ASTM D3231

The ASTM D3231 test method covers the determination of phosphorus generally present as pentavalent phosphate esters or salts, or both, in gasoline. Organic matter in the sample is decomposed by ignition in the presence of zinc oxide. The residue is dissolved in sulfuric acid and reacted with ammonium molybdate and hydrazine sulfate. The absorbance of the Molybdenum Blue complex is proportional to the phosphorus concentration in the sample and is read at approximately 820 nm in a 5 cm cell. Phosphorus in gasoline will damage catalytic convertors used in automotive emission control systems, and its level therefore is kept low. For this test method, Tamson supplies a constant temperature bath, equipped with a levelling platform to hold up to eight 100 mL volumetric flasks submerged to the mark to keep the temperature at 82.2 °C to 87.8 °C (180 °F to 190 °F) during the entire period of sample heating.