Acid Number (AN, formerly TAN) – On-site and Off-Site

AN is one of two common Neutralisation Numbers. It is performed for purposes of determining the amount of acidic constituents in an oil, expressed as a concentration.



A few grams of oil are diluted with an appropriate solvent and the subsequent mixture titrated with an alkaline reagent, potassium hydroxide (KOH). The resulting answer at the completion of the titration is called the “Acid Number”. The history of this test includes a number of changes in the method, some of them ad hoc, such that the term, TAN (Total Acid Number), is being phased out. The original method allowed for Strong Acid Number and Total Acid Number - and Weak Acid Number, by inference, if one subtracted Strong from Total. None of these is commonly separated anymore, thus the test simply became “Acid Number”, and it is ‘total’.

On-Site: A titration test suitable for Strong Acid Number, typically not very user friendly Including the chemical test from Kittiwake).  However new portable IR device (FTIR3) is now available and acurately mirrors ASTM methodology in a format suitable for use in the field.  These types of devices have been around for a while but true ASTM replicability is an industry first!

Off-Site: Auto-titrators comprise the predominant method in use

A rather new one-step procedure that involves use of FTIR and new chemistries is now an approved ASTM method
Any of these methods should prove adequate for in-service analysis needs.

Reporting Units: Lube oil acid number is reported as milligrams of KOH/gram of oil. The most referenced method is ASTM D664. NOTE: It is imperative to have a New Lube baseline as additives can impart AN componentry to an oil.



Above: Failure due to acidic constituents in oil

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General information regarding AN:

Lube oil acid number can indicate these possibilities, among others:

• Oxidation (best if correlated with VIS)
• Corrosive tendency of the lubricant (may need to know strong acid number to glean better corroboration)
• Depletion of BN (Base Number, formerly TBN)

Lube oil acid number does not usually allow one to distinguish between ‘strong’ and ‘weak’ acids, but it is possible to determine the difference when it is deemed important to do so. In such case a special section of the D664 method would have to be strictly followed.