5. Significance and Use
5.1 New and used petroleum products, biodiesel and blends of biodiesel may contain acidic constituents that are present as additives or as degradation products formed during service,such as oxidation products. The relative amount of these
materials can be determined by titrating with bases. The acid number is a measure of this amount of acidic substance in the oil, always under the conditions of the test. The acid number is used as a guide in the quality control of lubricating oil
formulations. It is also sometimes used as a measure of lubricant degradation in service. Any condemning limits must
be empirically established.
5.2 Since a variety of oxidation products contribute to the acid number and the organic acids vary widely in corrosion
properties, the test method cannot be used to predict corrosiveness of oil or biodiesel and blends under service conditions. No general correlation is known between acid number and the corrosive tendency of biodiesel and blends or oils toward
metals.