TERM-TOTAL-ACID-NUMBER
Total Acid Number
Chemical Degradation
Also: TAN · acid number · AN · acidity
Abbrev: TAN · AN
Canonical Definition
A measure of the total acidity in a lubricating or hydraulic oil, expressed in milligrams of potassium hydroxide required to neutralise all acidic components in one gram of oil (mg KOH/g). TAN includes both strong inorganic acids (products of sulfur combustion) and weak organic acids (oxidation byproducts). Rising TAN during service indicates oil oxidation and additive depletion. For hydraulic fluids, a TAN increase of more than 0.5–1.0 mg KOH/g above the fresh oil baseline indicates significant degradation and typically triggers oil replacement.
Engineering Context
TAN is used primarily for hydraulic oil condition assessment and fire-resistant fluid monitoring. For engine oils, TBN is the primary indicator because combustion-derived acids dominate. High TAN values indicate risk of corrosive attack on zinc, copper, and lead bearing overlays. TAN is measured by ASTM D664 potentiometric titration.
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| Permanent ID | TERM-TOTAL-ACID-NUMBER |
| Category | Chemical Degradation |
| Status | Published |
| Version | 1.0 |
| Last Reviewed | 2026-07-05 |