Title: Determination of Hydroxyl Value
1. Objective: To determine the Hydroxyl Value of the sample.
- Principle: 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.
3. Procedure:
Use Method A unless otherwise directed.
- Method A:
Unless otherwise specified in the individual monograph, weigh accurately the quantity of the substance being examined, stated in Table 1, in a 150-ml acetylation flask fitted with a condenser and add the quantity of Pyridine-Acetic Anhydride Reagent stated in Table 1. 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, re-place 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 %), previously neutralised to dilute Phenolphthalein solution. Titrate with 0.5 M Ethanolic Potassium Hydroxide using dilute Phenolphthalein solution as indicator (end point pink to colourless). Perform a blank determination.
Preparation of Pyridine-Acetic Anhydride Reagent: Just before use, add cautiously with cooling 25 ml of freshly distilled acetic anhydride to 50 ml of freshly distilled anhydrous Pyridine, cool and dilute with freshly distilled anhydrous Pyridine to 100 ml.
TABLE 1
| Presumed hydroxyl value | Quantity of substance (g) | Volume of pyridine acetic anhydride reagent (ml) |
| 10 to 100 | 2.0 | 5.00 |
| 101 to 150 | 1.5 | 5.00 |
| 151 to 200 | 1.0 | 5.00 |
| 201 to 250 | 0.75 | 5.00 |
| 251 to 300 | 0.60 or 1.20 | 5.00 or 10.00 |
| 301 to 350 | 1.00 | 10.00 |
| 351 to 700 | 0.75 | 15.00 |
| 701 to 950 | 0.5 | 15.00 |
Calculate the Hydroxyl value from the expression:-
Hydroxyl value = Acid Value + 28.05 x ( 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 being examined 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 1M Potassium Hydroxide using dilute Phenolphthalein solution as indicator. Perform a blank determination.
Calculate the Hydroxyl value from the expression:-
Hydroxyl value = 56.11 x v
w
Where, v = difference, in ml, between the titrations;
w = weight, in g, of the substance.