Simultaneous determination of trace amounts of sulfite and thiosulfate in petroleum and its distillates by extraction and differential pulse polarography

Simultaneous determination of sulfite and thiosulfate at sub-ppm levels in petroleum and its distillates was investigated using a convenient, accurate and sensitive procedure. This method involved preliminary extraction of the sample followed by detection via differential pulse polarography (DPP) at a dropping mercury electrode. In this procedure, an appropriate amount of sample was shaken with a recommended volume of 0.25% (w/w) sodium acetate solution. The mixture was filtered in two steps and was then ready for DPP. The method was free from interferences from hydrogen sulfide, elemental sulfur, organic sulfides, and thiophene. Various instrumental factors such as scan rate, pulse height, initial and final potential, and purge time were optimized. The 3(sigma) detection limits were 410 and 125 ng g^-1 for sulfite and thiosulfate, respectively. At 5 (mu)g g^-1 level in samples, the relative standard deviations (n=4) were 2.51 and 1.15% for sulfite and thiosulfate, respectively. The proposed method was applied to real samples containing input feeds, distillates and fuel oils from Abadan Petroleum Refinery in the south of Iran.