Acid value measures the milligrams of potassium hydroxide required to neutralize free fatty acids in one gram of a substance, serving as a key indicator of freshness in oils, fats, and industrial lubricants. It is determined via titration, with higher values signifying increased degradation, rancidity, or the need for oil replacement. For a detailed overview, see the ScienceDirect Topics article . Acid Number: A Comprehensive Guide - Machinery Lubrication