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 laboratory sample, previously ground and sifted. Addition of formaldehyde solution to transform any ammonium ions present into hexamethylenetetramine, and of ethylenedinitrilotetraacetic acid, disodium salt, to complex any extraneous cations present which could cause excess errors. These additions in no case have a detrimental effect on its precision or reproducibility. Precipitation of the potassium by sodium tetraphenylborate in a weakly alkaline medium, drying and weighing of the precipitate.