ASTM D5305 Test Method for Determination of Ethyl Mercaptan in LP-Gas Vapor
3. Summary of Test Method
3.1 Using a manually-operated vacuum pump, a sample of LP-gas from the vapor space of an LP-gas cylinder, storage tank or other closed containment system is drawn through a detector tube made specifically for detection of mercaptans. The length of stain (color change) produced in the detector tube when exposed to a measured volume of sample is directly proportional to the amount of ethyl mercaptan present in the sample being tested. The length of stain produced in the detector tube is converted to concentration, in parts per million volume (ppmv), by comparison with a calibration scale provided by the manufacturer of the stain tube.
4. Significance and Use
4.1 LP-gas is colorless and odorless, and not detectable by normal human senses. To provide an olfactory warning in the event of a leak, LP-gas intended for domestic or commercial fuel use is intentionally odorized so as to be readily detectable well below flammable or suffocating concentration levels of LP-gas in air. (See Appendix X1.) The most common odorant for LP-gas is ethyl mercaptan. The field use of this test method will rapidly determine the presence and concentration of ethyl mercaptan in LP-gas vapor without the necessity for complex laboratory equipment.
5. Interferences
5.1 Detector tubes can be subject to interferences from materials other than the target substance. Methyl mercaptan will likely interfere with tubes designed to measure ethyl mercaptan. Because of different detection chemistry by different manufacturers, interferences can vary. Consult the manufacturer's instructions for specific interference information and observe any instructions given.
5.2 Propylene (propene) will cause an interfering (gray) discoloration with some tubes designed for ethyl mercaptan. LP-gas from natural gas sources usually does not contain propylene (propene). However, LP-gas produced in refinery operations often does contain propylene (propene). Detector tubes calibrated for t-butyl mercaptan eliminate this interference, and should be used if the presence of propylene (propene) is suspected. Some tubes designed for measurement of t-butyl mercaptan are calibrated in milligrams per cubic metre (mg/m3) and should be converted to ppmv ethyl mercaptan as shown in Annex A1.
5.3 The validity of this test method depends on the ethyl mercaptan in the LP-gas vapor phase being in equilibrium with ethyl mercaptan in the LP-gas liquid phase. If LP-gas vapor has recently been vented, or if a significant volume of vapor relative to the total volume of the vapor phase is vented during this test procedure, the concentration of ethyl mercaptan in the vapor phase sample can be lower than the equilibrium concentration.
