IP 139: Petroleum products and lubricants - Determination of acid or base number - Colour-indicator titration method

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  • Method adopted/last revised: 1998
  • Method reapproved: 2017
  • REF/ISBN: IP139-2938621
  • Status: Current
  • First printed in STM books: April 2018


This International Standard specifies a colour-indicator titration method for the determination of acid or basic constituents in petroleum products and lubricants soluble in mixtures of toluene and propan-2-ol. It is applicable for the determination of acids or bases whose dissociation constants in water are greater than 10-9; extremely weak acids or bases whose dissociation constants are less than 10-9 do not interfere. Salts react if their hydrolysis constants are greater than 10-9.

NOTE 1 In new and used oils, the constituents considered to have acidic characteristics include inorganic acids, esters, phenolic compounds, lactones, resins, salts of heavy metals, and additives such as inhibitors and detergents. Similarly, constituents considered to have basic properties include organic and inorganic bases, amino compounds, salts of weak acids (soaps), basic salts of polyacidic bases, salts of heavy metals, and additives such as inhibitors and detergents.

The method is not suitable for measuring the basic constituents of many basic additive-type lubricating oils, for which IS0 3771 can be used.

This International Standard indicates relative changes that occur in an oil during use under oxidizing conditions. Although the titration is made under definite equilibrium conditions, the method does not measure an absolute acidic or basic property that can be used to predict performance of an oil under service conditions.

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