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.
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.