The method is applicable to products having a minimum KCl content of about 95 % (m/m). The principle consists in dissolution of a test portion taken from the previously ground and sifted laboratory sample and elimination of many ammonium ions present. Precipitation of the potassium, in a slightly acid medium, by an excess of an standard volumetric sodium tetraphenylborate solution. Filtration and back-titration of this excess in the filtrate, with a standard volumetric solution of a quaternary ammonium salt, using bromphenol blue as indicator. Replaces ISO Recommendation R 2052-1971.