ISO 17319 Fertilizers and soil conditioners — Determination of water-soluble potassium content — Potassium tetraphenylborate gravimetric method

ISO 17319:2015 specifies the determination of water-soluble potassium content in fertilizers and soil conditioners by the potassium tetraphenylborate gravimetric method. It was published on 15 May 2015.

The method precipitates potassium as potassium tetraphenylborate from the sample solution and weighs the precipitate. Interference from ammonium ions — the principal interference, since ammonium is omnipresent in nitrogen-containing fertilizers — is removed by heating with alkali under controlled conditions before precipitation, and other interfering cations are masked with EDTA.

The distinction between water-soluble and total potassium is the reason this standard exists as a separate document. Potassium declared on a label is normally water-soluble potassium, because that is the fraction available to the plant. A product can contain substantial potassium in forms that will not dissolve under field conditions, so measuring total potassium would overstate its fertilizing value.

Gravimetric methods of this type are slower and more demanding than instrumental alternatives, but they are the reference against which faster methods are calibrated. For high-value products, contract disputes and regulatory enforcement, the gravimetric result is the one that settles the question.