Potentiometric determination of trace amounts of elemental sulphur in crude oil and petroleum fractions

Abstract

A potentiometric method is described for the determination of elemental sulphur in petroleum and petroleum fractions over all concentration ranges. The method involves the conversion of elemental sulphur into a homogeneous thiosulphate solution, followed by potentiometric titration using mercury(II) chloride as titrant with a sulphide-selective indicator electrode.

The accuracy and reproducibility of the method were established by titrating several synthetic standards dissolved in heptane in which the free sulphur, at levels ranging between 1.0 × 10^–3 and 1.0 × 10^–1 g, was converted quantitatively into homogeneous thiosulphate solutions containing from 4.0 × 10^–6 to 4.0 × 10^–4 g ml^–1 of elemental sulphur. The results obtained, expressed in the form of a Gran plot, showed an error ranging from 0.88 to 2.33% by mass. The range of concentrations of elemental sulphur in crude oil and eleven petroleum fractions ranged from 1.83 × 10^–3% by mass in heavy naphtha to 4.62 × 10^–5% by mass in hydrotreater kerosene, with a relative standard deviation ranging from 1.20 to 5.61%.