ASTM D664 Acid number of Petroleum Products by Potentiometric Titration
Acids are frequently formed as a result of lubricant oxidation. These acids may change the viscosity of the lubricant, increase corrosion and wear of system components, and cause the formation of sludge, varnish and piston deposits, potentially shortening the life of the lubricant. The acid number indicates the quantity of acid in a sample – a low acid number indicates a small amount of acid, a high acid number indicates a large amount of acid. This test determines the acid number of petroleum products, lubricants, biodiesel fuels and biodiesel blends.
The sample is weighed into the titration vessel, dissolved in solvent and potentiometrically titrated with potassium hydroxide. In the classic method, the data is graphed, the inflection points determined and a blank corrected applied. For the simpler pH11 method, the titration is taken to an endpoint at pH11 when no inflections are observed. The acid number is reported in milligrams KOH per gram of sample.