Hidroxyl and Acetyl numbers
The hydroxyl number, defined as the number of milligrams of potassium hydroxide equivalent to the hydroxyl content of 1 gram of fat, oil, or wax. Acetyl value is expressed as number of milligrams of potassium hydroxide required to neutralize the acetic acid produced hydrolysis of 1 g of acetylated substance.
Hydroxyl value
The hydroxyl value is a measure of the content of free hydroxyl groups in a compound, typically a fat, oil, natural or synthetic ester. In determining the hydroxyl value, the hydroxyl group is acetylated with acetyl chloride or by using acetic anhydride-pyridine reagent. The excess of acetyl chloride or anhydride is converted to acetic acid by reacting with water. This acid is titrated with standard alcoholic potassium hydroxide solution.
Method
USP 401 Pyridine-Acetic Anhydride Reagent- Just before use, mix 3 volumes of freshly opened or freshly distilled pyridine with 1 volume of freshly opened or freshly distilled acetic anhydride. Transfer a quantity of the substance, determined by reference to the accompanying table accurately weighed, to a glass-stoppered, 250 mL conical flask, and add 5.0 mL of Pyridine-Acetic Anhydride Reagent. Transfer 5.0 mL of Pyridine-Acetic Anhydride Reagent to a second glass-stoppered, 250 mL conical flask to provide the reagent blank. Fit both flasks with suitable glass-jointed reflux condensers, heat on a steam bath for 1 hour, add 10 mL of water through each condenser, heat on a steam bath for 1 hour, add 10 mL of water through each condenser, and heat on steam bath for 10 minutes more. Cool, and to each add 25 mL of butyl alcohol, previously neutralized to phenolphthalein TS with 0.5 N alcoholic potassium hydroxide, by pouring 15 mL trough each condenser and, after removing the condensers, washing the sides of both flasks with the remaining 10-mL portions. To each flask add 1 mL of phenolphthalein TS, and titrate with 0.5 N alcoholic potassium hydroxide VS, recording the volume, in mL, consumed by the residual acid in the test solution as T and that consumed by the blank as B. In a 125-mL conical flask, mix about 10 g of the substance, accurately weighed, with 10 mL of freshly distilled pyridine, previously neutralized to phenolphthalein TS, add 1 mL of phenolphthalein TS, and trite with 0.5 N alcoholic potassium hydroxide VS, recording the volume, in mL, consumed by the free acid in the test specimen as A, or use the Acid Value to obtain A. Calculate the Hydroxyl Value taken by the formula:
(56.11N/W)[B + (WA/C)-T] In which W and C are the weights, in g, of the substance taken for the acetylation and for the free acid determination, respectively; N is the exact normality of the alcoholic potassium hydroxide; and 56.11 is the molecular weight of potassium hydroxide. [1]
| Hydroxyl Value Range | Weight of test specimen, g |
|---|---|
| 0 to 20 | 10 |
| 20 to 50 | 5 |
| 50 to 100 | 3 |
| 100 to 150 | 2 |
| 150 to 200 | 1.5 |
| 200 to 250 | 1.25 |
| 250 to 300 | 1.0 |
| 300 to 350 | 0.75 |
Acetyl value
The determination of acetyl number is the commonly accepted procedure for the estimation of hydroxylated esters, free alcohols (including sterols), and free hydroxyl acids in an unhydrolysed wax. If the wax is saponified and acetyl number is determined on the unsaponifiable material, it becomes a measure of the total amount of alcohols, free and combined, since the total amount of free and combined hydroxyl acids will have been eliminated in the separation of the total acids fraction. [2] The acetyl value is defined as the number of milligrams of potassium hydroxide required to neutralize the acetic acid formed when one gram of acetylated fat is saponified.[3]
Method
Boil 10 grams of the wax and add 20 mL of acetic anhydride in a round-bottomed flask under a reflux condenser for two hours; pour the resulting mixture into a large beaker containing 500 mL of hot water and boil for half and an hour. Allow the mixture to separate into two layers, siphon off the water, and boil the oily layer with three successive portions of fresh water. All free acetic acid removed. The acetylated wax is carefully separated from water and further dried by filtering through anhydrous paper in a drying oven. Weigh two grams of the acetylated wax and saponify with a measured volume of standard alcoholic potassium hydroxide as in the determination of the saponification number; evaporate nearly to dryness to expel the alcohol, dissolve the soap in water, and add an amount of standard sulfuric acid exactly equivalent to the alkali used for saponification. Warm gently until the fatty acids separate as a layer at the top. Filter through wet paper, wash with boiling water until filtrate is no longer acid, and titrate the filtrate and washings with tenth-normal alkali, using phenolphthalein as an indicator. Deduct the amount of alkali required to neutralize any soluble fatty acids in the original substance and calculate the acetic acid found in terms of acetyl number as defined above.
Acetyl number = (mL N/10 KOH used to neutralize HA)(5.61) / Wt. of acetylated product taken
References:
1. Pharmacopedia/National Formulary. US. (2009) Vol. 1, p. 151 2. Warth, A.H. The chemical and technology of waxes. (1956) Second edition, p. 590 3. Ranken, M.D., Kill, R.C. and Baker C.G.J. Food Industries Manual. (1997) 24th edition, p. 299