IP 177: Determination of weak and strong acid number - Potentiometric titration method

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  • Method adopted/last revised: 2013
  • Method reapproved: None
  • REF/ISBN: IP177-2966450
  • Status: New
  • First printed in STM books: March 2013


This document specifies procedures for determining the acidic constituents in petroleum products and lubricating oils (see Note 1). The method resolves these constituents into groups having weak-acid and strong-acid ionization properties provided the dissociation constants of the more strongly acidic compounds are at least 1 000 times those of the next weaker groups.

NOTE 1 - In new and used lubricating oils, the constituents that may be considered to have acidic characteristics include organic and inorganic acids, esters, phenolic compounds, lactones, resins, salts of heavy metals, salts of ammonia and other weak bases, acid salts of polybasic acids and additives such as inhibitors and detergents.

The method may be used to indicate relative changes that occur in a lubricating oil during use under oxidizing conditions regardless of the colour

or other properties of the resulting oil (see Note 2). Although the titration is made under definite equilibrium conditions, the method is not intended to measure an absolute acidic property that can be used to predict performance of a lubricating oil

under service conditions. No general correlation is known between acid number and the corrosive tendency of oils toward metals.

NOTE 2 - The acid number obtained by this standard may or may not be numerically the same as that obtained in accordance with IP 1391).

NOTE 3 - For the purposes of this standard, the terms "% (m/m)" and "% (V/V)" are used to represent respectively the mass fraction and the volume fraction.

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