29—A SEMI-MICRO METHOD OF HIGH ACCURACY FOR THE DETERMINATION OF SULPHUR IN WOOL

Abstract

An accurate method, based on many hundred assays, is given for the determination of sulphur in samples of approximately 0·2 g of wool. It gives highly repeatable results, as evidenced by the small range of 3·21—3·32% sulphur with a standard error of 0·003%, for 61 determinations on a Merino top and the quantitative recovery of wool sulphur in the presence of potassium sulphate. The analytical procedure involves the wet oxidation of the sulphur to sulphuric acid by a nitric–perchloric acid mixture, and the subsequent precipitation and weighing of barium sulphate. The use of potassium dichromate as a catalyst for the oxidation, and as an indicator for its end point is discussed. The rate of the oxidation and the quantity of nitric acid required are critical as slow oxidations produce low results. Excess perchloric acid or potassium dichromate interferes with the precipitation of barium sulphate through solubility and co-precipitation effects. The methods for addition of barium chloride, aggregation and filtration of the precipitate, and the prevention of co-precipitation effects are fully discussed.