The nuts and bolts of TBN and TAN.

Good Morning,

Recently we have been asked many questions about TBN and its relationship with various other parameters, especially in engine oil. To provide some scientific basis for TBN and TAN I have attached a recent paper on the topic by our Chief Chemist in Indonesia, Ibu Eka Karmila. Please don't hesitate to call us to discuss further. But it seems that a healthy TBN is very important to the health of your engine. THEORY:

TBN ( Total Base Number) value indicates the alkali value of the additive in the oil. TBN values are the  amount of base required to neutralize  acid that is contained in 1 gram of oil, and conversion to mg of potassium hydroxide (KOH). This value is expressed in units of mg.KOH / g. Values for new oil in general is 6.0-13.0 mg.KOH / g.
When the number  is  below 50% of the originalTBN value, the acid neutralizing performance of oil is reduced and corrosive wear occurs. There are two main methods to determine TBN

  1. Hydrochloric acid (ASTM 4739) and
  2. Perchloric acid method (ASTM D2896).

Because perchloric acid method takes into account weak base numbers, a higher TBN is obtained. Therefore, it is necessary to establish which method of calculation is used. If the value of TAN is over the limit, the engine oil should not be used even if the remaining TBN value high. Sulfur contained in the fuel in the combustion process will oxidize (react with oxygen O2) to form gaseous SO2 (sulfur dioxide), and some will turn into SO3 (sulfur trioxide) if the combustion temperatures drop rapidly Furthermore, SO3  gas will react with moisture and combustion produced H2O to form sulfuric acid (H2SO4) which is very corrosive.


The resulting sulfuric acid may form within the combustion chamber and / or outside the combustion chamber. If the process takes place in the crankcase, during blow-by (leakage of combustion gases past the piston rings), sulfuric acid formed will contaminate the engine oil. As a result, the TBN value of oil drops and function and if lower enough acidic corrossion will occur.


Oxidation of the lubricant causes an increase in acid number( TAN) which results in corrosion of theequipment.

TAN is usually analysed for non engine oils and TBN for crankcase (engine) oils.
TAN analysis is to determine :

  • Trend
  • of depletion of certain additive such as ZDDP, overbase detergent and rust inhibitor.
  • Identifies the onset of basestock oxidation
  • Measures the level of corrosive acids
  • Measures the level of reserve alkalinity

TAN analysis was also used to detect the corrosive acid:


Notes on TAN Monitoring

  • For Mineral – based turbine oils, an increase in TAN of just 0.01 mgKOH/g is cause for concern.
  • In high temperature oils, acids from oxidation can vaporize or polymerize resulting in TAN being an unreliable indicator of oxidation. Solution is  to monitor oil oxidation by FTIR.
  • Water is often the cause of acid formation. When water is mixed with oil, it gives the acids greatest corrosion potential.
  • TAN measures acid concentration not acid strength
  • EP and engine oil additive can interfere with TAN trending
  • For mineral oils, a TBN above four isconsidered highly corrosive.

How to Monitor TAN trends



Increasing TBN Trends Are Influenced by :

  1. Makeup oil added
  2. Loss of base oil volatiles

Decreasing TBN are influenced by:

  1. Fuel/water dilution
  2. Thermal Degradation
  3. Reaction with acidic combustions products

How to monitor TBN Trends