Accuracy and sensitivity of the potentiometric determination of total sulphur in petroleum

Abstract

A potentiometric method is described for the determination of total sulphur in crude oil and its fractions over all concentration ranges. The method involves the quantitative conversion of sulphur to sulphate using an oxygen flask method followed by potentiometric titration using lead nitrate as titrant with a lead-selective indicator electrode. The sensitivity, accuracy and reproducibility of the method were investigated by analysing a series of standard sodium sulphate solutions and synthetic standards. The limits of detection ranged from 3.2 × 10–4 to 1.6 × 10–6 g ml–1. The results were plotted on Gran plots and showed an error ranging from 0.51 to 3.0%, while the relative standard deviation did not exceed 3.13%. The method was applied successfully to the determination of total sulphur in crude oil and its fractions, before and after treatment, in addition to premium and super gasoline with and without tetraethyllead (TEL). The results showed that the amount of sulphur present in these fractions ranged from 0.035% m/m in super gasoline with TEL to 0.73% m/m in gas oil, and amounted to 1.99% m/m in crude oil with a relative standard deviation of between 0.9 and 5.17%.