Determination of Hydroxyl Value

The hydroxyl value is the number of milligrams of potassium hydroxide required to neutralize the acid combined by acylation in 1 g of the substance.
Use method A unless otherwise specified.

Method A

Unless otherwise specified in the individual monograph, weigh accurately the quantity of the substance under examination, stated in the table (see below), in a 150-ml acetylation flask fitted with a condenser and add the quantity of pyridine acetic anhydride reagent stated in the table. Boil for 1 hour on a water-bath, adjusting the level of the water to maintain it 2 to 3 cm above the level of the liquid in the flask all through.

Cool, add 5 ml of water through the top of the condenser; if this causes cloudiness, add sufficient pyridine to produce a clear liquid. Shake, replace in the water-bath for 10 minutes, remove and cool. Rinse the condenser and the walls of the flask with 5 ml of ethanol (95 percent), previously neutralized to dilute phenolphthalein solution. Titrate with 0.5 M ethanolic potassium hydroxide using dilute phenolphthalein solution as indicator. Perform a blank determination.

Calculate the hydroxyl value from the expression
Hydroxyl value = Acid Value + 28.05 v/w
where, v = difference, in ml, between the titrations;
w = weight, in g, of the substance.

Method B

Weigh accurately the specified quantity of the substance under examination into a flask fitted with a reflux condenser, add 12 g of stearic anhydride and 10 ml of xylene and heat under reflux for 30 minutes. Cool, add a mixture of 40 ml of pyridine and 4 ml of water, heat under reflux for a further 30 minutes and titrate the hot solution with 1M potassium hydroxide using dilute phenolphthalein solution as indicator. Perform a blank determination.

Calculate the hydroxyl value from the expression
Hydroxyl value = 56.11 v/w
where, v = difference, in ml, between the titrations;
w = weight, in g, of the substance.