Simultaneous potentiometric determination of polysulphide and thiol sulphur in petroleum fractions

Abstract

A method is described for the simultaneous determination of polysulphide, disulphide and thiol sulphur in petroleum fractions. The method is based on the quantitative transformation of polysulphides and disulphides into thiosulphate and the selective potentiometric titration of thiosulphate and thiols using mercury(II) chloride as titrant with a sulphide-selective indicator electrode.

The accuracy and sensitivity of the method were established by titrating several synthetic standard samples containing both dimethyl disulphide in the range 2.12 × 10^–4-2.83 × 10^–6 g ml^–1 and ethanethiol in the range 2.80 × 10^–4-2.24 × 10^–6 g ml^–1. The results obtained, expressed in the form of a Gran plot, showed an error ranging from 1.41 to 4.95% by mass for the former and from 0.71 to 4.02% by mass for the latter.

The method was applied successfully to the simultaneous determination of di-and polysulphide and thiol sulphur in four petroleum fractions with a relative standard deviation ranging from 0.05 to 1.26%.