Determination of total sulphur in natural iron sulphides by potentiometric titration as lead(II) sulphate or as lead(II) potassium sulphate

Abstract

After oxidative decomposition of a sample of a sample of a natural iron sulphide with reversed aqua regia, sulphate and selenite were successively separated from all the cations using Bio-Rad AG 50W-X8 cation exchanger. Selenite was measured spectrophotometrically or determined by an atomic-absorption hydride generation technique.

The direct titration of sulphate was unsatisfactory because of interference from chloride. Therefore, to the combined effluents containing sulphuric acid together with interfering amounts of hydrochloric acid, enough sodium chloride was added to avoid loss of sulphuric acid and the solution was evaporated to a dry residue. The residue was dissolved in 50% ethanol, and the sulphate was titrated with lead perchlorate as lead(II) sulphate using a lead ion-selective lead electrode as an indicator.

During this work, it was observed that potassium interferes because of the formation of the double salt, K₂Pb(SO₄)₂. On studying this interference it was found that if the effluent containing the sulphuric acid was neutralised with KOH and saturated (with respect to potassium sulphate) with ethanol, the sulphate could be quantitatively titrated as the double salt, lead(II) potassium sulphate.