Hydroxyl value

The hydroxyl value ( OHV) is a measure of the content of hydroxy groups in organic materials, such as resins, coatings, polyesterols, fats and solvents.

The hydroxyl value indicates the amount of potassium hydroxide in milligrams that is bound by one gram of the substance at an amount of acetic acid equivalent to the acetylation.

Implementation

To determine the hydroxyl number experimentally, a corresponding mass of the material under investigation is accurately weighed ( eg hard fat). Subsequently, an acetylation mixture of acetic anhydride and anhydrous pyridine is added in excess ( 1:3 ) and heated for one hour in a water bath. The one hydroxyl group ( the fat ) reacts with acetic anhydride during which the acetylated hydroxy group and one molecule of acetic acid. All this unused acetic anhydride molecules are reacted in the next step with water to form two molecules of acetic acid. Followed by neutralization with ethanolic KOH solution and phenolphthalein.

The same is repeated without the analyte ( blank value ), then each molecule of acetic anhydride, acetic acid is converted to two molecules - in accordance with one more of KOH required to neutralize the acetic acid formed. In the main experiment with the analyte is created for each hydroxy group present in it, exactly one molecule of acetic acid less. About this context, the OHN can be determined. Important is the relationship: n ( OH groups) = n ( acetic acid, which produced less) = n (KOH that is less consumed for the acetic acid in the main study ).

Calculation according to DIN

This is