The acidic content of a lubricant may be measured and expressed as a neutralization number obtained from an acid titration. Acid Number (AN or TAN – Total Acid Number) measures the total amount of acidic material present in the lubricant. An increase in AN above that of the new product indicates oil oxidation, or mixing with a more acidic product or contaminant. The results are expressed as a numeric value corresponding to the amount of potassium hydroxide required to neutralize the acid per one gram of sample.