Revision as of 10:32, 1 March 2013

The hydroxyl number, defined as the number of milligrams of potassium hydroxide equivalent to the hydroxyl content of 1 gram of fat, oil, or wax. Acetyl value is expressed as number of milligrams of potassium hydroxide required to neutralize the acetic acid produced hydrolysis of 1 g of acetylated substance.

Hydroxyl value

The hydroxyl value is a measure of the content of free hydroxyl groups in a compound, typically a fat, oil, natural or synthetic ester. In determining the hydroxyl value, the hydroxyl group is acetylated with acetyl chloride or by using acetic anhydride-pyridine reagent. The excess of acetyl chloride or anhydride is converted to acetic acid by reacting with water. This acid is titrated with standard alcoholic potassium hydroxide solution.

Method

USP 401 Pyridine-Acetic Anhydride Reagent- Just before use, mix 3 volumes of freshly opened or freshly distilled pyridine with 1 volume of freshly opened or freshly distilled acetic anhydride. Transfer a quantity of the substance, determined by reference to the accompanying table accurately weighed, to a glass-stoppered, 250 mL conical flask, and add 5.0 mL of Pyridine-Acetic Anhydride Reagent. Transfer 5.0 mL of Pyridine-Acetic Anhydride Reagent to a second glass-stoppered, 250 mL conical flask to provide the reagent blank. Fit both flasks with suitable glass-jointed reflux condensers, heat on a steam bath for 1 hour, add 10 mL of water through each condenser, heat on a steam bath for 1 hour, add 10 mL of water through each condenser, and heat on steam bath for 10 minutes more. Cool, and to each add 25 mL of butyl alcohol, previously neutralized to phenolphthalein TS with 0.5 N alcoholic potassium hydroxide, by pouring 15 mL trough each condenser and, after removing the condensers, washing the sides of both flasks with the remaining 10-mL portions. To each flask add 1 mL of phenolphthalein TS, and titrate with 0.5 N alcoholic potassium hydroxide VS, recording the volume, in mL, consumed by the residual acid in the test solution as T and that consumed by the blank as B. In a 125-mL conical flask, mix about 10 g of the substance, accurately weighed, with 10 mL of freshly distilled pyridine, previously neutralized to phenolphthalein TS, add 1 mL of phenolphthalein TS, and trite with 0.5 N alcoholic potassium hydroxide VS, recording the volume, in mL, consumed by the free acid in the test specimen as A, or use the Acid Value to obtain A. Calculate the Hydroxyl Value taken by the formula:

(56.11N/W)[B + (WA/C)-T] In which W and C are the weights, in g, of the substance taken for the acetylation and for the free acid determination, respectively; N is the exact normality of the alcoholic potassium hydroxide; and 56.11 is the molecular weight of potassium hydroxide. [1]