Sulfated ash is defined as the residue remaining after a biodiesel sample has been carbonized (i.e. combusted), and the residue subsequently treated with sulphuric acid and heated to constant weight. The primary ash-forming materials in biodiesel include calcium, magnesium, sodium and potassium. These materials may be present in abrasive solids, soluble metallic soaps and any remaining catalyst. Abrasive solids and catalysts can lead to wear on injectors, fuel pumps, pistons and rings, as well as engine deposits. Soluble metallic soaps can also lead to engine deposits, as well as filter plugging. ASTM D6751 allows for a maximum sulfated ash level of 0.02 (% mass).
Sulfated ash is determined using ASTM test method D874. A sample of biodiesel is first heated in a crucible and ignited to burn off everything until only ash and carbon remain. After the sample is cooled, sulphuric acid is added to the ash and carbon and heated again until all fumes are removed. The crucible is then placed in a muffle furnace set at 775°C and further heated until oxidation of the carbon is complete. Once the crucible is removed and allowed to cool down, water and sulphuric acid is again added to the sample and heated until no fumes remain. It is then placed back in the furnace at 775°C for 30 minutes, removed and weighed. This is then repeated until two successive weighings differ by less than 1 mg.