Oxidative coulometric trace determination of sulphur in hydrocarbons

A new combustion system for the oxidative coulometric determination of sulphur in liquid hydrocarbons is described. The conditions were selected so that the recovery as SO(2) was close to 100%. The relative standard deviation was < 1% for sulphur in the range 2-1000 mg/l., with thiophene in cyclohexane as a test substance. Thermodynamic data on the equilibrium between SO(2), O(2) and SO(3) were used to select the operating conditions. To increase the recovery of SO(2) the combustion gas mixture was diluted with an inert gas to lower the partial pressure of oxygen. A temperature of 1000 degrees in the equilibrium zone resulted in a recovery of 99%. The SO(2) was titrated with coulometrically generated iodine, the concentration of which was controlled by a Pt-redox electrode. The response of this electrode has been examined. A rather high concentration of I(-) was used to suppress iodine losses during the analysis. The time of analysis was 2-5 min, and sample sizes were 3-7 mul. An LKB 16300 Coulometric Analyzer governed the titration procedure.