Total Acid Number(TAN)

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Total Acid Number is an analytical test which determines the level of acidity in the oil and the level of detetriotation of the lubricant. The more acidic a lubricant is, the further its degradation has proceeded. When oils or hydraulic oils breakdown, they generally form acidic by-products which can :

  • Accelarate wear
  • Form deposits – Varnish, sludge and sedimentation formation
  • Increase viscosity
  • Base oil breakdown
  • Additive depletion
  • Loss of lubricity
  • Rust formation and corrosion
  • Fliter plugging
  • Danger of component failure

Oils in the system continously react with atmospheric oxygen to create organic oxidation products which are acidic in nature. At normal or ambient temperature, this reaction is very slow and thus has little or minor effect upon the oil. But at higher or elevated temperatures which exist within the system or engine these rates are compounded and are much higher and can generate high level of weak organic acids.

Total Acid Number or TAN is the measure of both the weak organic and strong inorganic acids present in oil. It is applicable to :

  • Hydraulic oils
  • Gas engine
  • Gear box
  • Gas turbine

Acidity is of different types, mainly weak acids and strong acids and can be further classified as Organic, Hydrochloric, Hydrofloric, Sulphuric, Nitric and Phosphoric as shown below :

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The initial Acid Number of the new oil is determined by the amount of acid present in it. So supposing the new oil or fresh oil has a starting acid number between 0.5 to 1.00 mg KOH/g then it would be generally recommended that the oil be changed when the acid number is four times greater than the acid number of fresh oil. Thus an oil with a initial or starting acid number of 1.00 mg KOH/g should be changed when the acid number test result is 4.00 mg KOH/g or greater.

However in most cases, it is generally recommended that the oil be changed if the acid number is greater than 2.50 mg KOH/g.


ASTM D-644 is the test method which we are using for monitoring the degradation acid content of in-service oil or used oil. The acid number test is performed by diluting the sample with a mixed solvent in a reaction beaker. After mixing, potassium hydroxide is titrated to chemically react with acidic components in the sample. When the end point of the chemical reaction is reached, the amount of titrant used is reported in milligrams of potassium hydroxide equivalent per gram of sample (mg of KOH/g).

Testing Equipment used by us in our laboratory: Mettler Toledo G20

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