ASTM D7621 Standard Test Method for Determination of Hydrogen Sulfide in Fuel Oils by Rapid Liquid Phase Extraction
4. Summary of Test Method
4.1 A weighed test specimen is introduced into a heated test vessel containing a diluent base oil. Air is bubbled through the oil to extract the H2S gas. The air with the extracted H2S is passed, via a vapor phase processor (Procedure A only), to an H2S specific electro-chemical detector enabling the H2S content of the air to be measured and the amount in the liquid phase to be calculated in mg/kg. The filter cartridge (see 7.4) is not required for Procedure B.

5. Significance and Use
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.