Applicable to products having contents, expressed as chlorine, between 2 mg/kg and 50 mg/kg. For contents exceeding 50mg/kg the method specified in ISO 992 shall be used. The principle consists in quantitative displacement of the thiocyanate (SCN- ) ions of the mercury(II) thiocyanate by the cloride (Cl- ) 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.