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.

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.

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.

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.

1.1 This test method covers a method suitable for measuring the total amount of hydrogen sulfide (H₂S) in heavy distillates, heavy distillate/residual fuel blends, or residual fuels as defined in Specification D396 Grade 4, 5 (Light), 5 (Heavy), and 6, when the H₂S concentration in the fuel is in the 0.01 to 100 μg/g (ppmw) range.

1.2 The values stated in SI units are to be regarded as standard. No other units of measurement are included in this standard.

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. For specific warning statements, see 7.5, 8.2, 9.2, 10.1.4, and 11.1.