Determination Method for Relative Density of Undecylenic Acid

Determination Method for Relative Density of Undecylenic Acid

Undecylenic acid, also known as 10undecylenoic acid, is an organic compound with the molecular formula C₁₁H₂₀O₂. It is a pale yellow to yellow liquid at room temperature and atmospheric pressure. When cooled, it forms milky white crystalline masses with a characteristic odor. It is miscible with ethanol, chloroform, fatty oils, or volatile oils in all proportions, and insoluble in water. It is widely used in the synthesis of perfumes, pharmaceutical antifungal agents, and other products.

Experimental Purpose

• As a physical constant, relative density can objectively reflect the intrinsic quality of undecylenic acid and is one of the important indicators for evaluating its efficacy and safety.
• Reflect purity and consistency: According to the Relative Density Test (Method 0601) in the Chinese Pharmacopoeia, the relative density of undecylenic acid is 0.910–0.913 g/cm³ at 25 °C. By measuring its relative density, it can be determined whether the undecylenic acid sample is adulterated.

③ Assist in authenticity identification: Combined with other physical constants such as refractive index and iodine value, relative density helps to verify whether the sample is genuine undecylenic acid.

④ Ensure compliance in industrial applications: In perfumes, pharmaceuticals (e.g., antifungal preparations), and cosmetics, the concentration and purity of undecylenic acid directly affect product safety and efficacy. Relative density is a key parameter for raw material acceptance.

The experiment is performed according to Method 3 (Oscillating Density Meter Method) for the Relative Density Test (0601) in the Chinese Pharmacopoeia 2020.The ST217A Automatic Pharmaceutical Relative Density Meter (Shengtai Instrument) complies with this method and is therefore used for the test.

Experimental Apparatus

①ST217B Automatic Touchscreen Pharmaceutical Relative Density Meter

② Auxiliary items: purified water, stirring rod, beaker, etc.

Experimental Procedures

① Inspect the instrument: Verify that the power supply, sensor, temperature control system (e.g., Peltier thermostat), Ushaped oscillating tube, and other components are functioning normally.

② Clean the measuring cell: Wash the Ushaped tube or sample cell with anhydrous ethanol or distilled water, then dry it to avoid interference from residues.

③ Calibrate the instrument: Inject pure water at 25 °C (density = 0.997043 g/mL) into the measuring cell, remove air bubbles, press the “Calibrate” key, and allow the instrument to complete calibration automatically. Then take out the measuring cell and clean it.

④ Sample introduction: Filter to remove bubbles and impurities. Start the automatic program; the builtin sampling pump completes sampling, cleaning, and drying automatically. Results are output within 2–10 minutes.

⑤ Read the results and repeat the test 1–3 times.

Experimental Results and Analysis

According to the requirement of the Chinese Pharmacopoeia 0601 Relative Density Test, the relative density of undecylenic acid at 25 °C shall be 0.910–0.913 g/cm³.The values obtained in three replicate tests all fall within the specified range, meeting the standard requirements.