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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.

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    ASTM D3231 Constant temperature bath
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General
ASTM D3231 is a standard test method for determining the phosphorus content in gasoline. The method is applicable to phosphorus concentrations ranging from approximately 0.2 to 40 mg/L. Measuring phosphorus is important because excessive phosphorus can poison automotive catalytic converters and increase vehicle emissions.

Test
The sample is ashed in the presence of zinc oxide to decompose the organic material. The residue is dissolved in sulfuric acid and reacted with ammonium molybdate and hydrazine sulfate to form a molybdenum blue complex. The absorbance is measured spectrophotometrically at approximately 820 nm, and the phosphorus concentration is calculated from a calibration curve.

Precision
ASTM D3231 provides established repeatability and reproducibility values based on interlaboratory testing. The precision depends on the phosphorus concentration, providing reliable and consistent results for quality control and regulatory compliance within the specified measurement range.

P Range, mg/L

Repeatability

Reproducibility

0.2 to 1.3

0.05

0.13

1.3 to 40

7 % of the Mean

13 % of the Mean

 

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