ASTM D6021-12(2017)

5.1 Residual fuel oils can contain H_{5.1.1 The measurement of H5.2 This test method was developed so refiners, fuel terminal operators and independent testing laboratory personnel can analytically measure the amount of HNote 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 μL/L(ppmv) 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.