ISO 5318:1983

Fertilizers — Determination of potassium content — Potassium tetraphenylborate gravimetric method (Reference method)

The method consists in preparing of a test solution as specified in ISO 5317, precipitating of potassium ions by sodium tetraphenylborate in a weakly alkaline medium in the presence of disodium ethylenediaminetetraacetate dihydrate (EDTA, disodium salt) and formaldehyde, followed by filtrating of the precipitate, drying and weighing.