This test determines hydroxyl number, which gives an indication of the total number of free hydroxyl groups in a sample – a large hydroxyl number indicates a large number of free hydroxyl groups, a small number indicates a small number of free hydroxyl groups. This test is intended for use with castor oil, dehydrated castor oil and castor oil derivatives, fatty alcohols, mono and di-gycerides, hydroxysteric acid and other fatty samples.
As an example of the usefulness of this test, consider castor oil. Castor oil is a natural product derived from the castor bean. It is biodegradable, has excellent low temperature viscosity properties, does not react with rubber seals and has a large dielectric constant. Castor oil and its derivatives are therefore attractive for use in electrical capacitor oils, diesel fuels and lubricants – but has one major draw back – it has free hydroxyl groups that tend to react with isocyanates to form varnish, potentially clogging filters, valves and other system components. The hydroxyl number indicates if the castor oil or castor oil derivative is appropriate for the intended application.
The sample is accurately weighed into two flasks. In the first flask, acetylating agents are added and the sample is refluxed to allow acylation to take place. The second flask is subjected to the same conditions with no acylation agent added (this determines acid number). Both flasks are titrated with potassium hydroxide (KOH). The hydroxyl number is the mg KOH per gram of acetylated sample (flask 1) minus the acid number (flask 2). Reported is the hydroxyl number in mg KOH per gram of sample.