The fuel treatment acidity or acid number is a measure of the concentration of hydrogen ions in fuel. It is measured by titration with an indicator solution, usually phenolphthalein. The higher the value, the more acidic the fuel is.

Examples: Some fuel treatment additives reduce the acid number in diesel to less than 100ppm, equivalent to a pH value of about 7.5. Other fuel additive packages are used to increase the acid number in diesel so that it is between 150 and 400ppm, or as high as 500 ppm (hydrogen sulfide). or 700 ppm (sulfuric acid).

A fuel treatment additive package is an additive package for fuel which usually consists of a combination of chemicals. These packages are generally designed to reduce the corrosiveness and/or viscosity of oil, as well as speed its oxidation and combustion in an engine’s internal combustion chamber. The additive package is generally mixed with the fuel to be treated, then used in the engine.