Determination of trace elemental sulfur and hydrogen sulfide in petroleum and its distillates by preliminary extraction with voltammetric detection

Abstract

Determination of elemental sulfur and hydrogen sulfide at sub-ppm levels in petroleum and its distillates was individually investigated using two convenient, accurate and sensitive procedures. These involved preliminary extraction of sample species, which were then detected by differential pulse polarography (DPP) and square wave voltammetry (SWV) on dropping mercury and static mercury drop electrodes, respectively. In one procedure, the required amount of sample was diluted with isopropyl alcohol–toluene (2 + 1 v/v) and was heated to 80 °C. Hydrogen sulfide available in the sample was then delivered through a designed system to an absorber cell by a controlled flow of helium gas in order for its concentration to be determined by SWV detection. In another procedure, an appropriate amount of sample was shaken with a recommended volume of extraction solution. The mixture was filtered in two steps and was then ready for its elemental sulfur content to be measured by DPP detection. The effects of extraction parameters such as volume ratio of extraction solution to sample, type of extraction solution and extraction time were studied in order to achieve a high recovery of sulfur. Various instrumental factors such as scan rate, pulse height, initial and final potential and purge time were optimized. The method was free from interferences from organic sulfides, disulfides and thiophene. The 3ς detection limits were 10 and 100 ng g⁻¹ for H₂S (by use of SWV) and S⁰ (by use of DPP), respectively. At a 1 μg g⁻¹ level in samples, the relative standard deviations (n = 4) were 1.7 and 3.3% for SWV and DPP, respectively