The total acid number, or TAN, of a mineral oil measures the amount of acid present in the oil. TAN is calculated not by the parts per million of acidic molecules, but by the overall acidity of the oil.
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The total acid number is expressed as the number of milligrams of the strong base potassium hydroxide (KOH) needed to bring one gram of the oil to a neutral pH.
TAN is calculated by means of a process called titration, in which a reagent (potassium hydroxide) is added to a sample in small, measured amounts. After each addition, the sample is observed for the desired change (in this case, neutral pH).
Any naturally occurring oil typically contains some weak organic acids such as fatty acids. Diesel engines can also generate strong acids via combustion; these acids may be present in lubricating mineral oils.
TAN is an important consideration because it causes corrosion of engine parts and can leave deposits on the engine's surfaces. The most premium oils are those with the lowest total acid number.
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