Determination of Acetyl Value

The acetyl value is the number which expresses in milligrams the amount of potassium hydroxide required to neutralize the acetic acid liberated by the hydrolysis of 1 g of the acetylated substance.

Method

Determination of the saponification value of the substance under examination. Acetylate the substance under examination by the following method. Place 10 g with 20 ml of acetic anhydride in a long-necked, round-bottomed 200-ml flask attached to a reflux air condenser. Support the flask on a sheet of heat-resistant material in which a hole of about 4 cm in diameter has been cut and heat it with a small, naked flame, not more than 25 mm in height and which does not impinge on the bottom of the flask.

Boil gently for 2 hours, allow to cool, pour into 600 ml of water contained in a large beaker, add 0.2 g of pumice powder and boil for 30 minutes. Cool, transfer to a separator and discard the lower layer. Wash the acetylated product with three or more quantities, each of 50 ml, of a warmed saturated solution of sodium chloride until the washings are no longer acid to litmus paper. Finally, shake with 20 ml of warm water and remove the aqueous layer as completely as possible. Pour the acetylated substance into a small dish, add 1 g of powdered anhydrous sodium sulfate, stir thoroughly and filter through a dry pleated filter. Determine the saponification value of the acetylated substance.

Calculate the Acetyl value from the expression
Acetyl value = 1335(b - a)/(l335 - a)
where a = saponification value of the substance;
b = saponification value of the acetylated substance.

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