parameter to judge the stability of ester oil products. The larger the iodine value, the higher the content of unsaturated hydrocarbons. Under the action of air and high temperature, it is easy to produce colloidal substances and the stability is low; on the contrary, the chemical properties of the product are relatively stable and easy to store. Iodine value can also be used as a technical indicator for the performance acceptance of ester oil raw materials. The size and range of iodine value of various substances are certain. By detecting their iodine value, it can be judged whether the purity and composition of the raw materials are normal, whether there is any adulteration, etc.

[0003] The existing methods for determining iodine value mainly include infrared spectroscopy, chemiluminescence and chemical titration. The principle of infrared spectroscopy is to use the wavelength of 3000-3100 cm -1 The absorption peak of vinyl carbon-hydrogen stretching vibration at the position is correlated with its iodine value, and the iodine value can be quickly detected. However, this method is prone to overlap with other absorption peaks of oil products to produce interfering absorption peaks. At the same time, a large amount of data is required to establish a relevant curve or model. The chemiluminescence method uses the principle of chemiluminescence and has a fast analysis speed, but this method is easily interfered by the color of the product and is not suitable for dark products. In terms of chemical titration, commonly used determination methods include Wijs method (Wijs), Hanus method (Hanus) and catalyst rapid determination method, among which Wijs method is the most commonly used. The chemical titration method has high measurement accuracy and is easy to operate, but the accuracy of the endpoint determination is directly related to personal experience, especially the endpoint judgment of dark samples is difficult, and it is not suitable for the detection of dark samples. Summary of the invention

[0004] In view of this, in order to solve the above problems existing in the prior art, the present invention provides a method for determining the iodine value of ester oil by potentiometric titration. This method can reduce the error in determining the titration endpoint caused by manual counting errors, make the titration measurement more accurate, and realize automated continuous operation, and has the advantages of high sensitivity and good accuracy.

[0005] The present invention provides a method for determining the iodine value of ester oil by potentiometric titration, comprising the following steps:

[0006] S11: Providing a sample: Weighing 0.010-5.000 g of a sample, adding a mixed solution I containing the sample, a solvent and Wei's reagent into a titration cup I of a potentiometric titrator, sealing the mixture, and placing the mixture in a dark place for 1-3 hours to obtain a sample; the sample refers to an ester oil;

[0007] S21: Provide a blank sample: add the mixed solution II containing the solvent and the Wei's reagent into the titration cup II of the potentiometric titrator, seal it, and place it in a dark place for 1 to 3 hours to obtain a blank sample;

[0008] S12: Titration operation of the sample: add potassium iodide solution and water into the titration cup I; insert the electrode and the titration head into the titration cup I, adopt the equal titration mode (MET), run the titration program, perform titration and observe the titration curve and potential change, and after reaching the titration endpoint, record the volume V of the standard solution consumed at the titration endpoint;

[0009] S22: Titration operation of blank sample: insert the electrode and titration head into the titration cup II, adopt the equal volume titration mode (MET), run the titration program, perform titration and observe the titration curve and potential change. After reaching the titration endpoint, record the volume V0 of the standard solution consumed at the titration endpoint;

[0010] S3: Calculate the iodine value based on the amount of standard solution consumed in titration;

[0011] The standard solution is sodium thiosulfate solution;

[0012]

[0013] In the formula:

[0014] V0——the volume of standard solution consumed by the blank sample, in mL;

[0015] V——the volume of standard solution consumed by the sample, in mL;

[0016] C——Concentration of standard solution, unit mol·L -1 ;

[0017] m——mass of the sample, in g.

[0018] The invention adopts an automatic potentiometric titration method to determine the iodine value of ester oil. The sample is dissolved in a solvent and a Wijs reagent is added, and excess iodine chloride reacts with unsaturated double bonds. After the reaction is complete, a potassium iodide solution is added, and excess iodine chloride reacts with potassium iodide to generate free iodine. An automatic potentiometric titrator is used to titrate the precipitated iodine with a sodium thiosulfate standard solution, and the iodine value is calculated according to the amount of the sodium thiosulfate standard solution consumed in the titration.

[0019] The present invention refers to the sample weighing mass in GB / T 5532 "Determination of iodine value of animal and vegetable oils and fats", and weighs an appropriate amount of sample according to the estimated iodine value, accurate to 0.001g.

[0020] Preferably, in step S11, the mass of the sample weighed must ensure that the added Wijs reagent is 50% to 60% in excess, that is, 100% to 150% of the absorption amount. If the color of the mixture in the iodine volume flask is very light after the sample reacts with the Wijs reagent, it indicates that the sample is weighed too much and the weighing should be reduced. If the sample volume is too large and cannot be shaken to mix, the sample to be tested is extracted from the bottom of the container. The reference for the sample weighing mass is shown in Table 1;

[0021] Table 1 Sample weight

[0022] Estimated iodine value / (g / 100g) Sample reference mass / g ＜1.5 3.75 1.5～2.5 2.50 2.5～5 0.75 5～20 0.25 20～50 0.10 50～100 0.05 100～150 0.03 150～200 0.02

[0023] As a preferred embodiment, in step S11, the weighed sample is first added to the titration cup of the potentiometric titrator, and then a solvent is added to dissolve the sample, and then Wijs reagent is added, and the sample is sealed and placed in a dark place after shaking. For samples with an iodine value lower than 150, the sample is sealed after shaking and placed in a dark place for at least 1 hour; for samples with an iodine value higher than 150 and oxidized to a considerable degree, the sample is sealed after shaking and placed in a dark place for at least 2 hours;

[0024] As a more preferred embodiment, in step S11, the weighed sample is first added into the titration cup of the potentiometric titrator, and then 5 mL of solvent is added to dissolve the sample, and then 6 mL of Wijs reagent is added, and after shaking, the mixture is sealed and allowed to stand in a dark place.

[0025] As a preferred technical solution, the titration parameters of the titration program are: signal drift 50mV / min, titration speed 0.2mL / drop to 0.4mL / drop, stirring speed 6r / min, equivalence point recognition standard 30mV, and equivalence point recognition as maximum.

[0026] As a preferred technical solution, the solvent contains cyclohexane and glacial acetic acid, and preferably the volume ratio of the cyclohexane to the glacial acetic acid is 1:1 to 1:2; more preferably, the solvent is a mixture of cyclohexane and glacial acetic acid in equal volumes.

[0027] As a preferred technical solution, the volume ratio of the solvent to the Wei's reagent is 1:1 to 1:2.

[0028] As a preferred technical solution, the concentration of the potassium iodide solution is 100 g / L and does not contain iodate or free iodine. The volume ratio of the potassium iodide to water is 1:10 to 1:15. Preferably, in S12, 5 mL of potassium iodide solution (100 g / L, the solvent is water) and 60 mL of water are accurately added to the titration cup.

[0029] Preferably, the Wijs reagent is an acetic acid solution containing iodine monochloride with a concentration of 0.1 to 0.2 mol·L -1 ; The ratio of I / CI in Wijs' reagent should be controlled within the range of 1.10±0.1. The preparation method of acetic acid solution containing iodine monochloride can be as follows: 25g of iodine monochloride is dissolved in 1500mL of glacial acetic acid. Wijs' reagent has poor stability. In order to make the determination result accurate, a blank sample should be used for control determination. The glacial acetic acid used to prepare Wijs' reagent should meet the quality requirements and should not contain reducing substances. The method for identifying whether it contains reducing substances: take 2mL of glacial acetic acid, dilute it with 10mL of distilled water, add 0.1mL of 1mol / L potassium permanganate, and the color presented should remain unchanged within 2h. If the red color fades, it means that reducing substances exist. It can be refined by the following method: take 800mL of glacial acetic acid and put it into a round-bottom flask, add 8g~10g of potassium permanganate, connect a reflux cooler, heat and reflux for about 1h, transfer it into a distillation flask for distillation, and collect the distillate between 118℃ and 119℃.

[0030] Preferably, the sodium thiosulfate standard solution has a concentration of 0.1 mol / L and is used within 7 days after calibration.

[0031] Preferably, the potentiometric titrator has an accuracy of ±2mV, can display the potential value-titration volume change curve of the titration process in real time, is equipped with a composite platinum ring electrode and a burette, and the burette has an accuracy of 0.002mL. The iodine value is determined by potentiometric titration, and the sample should be avoided from being measured under direct sunlight.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1. The present invention adopts potentiometric titration to determine the iodine value of ester oil, which can realize automatic and continuous operation, and can make up for the error of endpoint color judgment by human eyes. In addition, the detection process does not require an indicator and is not affected by the color or turbidity of the sample, and the titration measurement is more accurate.

[0034] 2. Compared with the national standard method, this method also has the advantages of less reagent usage, shorter time consumption, lower detection cost, safety and environmental protection.

[0035] 3. The method for determining the iodine value of ester oils by potentiometric titration of the present invention was used to test a variety of ester oils. The repeatability and reproducibility test results were good. There was no significant difference between the test results and the national standard GB / T 5532 test results. The method can meet the requirements of the determination of the iodine value of various ester oils, and the results are accurate and reliable. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a typical iodine value titration curve of the blank sample in Example 1 of the present invention;

[0037] Figure 2 This is a typical iodine value titration curve of the sample in Example 1 of the present invention. DETAILED DESCRIPTION

[0038] The present invention will be further described below in conjunction with specific embodiments, but they do not constitute any limitation to the present invention.

[0039] In each of the embodiments and comparative examples of the present invention, samples No. 1 to No. 9 are all ester oil samples to be tested.

[0040] The raw materials in the present invention can be prepared in-house or commercially available, and the present invention is not particularly limited thereto.

[0041] Example 1

[0042] This embodiment provides a method for testing the iodine value of ester oil, which is used to test a No. 1 sample of ester oil. The specific details are as follows:

[0043] S11: Refer to GB / T 5532 "Determination of iodine value of animal and vegetable oils and fats", the iodine value of sample No. 1 is estimated to be 100-150g / 100g, and 0.030g is weighed; the weighed sample is added to the titration cup I of the potentiometric titrator, and 5mL of cyclohexane-glacial acetic acid mixture (1:1, v / v) is added, and the sample is shaken to completely dissolve, and then 6mL of Wei's reagent is added to the titration cup, and after shaking, it is sealed and placed in a dark place for 1h to prepare the sample.

[0044] S21: Add 5 mL of cyclohexane-glacial acetic acid mixture (1:1, v / v) and 6 mL of Wei's reagent to the titration cup II of the potentiometric titrator, shake well, seal and place in a dark place for 1 h as a blank sample;

[0045] S12: After the specified reaction time is reached, accurately add 5 mL of potassium iodide solution and 60 mL of water to the titration cup I. Start the magnetic stirrer, insert the electrode and the titration head, set the titration parameters (signal drift 50 mV / min, titration speed 0.2 mL / drop to 0.4 mL / drop, stirring speed 6 r / min, equivalence point recognition standard 30 mV, equivalence point recognition as maximum, and equal titration mode MET), run the titration program, perform titration and observe the titration curve (see Figure 1 ) and potential changes, after reaching the titration endpoint, record the volume V of the standard solution consumed at the titration endpoint;

[0046] S22: Perform a blank test according to step S4, and accurately add 5 mL of potassium iodide solution and 60 mL of water to the titration cup II. Start the magnetic stirrer, insert the electrode and the titration head, set the titration parameters (signal drift 50 mV / min, titration speed 0.2 mL / drop to 0.4 mL / drop, stirring speed 6 r / min, equivalence point recognition standard 30 mV, equivalence point recognition as maximum, and use equal titration mode MET), run the titration program, perform titration and observe the titration curve (see Figure 1 ) and potential changes, and when the titration endpoint is reached, record the volume V0 of the standard solution consumed at the titration endpoint.

[0047] S3: The iodine value calculated based on the amount of sodium thiosulfate consumed by titration is 128 g / 100 g.

[0048]

[0049] In the formula:

[0050] V0——The volume of sodium thiosulfate standard solution consumed by the blank sample, which was measured to be 12.2846mL;

[0051] V——The volume of sodium thiosulfate standard solution consumed by the sample, which is measured to be 9.2540mL;

[0052] C——Concentration of sodium thiosulfate standard solution, 0.1 mol·L -1 ;

[0053] m——mass of the sample, 0.030g.

[0054] Example 2-Example 9

[0055] Examples 2 to 9 provide repeatability determination of the iodine value of ester oils. Using a method basically the same as that of Example 1, the repeatability of 8 different types of actual ester oil samples is tested, wherein Example 2 uses a certain ester oil No. 1 sample, Example 3 uses a certain ester oil No. 2 sample, Example 4 uses a certain ester oil No. 3 sample, Example 5 uses a certain ester oil No. 4 sample, Example 6 uses a certain ester oil No. 5 sample, Example 7 uses a certain ester oil No. 6 sample, Example 8 uses a certain ester oil No. 7 sample, and Example 9 uses a certain ester oil No. 8 sample. The results are shown in Table 2.

[0056] Table 2 Repeatability test results

[0057]

[0058] The repeatability results all meet the requirements of GB / T 5532-2008 standard, and it can be considered that the potentiometric titration method for determining the iodine value of ester oil has good repeatability.

[0059] Comparative Example 1-Comparative Example 5

[0060] Comparative Examples 1 to 5 provide the prior art method GB / T 5532 "Determination of iodine value of animal and vegetable oils and fats" to determine the iodine value of 5 kinds of oils, wherein Comparative Example 1 uses a certain ester oil No. 2 sample, Comparative Example 2 uses a certain ester oil No. 3 sample, Comparative Example 3 uses a certain ester oil No. 5 sample, Comparative Example 4 uses a certain ester oil No. 6 sample, and Comparative Example 5 uses a certain ester oil No. 7 sample, and then compares the measurement results with the present method, and the comparison results are shown in Table 3:

[0061] Table 3 Comparison of the results of iodine value determination by two methods

[0062]

[0063] There is no significant difference between the results of this method and those of GB / T 5532.

[0064] Relative error calculation formula:

[0065]

[0066] Wherein, a is the value measured according to GB / T 5532, and b is the value measured by the method of Example 1 of the present invention.

[0067] Comparative Example 6-Comparative Example 11

[0068] Comparative Examples 6 to 11 provide reproducibility determination of iodine value of ester oils. The method substantially the same as that in Example 1 is used and sent to an external laboratory (using the method of GB / T 5532) to test the reproducibility of 6 different types of actual ester oil samples. Among them, Comparative Example 6 uses a certain ester oil No. 1 sample, Comparative Example 7 uses a certain ester oil No. 3 sample, Comparative Example 8 uses a certain ester oil No. 4 sample, Comparative Example 9 uses a certain ester oil No. 5 sample, Comparative Example 10 uses a certain ester oil No. 7 sample, and Comparative Example 11 uses a certain ester oil No. 9 sample. The results are shown in Table 4:

[0069] Table 4 Reproducibility test results

[0070]

[0071]

[0072] The reproducibility results all meet the requirements of GB / T 5532-2008 standard, and it can be considered that the potentiometric titration method for determining the iodine value of ester oil has good reproducibility.

[0073] Comparison between Example 1-2 and Comparative Example 1-2 shows that the potentiometric titration method of the present invention is feasible for determining iodine value and can meet the requirements for feasibility, repeatability and accuracy during determination.

[0074] In summary, the test method for the iodine value of ester oil of the present invention is to dissolve the sample in a solvent and add Wijs reagent, and the excess iodine chloride reacts with the unsaturated double bond. After the reaction is complete, potassium iodide solution is added, and the excess iodine chloride reacts with potassium iodide to generate free iodine. The precipitated iodine is titrated with a sodium thiosulfate standard solution by an automatic potentiometric titrator, and the iodine value is calculated according to the amount of the sodium thiosulfate standard solution consumed by the titration. If the expected iodine value of the sample is 90g / 100g, the relative error of the result can be reduced to 0.78% by using this test method.

[0075] The present invention adopts automatic potentiometric titration to determine the iodine value of ester oil, dissolves the sample in a solvent and adds Wijs reagent, and the excess iodine chloride reacts with the unsaturated double bond, and potassium iodide solution is added after the reaction is complete, and the excess iodine chloride reacts with potassium iodide to generate free iodine, and the precipitated iodine is titrated with sodium thiosulfate standard solution by an automatic potentiometric titrator, and the iodine value is calculated according to the amount of sodium thiosulfate standard solution consumed by the titration. The present invention adopts potentiometric titration to determine the iodine value of ester oil, can realize automatic continuous operation, can make up for the error of endpoint color judgment of human eyes, and the detection process does not need an indicator, is not affected by the color or turbidity of the sample, and the titration measurement is more accurate. At the same time, the method also has the advantages of less reagent usage, short time consumption, low detection cost, safety and environmental protection, etc.; the method for determining the iodine value of ester oil by potentiometric titration of the present invention is used to test a variety of ester oils, and the repeatability and reproducibility test results are good. The detection results have no significant difference with the determination results of the national standard GB / T 5532, and can meet the determination of the iodine values ​​of various ester oils, and the results are accurate and reliable.

[0076] Any numerical value mentioned in the present invention, if there is only an interval of two units between any minimum value and any maximum value, includes all values ​​from the minimum value to the maximum value each time increasing by one unit. For example, if the amount of a component, or the value of a process variable such as temperature, pressure, time, etc. is declared to be 50-90, in this specification it means that 51-89, 52-88... and 69-71 and 70-71 are specifically listed. For non-integer values, 0.1, 0.01, 0.001 or 0.0001 can be appropriately considered as a unit. These are just some specially 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.

[0077] It should be noted that the embodiments described above are only used to explain the present invention and do not constitute any limitation to 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 words, rather than restrictive words. The present invention may be modified as specified within the scope of the claims of the present invention, and the present invention may be revised without departing from the scope and spirit of the present invention. Although the present invention described therein relates to specific methods, materials and embodiments, it does not mean that the present invention is limited to the specific examples disclosed therein, on the contrary, the present invention can be extended to all other methods and applications with the same functions.

Claims

1. A method for testing the iodine value of ester oil, characterized in that: Includes steps: S11: Providing a sample: Weighing 0.010-5.000 g of a sample; adding a mixed solution I containing the sample, a solvent and Wei's reagent into a titration cup I of a potentiometric titrator, sealing the mixture, and placing the mixture in a dark place for 1-3 hours to obtain a sample; S21: Provide a blank sample: add the mixed solution II containing the solvent and the Wei's reagent into the titration cup II of the potentiometric titrator, seal it, and place it in a dark place for 1 to 3 hours to obtain a blank sample; S12: Titration operation of the sample: add potassium iodide solution and water into the titration cup I; insert the electrode and the titration head into the titration cup I, adopt the equal volume titration mode, run the titration program, perform titration and observe the titration curve and potential change, and after reaching the titration end point, record the volume V of the standard solution consumed at the titration end point; S22: Titration operation of blank sample: insert the electrode and titration head into the titration cup II, adopt the equal volume titration mode, run the titration program, perform titration and observe the titration curve and potential change, and after reaching the titration endpoint, record the volume V0 of the standard solution consumed at the titration endpoint; S3: Calculate the iodine value based on the amount of standard solution consumed in titration; The standard solution is sodium thiosulfate solution; In the formula: V0——the volume of standard solution consumed by the blank sample, in mL; V——the volume of standard solution consumed by the sample, in mL; C——Concentration of standard solution, unit mol·L -1 ; m——mass of the sample, in g.

2. The testing method according to claim 1, characterized in that: The titration parameters of the titration program are: signal drift 50mV / min, titration speed 0.2mL / drop to 0.4mL / drop, stirring speed 6r / min, equivalence point recognition standard 30mV, and equivalence point recognition as maximum.

3. The testing method according to claim 1, characterized in that: The solvent contains cyclohexane and glacial acetic acid, and preferably the volume ratio of the cyclohexane to the glacial acetic acid is 1:1 to 1:

2.

4. The testing method according to claim 1, characterized in that: The Wei's reagent is an acetic acid solution containing iodine monochloride with a concentration of 0.1 to 0.2 mol·L -1 .

5. The testing method according to claim 1, characterized in that: The volume ratio of the solvent to the Wei's reagent is 1:1 to 1:

2.

6. The testing method according to claim 1, characterized in that: The volume ratio of potassium iodide to water is 1:10 to 1:

15.

7. The testing method according to claim 1, characterized in that: The concentration of the potassium iodide solution is 100 g / L.

8. The testing method according to claim 1, characterized in that: The concentration of sodium thiosulfate in the standard solution is 0.1 mol / L and it should be used within 7 days after calibration.

Images