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Determination of Hydroxyl Value


Learn how to determine the Hydroxyl Value of any Pharmaceutical substance.

The hydroxyl value is the number of milligrams of potassium hydroxide required to neutralise 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 pyridineacetic 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 neutralised to dilute phenolphthalein solution. Titrate with 0.5 M ethanolic potassium hydroxide using dilute phenolphthalein solution as indicator. Perform a blank determination.
Quantity of Substance for Analysis

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 1 M 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.



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