ISO 7058:1983

Phosphoric acid for industrial use — Determination of higher soluble sulfate contents — Titrimetric method after reduction

The sulphate present in a test portion is reduced to sulphide by a mixture of hydriodic and phosphinic (hypophosphorous) acids. The hydrogen sulphide evolved is absorbed in an excess of cadmium acetate solution. The cadmium sulphide formed is oxidized by an excess of standard volumetric potassium iodate/iodide solsution, and the excess iodine is titrated with standard volumetric sodium thiosulphate solution, using starch as indicator. The method is applicable to products having sulphate contents between 0.2 and 6 % (m/m).