Overview
ISO 17319:2015 describes a gravimetric reference method for measuring the water‑soluble potassium content of fertilizer test solutions using the potassium tetraphenylborate (NaTPB) precipitation technique. The standard is intended for arbitration and reference laboratories and applies to fertilizers containing more than 1.0 % K2O (or equivalent K). It defines sample preparation (per ISO 5317), reagents, apparatus, the precipitation, filtration, drying and weighing steps, and requirements for blank and quality‑control tests.
Key topics and technical requirements
- Principle: Precipitation of K+ from an aliquot of the test solution by sodium tetraphenylborate in a weakly alkaline medium; EDTA (disodium salt) is added to eliminate interference from ammonium and other metal cations. The K‑tetraphenylborate precipitate is filtered, dried and weighed.
- Scope: Suitable for arbitration and reference purposes; applicable to fertilizers with >1.0 % K2O.
- Test solution: Prepared according to ISO 5317 (boil, dilute and filter); discard first 50 mL of filtrate.
- Sample treatment: If cyanamide or organic material is present, pre‑treat with bromine water and activated charcoal before precipitation.
- Reagents & apparatus: Sodium tetraphenylborate solution, EDTA disodium salt, NaOH, phenolphthalein, bromine water, activated charcoal; Gooch crucibles (porosity P10/P16), grade A glassware, drying oven at 120 °C ± 5 °C.
- Procedure highlights: Add EDTA and adjust to alkaline (phenolphthalein indicator), boil briefly, add NaTPB solution dropwise, allow to settle then decant and wash precipitate; dry crucible + precipitate and weigh (oven 120 °C ± 5 °C, 90 min).
- Quality control: Carry out blank tests and periodic checks using K2SO4 standard material.
- Precision: Repeatability limit r = 0.19 %; reproducibility is specified in Clause 8.2.3 of the standard.
Applications and who uses it
- Routine and reference laboratories performing fertilizer analysis where legally defensible or arbitration results are required.
- Fertilizer manufacturers for product specification, batch control and labeling verification.
- Regulatory bodies and inspection laboratories enforcing nutrient declarations and compliance.
- Research and standards organizations comparing methods or validating alternative analytical approaches.
- Suitable when a gravimetric (weighing) reference is preferred over volumetric/titrimetric techniques for robustness at higher potassium contents.
Related standards
- ISO 5317 - Preparation of the test solution for fertilizer potassium determination (normative reference).
- ISO 3696 - Water for analytical laboratory use (reagent water quality).
- Mentioned comparative methods: ISO 5310 (titrimetric method); previous gravimetric method ISO 5318 (withdrawn).
Keywords: ISO 17319:2015, potassium content, water‑soluble potassium, potassium tetraphenylborate gravimetric method, K2O, fertilizer analysis, gravimetric method, EDTA, sodium tetraphenylborate, laboratory quality control.
Open preview (PDF)Fertilizers and soil conditioners — Determination of water-soluble potassium content — Potassium tetraphenylborate gravimetric method
