ASTM D6021 Standard Test Method for Measurement of Total Hydrogen Sulfide in Residual Fuels by Multiple Headspace Extraction and Sulfur Specific Detection
4. Summary of Test Method
4.1 A representative sample of residual fuel oil is obtained in sufficient quantity to completely fill the sample container. The sample is taken to the laboratory preferably within one to 4 h, within 24 h maximum and placed in a refrigerator until the hydrogen sulfide analysis can be run. At that time, the sample is removed from the refrigerator and allowed to sit at ambient temperature until it flows freely.
4.2 A 0.05 to 5.0 g test specimen (aliquot) is placed in a headspace vial and heated in an oven at 60°C for more than five but less than 15 min. The headspace gas is sampled and injected into either of two types of apparatus capable of measuring the hydrogen sulfide concentration in the gaseous sample. The two types of apparatus are those using the reaction of lead acetate with H2S (see Test Method D4084 or Test Method D4323) and those based on chemiluminescence (see Test Method D5504).
4.3 The remaining contents of the headspace vial are cooled for 5 min, then again heated in the oven. The headspace contents are again transferred to the hydrogen sulfide measuring instrument. The procedure is repeated for a third time. This is known as multiple headspace extraction procedure (MHE).
4.4 A linear plot of the natural logarithm of the area or peak height difference of the instrument reading against the number of injections is indicative of the correctness of the extraction procedure. The difference in area or peak height of the first two injections is used to calculate a total area or total peak height difference. The total area or total peak height difference is multiplied by a response factor determined from a direct gas calibration mixture and divided by the weight of the test specimen to determine the concentration of H2S in the residual fuel in µg/g (ppmw).
5. Significance and Use
5.1 Residual fuel oils can contain H2S in the liquid phase, and this can result in hazardous vapor phase levels of H2S in storage tank headspaces. The vapor phase levels can vary significantly according to the headspace volume, fuel temperature, and agitation. Measurement of H2S levels in the liquid phase provides a useful indication of the residual fuel oil's propensity to form high vapor phase levels, and lower levels in the residual fuel oil will directly reduce risk of H2S exposure. It is critical, however, that anyone involved in handling fuel oil, such as vessel owners and operators, continue to maintain appropriate safety practices designed to protect the crew, tank farm operators and others who can be exposed to H2S.
5.1.1 The measurement of H2S in the liquid phase is appropriate for product quality control, while the measurement of H2S in the vapor phase is appropriate for health and safety purposes.
5.2 This test method was developed so refiners, fuel terminal operators and independent testing laboratory personnel can analytically measure the amount of H2S in the liquid phase of residual fuel oils.
NOTE 1 - Test Method D6021 is one of three test methods for quantitatively measuring H2S in residual fuels:
1) Test Method D5705 is a simple field test method for determining H2S levels in the vapor phase.
2) Test Method D7621 is a rapid test method to determine H2S levels in the liquid phase.
5.3 H2S concentrations in the liquid and vapor phase attempt to reach equilibrium in a static system. However, this equilibrium and the related liquid and vapor concentrations can vary greatly depending on temperature and the chemical composition of the liquid phase. A concentration of 1 mg/kg (µg/g) (ppmw) of H2S in the liquid phase of a residual fuel can typically generate an actual gas concentration of >50 to 100 µL/L(ppmv) of H2S in the vapor phase, but the equilibrium of the vapor phase is disrupted the moment a vent or access point is opened to collect a sample.
NOTE 2 - Because of the reactivity, absorptivity, and volatility of H2S any measurement method only provides an H2S concentration at a given moment in time.
