12. Chemical characteristics and analysis

12.1 Neutralization index (acidity and/or basicity of oils)

• Acid Number (AN), formerly Total Acid Number (TAN), is, by definition, the number of milligrams of potash required to neutralize the organic or mineral acids contained in one gram of oil (mg KOH/g).

  Pure hydrocarbon base oils (conventional mineral bases, hydrotreated bases, PAO, etc.) must be acid-free when new. The appearance of acidity can only result from their oxidation, except in exceptional cases of pollution. On the other hand, formulated oils may initially have a significant acidity index. This is due to the presence of certain additives (e.g. zinc dithiophosphate) and is not dangerous. In all cases, an increase in the acid number makes it possible to monitor the...