ASTM D974 Acid Number Test – Key to Determining Lubricant Condition

While Acid Number (AN) and Base Number (BN) should be run together in a limited number of circumstances, including testing lubricants for natural gas engines, they generally each have their own set of applications.

                Eurofins TestOil Analyst III Field Technician Matt McMahon explains, “Actually, it’s pretty rare that they are run together. AN is the correct test for industrial oils and BN is more for engine oils.” Both tests are included in the Eurofins TestOil basic test package.

AN is defined as the weight (in milligrams) of a standard base (e.g. potassium hydroxide, KOH) that’s required to neutralize all acidic components in the oil. AN used to be referred to as TAN (Total Acid Number), which was misleading since AN does not indicate the total acid concentration of the lubricant.

                Antioxidant additives are consumed with use, which can lead to oxidation and corrosion. Oxidation severely affects an oil’s ability to protect internal components and can also affect the viscosity. Even so, the primary purpose of AN testing is to determine general lubricant condition, not corrosion.

                Eurofins TestOil conducts ASTM D974 to determine the level of acidity in a lubricant. During the test, a portion of the sample is diluted and then exactingly titrated with a basic solution. The resulting AN indicates how much titrant is needed per amount of sample. Higher numbers result in a higher AN. This test can be used on new oils and any oil that is not excessively dark.

                A healthy AN is critical to maintaining the operational integrity of equipment and to prevent internal damage to components. However, no matter how healthy the oil, AN will increase over time, especially if the lubricant is exposed to high running temperatures.  

                While a lubricant with an AN in alarm needs to be addressed immediately, the correct course of action usually isn’t the addition of more antioxidant additives. “I would not recommend trying to resolve a high AN by replenishing the additives.”, Matt says. “The recommended action would be a complete oil change and in severe cases a flush.”  

                Eurofins TestOil Analysts are highly experienced with AN testing,  BN testing and a full range of other tests for industrial oils and motor oils. For more information on working with Eurofins TestOil for oil analysis and training visit Contact: 216-251-2510;

About Eurofins TestOil

With more than 30 years of experience in the oil analysis industry, Eurofins TestOil focuses exclusively on assisting industrial facilities with reducing maintenance costs and avoiding unexpected downtime through oil and fuel analysis program implementation. As industry experts in diagnosing oil-related issues in equipment such as turbines, hydraulics, gearboxes, pumps, compressors and diesel generators, Eurofins TestOil provides customers with same-day turnaround on routine oil analysis testing.   For more information on partnering with Eurofins TestOil on oil analysis programs or training opportunities visit Contact: Michael Barrett 216-251-2510;

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