Applicable to products having contents, expressed as chlorine, between 2 mg/kg and 50 mg/kg. For contents exceeding 50 mg/kg the method specified in ISO 981 shall be used. The principle consists in the quantitative displacement of the thiocyanate ions of the mercury(II) thiocyanate by the cloride ions contained in the test portion. Reaction of the thiocyanate ions with iron(III) nitrate to form iron(III) thiocyanate (red). Photometric measurement of the colour at a wavelength of about 450 nm.