ASTM D7638-10 Determination of Fatty Acids and Esters in Glycerin

ASTM D7638-10 Determination of Fatty Acids and Esters in Glycerin

The fatty acid and ester concentration in pure glycerin is determined quantitatively using ASTM D7638-10, a titrimetric approach. The values are considered as a standard when expressed in SI units.


    Fatty acids and esters (FA&E) should only be present in trace amounts in refined glycerin, which ASTM D7638-10 determines. This approach necessitates adding a quantity of sodium hydroxide that is more than the amount required to react with FA&E in the glycerin sample.  


    In a 200 to 250 mL Erlenmeyer flask, 50 ± 0.1 g of a well-mixed glycerin sample was added, and 50 mL freshly boiled water was used to dilute the sample. About 5.00 ± 0.1 mL of 0.5 N sodium hydroxide solution is added to the flask using a pipette, the flask is gently swirled to ensure proper mixing. The flask is then fixed with a condenser and flask and placed on a heat source and. The solution is refluxed for at least 5 minutes. The flask is cooled to room temperature and condenser assembly is removed from the heating unit.  The flask is filled with 1 mL of phenolphthalein indicator solution. Using the standardized 0.5 N hydrochloric acid solution, the test solution is ttitrated until the pink hue of the indicator vanishes. The amount of hydrochloric acid needed to titrate the sample in mL is calculated.

    Specimen size:

    50 ± 0.1 g of a well-mixed glycerin sample is used.


    The following equation is used to calculate the fatty acid, and ester equivalent in milliliters of 0.5 N NaOH used per 50 g of sample.

    (B-S) × N /0.5 = FA&E as mL of 0.5 N NaOH per 50 g sample

    B = The volume of 0.5 N hydrochloric acid solution, in mL (±0.01 mL) used to titrate the blank.
    S = The volume of 0.5 N hydrochloric acid solution, in mL (±0.01 mL) required to titrate the sample.
    N = Normality of the HCl solution reported to 60.001 mL.


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