ASTM D6428 Test Method for Total Sulfur in Liquid Aromatic Hydrocarbons and Their Derivatives by Oxidative Combustion and Electrochemical Detection
4. Summary of Test Method
4.1 The sample of liquid aromatic hydrocarbon is injected, at a controlled rate, into a stream ofinert gas (helium or argon) or inert gas mixed with oxygen. The sample is vaporized and carried into a high temperature zone (>900°C) where oxygen is introduced. Sulfur compounds, present in the specimen are converted to sulfur dioxide (SO2). Sulfur dioxide is reacted with the sensing electrode in a 3-electrode electrochemical cell. This reaction produces a measurable current that is directly proportional to the amount of sulfur in the original sample material. The reaction that occurs is as follows:
Combustion: R-S + O2 >900°C + > CO2 + H2O + SO2 + oxides
Detection: SO2 + 2H2O electrolyte H2SO4 + (2H+) + (2e−)

5. Significance and Use
5.1 Some process catalysts used in petroleum and chemical refining may be poisoned when even trace amounts of sulfurous materials are contained in the feedstocks. This test method can be used to determine total sulfur in process feeds and may also be used to control sulfur compounds in finished products which fall within the scope of this test method.

NOTE 1 - Virtually all sulfur compounds will be detected by this technique.

6. Interferences
6.1 Moisture produced during the combustion step can interfere if not removed prior to the detector.