D5705-20 – Standard Test Method Technical Guide

🔬 Scope and Significance of D5705-20

This test method provides a standardized field procedure for the direct measurement of hydrogen sulfide (H₂S) in the vapor phase (equilibrium headspace) above residual fuel oil samples. It is critically important for assessing the acute toxic hazard present in storage tank headspaces, where dissolved H₂S in the liquid phase can accumulate to dangerous vapor phase concentrations.

The standard is applicable to fuels conforming to ASTM Specification D396 Grade Nos. 4, 5 (Heavy), and 6. The applicable viscosity range for the test is 5.5 mm²/s at 40 °C to 50 mm²/s at 100 °C. The detection range of the method spans from 5 µmol/mol to 4000 µmol/mol (5 ppm v/v to 4000 ppm v/v).

⚙️ Standard Test Procedure and Conditions

The test method utilizes a 1 L H₂S-inert glass test bottle, filled to exactly 50% by volume with the fuel oil sample directly from an inert sample bottle. The vapor headspace is purged with nitrogen to remove ambient air. The sealed container is then heated in an oven to 60 °C and agitated on an orbital shaker at 3.67 s⁻¹ ± 0.08 s⁻¹ (220 rpm ± 5 rpm) for 3 minutes. This rigorous controlled environment ensures the H₂S partitions into the vapor phase and reaches equilibrium.

The measurement is performed using a length-of-stain detector tube and a hand-operated pump. The tip of the detector tube is inserted into the headspace, close to but not in contact with the liquid surface, and a specific volume of vapor is drawn through the tube. The resulting length of the stain is directly proportional to the H₂S concentration in the vapor phase.

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