ISO 3771:2011 specifies a method for the determination of basic constituents in petroleum products by potentiometric titration with perchloric acid in glacial acetic acid.

The constituents that can be considered to have basic characteristics include organic and inorganic bases, amino compounds, salts of weak acids (e.g. soaps), basic salts of polyacid bases, and salts of heavy metals.

The ranges of base number values for which precision values for the method have been established are:

• unused oils: base numbers from 3 to 45;
• additive concentrates: base numbers from 5 to 45;
• used oils: base numbers from 3 to 30.