Biodiesel, defined as fatty acid methyl esters produced from plant oils and animal fats by transesterification reactions, plays a very important role as an alternative to conventional petroleum diesel. However, the relatively simple production process of transesterification does not ensure high quality biodiesel.  Any amount of reactants or by-products from the reaction can contaminate the final product, including water, glycerine, catalyst, residual alcohol, unsaponifable matter, soaps and free fatty acids.  These minor components can cause several problems, including engine deposits, filter clogging or fuel deterioration in an engine, or corrosion during long-term storage and/or transportation. The total acid number is an indication of the degree of oxidation and hydrolysis and an easy method for monitoring fuel quality.  Of the products listed above, free fatty acids are the main cause of a high acid number in biodiesel

Significance

There are two standard test methods for quantifying the acid number in biodiesel: ASTM D664 and D974.  For both methods, the acid number is expressed as the mass of KOH (potassium hydroxide) in milligrams required to neutralize the acids in one gram of sample.  ASTM D664 measures the amount of KOH added through potentiometric titration, a more applicable and accurate method compared to the use of a colour indicator in D974.