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Acid Number

ASTM method - D974

Measures the level of acidity in a lubricant.

Operating Principle

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.

Significance

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.

Applications

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.