I have seen articles/analysis regarding Acid Number (AN)& Total Acid Number (TAN). I can find no distinguishing factors that seperate the two.
I was recently asked the question what is the difference between the two. I believe they are the same but would like feedback from those willing to share their knowledge.
Is there a difference between AN & TAN?

Looking at "Practical Lubrication for Industrial Facilities" by Heinz P. Bloch P.E.
The Glossary of Terms lists Acid Number and refers one to Strong Acid Number (SAN) and Total Acid Number (TAN)reference for Acid Number. Both of which say basicly the same thing - "The quantity of base, expressed in milligrams of potassium hydroxide, that is required to titrate all acidic constituents present in a 1 gram sample of oil" (TAN)
From this I would be inclined to conclude that AN and TAN are the same TAN and SAN are only different in the range of measurement. The TAN test kits I use only go up to 2.0 mg KOH.

The method most often currently used in US for measuring acidity in oils is ASTM D 664, and results are referred to as the AN (Acid Number). The term TAN (Total Acid Number) has been used in the past (and it meant the same as AN). However, this term (TAN) has been abandoned because of the difficulty to reach and establish final and definite inflection point during a titration (curve creeps up forever during titration because of a slow process of neutralization of very weak organic acids). According to the ASTM, precision data for Strong Acid Number measurement has not been established because of its rear occurrence in sample analysis.