scope:

This International Standard specifies a method for the determination of acidic constituents in petroleum products and lubricants soluble or nearly soluble in mixtures of toluene and propan-2-ol. It is applicable for the determination of acids whose dissociation constants in water are larger than 10^-9; ex tremely weak acids whose dissociation constants are smaller than 10^-9 do not interfere. Salts react if their hydrolysis constants are larger than 10^-9.

NOTE - In new and used 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 base constituents, acid salts of polybasic acids, and additives such as inhibitors and detergents.

This method may be used to indicate relative changes that occur in an oil during use under oxidizing conditions regardless of the colour or other properties of the resulting oil. 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. No general relationship between bearing corrosion and acid or base number is known.

NOTE - A colour-indicator titration method is also available in ISO 6618 (see bibliography in annex A). The acid numbers obtained by the potentiometric method may or may not be numerically the same as those obtained by ISO 6618, but they are generally of the same order of magnitude.

New and used petroleum products may contain acidic constituents that are present as additives or as degradation products formed during service, such as oxidation products. The relative amounts of these materials can be determined by titrating with bases. The acid number is a measure of this amount of acidic substances in the oil under the conditions of the test. The acid number is used as a guide in the quality control of lubricating oil formulations. It is also sometimes used as a measure of lubricant degradation in service. Any condemning limits must be empirically established.

As a variety of oxidation products contribute to the acid number and the organic acids vary widely in corrosion properties, the test cannot be used to predict corrosiveness of an oil under service conditions. No general correlation is known between acid number and the corrosive tendency of oils toward metals. Compounded engine oils can, and usually do, have an acid number in this test.