ASTM D2158 Standard Test Method for Residues in Liquefied Petroleum (LP) Gases
11. Procedure
11.1 Residue Measurement:
11.1.1 If the LPG sample is expected to be clean, with no residues or contaminants that could result in an oil stain, the procedure may be performed without a cooling coil. However, if a residue or oil stain is detected using this screening procedure (that is, a residue is measured at greater than or equal to 0.05 mL in 11.1.7, or an oil stain is generated with 1.5 mL of test solution in 11.2.4), the test shall be repeated using a cooling coil.
11.1.2 Attach the cooling coil to the LPG sample source, cool the coil to below the boiling point of the sample, and flush the coil and sampling line.
11.1.3 Rinse and cool the centrifuge tube with the material to be sampled and then fill it to the 100-mL mark with a representative sample of LPG.
11.1.4 Immediately insert the copper wire through a clean, slotted cork or a clean, loose-fitting plug of cotton or cleansing tissue in the mouth of the centrifuge tube. The wire helps to prevent superheating and resulting bumping (erratic or excessive boiling), and the cork (or plug) will keep out air or moisture while the sample is weathering.
11.1.5 If more than 10 mL of the sample is lost because of bumping, obtain a new sample and repeat the test.
11.1.6 Allow the sample to weather, using artificial heating if the ambient temperature or type of sample requires it. If, when weathering has ceased and the tube has reached ambient temperature, a visible residue remains, place the tip of the tube in a water bath at 38°C for 5 min.
11.1.7 Record the volume of any remaining residue to the nearest 0.05 mL, and the presence of extraneous matter, if observed.
11.1.8 Perform the oil stain observation described in 11.2 even if there is no apparent or visible residue in the centrifuge tube. Experience has shown that there can be a thin film of oil on the inner surface of the centrifuge tube that is difficult to see and does not give a measurable volume, but can still give a ring or stain in the Oil Stain Observation procedure.
11.2 Oil Stain Observation:
11.2.1 Add sufficient solvent to the centrifuge tube containing the residue described in 11.1.6 to restore the volume to 10 mL. Add the solvent from the wash bottle and carefully wash down the sides of the tube. Stir well so that any residue at the bottom of the tube is dissolved uniformly in the solvent. Stirring with the syringe needle (see 7.3) or pipette has been found satisfactory. This mixture will be referred to as the solvent-residue mixture.
11.2.2 Mark the center of a clean white filter paper.
11.2.3 Fill the syringe or equivalent liquid dispensing device (see 7.3), and direct 1.5 mL of the solvent-residue mixture at the center of the paper at an appropriate rate such that the wetted circle is maintained at about 30 to 35 mm in diameter. If desired, steps 11.2.3 and 11.2.4 may be omitted, and the test continued according to 11.2.5.
11.2.4 After directing the entire 1.5 mL of residue solution onto the filter paper and at the end of a 2-min waiting period, if no oil ring persists when holding the dry filter paper between the eye and a bright incandescent light or strong daylight, discontinue the test and go to 12.1.
11.2.5 If a stain or ring is discernible, determine the volume of the solvent-residue mixture at which the oil stain or ring first persists for 2 min on a new filter paper by adding the solvent-residue mixture in 0.1-mL increments, waiting 2 min after each addition, and inspecting the paper for an oil stain at the end of the 2-min period.
11.2.6 Record the volume in millilitres of the solvent-residue mixture required to yield a persistent oil stain or ring as the oil stain observation.
11.2.7 Alternatively, if a product specification calls for adding a specified amount of solvent-residue mixture to the filter paper, add the specified quantity in 0.1 mL increments, and report the result at the specified total amount.
11.2.7.1 Specification D1835 specifies that 0.3 mL of solvent-residue mixture shall be deposited on the filter paper in 0.1-mL increments.
11.2.7.2 If there is no oil stain after the addition of 0.3 mL and a 2-min waiting period, report the result as "Pass".
11.2.7.3 If an oil stain is observed at 0.3 mL after a 2-min waiting period, report the result as "Fail".
11.3 Any solvent transferred to the wash bottle for purposes of running the test shall either be used in testing during the same day or discarded.
11.4 It has been noted that at low ambient temperatures (below about 5°C) materials in the gasoline boiling range will leave an oil stain or ring that persists after 2 min. Oil stain determinations should be made in a protected area where the temperature is above 5°C. If it is necessary to determine the oil stain at temperatures below 5°C, allow 10 min for oil stain persistence.
