Direct determination of sulphate, sulphide, pyritic and organic sulphur in a single sample of coal by selective, step-wise oxidation with perchloric acid

Abstract

A procedure was developed for the direct determination of each of the sulphur forms in coal by using various strengths of perchloric acid as the selective oxidizing agent. Sulphate, sulphide, pyritic and organic sulphur were each determined sequentially on a single sample of coal. The oxidizing ability of perchloric acid was varied by changing the boiling point and thus the concentration of perchloric acid. Sulphate sulphur was extracted by boiling perchloric acid at 120 °C. Sulphide sulphur was simultaneously evolved as hydrogen sulphide, trapped in hydrogen peroxide, and converted to sulphate. Perchloric acid boiling at 155 °C was used to oxidize pyritic sulphur to sulphate and a sulphur-containing gas, which was also trapped in hydrogen peroxide. Organic sulphur was oxidized to sulphate and a sulphur-containing gas by boiling at 205 °C in a 9:1 solution of concentrated perchloric and concentrated phosphoric acids. The sulphur produced in each of the step-wise fractions was measured as sulphate. Total sulphur was determined by using boiling concentrated perchloric acid. The results were comparable with those obtained by the ASTM procedure. The relative standard deviations for sulphate, pyritic, organic and total recovered sulphur were 2.7, 3.4, 2.4 and 2.4%, respectively.