Dissolution of a test portion taken from the laboratory sample, previosly ground and sifted. Addition of formaldehyde to transform any ammonium ions present into hexamethylenetetramine and of the disodium salt of ethylenediamin tetra-acetic acid to complex any extraneous cations present which could cause excessive errors. These additions have no detrimental effect on the precision or reproduciblility of the method. Precipitation of the potassium by sodium tetraphenylborate in a weakly alkaline medium. Drying and weighing of the precipitate.