5.1 Excessive levels of hydrogen sulfide in the vapor phase above residual fuel oils in storage tanks can result in health hazards, violation of local occupational health and safety regulations, and public complaint. An additional concern is corrosion that can be caused by the presence of H₂S during refining or other activities. Control measures to maintain safe levels of H₂S require a precise method for the measurement of potentially hazardous levels of H₂S in fuel oils. (Warning—Safety. Hydrogen sulfide (H₂S) is a very dangerous, toxic, explosive and flammable, colorless and transparent gas which can be found in crude oil and can be formed during the manufacture of the fuel at the refinery and can be released during handling, storage, and distribution. At very low concentrations, the gas has the characteristic smell of rotten eggs. However, at higher concentrations, it causes a loss of smell, headaches, and dizziness, and at very high concentrations, it causes instantaneous death. It is strongly recommended that personnel involved in the testing for hydrogen sulfide are aware of the hazards of vapor-phase H₂S and have in place appropriate processes and procedures to manage the risk of exposure.)

5.2 This test method was developed so refiners, fuel terminal operators, and independent testing laboratory personnel can rapidly and precisely measure the amount of H₂S in residual fuel oils and distillate blend stocks, with a minimum of training, in a wide range of locations.

5.3 Test Method D5705 provides a simple and consistent field test method for the rapid determination of H₂S in the residual fuel oils vapor phase. However it does not necessarily simulate the vapor phase H₂S concentration of a fuel storage tank nor does it provide any indication of the liquid phase H₂S concentration.

5.4 Test Method D6021 does measure the H₂S concentration of H₂S in the liquid phase, however it requires a laboratory and a skilled operator to perform the complex procedure and calculations, and does not offer any reproducibility data. This test method (D7621) offers a 15 min automated test, simplicity, full precision, and a degree of portability.

5.5 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. The equilibrium of the vapor phase is disrupted the moment a vent or access point is opened to collect a sample.

1.1 This test method covers procedures (A and B) for the determination of the hydrogen sulfide (H₂S) content of fuel oils such as marine residual fuels and blend stocks, with viscosity up to 3000 mm²s^-1 at 50 °C, and marine distillate fuels, as measured in the liquid phase.

Note 1: Specification fuels falling within the scope of this test method are: ASTM Specification D396, MIL-DTL-16884, and ISO 8217.

1.2 Procedure A has been shown to eliminate interferences such as thiols (mercaptans) and alkyl sulfides. Procedure B can give elevated results if such interferences are present (see Annex A2).

Note 2: A procedure for measuring the amount of hydrogen sulfide in crude oil can be found in Appendix X1. Full precision for Appendix X1 has not yet been determined.

1.3 Valid ranges for the precision are given in Table 2 and Table 3. Measurements can be made outside these ranges however precision has not been determined.

1.4 Samples containing FAME do not affect the measurement of hydrogen sulfide by this test method.

1.5 The values stated in SI units are to be regarded as standard. Non-SI units given in parentheses are for information only.

1.6 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.