The Acid Number test measures the level of acidity in a lubricant
Acid Number (AN) is an indicator of oil serviceability. It is useful in monitoring acid buildup in oils due to depletion of antioxidants. Oil oxidation causes acidic byproducts to form. High acid levels can indicate excessive oil oxidation or depletion of the oil additives and can lead to corrosion of the internal components. By monitoring the acid level, the oil can be changed before any damage occurs.
A portion of the sample is diluted and then titrated with a basic solution to a certain end point. The acid number result is an expression of how much titrant is needed per amount of sample. Higher numbers indicate a higher acidity level. ASTM method - D974
As antioxidants are consumed during use, generated acids will not be neutralized and will start to increase in concentration. By monitoring acid number results, a component of lubricant health can be ascertained.
All industrial lubricating oils and a limited amount of combustion engine oils. This test is not appropriate for high water content fluids such as water glycols and oil / water emulsions.