Application of automatic potentiometric titration to determine the acid value content of ARA oil

1. Project background
ARA oil is an important nutrient enhancer, and its acid value is a key index to measure its quality. According to the national standard GB 5009.229-2016, the accurate determination of the acid value of ARA oil is of great significance for evaluating product quality and ensuring food safety. The automatic potentiometric titrator of Shanghai Jiahang Instrument Co., Ltd. can accurately determine the acid value of ARA oil, effectively avoid the visual judgment error, and ensure the accuracy and repeatability of the data.
2. Instruments and reagents
(1) Instruments
1. Automatic potentiometric titrator: Select the [specific model] automatic potentiometric titrator from Shanghai Jiahang Instrument Co., LTD., which has a high-precision titration system and can precisely control the amount of titration liquid added; With sensitive potential detection module, can monitor potential changes in real time; Equipped with intelligent data processing system, it can automatically record data, draw titration curve and calculate the result.

2.pH non-aqueous composite electrode: suitable for potential measurement of non-aqueous system, to ensure stable and accurate acquisition of potential data in the experimental environment of determining the acid value of ARA oil. Before use, it is necessary to check whether the appearance of the electrode is intact, whether there is damage, pollution, etc., and to activate it according to the instruction manual to ensure the measurement accuracy.

3. Burette: According to the experimental requirements, equipped with appropriate specifications (such as 20mL) burette for accurate measurement of sodium hydroxide standard solution. It is necessary to calibrate the burette before use to ensure that the calibration is accurate and avoid affecting the experimental results due to burette errors.

(2) Reagents
1. Sodium hydroxide standard solution (0.1mol/L) : as a titrant, the accuracy of its concentration directly affects the acid value determination results. Preparation and calibration should be carried out in accordance with standard methods to ensure accurate concentrations. When stored, pay attention to sealing to prevent the absorption of carbon dioxide in the air and lead to concentration changes.

2. Anhydrous ether: used to dissolve ARA oil samples in the experiment. The analytical purity level should be selected to ensure its purity and avoid interference of impurities on the experimental results. After use, it should be sealed and stored in a cool and ventilated place to prevent volatilization and oxidation.

3. Isopropyl alcohol: Mixed with anhydrous ether in 1:1 ratio to enhance the solubility of ARA oil. The same selection of analytical pure grade, pay attention to fire prevention and explosion prevention during use, and properly store after use.

3. Experimental steps
(1) Sample preparation
1. Use analytical balance to accurately weigh the sample 20g (accurate to 0.00001g) to ensure the accuracy of weighing. Place the measured sample in a clean, dry titration cup.

3. Add 50mL diethyl ether-isopropyl alcohol (1:1) mixture into the titration cup to ensure that the solution can not exceed the electrode, providing a good environment for subsequent potential measurement. The titration cup is placed on the potentiometric titration table, and the stirring device is turned on to dissolve the sample fully and ensure the uniform reaction system.

(2) Instrument parameter setting
1. Turn on the power source of the automatic potentiometric titration instrument, start the instrument, and wait for the instrument to complete the self-test.

2. Enter the instrument parameter setting interface, and set the following parameters according to the experimental requirements:

* Titration type: Select dynamic titration mode, so that the instrument can automatically adjust the titration speed according to potential changes, to ensure that the titration process is more accurate and efficient.

* Buret volume: Set to the buret specification actually used (e.g. 20mL).

* Working electrode: Select pH non-aqueous composite electrode.

* Reference electrode: Set it correctly according to the instrument instructions and experimental requirements (if no additional reference electrode is required in this experiment, set it as none according to the document).

* Sample unit of measurement: Set to "g".

* Titration display unit: Set to "pH" for visual observation of potential changes.

* Replenishment speed: Set to an appropriate value (such as 5, which can be fine-tuned according to the actual situation to ensure the timeliness and stability of titrant replenishment).

* Stirring speed: set to 5 to ensure that the solution is evenly mixed, but care should be taken to avoid bubbles resulting from too fast stirring speed affecting the experimental results.

* Electrode balance time: set to 4s, so that the electrode reaches a stable state before measurement, to ensure the accuracy of potential measurement.

* Electrode balance potential: set to 1 (the specific value is determined according to the instrument instructions and experimental conditions to ensure that the electrode is in the best working condition).

* Pre-mixing time: set to 6s, so that the added reagent is fully mixed.

* Minimum addition volume: set to 0.02mL to ensure the accuracy of titration solution addition during titration.

* End volume: Set to 20mL (can be adjusted according to the expected titration volume and experimental requirements).

* Titration speed: Select the standard speed, taking into account adequate reaction and experimental efficiency.

* Pre-titration equilibrium potential: Set to 6 (ensure that the potential is stable before titration starts).

* Potential jump: Set to 100 (this value is determined according to experimental experience and instrument performance, for accurate determination of titration endpoint).

* Correlation coefficient result unit: set to "CV1" (set according to experimental requirements and data processing requirements).

* Calculation formula: Used to automatically calculate acid value

Jiahang

Where XK is the acid value (unit is mg per gram, mg/g);

V1 and V0 are the volumes of titrant consumed by titrating sample and blank respectively.

56.1 is the molar mass of potassium hydroxide (g/mol);

m is the mass of the sample (g).

(3) titration operation
1. Carefully insert the pH non-aqueous composite electrode and titration head into the titration cup, ensuring that the electrode head is completely immersed in the solution to avoid contact with the cup wall or the formation of bubbles.

2. Start the titrator and start titration with sodium hydroxide (0.100mol/L) standard solution. During the titration process, the change of potential and the trend of titration curve were closely observed. When the potential jump is obvious and the pre-set potential jump is reached, the titrator automatically determines the titration end point and records the titration volume.

3. Perform blank test at the same time, that is, without adding ARA oil sample, add the mixed solution of ether and isopropyl alcohol according to the same operation steps, titration with sodium hydroxide standard solution, and record the volume of the standard titration solution of sodium hydroxide consumed in the blank.

Iv. Data processing and result analysis
(1) Data recording
After each titration is completed, the experimental data such as titration liquid concentration, sampling amount, titration volume and blank volume are recorded in time. Conduct multiple parallel experiments (3-5 times is generally recommended) to improve the reliability of the experimental results.

(2) Concentration calculation
According to the set calculation formula, the titrator is used to automatically calculate the acid value of each titration. The average value of the results of multiple parallel experiments was calculated as the final acid value of ARA oil. At the same time, the relative deviation between each measurement value and the average value was calculated to evaluate the repeatability of the experimental data.

(3) Evaluation of results
1. Repeatability evaluation: If the relative deviation of multiple parallel experiments is within the allowable range (such as the relative deviation is not more than ±2%), it indicates that the experimental data is good in repeatability, and the experimental operation and instrument performance are stable and reliable.

2. Accuracy assessment: Compare the test results with standard samples (if any) with known acid values, or with industry-recognized reference values to assess the accuracy of the test results. If the deviation is within a reasonable range, it indicates that the experimental method and instrument operation are accurate. If the deviation is large, it is necessary to check the experimental process, instrument parameter setting and other links, find the cause and re-perform the experiment.

5. Precautions
1. Before the experiment, be sure to ensure that all instruments and equipment are clean and dry, and the electrode performance is good. Calibrate and maintain the titrator regularly to ensure the accuracy and stability of the instrument.

2. The standard sodium hydroxide solution is corrosive. Wear protective equipment such as protective gloves and goggles to avoid contact with skin and eyes. In case of contact, rinse immediately with plenty of water and seek medical attention.

3. Anhydrous ether and isopropyl alcohol are volatile and flammable, and the experiment should be carried out in a well-ventilated environment, away from fire and heat sources. Avoid static electricity during use to prevent fire.

4. During the titration process, avoid solution splashing or electrode collision. If the solution has abnormal conditions (such as discoloration, precipitation, etc.), the titration should be stopped in time, the reason should be analyzed and the corresponding measures should be taken.

5. After the end of the experiment, clean the titration cup, electrode and other experimental equipment in time, and properly store the equipment and reagents. The waste liquid generated from the experiment should be treated in accordance with environmental protection requirements to avoid polluting the environment.