The oil in an engine or a turbine does not stay the same. It works hard, it gets hot, it meets air and water, and over time it changes. One of the ways labs keep track of that change is a pair of numbers you will see on many oil analysis reports: the acid number and the base number.
What the numbers actually are
The acid number tells you how much of a standard base it takes to neutralise the acidic material in one gram of oil. It is written in milligrams of potassium hydroxide per gram, which is just the standard way of keeping score. It does not name any particular acid. It counts everything acidic enough to react in the test, all added together.
The base number is the mirror-image count for an oil's basic ingredients, the additives engine oils carry to neutralise acids as they form. It is reported in the same units, so the two can be compared on one report. And yes, the same oil can have both numbers at once. They are not two ends of one scale; they are two different titrations, one counting what reacts with a base and one counting what reacts with an acid.
The number belongs to the method
Here is the part that surprises people. Because these numbers count "whatever reacts under the conditions of the test", change the test and you change the number. That is not a flaw; it is what the number is.
For base number, there are two common methods. One uses a stronger acid in a different solvent and counts weaker bases too, so it reads higher. The other counts mostly the stronger bases. The standards themselves say which to use when: the first for checking a new oil, the second for an oil in service, because a used oil can contain weak breakdown products that the first method would wrongly count as protection still left. Two good laboratories can report different base numbers for the same oil simply because they used different methods. Neither is wrong. They are answering slightly different questions.
Why oils make titration harder
Oil does not mix with water, so these titrations are done in a solvent blend. The probe that reads the solution has to be kept clean; a film of oil on it makes readings drift. And a used oil often does not give the sharp jump a clean titration has. When that happens, the method says to stop at a fixed reading taken from a reference solution instead. Even that reference has changed over the years, which is why a lab's numbers can shift slightly when its method is updated, with no change in the oil at all.
Why this matters to you
If you manage equipment, these numbers can save a machine. But only if they are read properly: always alongside the method that produced them, and never by switching methods partway through an oil's history. A change in method can look exactly like a change in the oil.
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References
- ASTM D664-24, Acid Number of Petroleum Products by Potentiometric Titration (scope and significance).
- ASTM D4739-26, Base Number Determination by Potentiometric Hydrochloric Acid Titration, 5.3.
- "Interview Helps Clarify Questions Surrounding AN/BN Test Methods in Used Oil Samples", Southwest Research Institute, Machinery Lubrication.