ASTM D3120 test method for Trace Quantities of Sulfur in Light Liquid Petroleum Hydrocarbons by Oxidative Microcoulometry
3. Summary of Test Method
3.1 A liquid sample is introduced into a pyrolysis tube maintained at a temperature between 900-1200°C, having a flowing stream of gas containing 50-80% oxygen and 20-50% inert gas (for example, argon, helium, etc.) Oxidative pyrolysis converts the sulfur to sulfur dioxide, which then flows into a titration cell where it reacts with triiodide ion present in the electrolyte. The triiodide ion consumed is coulometrically replaced and the total current (I x t) required to replace it is a measure of the sulfur present in the sample.
3.2 The reaction occurring in the titration cell as sulfur dioxide enters is:
The triiodide ion consumed in the above reaction is generated coulometrically thus:
3.3 These microequivalents of triiodide ion (iodine) are equal to the number of microequivalents of titratable SO2 ion entering the titration cell.
4. Significance and Use
4.1 This test method is used to determine the concentration of sulfur in light liquid hydrocarbons, gasoline, and diesels and their additives, where such concentrations of sulfur can be detrimental to their production, performance, and use. The measurement of sulfur in the production and final product of gasoline and diesel is required for both regulatory purposes and to ensure maximum life expectancy of catalytic converters used in the automotive industry.
5. Interferences
5.1 This test method is applicable in the presence of total halide concentrations of up to 10 times the sulfur level and total nitrogen concentrations of up to 1000 times the sulfur level.
5.1.1 Stringent techniques shall be employed and all possible sources of sulfur contamination eliminated to attain the quantitative detectability capable with this test method.
5.2 This test method is not applicable in the presence of total heavy metal concentrations (for example, Ni, V, Pb, etc.) in excess of 500 mg/kg (ppm).