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5.9 HYDROXYL VALUE

 ​           The hydroxyl value of a substance is the number of mg of potassium hydroxide required to neutralize the acid combined by acylation in 1 g of the substance.

 ​           Unless otherwise directed in the individual monograph, use the following quantities of the substance:

Procedure

 ​           Transfer a quantity of the substance, determined by reference to the accompanying table and accurately weighed, to a glass-stoppered, 250-ml conical flask, and add 5.0 ml of acetic anhydride-pyridine TS. Transfer 5.0 ml of acetic anhydride-pyridine TS to a second glassstoppered, 250-ml conical flask to provide the reagent blank. Fit both flasks with suitable glass-jointed reflux condensers, heat on a water-bath for 1 hour, add 10 ml of water through each condenser, and heat on the waterbath for 10 minutes more. Cool, and to each add 25 ml of butanol, previously neutralized to phenolphthalein TS with 0.5 M ethanolic potassium hydroxide, by pouring 15 ml through 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 M ethanolic 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 titrate with 0.5 M ethanolic potassium hydroxide VS, recording the volume, in ml, consumed by the free acid in the test substance as A, or use the acid value to obtain A.

 ​           Calculate the hydroxyl value from the expression:

in which W and C are the weights, in g, of the substances taken for the acetylation and for the free acid determination, respectively; M is the exact molarity of the ethanolic potassium hydroxide; and 56.11 is the molecular weight of potassium hydroxide.