Question

Over time, as their free fatty acid (FFA) content increases, edible fats and oils become rancid. To measure rancidity, the fat or oil is dissolved in ethanol, and any FFA present is titrated with KOH dissolved in ethanol. In a series of tests on olive oil, a stock solution of 0.050 M ethanolic KOH was prepared at 25°C, stored at 0°C, and then placed in a 100-mL buret to titrate oleic acid [an FFA with formula CH₃(CH₂)₇CH w CH(CH₂)₇COOH] in the oil. Each of four 10.00-g samples of oil took several minutes to titrate: the first required 19.60 mL, the second 19.80 mL, and the third and fourth 20.00 mL of the ethanolic KOH.
(a) What is the apparent acidity of each sample, in terms of mass % of oleic acid? (Note: As the ethanolic KOH warms in the buret, its volume increases by a factor of 0.00104/°C.)

Answer 1

By titrating any acid against a standardized alkali solution, the perceived acidity of the substance is determined. It is difficult to evaluate the exact acidity or strength of long chain fatty acids like oleic acid. Since alcohol is a polar substance that long chain fatty acids are soluble in, it is used as a solvent to dissolve fatty acids, and its acidity is consequently determined by titrating it against an alcoholic potassium hydoxide solution.

What is apparent acidity?

Fresh milk reacts with an acid due to its alleged apparent acidity. It is now generally accepted that it is caused by the carbon dioxide, acid phosphates, and casein present in fresh milk, all of which have an acidic nature.

What is oleic acid?

Various animal and vegetable fats and oils naturally include the fatty acid oleic acid. It is an odorless, colorless oil, however commercial examples may be yellowish. Oleic acid, whose lipid number is 18:1 cis-9, is a monounsaturated omega-9 fatty acid according to chemical classification.

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