Oxidimetric determination of thiocarbonate sulphur with chloramine-T, potassium ferricyanide and potassium permanganate

Abstract

Oxidimetric methods for determining the concentration of aqueous solutions of potassium thiocarbonate by using chloramine-T, potassium ferricyanide and potassium permanganate are discussed. The chloramine-T method is based on the reaction of potassium thiocarbonate with a known excess of chloramine-T in alkaline medium at 60° C and back-titration of the unreacted chloramine-T against a standardised solution of sodium thiosulphate, with starch as indicator. Twenty-four equivalents of the oxidant per mole of potassium thiocarbonate are consumed, showing that the three sulphur atoms of thiocarbonate are oxidised to sulphate. When ferricyanide is used twenty-four equivalents of the oxidant per mole of potassium thiocarbonate are also consumed in an alkaline medium at 60° C. However, in acidic medium, potassium permanganate is found to oxidise the three sulphur atoms of thiocarbonate to elemental sulphur. The following molar relationships are established—