TAN (Total Acid Number)

is the amount of potassium hydroxide (KOH) in milligrams (mg) that is needed to neutralize the acids in one gram of oil. It is an important quality measurement of a lubricants propensity to form Filming (Varnish & Lacquer) compounds. The TAN value can be deduced by a couple of different methods, eg: by color indicator titration, by potentiometric titration.

Color Indicating Titration:  An appropriate pH color indicator eg: Phenolphthalein, is used. Titrant is added to the sample by means of a burette. The volume of titant used to cause a permanent color change in the sample is recorded and used to calculate the TAN value.

Potentiometric Titration:  The sample is normally dissolved in toluene and propanol with a little water and titrated with alcoholic potassium hydroxide (if sample is acidic). A glass electrode and reference electrode is immersed in the sample and connected to a voltmeter/potentiometer. The meter reading (in millivolts) is plotted against the volume of titrant. The end point is taken at the distinct inflection of the resulting curve corresponding to the basic buffer solution

Also see neutralization number.

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