The determination of the Total Acidity Number (TAN- ASTM D974, D3339) by colorimetry can be used as a measure of the degradation of a lubricant (depletion of additives, oxidation and contamination).
TAN is a measure of the total acid concentration in a lubricating oil and values are expressed as the volume of potassium hydroxide (KOH) needed to neutralize all acidic components in a gram of oil sample (mgKOH/g).
With the aging of the oil and its oxidation, small amounts of acid products are formed that cause an increase in the Acidity Number (AN).
A high TAN is a clear indication that the lubricant has reached its end of life and needs to be replaced immediately.