HYDROXYL NUMBER: ASTM D1957
EXPLANATION
Hydroxyl value is important in establishing reactivity with acids and isocyanates. It is also a measure of the degree of dehydration of castor oil.
This test method determines the total amount of residual hydroxyl groups present in oils and other fatty acid-containing materials, reported as hydroxyl value.
This test method involves the acetylation of hydroxyl-containing fatty oils and acids using pyridine as solvent. Other groups that will react with acetic anhydride such as primary and secondary amines under the conditions of the method will be reported as hydroxyl. Hydrocarbons, acetals, esters and water do not interfere. However, mercaptans, amines, some esters, and aldehydes interfere. Tertiary alcohols are incompletely determined. The hydroxyl value is expressed as milligrams of potassium hydroxide equivalent to the hydroxyl content of 1 g of the oil. A correction is applied for acid groups present.
This test method can be used for the determination of hydroxyl content of fatty oils, fatty alcohols, mono- and di-glycerides, and hydroxystearic acid.
TEST SUMMARY
A sample is acetylated with pyridine-acetic anhydride solution by refluxing for 1 h. Later neutralized n-butyl alcohol is added and the solution is titrated with alcoholic KOH solution to phenolphthalein end point.
TEST PRECISION
Repeatability: 2.4
Reproducibility: 3.0
