AN is the measure of acid concentration in a nonaqueous solution. It is determined by the amount of potassium hydroxide (KOH) base required to neutralize the acid in one gram of an oil sample. The standard unit of measure is mg KOH/g. AN does not represent the absolute acid concentration of the oil sample. The AN measurement detects both weak organic acids and strong inorganic acids. A change in the acid concentration of an oil can originate from multiple sources. Acidic contaminants, wrong oil, alkaline-reserve depletion and oxidation by-products can cause an increase in acid concentration. Table 1 lists common acids that can be detected.
Understanding the extent of additive depletion is key in determining the RUL of an oil. Some additives are weakly acidic and can elevate the oil's initial AN. As the lubricant ages these additives deplete, thereby reducing the acidity created by the additives. The common antiwear additive, zinc dialkyl dithiophosphate (ZDDP), produces certain AN trends during lubricant aging. Concurrently, the oil is possibly being contaminated with acidic constituents, increasing the acid content in the oil. The combined effects of additive depletion, acidic contamination and other acidic-affecting events create a challenge in determining what the AN represents. Figure 1 shows the underlying components that affect the AN during lubricant aging. It can be seen that during an induction period the antioxidant additives are depleting; once these additives are depleted, the base oil begins to oxidize if the stressing conditions are sufficiently high. By trending the AN, this increase can be detected.