Applicable to products having SO4 contents of more than 0.05 % (m/m). Covers the dissolution of the test portion in hydrochloric acid and evaporation in hydrochloric acid solution for drying. Dissolution of the residue in a hydrochloric acid solution. Measurement of the turbidity obtained by precipitation, under specified conditions, of barium sulphate using a spectrophotometer at a wavelength of about 470 nm or a photoelectric absorptiometer.