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.

Recommended procedures

Method A

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.

Method B

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.

Presumed hydroxyl value

Quantity of substance
(g)

Volume of
pyridine/acetic
anhydride TS
(ml)

10-100

2.0

5.0

100-150

1.5

5.0

150-200

1.0

5.0

200-250

0.75

5.0

250-300

0.60 or 1.20

5.0 or 10.0

300-350

1.0

10.0

350-700

0.75

15.0

700-950

0.5

15.0

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.

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