Journal
JOURNAL OF THE AMERICAN OIL CHEMISTS SOCIETY
Volume 83, Issue 6, Pages 567-570Publisher
WILEY
DOI: 10.1007/s11746-006-1241-8
Keywords
acid number measurement; biodiesel standards; fatty acid methyl esters
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Commercial biodiesel is composed of FAME. It may also contain small amounts of FA, which are quantified by an acid number, expressed as milligrams of potassium hydroxide required to neutralize 1 g of sample. In 2006, the ASTM D 6751 biodiesel acid-number limit was harmonized with the European biodiesel value of 0.50. ASTM D 664 is the standard reference method for measuring the acid number of both ASTM biodiesel and petroleum-derived diesel. This potentiometric method cites acceptable repeatability and mediocre reproducibility, but no information on accuracy. ASTM D 974 is a non-aqueous colorimetric titration that uses potassium hydroxide in isopropanol as the titrant and p-naphtholbenzein as indicator. It was designed for petroleum products and is suitable for colored samples. It has been tested on nine palmitic acid/soy-bean oil standards in the acid-number range of 0.198 to 1.17. All accuracies were within 3.3%. The repeatability was approximately 6% at an acid number of 0.5. The reproducibility appears to be only slightly greater than the repeatability at an acid number of 0.5. It is concluded that ASTM D 974 is a good method for evaluating the acid-number compliance of biodiesel samples.
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