Applicable to products the SO4 content of which is equal to or more than 0.0001 % (m/m). Heating of a test portion to dryness in the presence of sodium carbonate. Reduction of the sulphureous residue to sulphides by a mixture of hydriodic acid and hypophosphorus acid. Absorption of the hydrogen sulphide evolved in a mixture of sodium hydroxide solution and acetone. Titration with standard volumetric mercury(II) acetate solution in the presence of dithizone as indicator. A typical apparatus for reduction and distillation is shown in a figure.