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

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  • Method adopted/last revised:
  • Method reapproved: 2019
  • Status: Current
  • First printed in STM books:

This document specifies procedures for the determination of the acidic constituents in petroleum products, lubricating oils, biodiesel and blends of biodiesel (see Note 1). 1.1 Method A: For petroleum products and lubricants soluble or nearly soluble in mixtures of toluene and propan-2-ol. It is applicable to the determination of acids whose dissociation constants in water are larger than 10–9; extremely weak acids whose dissociation constants are smaller than 10–9 do not interfere. Salts react if their hydrolysis constants are larger than 10–9. The range of acid numbers included in the precision
statement is 0.1 mg/g to 150mg/g KOH. 1.2 Method B: Developed specifically for biodiesel and biodiesel blends with low acidity and slightly different solubility. This test requires the use of an automatic titrator with automatic endpoint-seeking capability.

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