Methods of Analysis: 4. Methods for materials of plant origin: 4.7 Determination of hydroxyl value
The hydroxyl value of a substance is the amount, in milligrams, of potassium hydroxide required to neutralize any acid when combined by acylation in 1 g of the substance under examination.
To the quantity of the substance being examined (as specified in the individual monograph) add 12 g of stearic anhydride R and 10 mL of xylene R and heat gently under a reflux condenser for 30 minutes. Allow to cool, add a mixture of 40 mL of pyridine R and 4 mL of water, and heat again under a reflux condenser for 30 minutes. Titrate the hot solution with carbonate-free sodium hydroxide (1 mol/l) VS, using phenolphthalein/ethanol TS as indicator. Repeat the procedure, omitting the substance under examination.
The hydroxyl value is calculated from the expression 56.10 v/m, where v is the difference, in mL, between the two titrations and m is the quantity, in g, of the substance taken.
Unless otherwise indicated in the individual monograph, weigh accurately the quantity of the substance to be examined shown in the table under 4.7 Determination of hydroxyl value, place it in a 150-mL acetylation flask fitted with an air condenser and add the corresponding volume of pyridine/acetic anhydride TS.
Heat the flask for 1 hour in a water-bath, maintaining the level of the water 2-3 cm above the level of the liquid in the flask. Remove the flask and condenser, allow to cool, and add 5 mL of water through the top of the condenser. If a cloudiness appears, add sufficient pyridine R to produce a clear liquid, noting the volume added. Shake the flask, place it in a water-bath for 10 minutes, remove, and allow to cool. Rinse the condenser and the walls of the flask with 5 mL of neutralized ethanol TS. Titrate with potassium hydroxide/ethanol (0.5 mol/l) VS, using 0.2 mL of phenolphthalein/ethanol TS as indicator. Repeat the procedure, omitting the substance under examination.
Calculate the hydroxyl value from the expression (a + 28.05) v/m, where v is the difference, in mL, between the two titrations, a is the acid value determined for the substance, and m is the quantity, in g, of the substance taken.