The hydroxyl value is defined as the number of milligrams of potassium hydroxide equivalent to the hydroxyl content of one gm. of the sample. Applicable to oils and epoxidized oils and their derivatives containing an oxirane ring structure.Â
Apparatus
(a) Erlenmeyer flasks 250 ml, standard glass stopper.
(b) Measuring Pipet 10 ml, Volumetric.
(c) Buret, 50 ml graduated in 0.1 ml divisions.
Reagents
(i) Pyridine R. G distilled at 144 - 115 °C.
(ii) LR grade Acetic Anhydride fresh.
(iii) Mix to pyridine and acetic anhydride reagent, this is freshly prepared before using. 3 Volumes of pyridine and 1 volume of acetic anhydride.
(iv) Normal butyl alcohol reagent grade and neutralized with 0.5N methanolic KOH to faint pink color, use phenolphthalein indicator.
(v) Methanolic potassium hydroxide solution, 05N and 1.0N with standardized.
(vi) Phenolphthalein indicator solution 1.0 gm in 100 ml ethanol.
Procedure
Weight to sample the nearest 0.1 mg. into the Erlenmeyer flask the indicated amount of sample for acetylation.
Pipet 5.0 ml of the pyridine-acetic anhydride reagent into the flask containing the sample for acetylation. (For samples having a 0.0 - 26.0 hydroxyl value add an additional 5.0 ml of pyridine to the flask) Thoroughly mix the contents by swirling. Let's stand at 25 oC for 24 Hours.
Pipet 5.0 ml of pyridine-acetic anhydride reagent in an empty flask for the reagent blank and add any extra pyridine that was added to the sample. Stopper same as the sample. Let's stand at 25Â oC for 24 hrs. (and check the acid value for this sample to add to the calculation)
After the sample and blank flask have stood for 24 hrs add 10 ml of water. Use water to wash the stoppers and the sides of the flask. and add 10 ml of pyridine again washing the stoppers and sides of the flask. stoppers and mix thoroughly by swirling. let flasks stand for 10 minutes after mixing before proceeding to the next step with an occasional swirling of the flask.Â
Now add 25 ml of butyl alcohol (neutralized to phenolphthalein) to both samples and blank again rinsing off stoppers and sides of the flask.
Add 1.0 ml of phenolphthalein indicator solution to both sample and blank. Titrate to a faint pink end-point with 0.5N alcoholic or methanolic potassium hydroxide solution.
Calculation
Uncorrected hydroxyl value = (B-S) *N*56.1 / W
or corrected Hydroxyl value = Uncorrected Hydroxyl value + Acid Value
%OH = Hydroxyl Value / 32.98
Where
B = ml of potassium hydroxide solution required for the reagent blank.
S = ml of potassium hydroxide solution required for titration of the acetylated sample.
W = Weight of sample used.
Precision
The method precision was determined by using five laboratories and three types of epoxidized products.
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