The acid number, also known as the acid value or neutralization number, measures the amount of acidic compounds present in a lubricating oil. An increasing acid number can indicate oil degradation, contamination, or oxidation.

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The ASTM D2074 standard provides test methods for determining the bleed oil and acid number of used lubricating oils. This standard is essential for evaluating the condition and quality of used lubricating oils in various industries, including automotive, aviation, and industrial applications.

Bleed oil refers to the oil that separates from a lubricating oil sample under specific conditions, indicating the oil's ability to maintain its viscosity and lubricating properties.